Geologic Map of the Denver West 30' x 60' Quadrangle, North-Central Colorado

Full text

Geologic Map of the
Denver West 30’ x 60’ Quadrangle,
North-Central Colorado
By Karl S. Kellogg, Ralph R. Shroba, Bruce Bryant, and Wayne R. Premo
Pamphlet to accompany
Scientific Investigations Map 3000
U.S. Department of the Interior
U.S. Geological Survey

U.S. Department of the Interior
DIRK KEMPTHORNE, Secretary
U.S. Geological Survey
Mark D. Myers, Director
U.S. Geological Survey, Reston, Virginia: 2008
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Kellogg, K.S., Shroba, R.R., Bryant, Bruce, and Premo, W.R., 2008, Geologic map of the Denver West 30’ x
60’ quadrangle, north-central Colorado: U.S. Geological Survey Scientific Investigations Map 3000, scale
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Contents
Description of Map Units ............................................................................................................................ 1
Geologic History of the Denver West Quadrangle ............................................................................... 25
Precambrian History ......................................................................................................................... 25
Paleozoic and Pre-Laramide Mesozoic History .......................................................................... 27
The Laramide Orogeny ..................................................................................................................... 27
The Colorado Mineral Belt .............................................................................................................. 28
Post-Laramide Cenozoic History of the Front Range .................................................................. 28
Paleogene ................................................................................................................................. 28
Neogene .................................................................................................................................... 29
Quaternary ................................................................................................................................ 30
Potential Geologic Hazards ............................................................................................................ 33
Mass Movement ...................................................................................................................... 33
Expansive Soils and Bedrock and Heaving Bedrock ......................................................... 34
Compactable and Compressible Soils .................................................................................. 34
Floods ........................................................................................................................................ 34
Abandoned Mines ................................................................................................................... 35
Seismicity .................................................................................................................................. 35
Radon ........................................................................................................................................ 35
Snow Avalanches .................................................................................................................... 35
Acknowledgments ..................................................................................................................................... 36
References Cited ....................................................................................................................................... 36
Table
1. Estimated age ranges and correlation of alluvial deposits ..................................................... 32
iii

iv
Conversion
____________
To convert
____________
centimeters (cm)
meter (m)
kilometer (km)
____________
Divisions of
Factors
____________
____________
____________
Quaternary
______________
Multiply by
______________
0.39
3.281
0.6214
______________
and Neogene
___________
To obtain
___________
inches (in.)
foot (ft)
mile (mi)
___________
Time Used
__
__
__
in
_____
_____
_____
This
________
________
________
Report
Period or
Subperiod
Epoch Age
Quaternary
Holocene
Pleistocene
late
middle
early
0–11.5 ka
11.5–132 ka
132–788 ka
788 ka–1.81 Ma
Neogene
Pliocene
Miocene
1.81–5.32 Ma
5.32–23.8 Ma
From Hansen (1991) with these exceptions: 11.5 (Holocene–late Pleistocene boundary from U.S.
Geological Survey Geologic Names Committee (2007); 132 ka (late-middle Pleistocene boundary)
and 788 ka (middle-early Pleistocene boundary) from Richmond and Fullerton (1986a); 1.81 Ma
(Pleistocene-Pliocene boundary) from Lourens and others (1996); 5.32 Ma (Pliocene-Miocene
boundary) and 23.8 (Miocene-Oligocene boundary) from Berggren and others (1995). Ages expressed
in ka for kilo-annum (thousand years) and Ma for mega-annum (million years).

DESCRIPTION OF MAP UNITS
SURFICIAL DEPOSITS
[Surficial deposits in the Denver West quadrangle record alluvial, mass-movement, glacial, and eolian
processes in the central part of the Front Range and the western margin of the Colorado Piedmont during
the Quaternary and late Neogene. Many of the surficial deposits are poorly exposed. Deposits that are of
limited extent (less than about 200 m wide) were not mapped, including (1) fill material in urban areas, (2)
mine-and-mill waste and dredge tailings in mining areas, (3) thin mass-movement deposits above present
treeline (such as block fields and block streams), and (4) thin sheetwash deposits that locally mantle gently
sloping map units.
Descriptions of surficial units on this map are based chiefly on information in maps and reports by
Birkeland and others (2003), Dethier and others (2003), Gable and Madole (1976), Lindsey and others
(2005), Lindvall (1978, 1979, 1980), Machette (1977), Machette and others (1976), Madole (1986, 1991a),
Madole and others (1998, 2005), Madole and Shroba (1979), Miller (1979), Moore and others (2001),
Muhs and others (1996, 1999), Scott (1960, 1962, 1963a, 1972, 1975), Shroba (1977, 1980, 1982), Shroba
and Birkeland (1983), Shroba and Carrara (1996), Trimble and Machette (1979), Van Horn (1972, 1976),
and Wells (1967). Crosby (1978) mapped some of the landslide deposits (Qls) depicted in the Colorado
Piedmont.
Most of the surficial units on this map are informal allostratigraphic units (discontinuity-bound
sequences) of the North American Stratigraphic Code (North American Commission on Stratigraphic
Nomenclature, 1983), whereas the other map units (“bedrock units”) are informal or formal lithostrati-
graphic units. Subdivisions of map units use time terms “late” and “early” where applied to surficial units,
but use position terms “upper” and “lower” where applied to lithostratigraphic units. Formal names for
fluvial and pediment deposits east of the mountain front are those established by Scott (1960, 1963a) for
the Colorado Piedmont. Informal names (such as young stream-terrace alluvium, Qg1, and lower pedi-
ment deposits, Qp1) are applied to deposits of similar origin near Fraser and in other valleys west of the
mountain front.
The mapped distribution of surficial units on the Berthoud Pass, East Portal, Empire, Fraser, Grays
Peak, Harris Park, Meridian Hill, Montezuma, Mount Evans, Nederland, and parts of the Bottle Pass,
Central City, Eldorado Springs, Golden, and Louisville quadrangles is based primarily on interpretation of
1:40,000-scale, color-infrared aerial photographs taken in 1988, 1990, and 1992.
Age assignments for surficial deposits within the map area are based chiefly on the relative degree
of modification of their original surface morphology, relative heights above present stream channels, and
degree of soil development and clast weathering. Soil-horizon designations are based on those of the Soil
Survey Staff (1999) and Birkeland (1999). Some of the surficial deposits contain secondary calcium car-
bonate of pedogenic (soil) origin. Stages of secondary calcium carbonate morphology (referred to as stages
I through IV) in Bk and K soil horizons are from Gile and others (1966) and Machette (1985).
Grain or particle sizes of surficial deposits are field estimates. Size limits for sand (0.05–2 mm),
silt (0.002–0.05 mm), and clay (<0.002 mm) are those of the Soil Survey Staff (1951). In descriptions of
surficial map units, the term “clasts” refers to granules and larger particles (>2 mm in diameter), whereas
the term “matrix” refers to sand and finer particles (≤2 mm in diameter). In descriptions of clast composi-
tion of surficial map units, the terms “granite” and “granitic” refer chiefly to various igneous or meta-igne-
ous rock types that are holocrystalline and felsic to intermediate in composition. The terms “gneiss” and
“gneissic” refer chiefly to quartz-feldspar gneiss, biotite gneiss, hornblende gneiss, and amphibolite.
In this report, the terms “alluvium” and “alluvial” refer to material transported by running water con-
fined to channels (stream alluvium) as well as by running water not confined to channels (sheetwash). The
term “colluvium” refers to all rock and sediment transported downslope chiefly by gravity (Hilgard, 1892;
Merrill, 1897). Colluvial material on slopes is transported chiefly by mass-movement (gravity-driven) pro-
cesses—such as creep, debris flow, and rock fall, locally aided by running water not confined to channels
(sheetwash).
Surficial map units that include debris-flow deposits probably also include hyperconcentrated-flow
deposits. These latter deposits are intermediate in character between stream-flow and debris-flow deposits
(Pierson and Costa, 1987; Meyer and Wells, 1997).
The term “mountain front” refers to the steep, eastward-sloping topography formed chiefly on the
easternmost exposures of Precambrian rock. The term “hogback belt” refers to prominent ridges, paral-
lel to and just east of the mountain front, that are composed of steeply tilted, resistant sedimentary strata

2 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado
(mostly rocks of the Dakota Group) that once covered the crystalline core of the Front Range (Hansen and
Crosby, 1982).
In much of the report the terms “soil” and “soils” refer to pedogenic soils formed in surficial deposits
(for example, Birkeland, 1999). However, in the section “Potential Geologic Hazards,” these terms are
used in the engineering sense (surficial deposits as well as the pedogenic soils formed in various surficial
deposits).
Oxygen isotope (δ18O) records in marine cores provide an index of global climate [as expressed in
global ice volume (for example, Shackleton and Opdyke, 1973, 1976)], and were used to estimate the ages
of the some of the older alluvial deposits in the Colorado Piedmont. Oxygen isotope variation in marine
cores may lag 500–3,000 years behind the corresponding changes in ice volume on the continents (Mix and
Ruddiman, 1984)]
Artificial fill deposits
af Artificial fill deposits (latest Holocene)—Compacted fill material composed mainly of silt,
sand, rock fragments, and, locally, trash. Mapped in an active landfill, about 8 km north
of Golden on east side of U.S. Highway 93, and in inactive landfills north of Interstate
70, about 3 km southeast of Golden, and east of Marshall Lake, about 4 km southeast of
Boulder. These landfills contain organic trash (such as plastic products and vegetation)
and inorganic trash (such as metal and concrete). Lateral extent of active landfill north
of Golden may change due to continued filling. Deposits of artificial fill (chiefly mine
waste from the Henderson mine) are also mapped in the valleys of Woods Creek and
West Fork Clear Creek, a few kilometers southwest of Berthoud Pass. Smaller deposits
of artificial fill in areas where land surface was modified by earth-moving equipment
are not mapped. Estimated thickness a few meters to more than 10 m
Alluvial deposits
Qa Post-Piney Creek alluvium and Piney Creek Alluvium, undivided (Holocene)—Mostly
sand and gravel along major streams in piedmont that head in mountains, and mostly
sand and locally sandy silt and silty clay along tributary streams that head in piedmont
east of mountain front. Locally cobbly and slightly bouldery along major streams near
mountain front and sandy and silty in mountain meadows. Post-Piney Creek alluvium
commonly lies within stream channels cut in Piney Creek Alluvium and consists chiefly
of silty to pebbly sand and pebble gravel that is subject to periodic stream flooding.
Piney Creek Alluvium commonly consists of sandy silt, silty sand, lenses of silty clay,
and detrital organic matter in upper part, and mostly sand and lenses of pebble gravel in
lower part. Forms one or two terraces less than 5 m above present streams, and locally
forms alluvial fans and sheetwash aprons along margins of major valleys. In valleys of
major streams that head in glaciated drainages, Piney Creek Alluvium forms much of
flood plain where it overlies Broadway Alluvium (Qb).
Grain size and content of organic matter in upper part of Piney Creek Alluvium
suggests that it was eroded from soils formed in loess (Qlo) and other fine-grained
deposits. Soils formed in upper part of post-Piney Creek alluvium have A/Cu profiles.
Soils formed in upper part of Piney Creek Alluvium are better developed and have A/
Bw/C profiles where formed in sand and A/Bw/Bk profiles with stage I carbonate mor-
phology where formed in finer grained sediments. Some of the soil A horizons formed
in Piney Creek Alluvium are over-thickened (cumulic) where A-horizon formation kept
pace with sedimentation.
Radiocarbon (14C) dating of organic material helps to constrain ages of post-
Piney Creek alluvium and Piney Creek Alluvium. Wood from logs in post-Piney Creek
alluvium near Fort Lupton, Colo., yielded radiocarbon ages of a few hundred years
(Lindsey and others, 1998), and charcoal at an archeological site (stratigraphic position
not specified) near Kassler, Colo., just south of map area, yielded an age of about 1,500
14C yr B.P. (carbon-14 years before present; Scott, 1962, 1963a). Radiocarbon ages of
organic material in Piney Creek Alluvium near Colorado Springs range from about 1.1
to 2.2 ka (Madole and others, 2005). Gravelly alluvium overlying Broadway Alluvium
near Longmont, Colo., was deposited between 1,900 and 3,900 14C years ago (Madole,

1976). Unit may locally contain pre-Piney Creek alluvium (Scott, 1960, 1962, 1963a).
Low-lying deposits are prone to periodic stream flooding. Thickness about 1.5–6 m
Qva Valley-floor alluvium (Holocene and late Pleistocene)—Mostly poorly sorted coarse sand
and pebbly to bouldery cobble gravel in stream channels, flood plains, and low terraces
in mountain valleys. Flood-plain and terrace deposits of Holocene age are commonly
about 1.3 m or less above present streams (Madole, 1996). Unit locally includes a
minor amount of young stream-terrace alluvium (Qg1), colluvium (Qc) and other
mass-movement deposits, fan deposits (Qf), and glacial deposits (Qtp and Qti) along
valley margins and organic-rich sediments in bogs, marshes, and meadows. Locally
may include minor lake sediments. Top of unit probably consists mostly of poorly
sorted sand and gravel in nonglaciated valleys. Unit is commonly less than 5 m above
the Fraser River and tributary streams near Fraser and Tabernash, Colo., and commonly
less than 5 m above present streams elsewhere within map area. Low-lying deposits are
prone to periodic flooding. Estimated thickness 5–10 m
Qb Broadway Alluvium (late Pleistocene)—Cobbly gravel along major streams in piedmont
near hogback belt; mostly pebble gravel and pebbly sand interbedded with sandy silt
along South Platte River; and sand to sandy silt along tributary streams. Uppermost
0.3–1 m of unit is commonly clayey silt to silty sand. Soils formed in upper part of unit
have A/Bw/Cox profiles where formed in coarse sand and A/Bt/Bk profiles with stage
I carbonate morphology where formed in finer grained sediments. Deposits of unit Qb
underlie unit Qa in flood plains along major streams and terraces about 8–12 m above
major present streams near hogback belt. Small deposits of unit Qb in flood plains of
major streams are exposed in gravel pits. Broadway Alluvium was deposited during
the Pinedale glaciation (Bryan and Ray, 1940; Hunt, 1954; Scott, 1960, 1975; Brad-
ley, 1987; Madole, 1991a, 1995), about 30–12 ka (Nelson and others, 1979; Madole,
1986; Schildgen and others, 2002; Benson and others, 2004, 2005). Upper part of the
Broadway probably was deposited after Pinedale glaciers reached their maximum extent
about 21 ka (Madole, 1986), and ceased deposition by about 11–10 ka (Holliday, 1987).
Broadway Alluvium, on east side of South Platte River in and near downtown Denver,
overlies terrace deposits composed of Louviers Alluvium (Qlv). The presence of unit
Qlv beneath unit Qb is indicated by a soil buried by unit Qb that (1) has a clayey,
reddish-brown B (argillic?) horizon formed in overbank(?) sediments and (2) overlies a
carbonate-enriched (Bk?) horizon formed in underlying cobbly gravel (Hunt, 1954, p.
101). The buried soil is similar to the surface soil formed in Louviers Alluvium. Unit
may locally include small deposits of pre-Piney Creek alluvium (Scott, 1960, 1962,
1963a). Unit Qb is a source of coarse aggregate. Thickness commonly about 1–9 m; as
much as 9 m in flood plain of South Platte River near downtown Denver; as much as 18
m in flood plain of Clear Creek east of Golden; possibly as much as 20 m beneath ter-
race on east side of South Platte River in and near downtown Denver
Qg1 Young stream-terrace alluvium (late Pleistocene)—Glacial outwash composed chiefly
of cobbly pebble gravel that underlies one terrace (locally two) along streams in major
mountain valleys near and below lower limit of glaciation and fluvial deposits of
nonglacial origin along Pole Creek near Tabernash. Amount and size of small boul-
ders (commonly 30–50 cm in diameter) and cobbles in deposits decrease downstream
from ice-contact deposits. Pebbly sand or silty overbank sediments less than about 50
cm thick locally overlie the gravel. Fluvial deposits of nonglacial origin north of Pole
Creek, about 1 km southwest of Tabernash, consist of slightly cobbly pebble gravel
overlain by about 35 cm of slightly pebbly, sandy, clayey silt overbank sediment.
Deposits of unit Qg1 in glaciated valleys are composed of gravelly glacial outwash
deposited during the Pinedale glaciation, about 30–12 ka (Nelson and others, 1979;
Madole, 1986; Benson and others, 2004, 2005). Cosmogenic dating (based on mea-
surement of isotopes 10Be and 26Al) of Pinedale outwash, 4–14 m above stream level
in Boulder Canyon east of Nederland, yielded exposure ages of 32–10 ka (Schildgen
and others, 2002). Unit underlies terrace treads commonly about 6 and 12 m above the
Fraser River between Winter Park and Fraser, about 5 m above the Fraser River near
Tabernash, about 5–9 m above streams tributary to the Fraser River, and commonly less
Description of Map Units 3

than 12 m above stream level elsewhere in map area. Unit locally may include a minor
amount of valley-floor alluvium (Qva). Along Swan River, near southwest corner of
map area, unit Qg1 includes extensive dredge tailings produced by placer mining during
the late 1880s. Unit Qg1 is a source of coarse aggregate. Estimated thickness 3–20 m
Qlv Louviers Alluvium (late and middle Pleistocene)—Pebbly coarse sand, cobbly gravel, and
lenticular masses of silt and clay along major streams in the piedmont; commonly sandy
to clayey alluvium along tributary streams. Uppermost 0.3–1 m of unit is commonly
clayey to pebbly silt. Some of silt and clay within upper part of unit may be of eolian
origin (Reheis, 1980). Soils formed in upper part of unit have A/Bt/Bk profiles with
stage II–III carbonate morphology. Deposits underlie terraces about 12–20 m above
major present streams near hogback belt and about 21–24 m above South Platte River
south of Denver. Farther downstream, unit Qlv underlies terrace deposits of unit Qb
on east side of South Platte River in and near downtown Denver (Hunt, 1954, p. 101).
Louviers Alluvium was deposited during the Bull Lake glaciation (Scott, 1972, 1975;
Bradley, 1987; Madole, 1991a), about 170–120 ka (Schildgen and others, 2002; Sharp
and others, 2003; Pierce, 2004). Unit Qlv is a source of coarse aggregate. Thickness
commonly about 3–7.5 m; at least 4.5 m thick below unit Qb on east side of South
Platte River in and near downtown Denver; about 6–14 m along Clear Creek between
Golden and valley of South Platte River
Qg2 Intermediate stream-terrace alluvium (late and middle Pleistocene)—Glacial outwash
composed chiefly of cobbly pebble gravel that underlies one terrace (locally two) along
Fraser River below lower limit of glaciation and fluvial deposits of nonglacial origin
along Crooked Creek near Tabernash. Deposits near lower limit of glaciation locally
contain small boulders about 30–35 cm in diameter, and rarely as large as 70–120 cm in
diameter. Large pebbles and cobbles commonly are subangular to subrounded granite
and gneiss; some are rounded to well rounded. Pebbly sand or sandy silt overbank
sediments about 1–3 m thick locally overlie the gravel. Fluvial deposits of nonglacial
origin along Crooked Creek, about 3 km southwest of Tabernash, consist of slightly
cobbly pebble gravel overlain by about 2.5–3 m of slightly pebbly and cobbly, sandy,
clayey silt overbank sediments that contain lenses of cobbly pebble gravel. The soil
formed in pebbly alluvium beneath the 24-m-high terrace west of Fraser has an argillic
B (Bt) horizon 35 cm thick. The proximity of till of the Bull Lake glaciation (Qtb) to
outwash terraces underlain by unit Qg2 suggests that unit Qg2 was deposited by glacial
meltwater during the Bull Lake glaciation (about 170–120 ka; Sharp and others, 2003;
Pierce, 2004). Recent cosmogenic dating (on 10Be and 26Al) of Bull Lake outwash, 16
m above stream level in Boulder Canyon east of Nederland, yielded an exposure age
of 130 ka (Schildgen and others, 2002). Unit underlies terrace treads commonly about
24 m above the Fraser River near Winter Park and Fraser, about 12 and 18 m above the
Fraser River east of Tabernash, and about 18–24 m above streams tributary to the Fraser
River. About 15 km downstream of Tabernash at Granby, Colo., outwash of Bull Lake
age underlies a terrace tread about 12 m above the Fraser River (Meierding, 1977). Unit
Qg2 is a source of coarse aggregate. Estimated thickness 3–20 m
Qs Slocum Alluvium (middle Pleistocene)—Deposited in piedmont east of mountain front
where it consists of silty sand and sandy to clayey silt in upper part and mostly pebbly
to cobbly gravel interbedded with pebbly to clayey silt in lower part. Some of the silt
and clay within upper part of unit may be of eolian origin (Reheis, 1980). Soils formed
in upper part of unit have A/Bt/Bk/K/Bk profiles with stage II–III carbonate morphol-
ogy. Amount of cobbles and boulders in lower part of unit decreases with increasing
distance east of mountain front. Unit Qs consists of terrace, pediment, and valley-fill
deposits locally at two levels, about 24–30 and 37–45 m above major streams near the
hogback belt. The Slocum is considered to be about 240 ka (Madole, 1991a), based on
a uranium-series age of 190±50 ka near Canon City, Colo. (Szabo, 1980). Based on
tentative correlation with marine oxygen isotope stages and a nonlinear rate of stream
incision in the Denver area since the deposition of the 640-ka Lava Creek B tephra,
younger deposits of unit Qs may have accumulated between about 300 and 220 ka, and
older deposits may have accumulated between about 390 and 320 ka (see discussion in
4 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

section, “Post-Laramide Cenozoic History of the Front Range”). Thickness commonly
about 3–6 m; locally as much as 12 m
Qg3 Old stream-terrace alluvium (middle and early? Pleistocene)—Glacial outwash and,
locally, fluvial deposits of nonglacial origin, composed chiefly of cobbly pebble gravel,
that underlie terrace remnants at five levels above the Fraser River, and at two levels
above Ranch Creek near Fraser. Pebbly sand or clayey silt overbank sediments about
1–1.5 m thick locally overlie the gravel. Large pebbles and cobbles commonly are
subangular to subrounded granite and gneiss. Deposits locally contain a few boulders
commonly as large as 30–50 cm in diameter near ice-contact deposits. Deposits beneath
one of the higher terrace treads may be equivalent in age to fluvial deposits beneath a
60-m-high terrace near Clark, Colo.; this terrace (about 125 km northwest of Fraser)
has a water-laid deposit of 640-ka Lava Creek B tephra at the top of the fluvial depos-
its (Madole, 1991b, c). Unit underlies treads of terrace remnants about 30, 50–55, 67,
85–90, and 120 m above the Fraser River, and about 55 and 67 m above Ranch Creek
near Fraser. The fluvial deposit about 120 m above the Fraser River is of nonglacial
origin, and those about 67 m above the Fraser River and Ranch Creek may also be of
nonglacial origin. Outwash deposits and probably deposits of nonglacial origin of unit
Qg3 were deposited during multiple pre-Bull Lake glaciations. Estimated thickness
3–16 m
Qv Verdos Alluvium (middle Pleistocene)—Unit was deposited in piedmont east of mountain
front where it consists of pebbly, sandy to clayey silt in upper part and mostly cobbly
gravel interbedded with pebbly sand and finer sediment in lower part. Some of the silt
and clay within upper part of unit may be of eolian origin (Reheis, 1980). Soils formed
in upper part of unit have A/Bt/Bk/K/Bk profiles with stage II–III and locally stage IV
carbonate morphology. Amount of cobbles and boulders in lower part of unit decreases
with increasing distance east of mountain front. Unit consists of younger (topographi-
cally lower) and older (higher) terrace, pediment, and valley-fill deposits locally at two
levels, about 55–60 and 65–75 m above major streams near hogback belt. Beds and
lenses of water-laid Lava Creek B tephra (about 640 ka; Lanphere and others, 2002)
are present at several localities from Golden southward to southern boundary of map
area. The tephra locally occurs at the base of older pediment deposits of unit Qv (Scott,
1963a), but is also present within or at top of older main-stream fluvial deposits of unit
Qv (Hunt, 1954; Scott, 1972; Baker, 1973; Van Horn, 1976; Kirkham, 1977; Bradley,
1987). Water-laid Lava Creek B tephra is locally exposed beneath terraces about 61
m above Clear Creek at Golden (Baker, 1973; Van Horn, 1976) and about 65 m above
Bear Creek near its confluence with the South Platte River in Denver (Hunt, 1954). Age
and stratigraphic position of the Lava Creek B tephra indicate that older deposits of
unit Qv are about 640 ka, possibly about 675–610 ka. Based on tentative correlation
with marine oxygen isotope stages and a nonlinear rate of stream incision in the Denver
area since the deposition of the 640-ka Lava Creek B tephra, younger deposits of unit
Qv may have accumulated between about 475 and 410 ka (see discussion in section,
“Post-Laramide Cenozoic History of the Front Range”). Unit Qv is a source of coarse
aggregate. Thickness about 4.5–6 m; locally as much as 12 m
Qg4 Oldest stream-terrace alluvium (middle or early Pleistocene)—Chiefly slightly bouldery,
cobbly pebble gravel that underlies terrace remnants about 145 m above the Fraser River
southeast of Fraser. Gravel deposits locally contain (1) lenses of granule-rich, medium
to very coarse sand, (2) a minor amount of boulders about 30–120 cm in diameter com-
posed of granite, pegmatite, and sandstone probably from the Dakota Group (Kd), and
(3) a few rounded rhyolite tuff (bt) and red siltstone pebbles. Granitic clasts commonly
weathered. Matrix is poorly sorted—mostly coarse to very coarse sand that contains
abundant granules. Large pebbles and cobbles commonly are subangular to subrounded
granite and gneiss as well as a minor amount of subangular pegmatite and sandstone
probably from the Dakota Group. Cobbles are commonly as large as 25 cm in diameter.
Unit probably is glacial outwash deposited during one or more pre-Bull Lake glacia-
tions. Estimated thickness 2–18 m
Description of Map Units 5

QNr Rocky Flats Alluvium (early Pleistocene and late Pliocene?)—Mostly quartzite-rich
cobble and pebble gravel that contains beds and lenses of pebbly sand, silty sand, and
pebbly sandy clay at its type area on Rocky Flats about 9 km south of Boulder; mostly
gravelly alluvium interbedded with pebbly silt and clay elsewhere in piedmont portion
of map area. Some of the silty and clayey sediment mantling or within upper part of
unit may be of eolian origin (Reheis, 1980; Shroba and Carrara, 1996). At Rocky Flats,
upper part (typically ≤5 m) of unit forms an alluvial fan that locally buries a hogback
composed of Fox Hills Sandstone (Kf), and lower part of unit (≤25 m) fills small basins
and paleovalleys that are cut in Cretaceous sedimentary rocks. Paleovalleys radiate
eastward away from apex of fan (Knepper, 2005; Lindsey and others, 2005). Elsewhere
in map area, unit forms pediment and valley-fill deposits. At its type section (Scott,
1960), the soil formed in upper part of unit has an A/Bt/K profile with stage III–IV
carbonate morphology. This soil directly overlies a buried soil that has a Btk/K/Btk pro-
file with stage I and stage III carbonate morphology. Amount of cobbles and boulders
in unit decreases with increasing distance east of mountain front. Unit consists of fan
deposits and valley-fill deposits that mantle a dissected landscape at Rocky Flats (Knep-
per, 2005); elsewhere in map area consists of relatively thin pediment and valley-fill
deposits. Soil properties and paleomagnetic data suggest that the Rocky Flats Alluvium
is at least 1.6–1.4 Ma (Birkeland and others, 1996) and could be about 2 Ma (Birkeland
and others, 2003). Cosmogenic dating (on 10Be) indicates that upper part of the Rocky
Flats Alluvium at Rocky Flats is about 1.5 Ma (Dethier and others, 2001); buried soils
formed in upper part of unit suggest that the underlying valley-fill deposits in lower part
of unit could be much older. Recent cosmogenic dating (on 10Be and 26Al) suggests that
incision of eastern part of Rocky Flats surface began about 2–1 Ma (Riihimaki and oth-
ers, 2006). A small area of Rocky Flats surface, which extends northeastward from the
mouth of Coal Creek Canyon (red circle pattern), displays a braided pattern produced
by subdued gravel bars and shallow channels. These features may have been produced
by a major flash flood on Coal Creek that flowed across the Rocky Flats surface. Age
of flash flood is inferred to predate the Verdos Alluvium (Qv), which is inset below
Rocky Flats surface along Coal Creek. Top of Rocky Flats surface is about 140–230 m
above most major streams near mountain front. Unit QNr is a source of coarse aggre-
gate. Thickness at Rocky Flats commonly less than 6 m but locally greater than 40 m
(Knepper, 2005); elsewhere in map area commonly about 3–6 m thick
QNpr Pre-Rocky Flats alluvium or debris-flow deposits (early Pleistocene or late Pliocene)—
Three small deposits along eastern margin of mountain front a few kilometers north
and south of mouth of Coal Creek Canyon consist chiefly of bouldery sand and gravel
and clayey sand. Boulders are as large as 2 m in diameter. Unit includes alluvial-fan
and channel-fill deposits. Top of unit is about 15–25 m above top of the Rocky Flats
Alluvium and about 225–235 m above South Boulder Creek. Unit could be as old as
Pliocene (Malde, 1955) and may be temporally equivalent, in part, to the Nussbaum
Alluvium (about 3 Ma) along the South Platte River in northeastern Colorado (Scott,
1978, 1982). Thickness about 3–4.5 m
Alluvial and mass-movement deposits
Qac Alluvium and colluvium, undivided (Holocene to middle? Pleistocene)—Chiefly undif-
ferentiated valley-floor alluvium (Qva), fan deposits (Qf), debris-flow deposits (Qdf),
colluvium (Qc), and other mass-movement deposits along minor streams and on adja-
cent lower slopes. Low-lying areas of unit adjacent to stream channels may be subject
to periodic stream flooding and debris-flow deposition. Estimated thickness 3–15 m
Qf Fan deposits (Holocene and late Pleistocene)—Mostly poorly sorted, slightly bouldery
pebble and cobble gravel and locally pebbly and cobbly silty sand. Deposited chiefly
by streams and debris flows in fan-shaped accumulations near base of moderate to
steep slopes. Unit locally includes sheetwash deposits, colluvium (Qc), and probably
hyperconcentrated-flow deposits. Near valley floors, areas underlain by map unit may
be subject to stream flooding and debris-flow deposition. Estimated thickness 3–15 m
6 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

Qp1 Lower pediment deposits (late Pleistocene)—Crudely stratified, matrix-supported small
boulders to granules in a sandy, clayey silt matrix that overlies northeast-sloping sur-
face formed on Troublesome Formation (Nbt) west of Crooked Creek southwest of
Tabernash. Matrix makes up much of unit and is chiefly derived from siltstone of the
Troublesome Formation. Unit probably deposited chiefly by debris flows and ephem-
eral streams under periglacial conditions during the Pinedale glaciation. Large pebbles
and cobbles commonly are angular to subangular and consist of sandstone probably
from the Dakota Group (Kd), gneiss, and pegmatite. Sandstone boulders are commonly
as long as 50 cm. Lower limit of unit is about 5 m above Crooked Creek. Estimated
thickness 2–5 m
Qp2 Higher pediment deposits (late and middle Pleistocene)—Crudely stratified, matrix-
supported small boulders to granules in a sandy, clayey silt matrix that overlies a north-
east-sloping surface formed on Troublesome Formation (Nbt) west of Crooked Creek
southwest of Tabernash. Matrix makes up much of unit and is chiefly derived from silt-
stone of the Troublesome Formation. Unit probably deposited chiefly by debris flows
and ephemeral streams under periglacial conditions during the Bull Lake glaciation.
Large pebbles and cobbles commonly are angular to subangular and consist of sand-
stone probably from the Dakota Group (Kd), gneiss, pegmatite, and, locally, a minor
amount of volcanic porphyry. Boulders composed of sandstone and, locally, volcanic
porphyry are commonly as long as 40 cm. Lower limit of unit is about 12 m above
Crooked Creek. Eastern margin of unit locally includes cobbly fluvial gravel (Qg2).
Estimated thickness 2–5 m
Qds Debris-flow deposits and Slocum Alluvium, undivided (middle Pleistocene)—Debris-
flow deposits, probably hyperconcentrated-flow deposits, and minor stream alluvium at
two or three levels near mouths of steep canyons along eastern margin of mountain front
north of Eldorado Springs. Lower limits of these deposits are about 15–25 m above
South Boulder Creek. Debris-flow deposits consist chiefly of nonsorted and nonstrati-
fied boulders to granules supported in sandy matrix. Clasts are angular to subangular
and commonly are randomly oriented. Boulders greater than 1 m in length are more
common in upper part of unit. Boulders composed of sandstone and conglomerate
of the Fountain Formation (Phf) on or near top of unit are as large as 3.1x7.0x8.1 m.
Hyperconcentrated-flow deposits and stream alluvium are locally present in eastern
(distal) part of unit. These deposits consist of poorly sorted and poorly stratified, grav-
elly sand and sandy gravel. Estimated thickness 3–8 m
QNdi Diamicton (middle? Pleistocene to Pliocene?)—Poorly exposed bouldery deposits that (1)
overlie the Troublesome Formation (Nbt) and, locally, rhyolite tuff (bt) on high ridges
beyond outer limits of tills of the Pinedale and Bull Lake glaciations a few kilometers
southeast, south, and southwest of Fraser and (2) mantle slopes above upper limit of
glacial ice on northwest flank of Saxon Mountain about 4 km northeast of Georgetown.
Deposits near Fraser consist of (1) glacial outwash(?) deposited by the Fraser
River, Elk Creek, and St Louis Creek, (2) till-like deposits probably deposited during
one or more pre-Bull lake glaciations, and (3) mass-movement deposits having clayey-
silt matrix on slopes steeper than about 6°. Glacial outwash(?) is poorly sorted, granite-
rich, matrix-supported and clast-supported, slightly bouldery, cobbly pebble gravel.
Deposits probably contain lenses of pebble gravel and sand. Matrix is mostly coarse to
very coarse sand that contains abundant granules. Large pebbles and cobbles commonly
are subangular to subrounded granite and gneiss. Boulders composed of granite, gneiss,
pegmatite, and sandstone probably from the Dakota Group (Kd) are commonly as large
as 40–90 cm in diameter. Dakota Group sandstone clasts are in deposits along Fraser
River; nearest known outcrops of the Dakota Group are about 10 km to the west and
southwest. Deposits along Fraser River also locally contain a few rounded, red siltstone
pebbles. Till-like deposits are locally present at elevations of about 9,300–9,500 ft
along Elk Creek and St Louis Creek. These deposits contain granite and gneiss boul-
ders as long as 1–2 m. Deposits along Fraser River contain clasts derived from glacial
outwash(?) and till-like deposits, and have a clayey-silt matrix derived chiefly from silt-
stone of the Troublesome Formation. Top of unit is about 35–170 m above Elk Creek,
Description of Map Units 7

about 50–195 m above St Louis Creek, and about 140–270 m above the Fraser River.
Small deposit about 270 m above the Fraser River, about 2 km northeast of Winter Park
ski area, may be of Pliocene age. Unit locally may include debris-flow deposits. Esti-
mated thickness of deposits near Fraser is as much as 15 m.
Deposit on northwest flank of Saxon Mountain near Georgetown is nonsorted
and bouldery and has a sandy matrix (Widmann and Miersemann, 2001). Lower limit
of this deposit is about 270 m above Clear Creek, and may be of Pliocene age. Esti-
mated thickness of deposit near Georgetown probably greater than 5 m
Qdv Debris-flow deposits and Verdos Alluvium, undivided (middle Pleistocene)—Debris-
flow deposits, probably hyperconcentrated-flow deposits, and minor stream alluvium
on mesas at two levels near mouths of steep canyons along eastern margin of mountain
front north of Eldorado Springs. Eastern lower limits of tops of upper and lower depos-
its are about 90 m and 35–45 m, respectively, above South Boulder Creek. Debris-flow
deposits consist chiefly of nonsorted and nonstratified boulders to granules supported in
a matrix of pebbly, slightly silty sand. Clasts are angular to subangular and commonly
are randomly oriented. Boulders greater than 1 m in length are more common in upper
part of unit. Boulders composed of sandstone and conglomerate of the Fountain Forma-
tion on or near top of unit are as large as 2.3x3.7x>3.7 m. Hyperconcentrated-flow
deposits and stream alluvium are locally present in eastern (distal) part of unit. These
deposits consist of poorly sorted and poorly stratified, gravelly sand and sandy gravel.
Thickness about 3–5 m
QNdr Debris-flow deposits and Rocky Flats Alluvium, undivided (early Pleistocene and late
Pliocene?)—Debris-flow deposits, probably hyperconcentrated-flow deposits, and
minor stream alluvium on high mesas near mouths of steep canyons along eastern mar-
gin of mountain front north of Eldorado Springs. Tops of these mesas project to the top
of the Rocky Flats Alluvium (QNr) at Rocky Flats, south of Eldorado Springs. Eastern
lower limits of tops of these mesas are about 140–200 m above South Boulder Creek.
Unit consists chiefly of nonsorted and nonstratified boulders to granules supported in
a matrix of pebbly, slightly silty sand. Clasts are commonly randomly oriented and
angular to subangular. Boulders greater than 1 m in length are more common in upper
half of unit. Angular to subangular boulders composed of sandstone and conglomerate
of the Fountain Formation on or near top of unit are as large as 2.8x3.2x5.8 m. Unit
locally consists of three superimposed depositional units. Thickness 4.5–6 m
Ng High-level gravel deposits (Miocene?)—Scattered deposits of coarse boulder gravel
mantle ridges and hills that range in elevation from about 2,715 m east of Nederland to
about 2,300 m east of Conifer. From north to south, top of gravel is as much as 185 m
above South Boulder Creek, 370 m above Clear Creek, 255 m above Bear Creek, and
70 m above Deer Creek. Unit consists of nonsorted, nonstratified to crudely stratified,
matrix-supported and locally clast supported, bouldery gravel and slightly bouldery,
coarse cobble gravel that locally contains lenses of boulder gravel, pebble gravel, and
sand. Matrix material is slightly silty and clayey, coarse sand. Some deposits near Bear
Creek have a clayey matrix (Sheridan and Marsh, 1976). Sediments in unit were depos-
ited by stream flow and probably locally by debris flow. Clasts are angular to rounded
and commonly subangular to subrounded. Biotite-rich clasts are partly or completely
disintegrated. Boulders are commonly as large as 2 m in diameter; some are as large as
4.5 m in diameter (Sheridan and others, 1972). Clasts of Silver Plume Granite (YgSP)
in deposits near Clear Creek indicate eastward transport of at least 10 km. Unit forms
thick aggradational fills in the east-trending Tungsten, Clear Creek, Evergreen, and
Wilds Peak paleovalleys. These paleovalleys are incised as much as about 240 m below
an extensive erosion surface (Epis and others, 1980) that truncates South Park Forma-
tion (59.7 Ma; Bryant and others, 1981a) and is capped by the Wall Mountain Tuff of
early Oligocene age (36.7 Ma; McIntosh and Chapin, 2004) south of map area (Scott
and Taylor, 1986).
Unit may represent proximal facies of the Ogallala Formation of Miocene age
(about 18–5 Ma; Swinehart and others, 1985) in the High Plains about 130 km east of
mountain front. Unit is younger than bouldery deposits of Oligocene age (Scott and
8 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

Taylor, 1986) on Niwot Ridge and other nearby ridges, about 5–10 km north of map
area (Madole, 1982; Gable and Madole, 1976). Maximum thickness 70 m near Boulder
Creek, 110 m near Clear Creek, 25 m near Bear Creek, and 6 m near Deer Creek
Mass-movement deposits
Qc Colluvium (Holocene to middle? Pleistocene)—Nonsorted deposits that consist of clay,
silt, sand, and angular to subrounded clasts that range in size from granules to large
boulders. Composition of deposits reflects that of upslope bedrock or sediment from
which colluvium was derived. Includes material transported by frost creep, solifluction,
and other periglacial processes, sheetwash, landslide, debris flow, hyperconcentrated
flow, and rock fall. Extensive periglacial deposits are locally present near or above
present treeline (about 3,500 m). Estimated thickness 3–50 m
Qdf Debris-flow deposits (Holocene to middle? Pleistocene)—Lobate and fan-shaped masses
of coarse debris deposited by sediment-charged flows. Deposits are chiefly very poorly
sorted and very poorly stratified boulders to granules supported in a matrix of silty sand
to slightly sandy clayey silt. Locally includes lenticular beds of poorly sorted, clast-
supported, bouldery cobbly pebble gravel. Clasts are commonly randomly oriented and
angular to subangular. Low-lying areas of unit adjacent to stream channels are prone
to periodic stream flooding and debris-flow deposition. Unit probably includes minor
stream-flow and hyperconcentrated-flow deposits. Mapped only in Eldorado Springs
7.5-minute quadrangle and at one locality in eastern part of Keystone 7.5-minute quad-
rangle. Estimated thickness 3–10 m
Qls Landslide deposits (Holocene to middle? Pleistocene)—Deposits of unsorted and unstrati-
fied debris, on slopes or at base of slopes, that are commonly characterized by hum-
mocky topography. Many of the landslides and landslide deposits form on unstable
slopes that are underlain by shale, siltstone, and claystone on east flank of hogback
belt and on slopes of North Table Mountain and South Table Mountain near Golden.
Younger deposits are commonly bounded upslope by crescent-shaped headwall scarps
and downslope by lobate toes. Unit locally includes material displaced chiefly by rota-
tional rock slides, rotational earth slides, debris slides, earth flows, and earth slide–earth
flows as defined by Varnes and Cruden (1996). Some deposits probably are formed by
translational slides and rock or earth creep. Sizes and lithologies of clasts and grain-size
distributions of matrices of these deposits reflect those of the displaced bedrock units
and surficial deposits. Landslide deposits are prone to continued movement or reacti-
vation due to natural, as well as human-induced, processes. Deposits in mountainous
terrain derived from Precambrian quartz-feldspar gneiss (Xf) and biotite gneiss (Xb)
locally contain blocks of rock as long as several meters. Landslide deposits derived
from Pierre Shale (Kp), Laramie Formation (Kl), and Denver Formation (bKd) are rich
in clay. Some of this clay is expansive and locally may have high potential for shrink-
ing and swelling (Shroba, 1982). Deposits on gentle slopes along and east of hogback
belt locally include minor sheetwash and creep-deformed deposits. Deposits in moun-
tains locally include minor rock-fall deposits. Estimated thickness 5–50 m
Qt Talus deposits (Holocene to middle? Pleistocene)—Angular pebbles to large boulders
deposited chiefly by rock and snow avalanche, rockfall, rock slide, and debris flow
at base of cliffs and steep slopes where debris forms aprons, cones, and fan-shaped
deposits. Locally includes debris flow and rubbly scree deposits. In cirques, locally
includes tills of the Satanta Peak and Triple Lakes advances of Benedict (1985), about
12–10 ka (Davis, 1988), and tills of Holocene age near cirque headwalls (Benson and
others, 2007). Much of the talus in glaciated valleys and cirques postdates the retreat
of Pinedale ice. Some of the talus derived from the Fountain Formation (Phf) south of
Boulder may be of middle Pleistocene age. Estimated thickness 3–15 m
Mass-movement and glacial deposits
Qrg Rock-glacier deposits (Holocene and latest Pleistocene)—Bouldery, lobate and tongue-
shaped masses along valley walls and on valley floors that commonly have steep fronts
and flanks. Deposits consist of a veneer of angular boulders that overlies a thick mass
Description of Map Units 9

of rock rubble that contains finer interstitial rock fragments. Lower part of unit locally
contains interstitial ice, ice lens, or an ice core. Lobate rock glaciers form along valley
walls and contain interstitial ice. Tongue-shaped rock glaciers form on valley floors.
East of the Continental Divide, tongue-shaped rock glaciers commonly have ice cores,
and are covered by debris. Those on the west side contain interstitial ice, and are known
as ice-cemented rock glaciers (Benedict, 1973; White, 1976). Ice-cored rock glaciers
commonly have depressions adjacent to headwall cliffs (where glacial ice melted),
longitudinal marginal-and-central meandering furrows, and collapse pits. Ice-cemented
rock glaciers commonly lack these surface features (White, 1976). Rock fragments on
and within rock-glacier deposits are derived from steep slopes chiefly by rockfall and
locally by rock slide and avalanche. Unit Qrg locally includes protalus ramparts, minor
talus deposits (Qt) displaced by post-depositional flowage, debris-flow deposits, col-
luvium (Qc), and other mass-movement deposits. Unit also includes till of the Satanta
Peak and Triple Lakes advances of Benedict (1985), about 12–10 ka (Davis, 1988), as
well as tills of Holocene age near cirque headwalls (Benson and others, 2007). Many
of the rock-glacier deposits in Colorado are of latest Pleistocene or early Holocene age
(Meierding and Birkeland, 1980). Rates of movement of active rock glaciers in the
Front Range are approximately 1–10 cm/yr (White, 1971, 1976). Estimated thickness
as much as 50 m
Qmg Mass-movement and glacial deposits, undivided (Holocene and latest Pleistocene)—
Unit commonly includes talus (Qt), colluvium (Qc) and other mass-movement depos-
its, rock-glacier deposits (Qrg), and till of Pinedale age (Qtp) near heads of glaciated
valleys. Unit locally includes tills of the Satanta Peak and Triple Lakes advances of
Benedict (1985), about 12–10 ka (Davis, 1988), and tills of Holocene age near cirque
headwalls (Benson and others, 2007). Unit also locally includes rotational rock-slide
deposits (Qls) derived from bedrock that is extensively faulted and (or) highly altered.
Estimated thickness 3–50 m; possibly as much as 100 m in large rotational rock-slide
deposits
Glacial deposits
[Mostly nonsorted and nonstratified till deposited from ice. Deposits locally include a minor amount of
stratified sand and pebble gravel (stratified drift) deposited by meltwater, mass-movement deposits, and
small areas of bedrock outcrops on steep slopes. Most glacial deposits are derived chiefly from Precam-
brian granitic and gneissic rocks]
Qtp Till of Pinedale age (late Pleistocene)—Mostly nonsorted and nonstratified, subangular
to subrounded boulders to granules in a silty sand matrix. Material less than 2 mm in
diameter is estimated to be 20–40 percent of unit. This material consists chiefly of
poorly sorted sand and ≤20 percent silt and ≤5 percent clay. Unit commonly forms
large prominent, sharp-crested lateral and end moraines that are very bouldery and have
distinct constructional morphology. Deposits in some areas have well-expressed knob-
and-kettle topography. Surface soils have A/Cox profiles on moraines with narrow
crests and A/Bw/Cox and A/Btj/Cox profiles on moraines with broad crests. Surface
and near-surface O and E soil horizons are locally present. Cambic (Bw) and weak
argillic (Btj) horizons are thin (10–40 cm) and commonly contain 1–5 percent more
clay than the underlying till (<2 mm size fraction). Most of the biotite-rich granitic and
gneissic clasts within the soil are unweathered, and disintegrated clasts are rare. Unit
locally includes deposits of stratified drift, mass-movement and glacial deposits, undi-
vided (Qmg), tills of the Satanta Peak and Triple Lakes advances of Benedict (1985),
about 12–10 ka (Davis, 1988), till of Bull Lake age (Qtb), colluvium (Qc) and other
mass-movement deposits, and valley-floor alluvium (Qva); locally may include till of
pre-Bull Lake age. Radiocarbon (14C) and cosmogenic-exposure ages indicate that till
of unit Qtp is about 30–12 ka (Nelson and others, 1979; Madole, 1986; Schildgen and
Dethier, 2000; Benson and others, 2004, 2005). Subsurface deposits and locally some
surface deposits of unit Qtp may be older than 30 ka, because uranium-series ages of
travertine in the northern Yellowstone area suggest an early advance of Pinedale ice
about 47–34 ka (Sturchio and others, 1994). Estimated thickness 1.5–30 m
10 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

Qtb Till of Bull Lake age (late and middle Pleistocene)—Mostly nonsorted and nonstrati-
fied, subangular to subrounded boulders to granules in a silty sand matrix. Material <2
mm in diameter is estimated to be 20–40 percent of unit. Unit Qtb commonly forms
prominent lateral moraines that have rounded crests beyond the outer limit of till of
Pinedale age (Qtp). Surface boulders typically are less abundant on moraines of Bull
Lake glaciation than on those of Pinedale glaciation. Till of Bull Lake age (Qtb) is
more weathered than till of Pinedale age (Qtp). Surface soils have A/E/Bt/Cox profiles.
Clay-enriched argillic (Bt) horizons are 35–75 cm thick and commonly contain 5–13
percent more clay than the underlying till (<2 mm size fraction). Many of the biotite-
rich granitic and gneissic pebbles and cobbles within the soil are weathered and are
partly to completely disintegrated. K-Ar and 230Th/U analyses that constrain the ages of
glaciofluvial deposits near the type area for the Bull Lake glaciation along north flank
of Wind River Range, Wyo., and ages of glacial deposits near West Yellowstone indicate
that the Bull Lake glaciation probably began prior to 167±6.4 ka (possibly 190 ka) and
may have continued until about 122±10 ka (Sharp and others, 2003; Pierce, 2004). 10Be
and 26Al analyses of surface boulders on moraines composed of till of Bull Lake age
near Nederland yielded minimum age estimates of 101±21 and 122±26 ka (Schildgen
and others, 2002). These age estimates are in accord with a uranium-trend age estimate
of 130±40 ka for till of Bull Lake age (Shroba and others, 1983) near Allenspark, Colo.
Unit locally includes deposits of stratified drift, such as those in the valley of Clear
Creek at Dumont, till of Pinedale age (Qtp), colluvium (Qc), other mass-movement
deposits, and thin deposits of loess or sheetwash deposits derived chiefly from loess;
locally may include till of pre-Bull Lake age. Estimated thickness 1.5–15 m
Qti Till of Pinedale age and till of Bull Lake age, undivided (late and middle Pleistocene)—
Unit consists of till chiefly of Pinedale age (Qtp) and a minor amount of till of Bull
Lake age (Qtb) in areas where till of Bull Lake age is of limited extent and is difficult
to distinguish from till of Pinedale age. In some areas, till of Bull Lake age may be
mantled by till of Pinedale age and is not exposed. Locally includes stratified drift,
mass-movement and glacial deposits, undivided (Qmg), colluvium (Qc), and other
mass-movement deposits; locally may include a minor amount of till of pre-Bull Lake
age that lacks depositional morphology. Estimated thickness 1.5–30 m
Eolian deposits
Qes Eolian sand (Holocene and late Pleistocene)—Slightly silty sand to silty and slightly
clayey sand derived from deflation of sediment from flood plains of major streams east
of mountain front by northwesterly paleowinds. Grain size decreases from northwest
to southeast. Unit locally may include a minor amount of loess (Qlo). Deposits east of
South Platte River probably contain less silt and clay than deposits of eolian sand else-
where within map area. Soils formed in upper part of deposits of late Pleistocene age
have A/Bt/Cox profiles where formed in slightly silty sand and A/Bt/Bk profiles with
stage I or II carbonate morphology where formed in finer grained sediment. Unit forms
sand sheets that mantle deposits as young as Broadway Alluvium (Qb) on east side of
valley of South Platte River. Deposits locally reworked by sheetwash contain a minor
amount of granules and a few pebbles.
Eolian sand in northeastern Colorado records at least three episodes of deposi-
tion between 27 and 11 ka, possibly between 11 and 4 ka, and within the past 1.5 ky
(Muhs and others, 1996). Most deposits in map area probably formed between about
27 ka (Muhs and others, 1996) and 4.5 ka (Scott and Lindvall, 1970). Thickness com-
monly less than 3 m west of South Platte River, and commonly less than 6 m east of
river
Qlo Loess (late and middle? Pleistocene)—Nonstratified, well-sorted, wind-deposited sandy
silt and locally sandy, clayey silt derived by wind erosion from flood plains and possibly
Cretaceous bedrock sources by westerly or northwesterly paleowinds. Soils formed
in upper part of unit have A/Bt/Btk/Bk profiles with stage I–II carbonate morphology.
Loess overlies deposits as young as Louviers Alluvium (Qlv) in map area, but locally
overlies Broadway Alluvium (Qb) along South Platte River about 15 km northeast
Description of Map Units 11

of map area (Lindvall, 1980). Thin (≈50 cm) layers of pebbly, clayey, sandy silt that
locally mantle Slocum Alluvium (Qs) and older alluviums in the piedmont may consist
in part of loess that contains clasts from the underlying gravelly alluvium. Deposits
of unit Qlo locally reworked by unconfined overland flow contain a minor amount of
coarse sand and granules, and locally a few pebbles. Unit Qlo locally contains deposits
of silty eolian sand (Qes) near Louisville and Lafayette.
Loess in northeastern Colorado records two episodes of deposition at about
20–14 ka and 13–10 ka (Muhs and others, 1999). Holocene loess is locally extensive in
eastern part of Colorado (Madole, 1995), but none has been recognized within or near
map area. Some loess near southeast corner of map area (older loess of Scott, 1962,
1963a) may be as old as 170–120 ka and may be correlative with eolian silt and sand
about 30 km northeast of map area that yielded thermoluminescence age estimates of
about 150 ka (Forman and others, 1995). Pre-Bull Lake till near Allenspark, about 25
km north of Nederland, is locally mantled by about 15 cm of loess (Madole and Shroba,
1979). Some of the closed depressions on till of Pinedale age (Qtp, Qti) may contain
thin (≤50 cm) deposits of loess or silty sheetwash deposits derived chiefly from loess.
Dust deflated from flood plains in the Colorado Piedmont has influenced properties of
soils downwind of flood plains (Reheis, 1980). Alpine soils in and near cirques on east
side of the Continental Divide, just north of map area, contain eolian dust (Birkeland
and others, 1987, 2003; Muhs and Benedict, 2006). Thickness about 1.5–3 m
LARAMIDE AND POST-LARAMIDE SEDIMENTARY AND VOLCANIC ROCKS OF FRASER BASIN
Nbt Troublesome Formation (Miocene and Oligocene)—Light-gray, light-brown, and light-
grayish-orange tuffaceous siltstone and silty, fine-grained sandstone; local white, water-
laid tuff; and light-tan and brown arkosic sandstone and conglomerate near base of unit.
Tuffaceous beds are rich in swelling clay. 40Ar/39Ar ages for interbedded tuffs just west
of quadrangle range from 11.0±05 Ma to 23.5±0.06 Ma (Izett and Obradovich, 2001).
Thickness probably greater than 245 m (Taylor, 1975)
bt Rhyolite tuff (Oligocene)—Rhyolitic crystal-lithic tuff and tuff breccia. Forms small
outcrops beneath Troublesome Formation (Nbt) along south margin of Fraser basin
(Bryant and others, 1981b). Contains conspicuous smoky quartz phenocrysts, sanidine,
and biotite. K-Ar sanidine age on tuff breccia of 29.8±3 Ma (Taylor and others, 1968).
Volcanic source may be either rhyolite porphyry at Red Mountain or volcanic center
near Mount Richthofen (O’Neill, 1981), about 45 km north of northwest corner of map
area
Middle Park Formation (Paleocene and Upper Cretaceous)
bKmu Upper member (Paleocene and Upper Cretaceous?)—Variegated gray, brown, maroon,
and rusty-orange siltstone, mudstone, sandstone, and conglomerate; predominantly
micaceous and arkosic (Taylor, 1975). Derived mostly from various Proterozoic
intrusive rocks and feldspathic gneiss shed during Laramide uplift of the Front Range.
Lower 150 m contains as much as 50 percent clasts of porphyritic andesite, trachyan-
desite, and other porphyries. Potentially unstable, even on gentle slopes. Maximum
thickness greater than 600 m
Kmw Windy Gap Volcanic Member (Upper Cretaceous)—Medium- to dark-gray, greenish-
gray, or purplish-gray volcanic breccia and conglomerate containing a trachyandesite
flow near base at one locality. Beds include massive lahar breccias containing andes-
ite and trachyandesite boulders as long as 1.2 m, bedded fluvial boulder and cobble
conglomerates, and volcanic sandstone containing a substantial fraction of Proterozoic
debris (Taylor, 1975) shed during Laramide uplift of the Front Range. Palynomorph
data suggest age is Late Cretaceous (Izett, 1968). Thickness 0–215 m
TERTIARY AND CRETACEOUS INTRUSIVE ROCKS
[Petrographic nomenclature follows Streckeisen (1976); geochemical nomenclature (calcic, calc-alkalic,
alkali-calcic, alkalic) follows De la Roche and others (1980)]
bKi Intrusive rock, undifferentiated (Oligocene to Late Cretaceous)—Dike rocks mapped
only in northeastern part of Idaho Springs 7.5-minute quadrangle (Widmann and others,
12 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

2000); probably mostly older felsic to intermediate porphyries of alkali-calcic group
(bKpc) or alkalic porphyries (bKpa)
brp Rhyolite porphyry (Oligocene and Eocene)—Mapped mostly in two areas: Red Mountain
and northern Montezuma 7.5-minute quadrangle. The Red Mountain rocks contain
quartz and sanidine in an aphanitic groundmass; rocks form dikes and a volcanic center
on Red Mountain; rock altered and has locally associated molybdenite mineralization
(Henderson ore body). All analyzed rocks on Red Mountain are alkali rhyolites (Ed
Dewitt, written commun., 2007); fission-track and 40Ar/39Ar analyses on these rocks
indicate age of 30–27 Ma (Geissman and others, 1992). Occurrences in Montezuma
quadrangle are predominantly rhyolitic, and include aplitic-textured rhyolite porphyry,
alkali-feldspar rhyolite porphyry, biotitic rhyolite porphyry, quartz latite porphyry, and
rhyodacite porphyry; dated rhyolite porphyries yield fission-track ages of 39.4–37.4
Ma (Bookstrom and others, 1987), and are probably derived from same magma that
produced the approximately coeval Montezuma stock
Rocks of Montezuma stock and vicinity (Eocene)
bma Aplite—Light-pinkish-gray to gray, fine-grained quartz-feldspar rock that forms two
irregular bodies and numerous unmapped dikes that intrude granodiorite porphyry of
Montezuma stock (bmm). Chemical analysis indicates composition is alkali-calcic
granite (Ed Dewitt, written commun., 2007). Undated, but considered about same age
as monzogranite porphyry (≈37 Ma)
bmm Monzogranite porphyry—Light-tan to gray, medium-grained, porphyritic, alkali-calcic
monzogranite and granodiorite with euhedral to subhedral phenocrysts of orthoclase and
quartz. Chemical composition indicates alkali-calcic granodiorite (Ed Dewitt, written
commun., 2007). Orthoclase phenocrysts form as much as 20 percent of rock and are
locally reported as long as 10 cm (Lovering, 1935). Quartz phenocrysts form as much
as 10 percent of rock and are commonly partially resorbed. Matrix contains orthoclase,
plagioclase, quartz, as much as 3 percent biotite, and trace amounts of hornblende,
magnetite, and sphene. Locally includes light-pink aplite dikes (bma) not mapped
separately. Includes several small intrusive bodies near Montezuma stock. Zircon
fission-track ages between 39.8±4.2 and 34.8±3.7 Ma (Bookstrom and others, 1987);
Cunningham and others (1994) reported a zircon fission-track age of 35.0±3.2 Ma; one
K-Ar age is 37.0±1.4 Ma (Marvin and others, 1989)
bpc Younger felsic to intermediate porphyries of alkali-calcic group (Eocene)—Occur
principally in three areas: (1) in southwestern part of Georgetown 7.5-minute quad-
rangle near Leavenworth Creek, (2) near Empire, where the Mad Creek monzogranite to
granodiorite stock intrudes the monzonitic Empire stock, and (3) in southwestern part of
Keystone 7.5-minute quadrangle, where granodiorite to monzogranite intrude the Pierre
Shale (Kp) as dikes, sills, and irregular-shaped bodies. Rhyolite porphyry near Leaven-
worth Creek is tan to pinkish gray and very fine grained, with phenocrysts of orthoclase,
biotite, and quartz (Widmann and Miersemann, 2001); it has a zircon fission-track age
of 36.6±4.2 Ma (Bookstrom and others, 1987). The Mad Creek stock consists of a very
fine grained matrix containing phenocrysts mostly of altered plagioclase and subordi-
nate sanidine, quartz, and biotite (Braddock, 1969); it has a fission-track age of 39.4±4.2
Ma (Bookstrom and others, 1987). Braddock (1969) reported monzogranite composi-
tion for the Mad Creek stock, but it also includes significant granodiorite (Ed Dewitt,
written commun., 2007). The Keystone occurrences are gray, porphyritic quartz-
plagioclase-orthoclase-biotite±hornblende granodiorite containing megacrysts of
orthoclase as long as 4 cm and smaller plagioclase phenocrysts in a medium- to
coarse-grained matrix (Widmann and others, 2003). Quartz phenocrysts are commonly
partially resorbed and embayed. Rock weathers light tan in blocky outcrops. The
Keystone rocks are almost identical mineralogically, texturally, and magnetically with
rocks in the Mt. Guyot stock, about 5 km south of the Keystone quadrangle (Ed Dewitt,
written commun., 2007). K-Ar biotite date on Mt. Guyot stock is 44.0±1.5 Ma (Bryant
and others, 1981a)
bKix Intrusive breccia (Eocene to Late Cretaceous?)—Occurs as a pipe-shaped body in Central
City mining district, as a fault-bounded body in southern Nederland 7.5-minute
Description of Map Units 13

quadrangle, and as a small body associated with intrusion of the Eocene Mad Creek
stock near Empire. Central City breccia pipe consists of altered fragments of biotite
gneiss, pegmatite, and quartzofeldspathic gneiss in a matrix of monzogranite (Sims,
1964, 1988; Sims and Gable, 1967); undated, but probably Paleocene based on age
of intrusive activity in area. Nederland occurrence consists of altered fragments of
Proterozoic rock in a matrix of altered porphyry (Gable, 1969) probably associated with
the 61 Ma Bryan Mountain (Eldora) stock. Breccia near Empire is composed of angular
fragments of Silver Plume Granite, monzonite of the Empire stock (Braddock, 1969),
and the Eocene granodiorite Mad Creek stock (Bookstrom and others, 1987)
bKdg Diorite, gabbro, and lamprophyre (Tertiary and Cretaceous?)—In Nederland and
Tungsten 7.5-minute quadrangles, consists of dikes and small stocks of biotite-
hornblende-pyroxene diorite, diorite porphyry, fine-grained andesite porphyry, diabase,
gabbro, lamprophyre, and pyroxenite (Gable, 1969, 1972). Pyroxenite, in particular, is
commonly associated with biotite-hornblende-pyroxene diorite in the Caribou stock. In
the Loveland Pass and Byers Peak 7.5-minute quadrangles, consists of north-striking,
fine-grained pyroxene diorite dikes containing phenocrysts of augite and plagioclase
(Eppinger and others, 1984). Undated, but in Nederland quadrangle intrudes, and prob-
ably closely postdates, rocks of the Caribou stock, part of the older felsic to intermedi-
ate porphyries of the alkali-calcic group (bKpc)
bKpc Older felsic to intermediate porphyries of alkali-calcic group (Paleocene to Late Cre-
taceous?)—Large stocks, adjacent smaller bodies, and dikes in Nederland area and
in southern Empire and northern Georgetown 7.5-minute quadrangles. Includes the
Caribou, Bryan Mountain (also called Eldora), and Apex stocks. Includes monzogran-
ite, monzogranite porphyry, biotite-hornblende±pyroxene monzogranite (Gable, 1969),
and granodiorite (Ed Dewitt, written commun., 2007). K-Ar age of Caribou stock is
68.2±4.1 Ma on hornblende and 65.5±2.4 Ma on biotite (Marvin and others 1989). K-
Ar age on Bryan Mountain stock is 60.6±1.8 Ma on biotite (McDowell, 1971; Marvin
and others, 1974)
bKpa Porphyries of the alkalic group (Paleocene and Late Cretaceous?)—Numerous small
bodies and northeast-trending dikes primarily in southern parts of Central City and
Empire 7.5-minute quadrangles and northern part of Idaho Springs 7.5-minute quadran-
gle. Includes trachyte porphyry, quartz trachyte porphyry, quartz latite porphyry, quartz
syenite porphyry, syenogranite porphyry, and dark- to light-gray monzonite porphyry.
In Central City and Idaho Springs area, includes leucocratic trachyte and quartz trachyte
porphyries, typically with a purplish tinge and locally containing garnet; commonly
called bostonite porphyry and quartz bostonite porphyry, respectively (for example, Har-
rison and Wells, 1959; Sims and others, 1963; Moench and Drake, 1966). K-Ar whole-
rock ages of 65.2±1.4 Ma and 61.6±1.3 Ma are from a bostonite dike in Central City
quadrangle (Rice and others, 1982). K-Ar whole-rock ages from syenite and quartz
syenite from Central City, Black Hawk, and Idaho Springs quadrangles range from
61.6±1.3 Ma to 60.0 Ma (Simmons and Hedge, 1978; Rice and others, 1982). Rocks
of the Empire stock are strongly silica undersaturated; include alkali gabbro, syenodio-
rite, essexite (feldspathoid-bearing monzodiorite or monzogabbro), syenite, and quartz
syenite (Ed Dewitt, written commun., 2007); commonly contain abundant hornblende or
pyroxene, and locally contain the feldsphathoids nepheline and (or) sodalite (Braddock,
1969). Rb-Sr whole-rock age of monzonitic Empire stock is 65.0 Ma (no uncertainty
given; Simmons and Hedge, 1978); fission-track ages on sphene are 67.8±4.5 Ma (Mar-
vin and others, 1974) and 66.0±6.2 Ma (Cunningham and others, 1994). The Lincoln
Mountain stock ranges from quartz syenite to monzogranite. Economic mineralization
in Idaho Springs–Central City mineral district is synchronous with or slightly younger
than alkalic intrusive activity in area
bbi Potassic basalt (shoshonite) intrusive of Ralston Buttes (Paleocene)—Very dark gray
porphyritic rock containing stubby phenocrysts of glassy plagioclase and green augite
generally as long as about 5 mm, in a very fine grained groundmass. At one locality, a
single plagioclase crystal 3 cm long and an augite crystal 2 cm long were reported (Van
Horn, 1976). Contains a few small xenoliths of metamorphic rock and sandstone. Rock
14 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

weathers light brown and crops out in rounded knobs. Forms a large dike (Ralston
dike), about 2,300 m long and 600 m wide, and several irregular plugs, all intruding
Pierre Shale (Kp), about 5 km north of Golden. Unit is probably the source for potassic
basalt lava flows (bdb) on North and South Table Mountains. Two K-Ar whole-rock
ages are 63.5±2.5 Ma (Marvin and others, 1974) and 61.9±2.5 Ma (Hobblitt and Larson,
1975); a Rb-Sr whole-rock age is 64.0 Ma (no uncertainty given; Simmons and Hedge,
1978). Paleomagnetic directions from the Ralston intrusive body are rotated, indicating
that unit was emplaced before major movement on the Golden fault (Hoblitt and Larson,
1975)
LARAMIDE SEDIMENTARY AND VOLCANIC ROCKS OF THE DENVER BASIN
[Sedimentary rocks contain Proterozoic, upper Paleozoic, Mesozoic, and (or) volcanic debris shed from
uplifting Front Range during Laramide orogeny, which began about 70 Ma and lasted for about 20 m.y.]
bg Green Mountain Conglomerate (Paleocene)—Well-rounded to subrounded cobble and
boulder conglomerate containing minor layers and lenses of sandstone, siltstone, and
claystone. Grain size increases upward. Andesite clasts form minor component of
lower part and decrease in amount upward. Other clasts include gneiss, pegmatite,
quartzite, and sandstone. Contains Paleocene pollen and plant remains in lower 135
m. A rhyolite tuff in upper part of unit has 40Ar/39Ar age on sanidine of 63.94±0.28 Ma
(Obradovich, 2002). Top of unit eroded; total exposed thickness 200 m (mostly consid-
erably thinner)
bKd Denver Formation (Paleocene and Upper Cretaceous)—Yellowish-brown to grayish-
brown fluvial claystone, siltstone, friable sandstone, and conglomerate. Sandstone and
finer grained rocks are tuffaceous and commonly weather to montmorillonitic clay with
high swelling potential. Clasts composed of about 95 percent andesite and 5 percent
granitic and metamorphic rocks. Locally contains fossil leaves, silicified wood, and
dinosaur and mammal bones. Unit susceptible to landsliding on steeper slopes. On
South Table Mountain near Golden, Cretaceous-Tertiary (K-T) boundary layer (65.4
Ma; Obradovich, 1993) is 71 m below base of lowest basalt flow (see bdb description).
Conformably overlies Arapahoe Formation. Total thickness in Morrison 7.5-minute
quadrangle 290 m (Scott, 1972)
bdb Potassic basalt (shoshonite) lava flows (Paleocene)—Dark-gray, porphyritic flow rock
containing small pheonocrysts of augite, plagioclase, magnetite, and olivine in a fine-
grained matrix. Called mafic latite by Van Horn (1976), although only upper flows
are latitic; basal flow is a trachytitic basalt. Formerly mapped as three flows on North
Table Mountain and two flows on South Table Mountain (Van Horn, 1976); four total
flows now recognized (Drewes, 2004). Clinkery bases and vesicular tops are visible,
except on upper flow where top is eroded. Locally forms prominent cliffs. Cavities
at some localities contain a variety of zeolite minerals (Ellemeier, 1947). Flows most
likely erupted from Ralston Buttes area, about 5 km north of Golden. Interlayered with
and considered here as members of Denver Formation. 40Ar/39Ar age of upper flow is
63.9±1.2 Ma (Obradovich, 2002). Flows are as thick as 30 m where they filled channels
Ka Arapahoe Formation (Upper Cretaceous)—Coarse- and fine-grained sandstone, siltstone,
claystone, and thin pebble beds in upper part; white, yellowish-gray, and yellowish-
orange, coarse-grained sandstone and poorly sorted pebble-and-cobble conglomerate
in lower part. Clasts include sandstone (as long as 0.5 m), shale (some more than 1 m
long), igneous and metamorphic rocks, and minor chert and petrified wood; most clasts
less than 3 cm long. Proportion of sedimentary clasts to Precambrian crystalline clasts
about 60:40 near base of unit, but decreases stratigraphically upward (Scott, 1972),
reflecting progressive stripping of Phanerozoic cover from Precambrian basement.
Contains ironstone and dinosaur bones in localized concentrations. Unconformably
overlies Laramie Formation (Kl). High potential for landslides in clayey beds of unit
(Shroba and Carrara, 1996). Total thickness in Morrison 7.5-minute quadrangle 120 m
(Scott, 1972)
bKda Denver Formation (Paleocene and Upper Cretaceous) and Arapahoe Formation (Upper
Cretaceous), undivided
Description of Map Units 15

PRE-LARAMIDE SEDIMENTARY ROCKS OF THE DENVER BASIN, BLUE RIVER VALLEY, AND FRASER BASIN
Kl Laramie Formation (Upper Cretaceous)—Upper 130–200 m is light-gray micaceous silt-
stone, light-olive silty claystone, grayish-brown lignitic claystone, minor white, friable,
resistant sandstone, and, near top of unit, thin layers of sedimentary-clast conglomerate;
lower 60 m of upper part contains subbituminous coal beds as thick as 2.5 m, mined
extensively in Marshall district. Commonly stained yellowish orange and contains
orange, sandy ironstone concretions. Lower 35 m of Laramie Formation consists of
sandstone and sandy shale, with minor claystone and coal. Gray and white claystone
beds are mined locally for ceramic clay and for brick making. Contains abundant fossil
leaves and wood fragments. Deposited in near-shore swamps and meandering stream
channels. High potential for landslides in claystones of unit (Shroba and Carrara,
1996). Mapped only in the Denver Basin. Conformable with underlying Fox Hills
Sandstone (Kf). Thickness in Morrison 7.5-minute quadrangle 165 m (Scott, 1972); in
Marshall area (in Louisville 7.5-minute quadrangle), thickness as much as about 240 m
(Spencer, 1961)
Kf Fox Hills Sandstone (Upper Cretaceous)—Upper 32 m is olive-gray to dark-yellow-
ish-brown, silty shale and interbedded friable, micaceous sandstone locally containing
flattened limestone concretions. Lower 23 m is yellowish-orange, massive to thin-bed-
ded, locally cross bedded, fine-grained, resistant sandstone and interbedded dark-olive-
gray shale and claystone. Reddish-brown, calcareous concretions near top of lower
sequence. Fossil pelecypods support interpretation of near-shore marine deposition
during regression of Cretaceous inland sea. Mapped only in the Denver Basin. Con-
formably overlies Pierre Shale (Kp). Total thickness in Morrison 7.5-minute quadrangle
55 m (Scott, 1972); due to intertonguing relationships with underlying Pierre Shale,
thickness in Louisville 7.5-minute quadrangle highly variable, ranging from 20 to 65 m
(Spencer, 1961)
Klf Laramie Formation (Upper Cretaceous) and Fox Hills Sandstone (Upper Cretaceous),
undivided
Kp Pierre Shale (Upper Cretaceous)—Predominantly olive-gray marine shale with subor-
dinate fine-grained, brown sandstone beds. Locally contains ironstone and limestone
concretions. Lower shale member, below Hygiene Sandstone Member (Kph), is
olive-gray, clayey shale with common, thin, bentonitic layers. Shale and bentonite beds
susceptible to swelling. Beds above Hygiene Sandstone Member similar to lower shale
member. Conformable above Niobrara Formation (Kn). Total thickness in Indian Hills
7.5-minute quadrangle about 1,750 m (Bryant and others, 1973) and in Morrison quad-
rangle about 1,885 m (Scott, 1972); near Louisville, thickness in one oil well test hole is
2,395 m (Malde, 1955). Thickness in two oil well test holes in Fort Logan 7.5-minute
quadrangle is 2,115 and 2,205 m (Lindvall, 1978).
In Blue River valley in southwest corner of quadrangle, Pierre Shale may have
been as thick as about 2,600 m (Izett and others, 1971), although at least the upper
1,000 m has been removed by erosion. Thickness greater than 1,400 m in and adjacent
to Fraser basin (Taylor, 1975)
Kpf Hornfels—Dense, black hornfels and metasandstone in window of Williams Range thrust
near Keystone; Pierre Shale metamorphosed by nearby Eocene Montezuma stock. Bed-
ding is preserved at most locations
Kph Hygiene Sandstone Member—Yellowish-gray or olive-brown, friable, massive sandstone
that grades upward into fine-grained, thinly bedded sandstone. Crops out in Denver
Basin about 500 m above base of Pierre Shale. Unit about 30 m thick in Louisville
7.5-minute quadrangle; about 18 m thick in Keystone 7.5-minute quadrangle where it is
exposed (but not shown separately on map) in a resistant cliff along western border of
map (Widmann and others, 2003)
Kn Niobrara Formation (Upper Cretaceous)—Upper part is Smoky Hill Shale Member,
which is a pale- to yellowish-brown, soft, thin-bedded calcareous shale with interbedded
thin limestone layers; weathers pale gray to white and platy; contains many bentonite
beds; thickness 125 m. Underlying Fort Hays Limestone Member is a gray, dense
16 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

limestone in beds as thick as 2 m; contains abundant large inoceramid bivalve (oyster)
fossils in Denver Basin, which are less abundant in Blue River valley; thickness about
10 m in Denver Basin. The Fort Hays is slightly less thick and less distinct from the
Smoky Hill in Blue River valley, where combined thickness of Niobrara is about 135
m (Kellogg and others, 2002). In Fraser basin, combined thickness about 125 m, but
thinned considerably by folding (Taylor, 1975)
Kb Benton Group (Upper and Lower Cretaceous)—Consists of three formations in Denver
Basin: in descending order, Carlile Shale (Upper Cretaceous), Greenhorn Limestone
(Upper Cretaceous), and Graneros Shale (Upper and Lower Cretaceous). Carlile Shale
consists of upper grayish-brown, hard calcarenite containing abundant shell fragments
(Juana Lopez Member), middle gray silty sandstone (Blue Hill Shale Member), and
lower yellowish-gray, soft calcareous shale (Fairport Chalky Shale Member). The
Carlile conformably overlies the Greenhorn Limestone, which consists of an upper gray,
dense limestone and hard calcareous shale (Bridge Creek Limestone Member), middle
gray, shaly calcareous sandstone (Hartland Shale Member), and lower grayish-brown,
thin beds of hard calcareous sandstone and shale with a marker bentonite bed as base
(Lincoln Limestone Member). Graneros Shale consists of dark-gray, hard, clayey shale
and siltstone. In Blue River valley, the Juana Lopez Member of the Carlile forms a
prominent marker above about 3 m of brown, soft sandstone sequence [Codell Sand-
stone Member of Carlile Shale of Berman and others (1980)] that, in turn, unconform-
ably overlies sequence of hard black shale of the Graneros; lower 25 m of this sequence
is a wavy black shale that contains abundant fish scales, equivalent to the Lower
Cretaceous Mowry Shale. Benton Group is about 136 m thick in the Eldorado Springs
7.5-minute quadrangle (Wells, 1967) and thickens to the south, where it is about 182
m thick just south of Chatfield Lake (Scott, 1963b). In Blue River valley, the Benton
is only about 80–100 m thick (Kellogg and others, 2002; Widmann and others, 2003).
In Fraser basin, thickness is about 140 m, but thinned considerably by folding (Taylor,
1975)
Kd Dakota Group (Lower Cretaceous)—Consists of South Platte Formation and underlying
Lytle Formation. South Platte Formation contains two or three yellowish-gray, well-
sorted, cross-stratified, porous, fine- to medium-grained quartz sandstone sequences
separated by dark-gray, silty, hard, locally carbonaceous shale, interbedded with thin
quartz sandstone beds and gray to white refractory clay or porcellanite layers [see Bry-
ant and others (1973) for more detailed description of members of South Platte Forma-
tion]. The South Platte is about 67 m thick. Lytle Formation consists of yellowish-gray,
medium- to fine-grained sandstone and conglomerate; locally contains reddish iron
stain. Well-rounded clasts in conglomerate composed of quartz, quartzite, chert, and
some petrified wood; conglomerate generally near base of unit. The Lytle is about 24 m
thick. In Fraser basin, combined thickness about 75 m (Taylor, 1975)
Jm Morrison Formation (Upper Jurassic)—Red siltstone and thin, brown sandstone beds
in upper part; green siltstone and claystone, with some interbedded sandstone and
limestone beds, in middle part; lower part contains several brown lenticular sandstone
beds and red jasper. Dinosaur bones locally contained in middle green siltstone beds
and in lower sandstone. Thickness in Morrison 7.5-minute quadrangle about 91 m
(Scott, 1972) and 105 m in Eldorado Springs 7.5-minute quadrangle (Wells, 1967).
Not exposed in Blue River valley in map area, but thickness just west of map area near
Dillon, Colo., about 70 m (Wahlstrom and Hornbeck, 1962). In Fraser basin, thickness
about 45–75 m, but thickened or thinned considerably by folding (Taylor, 1975)
Jmr Morrison Formation (Upper Jurassic) and Ralston Creek Formation (Upper and
Middle Jurassic), undivided—Morrison Formation described in previous paragraph.
Ralston Creek Formation is purplish-gray sandstone and siltstone, underlain by grayish-
yellow silty sandstone containing clayey limestone and shale beds with red jasper. Thin
layers of purple and white sandstone near base. Contains gypsiferous shale and white
gypsum as thick as 8 m south of Turkey Creek (Bryant and others, 1973). Crops out
only in Denver Basin. Thickness of Ralston Creek about 27 m in Morrison 7.5-minute
quadrangle (Scott, 1972), but thins to the north, where it is about 10 m thick in Eldorado
Description of Map Units 17

Springs quadrangle (Wells, 1967). Not mapped separately. Not recognized in either
Blue River valley or Fraser basin
dPl Lykins Formation (Triassic? and Permian)—Upper part is Strain Shale Member, which
consists of about 90 m of maroon, stratified, micaceous, fine-grained, silty sandstone
and siltstone with some green siltstone layers. About 2 m of light-brown, fine-grained
sandstone locally occurs at top of map unit. Middle part is Forelle Limestone Member,
which consists of 5 m of pink, wavy-laminated, sandy, marine, algal limestone. Lower
part is Bergen Shale Member and underlying Harriman Shale Member, which together
are about 40 m thick and consist of maroon and green siltstone separated by a thin (1 m
thick), laminated, red-weathering, gray and yellow crystalline limestone (Falcon Lime-
stone Member). Mapped only in Denver Basin
Pl Lyons Sandstone (Lower Permian)—In Morrison 7.5-minute quadrangle, composed of
yellowish-gray conglomerate with Proterozoic detritus as large as 5 cm. Grades down-
ward into pale-tan and yellowish-orange (iron stained from weathering), fine-grained,
calcite-cemented, cross-stratified eolian sandstone that also contains conglomerate near
base (Scott, 1972). Thickness about 58 m. Conglomerate beds, not reported in Indian
Hills 7.5-minute quadrangle (Bryant and others, 1973), disappear to the north where
formation thickens; in Eldorado Springs 7.5-minute quadrangle, formation is almost
entirely pink to pinkish-gray, cross-laminated, silica-cemented, fine- to coarse-grained
eolian sandstone as thick as 76 m (Wells, 1967). Mapped only in Denver Basin
Phf Fountain Formation (Lower Permian and Pennsylvanian)—Maroon and red, arkosic,
thick-bedded, coarse-grained to pebbly, cross-bedded fluvial sandstone and conglom-
erate containing thin layers of dark-maroon, hard (siliceous), micaceous siltstone and
silty, fine-grained sandstone. Coarser beds commonly fill shallow channels. Sandstone
locally bleached light tan to white. Clasts are well rounded to subrounded, as long as 25
cm, and composed mostly of Proterozoic rocks. Scott (1972) reported rare lower Paleo-
zoic clasts in lower part of formation in Morrison 7.5-minute quadrangle. Thickness
in Indian Hills quadrangle about 600 m; in Eldorado Springs quadrangle a measured
section is 305 m thick (Wells, 1967). Mapped only in Denver Basin
CAMBRIAN AND PROTEROZOIC INTRUSIVE ROCKS
[Note on unit-symbol nomenclature. Proterozoic intrusive rocks in the Denver West quadrangle were
emplaced during two principal time periods, recognized and formalized by Tweto (1987) as the Paleopro-
terozoic Routt Plutonic Suite (roughly 1,700±25 Ma) and the Mesoproterozoic Berthoud Plutonic Suite
(roughly 1,400±25 Ma). Formerly, some rocks of similar composition, texture, and age in separate plutons
in the Front Range have been given the same rock name. For example, Silver Plume Granite has been
applied to rocks of similar age and composition within several different batholitis (for example, Braddock
and Cole, 1979). However, because these batholiths represent distinctly different intrusive events, we
employ a unit-symbol nomenclature that differentiates the rocks of these plutons, as follows:
Age: X=Paleoproterozoic (2,500–1,600 Ma), Y=Mesoproterozoic (1,600–900 Ma).
Composition: g=granite, gd=granodiorite, d=diorite, qd=quartz diorite, and gb=gabbro; in addi-
tion, where appropriate, textural and (or) mineralogic variants that are mapped separately within an
intrusive complex are symbolized with an additional lower-case letter (for example, p=porphyritic phase,
h=hornblende bearing, and x=intrusion breccia).
Pluton: P=Pikes Peak batholith, SP=Silver Plume batholith, R=Rosalie Peak pluton, M=Mount
Evans batholith, T=Twin Spruce monzogranite of Boulder Creek batholith, and B=granodiorite of Boulder
Creek batholith.
Thus, the symbol for granodiorite of the Boulder Creek batholith is XgdB; the symbol for the fine-
grained porphyritic phase of the Pikes Peak Granite is YgPp.
Classification of intrusive rocks follows that of Streckeisen (1976), in which “granite” includes syeno-
granite and monzogranite]
eZk Kimberlite (Cambrian or Late Proterozoic)—Black to dark-green rock consisting of
olivine, ilmenite, garnet, diopside, and biotite in a groundmass of fine-grained olivine,
diopside, magnetite, ilmenite (with leucoxene rims), and pyrite (Kellogg, 1973). Oliv-
ine is rounded, variably serpentenized, and as long as 1 cm; the larger ilmenite grains
are unaltered and as long as 1 cm. The garnets commonly have dark kelyphite rims.
18 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

Rock crops out in a roughly circular diatreme about 30 m across on northern flank of
Green Mountain near Boulder. On the basis of paleomagnetic data, Kellogg (1973)
suggested the kimberlite is Proterozoic, which is supported by a recent Sm-Nd isochron
date of 572±49 Ma (Lester and others, 2001), although the uncertainty permits an earli-
est Cambrian age
YgP Pikes Peak Granite (Middle Proterozoic)—Pinkish-orange to light-gray, medium- to
coarse-grained biotite- and biotite-hornblende granite with feldspar crystals as long as
2.5 cm. Forms large, slabby outcrops and weathers to orange-brown grussy soil (con-
tains abundant feldspar and mica fragments). U-Pb zircon age about 1,080 Ma (Unruh
and others, 1995; Smith and others, 1999)
YgPp Fine-grained porphyritic phase—Pink, fine- to medium-grained, massive porphyritic
biotite granite with phenocrysts of spheroidal gray quartz, subhedral microcline, and
oligoclase
Ygi Gabbro dike (“Iron dike”) (Middle Proterozoic)—Dark-brown, medium- to fine-grained,
iron-rich ferrogabbro dike with chilled margins; contains augite and labradorite, and
lesser amounts of opaque minerals, biotite, secondary chlorite, and nontronite prob-
ably after olivine (Wells, 1967). Forms 10-m-wide, north-northwest-trending dike near
Magnolia, which marks southern end of a narrow dike swarm that has been traced north
to the Wyoming border (Braddock and Cole, 1990). Weathers to dark-brownish-orange,
fractured outcrops. Described in detail by Wahlstrom (1956), who considered it as
Cretaceous or Tertiary in age. Rb-Sr whole-rock age of 1,316±50 Ma (Braddock and
Peterman, 1989)
Ypd Pyroxene diorite dike (Middle? Proterozoic)—In Loveland Pass and Byers Peak 7.5-min-
ute quadrangles, consists of north-striking, dark-gray, fine-grained, unmetamorphosed
pyroxene diorite dikes containing phenocrysts of augite and plagioclase (Eppinger
and others, 1984). Undated, but are older than mineralization associated with rhyolite
porphyry (brp) and younger than Silver Plume Granite (YgSP); assigned Tertiary age
by Lovering (1935), but due to similarity to Middle Proterozoic dikes in northern Front
Range (for example, Kellogg, 1973), we interpret age as Middle Proterozoic
YXp Pegmatite and aplite (Middle and Early Proterozoic)—Pegmatite is coarse-grained to
very coarse grained, white to light-pink, inequigranular quartz-feldspar-mica rock that
forms irregular-shaped, commonly zoned dikes and intrusive bodies cutting Silver
Plume Granite (YgSP) and all older rocks; few pegmatites cut Pikes Peak Granite
(YgP and YgPp). Microcline may be longer than 1 m in some pegmatites; mica is
mostly biotite, but locally includes or is entirely muscovite. Accessory minerals include
tourmaline, garnet, and opaque minerals. Unit consists predominantly of pegmatite,
which commonly grades into and is intimately mixed with aplite, which also forms
separate dikes and bodies. Aplite is similar in composition to pegmatite but is a pink-
ish-tan, fine- to medium-grained, leucocratic, equigranular rock. Pegmatite and aplite
may be late-stage intrusions associated with rocks of either the Routt Plutonic Suite
(about 1,700 Ma) or the Berthoud Plutonic Suite (about 1,400 Ma) of Tweto (1987).
Muscovite±tourmaline-bearing varieties probably related to intrusions of the Berthoud
Plutonic Suite
YXgd Granodiorite and monzogranite of unknown age (Middle or Early Proterozoic)—Gray,
medium- to coarse-grained, equigranular to porphyritic, massive to weakly foliated
quartz-plagioclase-microcline-biotite±hornblende tonalite, granodiorite, and monzo-
granite. May include a younger, medium-grained monzogranite phase and an older,
medium- to coarse-grained granodiorite phase. Most rocks of unit similar to Boulder
Creek Granodiorite (XgdB), but here mapped separately due to uncertainty in age and
whether derived from same magma and intrusive event as Boulder Creek Granodiorite
(Tweto, 1987). Dated Middle Proterozoic rocks similar to Early Proterozoic Boulder
Creek Granodiorite, such as granodiorite of the Mount Evans batholith (YgdM) and
granodiorite and monzogranite unit (Ygd), attest to the difficulty in assigning a Middle
or Early Proterozoic age to some granodiorites and monzogranites
YgSP Silver Plume Granite (Middle Proterozoic)—Gray to pinkish-gray, medium- to coarse-
grained, equigranular, seriate, or porphyritic, massive to flow-foliated, biotite-muscovite
Description of Map Units 19

peraluminous syenogranite and monzogranite. Porphyritic varieties contain tabular
microcline phenocrysts. Locally includes muscovite-bearing pegmatite, alaskite, and
aplite. Muscovite in equivalent rocks north of quadrangle is almost entirely subsolidus,
indicating that it formed during retrograde crystallization (J.C. Cole, written commun.,
2005). Rb-Sr whole-rock age is 1,409±40 Ma (Hedge, 1969); preliminary U-Pb zircon
ages are 1,424±6 Ma (W.R. Premo, unpub. data, 2005) and 1,422 Ma (no error given;
Graubard and Mattison, 1990)
Yg Peraluminous monzogranite (Middle Proterozoic)—Rocks are very similar to those
of Silver Plume Granite (YgSP). Mapped by many people as Silver Plume Granite;
however, uncertainty as to whether they formed from same magma or intrusive episode
(Tweto, 1987) is reason unit is mapped separately. Undated
Ygd Granodiorite and monzogranite (Middle Proterozoic)—Gray to light-pinkish-gray,
medium- to coarse-grained, equigranular to porphyritic biotite-plagioclase-microcline-
quartz granitoid rocks. Unit consists of two distinct phases: medium-grained rock of
approximate monzogranite composition intrudes coarse-grained rock of approximate
granodiorite composition. Forms an irregular-shaped pluton near Empire and a larger
pluton along Continental Divide near north margin of quadrangle. Formerly considered
part of the Routt Plutonic Suite, but preliminary U-Pb zircon ages between 1,435 and
1,425 Ma on two granodiorite samples and one monzogranite sample demonstrate these
plutons are part of the Berthoud Plutonic Suite (W.R. Premo, unpub. data, 2006)
Ygb Younger gabbro (Middle Proterozoic)—Dark-gray, dark-greenish-gray, and black,
medium- to coarse-grained, massive, generally equigranular intrusive rocks ranging
from melagabbro to quartz diorite; most samples are either gabbro or pyroxene diorite
(Taylor and Sims, 1962). Contains calcic plagioclase, orthopyroxene, clinopyroxene,
magnetite, ilmenite, and secondary hornblende and biotite. Forms large body (Elk
Creek pluton of Taylor and Sims, 1962) and several nearby smaller bodies in northern
Central City 7.5-minute quadrangle (Sims and Gable, 1967). Prelininary U-Pb zircon
age is 1,436±6 Ma (W.R. Premo, unpub. data, 2007)
Yd Younger diorite and hornblendite (Middle Proterozoic?)—Dark-gray to mottled dark-
gray and white, medium- to coarse-grained plagioclase-hornblende diorite with minor
quartz and biotite; includes local hornblendite. Forms three small equant plutons in
northern part of Georgetown 7.5-minute quadrangle (Widmann and Miersemann, 2001).
Undated, but Middle Proterozoic age suspected by Widmann and Miersemann (2001)
because Spurr and others (1908) suggested unit derived by differentiation from magma
that generated granodiorite of Mount Evans
YgR Granite of Rosalie Peak (Middle Proterozoic)—Gray to light-pinkish-gray, coarse-
grained, equigranular to porphyritic biotite syenogranite and monzogranite (Bryant and
Hedge, 1978). Locally contains Carlsbad-twinned microcline phenocrysts as long as
4 cm and has weak foliation defined by aligned biotite. Two small plutons intrude the
Mount Evans batholith near Rosalie Peak. May be relatively felsic phase of granodio-
rite of Mount Evans batholith (unit YgdM). U-Pb zircon date is 1,448±9 Ma
(Aleinikoff and others, 1993a). Includes the granite of Sheep Mountain in southeastern
Keystone quadrangle (Widmann and others, 2003), which Lovering (1935) included
with the granite of Rosalie Peak
YgdM Granodiorite of Mount Evans batholith (Middle Proterozoic)—Gray, massive to strongly
foliated, coarse-grained, mostly porphyritic biotite-hornblende granodiorite, which
ranges in composition from monzogranite to tonalite (Aleinikoff and others, 1993a).
Locally contains as much as 5 percent of an aplite-pegmatite phase consisting of dikes
and irregular-shaped bodies of massive to weakly foliated aplite and magnetite-bearing
pegmatite. Formerly believed to be part of the Early Proterozoic Routt Plutonic Suite
of Tweto (1987), due to petrographic and textural similarities to rocks of that suite and
limited Rb-Sr data (for example, Bryant and Hedge, 1978). However, U-Pb zircon date
is 1,442±2 Ma (Aleinikoff and others, 1993a), confirming emplacement with Middle
Proterozoic Berthoud Plutonic Suite of Tweto (1987)
YXgT Twin Spruce Monzogranite (Middle? and Early Proterozoic)—Gray, fine- to medium-
grained, equigranular, massive to weakly foliated biotite-muscovite monzogranite and
20 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

rare granodiorite. Twin Spruce Monzogranite intrudes rocks of Boulder Creek Grano-
diorite (XgdB) in a complex pattern. Weathers tan in rounded outcrops. Preliminary
U-Pb zircon data suggest age about 1,704±6 Ma (W.R. Premo and K.S. Kellogg, unpub.
data, 2007), supporting the observation (Gable, 1980) that the unit is younger than
Boulder Creek Granodiorite. Two samples of rock petrographically similar to Twin
Spruce Monzogranite from within the Boulder Creek batholith have U-Pb zircon dates
of 1,443±19 Ma and 1,412±25 Ma (W.R. Premo, unpub. data, 2005), indicating a local
resurgence within the batholith of magmatism of the Berthoud Plutonic Suite. Twin
Spruce Monzogranite also referred to as Twin Spruce Quartz Monzonite (for example,
Gable, 1980); the lithic term “monzogranite” follows Streckeisen (1976)
Xgr Monzogranite of Elephant Butte (Early Proterozoic)—Fine- to medium-grained, massive
to moderately foliated biotite monzogranite and minor granodiorite. Forms stock-size
outcrop on and near Elephant Butte at intersection of Squaw Pass, Evergreen, Meridian
Hill, and Conifer 7.5-minute quadrangles, and elongate body in Indian Hills 7.5-min-
ute quadrangle (Bryant and others, 1973). Moderately foliated biotite monzogranite
from southeast corner of Squaw Pass 7.5-minute quadrangle has a U-Pb zircon date
of 1,684±9 Ma; sample from Indian Hills quadrangle has a U-Pb date of 1,678±6 Ma
(W.R. Premo, unpub. data., 2006)
Xgh Mafic granodiorite, quartz diorite, hornblende diorite, and hornblendite (Early
Proterozoic)—Quartz diorite is gray, black, or mottled black-and-white, medium- to
coarse-grained rock consisting mostly of plagioclase, hornblende, biotite, ±microcline,
and quartz. Hornblendite is a black to dark-green rock composed principally of horn-
blende with minor plagioclase and quartz. Unit includes mafic phases of granodioritic
rocks. Includes a quartz diorite pluton near St Marys Lake in Empire quadrangle that
has a gradational contact with surrounding granodiorite (YXgd) and may be older
than the granodiorite (Braddock, 1969). Includes hornblendite near Nederland (Gable,
2000). Two small bodies of mafic quartz diorite or hornblendite are mapped in south-
east corner of Empire quadrangle (Braddock, 1969). Preliminary U-Pb zircon date from
a biotite-hornblende diorite from Clear Creek is 1,702±8 Ma and a quartz diorite from
Turkey Creek drainage (Indian Hills quadrangle) is 1,708±4 Ma (W.R. Premo, unpub.
data, 2005), which indicates a 1,706±8 Ma quartz diorite intrusive event (Premo and
others, 2007; W.R. Premo, unpub. data, 2007)
Xgb Gabbro (Early Proterozoic)—Gray, dark-gray, and dark-greenish-gray, medium- to coarse-
grained, massive, generally equigranular intrusive rocks ranging from melagabbro to
pyroxene diorite. Composed mostly of calcic plagioclase (bytonite), clinopyroxene,
bronzite, magnetite, ilmenite, and variable amounts of secondary amphibole and mica
(Taylor and Sims, 1962). Hornblende surrounds most pyroxene grains. Forms a large,
irregular-shaped body, the Upson Creek pluton of Taylor and Sims (1962), in south-
west corner of Bottle Pass 7.5-minute quadrangle (Taylor, 1975). Intrudes granodiorite
(YXgd) that is probably similar in age to Boulder Creek Granodiorite (XgdB). Pre-
liminary U-Pb zircon date is 1,706±9 Ma (W.R. Premo, unpub. data, 2007)
XgdB Boulder Creek Granodiorite (Early Proterozoic)—Gray, coarse-grained, equigranular to
porphyritic, massive to weakly foliated biotite±hornblende±muscovite monzogranite,
granodiorite, and tonalite. Forms large intrusive body (Boulder Creek batholith) west
of Boulder (Gable, 1980). Typically weathers into grayish-tan rounded outcrops. Mean
U-Pb zircon date from numerous analyses is 1,716±3 Ma (W.R. Premo and K.S. Kel-
logg, unpub. data, 2007), which supports earlier U-Pb zircon dates of 1,725 Ma (Stern
and others, 1971) and 1,721±15 Ma (Premo and Fanning, 2000)
Xgg Granitic gneiss (Early Proterozoic)—Light- to medium-gray, fine- to medium-grained,
weakly to strongly foliated monzogranite, granodiorite, and trondhjemite. Composed
mostly of quartz, plagioclase, and microcline, with lesser amounts of biotite, ±horn-
blende, and ±muscovite. Locally contains metasedimentary and amphibolitic inclusions
indicating igneous origin, although protolith of some granitic gneiss is uncertain (Brad-
dock, 1969). Age inferred by Sheridan and others (1972) to be same as Boulder Creek
Granodiorite (XgdB) and may, in part, be equivalent to foliated facies of Boulder Creek
Granodiorite. However, most mapped granitic gneiss probably older than Boulder
Description of Map Units 21

Creek Granodiorite. Two preliminary U-Pb zircon ages from foliated granodiorite on
Mount Morrison in the Morrison 7.5-minute quadrangle are 1,772±10 Ma and 1,771±11
Ma, and a foliated monzogranite from near Deer Creek in the southern Harris Park 7.5-
minute quadrangle is 1,766±9 Ma (W.R. Premo, unpub. data, 2007)
PROTEROZOIC CATACLASTIC AND DUCTILEY DEFORMED ROCKS
YXcr Cataclastically and ductilely sheared rocks of the Idaho Springs–Ralston shear zone
(Middle and Early Proterozoic)—Gray, pinkish-gray, and pink, moderately to strongly
foliated, fine- to medium-grained, quartz-feldspar cataclastic gneiss. Contains small
white to pink feldspar porphyroclasts as long as 2.5 mm, quartz, and lesser amounts
of biotite, muscovite, and, locally, microantiperthite (Sheridan and others, 1967). In
some places, cataclastic gneiss is interlayered with biotite gneiss and schist, which
are included with map unit. Includes small (generally <10 m wide), discrete zones of
mylonite and ultramylonite. Mapped along southern part of Idaho Springs–Ralston
shear zone, in northern part of Ralston Buttes 7.5-minute quadrangle, where rocks are
sheared, granulated, and recrystallized and parent rock cannot be identified (Sheridan
and others, 1967). Includes rocks mapped as augen gneiss, containing small, lensoid,
feldspar porphyroclasts, in southern part of Eldorado Springs 7.5-minute quadrangle
(Wells, 1967). Monazite dating shows that age of shearing occurred in two discrete
stages: early, wide, high-temperature strain and amphibolite-grade metamorphism
about 1.71–1.63 Ma, producing closely spaced, upright compositional layering, and
localized mylonitic and ultramylonitic deformation about 1.45-1.38 Ma (Shaw and oth-
ers, 2001)
EARLY PROTEROZOIC METASEDIMENTARY AND META-IGNEOUS ROCKS
[Note on migmatite—Migmatite is mostly biotite gneiss (Xb) containing numerous layers of light-gray
to white granitic rock (leucosomes), which are typically 0.1–10 cm thick, although locally may be much
thicker. Some other units, such as quartz-feldspar gneiss (Xf), may locally be migmatitic. In most cases,
leucosomes form <50 percent of rock, have sharp contacts with the host rock, and are composed of equi-
granular, massive to weakly foliated, microcline-plagioclase-quartz-biotite rock (“granite”); leucosomes
may also contain minor or accessory muscovite, opaque minerals, sphene, apatite, garnet, and zircon.
Layers show much pinch and swell and in some places are strongly folded. Formation of leucosome may
be due to either injection from distant source, or in-situ partial melting (anatexis) (Olsen, 1982; Johannes
and Gupta, 1982); in the latter case, host rock adjacent to the leucosomes commonly has dark, biotite-rich
selvages (melanosomes). Distribution of mapped migmatite is highly subjective, as criteria for defining
mapped migmatite varies from place to place. For example, many felsic rocks in Squaw Pass (Sheridan
and Marsh, 1976), Central City (Sims, 1964; Sims and Gable, 1967), Black Hawk (Taylor, 1975), Ned-
erland (Gable, 1969), and Tungsten (Gable, 1972) 7.5-minute quadrangles are migmatitic; in these quad-
rangles only the rocks that are host to leucosomes were mapped. Most areas underlain by biotite gneiss
are variably migmatitic, but for the reasons cited above, migmatitic areas are not indicated on map. Two
preliminary U-Pb zircon dates from melt phase in migmatitic gneiss from Clear Creek are 1,698±3 Ma
and 1,693±35 Ma; one date from melt phase from just south of quadrangle (near Pine) is 1,692±6 Ma
(W.R. Premo, unpub. data, 2005). These data suggest peak metamorphism and partial melting in region at
1,693±5 Ma]
Xq Quartzite (Early Proterozoic?)—White, gray, and purplish-gray, medium- to coarse-
grained quartzite and quartz-rich gneiss and schist. Locally conglomeratic with clasts
of coarser grained, lighter colored quartzite that are commonly tectonically stretched.
Generally contains as much as a few percent muscovite and traces of garnet, magnetite-
ilmenite, and epidote. Unit includes muscovite-quartz gneiss and schist layers and some
calc-silicate gneiss lenses. Forms a large, synformal body (“Coal Creek quartzite”) in
southern part of Eldorado Springs 7.5-minute quadrangle (Wells, 1967) and northern
part of Ralston Buttes quadrangle (Sheridan and others, 1967); most contacts with Twin
Spruce Monzogranite (YXgT) are sheared, and relative age of quartzite and intrusive
rocks is uncertain. Gable (1980) and Wells and others (1964) described intrusive
contacts between rocks of the Boulder Creek batholith and quartzite. However, pre-
liminary detrital zircon evidence suggests quartzite is younger than the batholith (J.N.
22 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

Aleinikoff, unpub. data, 2007), a suggestion supported by structural and stratigraphic
evidence (McCoy and others, 2005). If this interpretation is correct, the depositional age
of quartzite is less than 1,704±6 Ma
Xqs Muscovite-quartz schist (Early Proterozoic?)—Mostly gray, well-foliated, fine- to
medium-grained schist composed of quartz, muscovite, biotite, and local occurrences of
andalusite, cordierite, garnet, plagioclase, and staurolite (Wells, 1967). At most places,
rock is conspicuously lineated and foliation planes are crinkled. Interlayered with
quartzite in Eldorado Springs 7.5-minute quadrangle (Wells, 1967) and Ralston Buttes
quadrangle (Sheridan and others, 1967)
Xbp Porphyroblastic quartz-biotite-muscovite schist of White Ranch (Early Protero-
zoic)—Silvery gray schist layers composed mostly of fine- to medium-grained, foliated,
locally porphyroblastic schist composed principally of quartz, biotite, and muscovite,
with small amounts of sillimanite and staurolite (Wells, 1967). Porphyroblasts include
andalusite, cordierite, and garnet; some andalusite crystals in central Ralston Buttes
7.5-minute quadrangle are as long as 30 cm (Sheridan and others, 1967). Where rock
is relatively unstrained, original sedimentary structures (graded bedding and cross
bedding) are visible. Locally contains lenses of meta-conglomerate and calc-silicate
rock (Sheridan and others, 1967). U-Pb zircon ages from clasts in metaconglomerate
are ≈1,750 Ma, which is maximum depositional age of sediments in basin that includes
rocks of this unit
Xlg Mixed layered gneiss (Early Proterozoic)—Interlayered gneiss of varied types, including,
but not limited to, quartz-feldspar gneiss (Xf), biotite gneiss (Xb), and hornblende-
plagioclase gneiss and amphibolite (Xh). Migmatitic in places. Mapped where indi-
vidual units too small to map
Xb Biotite gneiss (Early Proterozoic)—Gray, medium-grained, equigranular, well-foliated
gneiss typically containing approximately 25–50 percent quartz, 20–30 percent pla-
gioclase (approximately An30), 0–30 percent microcline, 10–15 percent biotite, 0–15
percent muscovite, 0–10 percent sillimanite, 0–5 percent hornblende, 1–2 percent
opaque minerals, and a trace zircon. Most outcrops contain 5–20 percent granitic layers
(leucosomes). Sillimanite, where abundant, occurs in fibrous, elongate, light-colored
aggregates or clots as wide as about 1 cm. Migmatitic biotite gneiss in Clear Creek
gave preliminary U-Pb zircon date on nonmelt phase of 1,773±18 Ma; near Pine, just
south of map area, similar rock gave U-Pb zircon date of 1,780±9 Ma (W.R. Premo,
unpub. data, 2005)
Xbhc Biotite gneiss, hornblende gneiss, and calc-silicate gneiss (Early Proterozoic)—Gray to
dark-gray, foliated, interlayered biotite gneiss (Xb), hornblende-plagioclase gneiss and
amphibolite (Xh), and calc-silicate gneiss (Xc). Mapped where individual units too
small to show separately
Xbg Biotite gneiss and schist with garnet (Early Proterozoic)—Gray, fine- to medium-grained,
well-foliated gneiss composed chiefly of quartz, plagioclase, and biotite; garnet crystals
in some layers as large as 1 cm in diameter. Locally schistose and contains muscovite.
Mapped in western Morrison 7.5-minute quadrangle (Scott, 1972) and northern Indian
Hills quadrangle (Bryant and others, 1973)
Xc Calc-silicate gneiss (Early Proterozoic)—Dark-gray, light-green, yellowish-green, white,
pink, or black, fine- to coarse-grained, compositionally layered rock commonly associ-
ated with hornblende-plagioclase gneiss and amphibolite (Xh). Wide variation in colors
due to relative amounts of constituent minerals, which include plagioclase, hornblende,
clinopyroxene, epidote, quartz, microcline, garnet, scapolite, vesuvianite, calcite, dolo-
mite, cummingtonite, tremolite, sphene, and magnetite-ilmenite (Sheridan and Marsh,
1976). May include layers of impure marble. Commonly occurs in lenses and pods.
Derived from metamorphosed carbonate-rich sedimentary rocks. Mapped mostly in
Ralston Buttes, Evergreen, and Byers Peak 7.5-minute quadrangles (Sheridan and oth-
ers, 1967, 1972; Eppinger and others, 1984)
Xbc Cordierite-biotite gneiss (Early Proterozoic)—Light- to dark-gray, fine- to medium-
grained, cordierite-bearing biotite gneiss. Composed principally of quartz, plagioclase,
biotite (phlogopitic in lighter varieties), as much as 20 percent cordierite, and variable
Description of Map Units 23

amounts of sillimanite. Cordierite inconspicuous in outcrop, so distinguished from
biotite gneiss (Xb) mostly by thin-section study. Typically has conspicuous, alternat-
ing, light- and dark-colored layers 2 mm to 2.5 cm thick. Mapped in Nederland and
Tungsten 7.5-minute quadrangles (Gable, 1969, 1972) and along a west-northwest trend
through the Morrison, Evergreen, and Squaw Pass quadrangles (Sheridan and others,
1972; Sheridan and Marsh, 1976). Due to difficulty in identifying cordierite, this unit
is probably much more common in map area, particularly where biotite gneiss (Xb) is
mapped
Xsr Rutile-sillimanite-quartz gneiss (Early Proterozoic)—Mostly white to light-gray, fine- to
medium-grained, rutile-bearing, biotite-quartz-plagioclase-sillimanite gneiss and silli-
manite-quartz gneiss in thin (<15 cm to as much as 150 m thick) but regionally exten-
sive layers and lenses. Rutile forms a few tenths of a percent to as much as 5 percent
of gneiss. Variants contain gahnite (zinc spinel), in which the rock commonly appears
bleached, and topaz, which locally forms as much as 67 percent of rock (Sheridan and
others, 1967). Mapped mostly in Squaw Pass 7.5-minute quadrangle (Sheridan and
Marsh, 1976); one small occurrence mapped in northern Indian Hills quadrangle (Bry-
ant and others, 1973)
Xhq Amphibolite, marble, and quartzite (Early Proterozoic)—Interlayered amphibolite,
quartzite, coarse-grained, locally siliceous, white to light-gray marble, quartz-rich calc-
silicate gneiss and schist, and quartz-sillimanite-muscovite-biotite gneiss and schist.
Marble and quartzite layers locally as thick as 10 m. Mapped only in Indian Hills 7.5-
minute quadrangle (Bryant and others, 1973)
Xaq Amphibolite and quartzite (Early Proterozoic)—Gray to dark-gray, interlayered amphib-
olite, hornblende gneiss, biotite-hornblende-plagioclase gneiss, and subordinate calc-
silicate gneiss and quartzite. Mapped only in Indian Hills 7.5-minute quadrangle (Bry-
ant and others, 1973), where it forms a structurally overlying carapace to two antiformal
bodies of quartz-feldspar gneiss (Xf). One U-Pb zircon age on a quartz-rich layer in a
hornblende-gneiss sequence from near Phillipsburg is 1,773±4 Ma (W.R. Premo, unpub.
data, 2006), which is minimum age of deposition of the quartz-rich sand protolith
Xf Quartz-feldspar gneiss (Early Proterozoic)—Gray, dark-gray, white, pinkish-gray, and
tan, moderately foliated to well-foliated, layered, fine- to coarse-grained (mostly
medium grained) quartz-plagioclase-microcline-biotite gneiss. Proportion of minerals
varies widely. Layers typically 10 cm to several tens of meters thick and commonly
wavy. Locally contains layers of foliated monzogranite and granodiorite. May con-
tain minor, thin layers of hornblende gneiss and amphibolite. Commonly migmatitic.
U-Pb date on weakly foliated rock of monzogranitic composition in northwestern part
of Indian Hills 7.5-minute quadrangle is 1,776±4 Ma. Weathers tan to pinkish tan in
rounded outcrops. Crops out widely throughout map area
Xfh Quartz-feldspar gneiss and hornblende gneiss (Early Proterozoic)—Interlayered
dark-gray and light-gray, well-foliated quartz-feldspar gneiss (Xf) and hornblende-pla-
gioclase gneiss and amphibolite (Xh), including amphibolite, in approximately equal
amounts. Some areas in Squaw Pass 7.5-minute quadrangle are equivalent to the mixed
layered gneiss unit (Xlg) of Evergreen quadrangle
Xh Hornblende-plagioclase gneiss and amphibolite (Early Proterozoic)—Dark-gray to
black, weakly to strongly foliated, layered, mostly medium grained, hornblende-pla-
gioclase gneiss and amphibolite containing variable amounts of biotite, quartz, and
augite. Commonly has black-and-white mottled texture due to weathered plagioclase
(white) and hornblende (black). Amphibolite contains greater than 50 percent amphi-
bole. Hornblende-plagioclase gneiss contains interlayered amphibolite, particularly in
Nederland 7.5-minute quadrangle, and minor calc-silicate gneiss (Xc) and cordierite-
biotite gneiss (Xbc) (Gable, 1969). Commonly intimately interlayered with more felsic
gneissic rocks, so many occurrences are included with other units (particularly Xlg,
Xbhc, Xaq, Xf, Xfh, Xh, and Xhc)
Xhc Hornblende gneiss and calc-silicate gneiss (Early Proterozoic)—Interlayered horn-
blende-plagioclase gneiss and amphibolite (Xh), with lesser amounts of interlayered,
commonly lensoidal or pod-shaped bodies of calc-silicate gneiss (Xc). Mapped widely
24 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado

in Ralston Buttes, Squaw Pass, and Evergreen 7.5-minute quadrangles (Sheridan and
others, 1972; Sheridan and Marsh, 1976; Sheridan and others, 1967) and in northwest
corner of quadrangle (Eppinger and others, 1984)
Xgc Mixed calc-silicate and biotite gneiss (Early Proterozoic)—Biotite schist and gneiss, cal-
careous quartzite, calc-silicate quartzite, biotite marble, and some pegmatites as much as
5 m thick that are concordant with layering. Mapped along north border of quadrangle
near Tabernash, Colo., and joins units in Strawberry Lake quadrangle to north (Schro-
eder, 1995)
Geologic History of the Denver West Quadrangle 25
Geologic History of the
Denver West Quadrangle
The Denver West quadrangle spans the entire axis of the
Front Range, one of numerous uplifts in the Rocky Mountain
region in which Precambrian rocks are exposed. The history
of the basement rocks in the Denver West quadrangle extends
as far back as 1,790 Ma. Along the east side of the range, a
sequence of sedimentary rocks as old as Pennsylvanian, but
dominated by Cretaceous-age rocks, overlies these ancient
basement rocks and was upturned and locally faulted dur-
ing Laramide (Late Cretaceous to early Tertiary) uplift of the
range. The increasingly coarser grained sediments up section
in rocks of latest Cretaceous to early Tertiary age record in
remarkable detail this Laramide period of mountain building.
On the west side of the range, a major Laramide fault (Wil-
liams Range thrust) places Precambrian rocks over Creta-
ceous marine sedimentary rocks. The geologic history of the
quadrangle, therefore, can be divided into four major periods:
(1) Precambrian history, (2) Pennsylvanian to pre-Laramide,
Late Cretaceous history, (3) Late Cretaceous to early Tertiary
Laramide mountain building, and (4) post-Laramide history.
Much of the geologic history herein is summarized from Kel-
logg and others (2004).
Precambrian History
Marine sediments, as well as mafic and felsic volcanic
rocks, that were generally metamorphosed to amphibolite
grade and intruded by calc-alkaline granitic rocks form the
core of the Front Range and are part of an Early Proterozoic
terrane called the Colorado province (Bickford and others,
1986). These rocks are interpreted to have formed over a long
orogenic episode, beginning at about 1,790 Ma and lasting
about 130 m.y., that may have accompanied Early Proterozoic
accretion of island arcs and back-arc basins to the southern
margin of an Archean continent (Reed and others, 1987;
Aleinikoff and others, 1993b).
Proterozoic rocks of the Front Range include complexly
folded and interlayered quartz-feldspar gneiss, amphibolite,
biotite schist, and partial-melt migmatite. The biotite-rich
rocks locally contain layers and lenses of marble, quartz-
ite, and conglomerate, indicating sedimentary protoliths.
Amphibolite (part of units Xh and Xhc) and quartz-feldspar
gneiss (Xf) are generally abundant in separate areas from
the metasedimentary rocks and probably represent original
volcanic complexes, although locally both the metasedimen-
tary and metavolcanic packages are complexly interlayered
on a regional scale. The rocks are commonly metamorphosed
to high-temperature, low-pressure, upper amphibolite assem-
blages; the metasedimentary rocks are mostly partially melted
(migmatitic) and chiefly contain potassium-feldspar, biotite,
sillimanite, ±garnet, and ±cordierite.
Details of the history of the Proterozoic rocks in Colo-
rado are only locally well known (see, for example, Bickford
and others, 1986, 1989; Braddock and Cole, 1979; Aleini-
koff and others, 1993b; Premo and Fanning, 2000; Premo
and others, 2007). Early metavolcanic and metasedimentary
rocks were deposited in the northern Front Range region
about 1,780–1,770 Ma (Premo and others, 2007). A younger
sedimentary basin, now occupied by porphyoblastic quartz-
biotite-muscovite schist of White Ranch (Xbp) and possibly
rocks of some adjacent units (for example, Xc, Xbhc, Xh, and
Xf), formed after ≈1,750 Ma.. Reliable facing directions have
been found in unit Xbp and in the large syncline in quartzite
(Xq, “Coal Creek quartzite”) in the Eldorado Springs and
Ralston Buttes 7.5-minute quadrangles (Wells, 1967; Sheri-
dan and others, 1967; McCoy and others, 2005; K.S. Kellogg
and Bruce Bryant, unpub. data, 2004). If the interpretation of
McCoy and others (2005) is correct, and Coal Creek quartzite
was deposited on rocks of the Boulder Creek batholith, then
a possible third depositional basin, younger than ≈1,704 Ma,
exists in the Denver West quadrangle.
At most places in the quadrangle, the Early Proterozoic
metasedimentary and metavolcanic rocks have been strongly
deformed in a ductile fashion, recrystallized, and partially
melted. Peak metamorphism, presumably during crustal
accretion to the Archean Wyoming province to the north near
the Wyoming border, is thought to have occurred ≈1,750 Ma
(Reed and others, 1993), although new single-crystal, U-Pb
zircon results from several localities in and near the Denver
West quadrangle indicate that a period of partial melting
(anatexis) and formation of migmatites occurred about 1,697
Ma (W.R. Premo, unpub. data, 2005). Coeval with or closely
following the ≈1,750 Ma metamorphic and deformational
event, extensive batholiths and smaller bodies of mostly grano-
diorite and monzogranite, referred to as the Routt Plutonic
Suite (Tweto, 1987), intruded the older layered rocks of the

26 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado
Front Range. The Boulder Creek Granodiorite (XgdB), dated
at 1,716±3 Ma (W.R. Premo and K.S. Kellogg, unpub. data,
2007), was emplaced synchronous with late folding (Gable,
2000) in these high-grade terranes. Numerous bodies of plu-
tonic rock have been correlated with this batholith, but many
of these rocks vary widely in age, with dates as old as ≈1,770
Ma and as young as ≈1,430 Ma (W.R. Premo and K.S. Kel-
logg, unpub. data, 2006).
The Early Proterozoic basement of the Front Range was
extensively modified by widespread intrusions, regional heat-
ing, and local deformation during a regional orogenic event
about 1,400 Ma. Large and small plutons of this age, called
the Berthoud Plutonic Suite (Tweto, 1987), are characterized
by mostly nonfoliated, commonly porphyritic biotite monzo-
granite and syenogranite.
The Silver Plume Granite (YgSP), a peraluminous
biotite-muscovite monzogranite, forms a batholith that extends
from the town of Silver Plume to west of the Continental
Divide. The Silver Plume Granite has a U-Pb zircon date
of 1,424±6 Ma (W.R. Premo, unpub. data, 2006) and was
derived by limited partial melting of lower crustal material
and emplaced possibly as shallow as 8 or 9 km (Anderson and
Thomas, 1985). This batholith and similar intrusions contain
many inclusions of country rock and have complex contacts
composed of numerous dikes and irregular bodies intruded
into the country rock.
The Mount Evans batholith of metaluminous granodiorite
and monzogranite (YgdM) superficially resembles the Boul-
der Creek batholith but has a U-Pb zircon date of 1,442 Ma
(Aleinikoff and others, 1993a). Similarly, granitic rocks near
Empire and along the Continental Divide in the northern part
of the quadrangle, formerly interpreted as part of the Routt
Plutonic Suite based on their similarity to the Boulder Creek
Granodiorite, have new U-Pb zircon dates between 1,435 and
1,425 Ma (W.R. Premo and K.S. Kellogg, unpub. data, 2006).
The Middle Proterozoic (≈1,400 Ma) magmatism reset
the rubidium-strontium and potassium-argon isotopic sys-
tems (Peterman and others, 1968; Shaw and others, 1999).
40Ar/39Ar dates on muscovite and biotite are all 1,400–1,340
Ma, reflecting cooling through closure temperature after 1,400
Ma. 40Ar/39Ar dates on hornblende range from 1,600 to 1,390
Ma and represent variable retention of radiogenic argon (Shaw
and others, 1999).
North-northwest-trending, ≈1,415 Ma diabase dikes
[not mapped; more extensive north of the quadrangle (Peter-
man and others, 1968)] and undated lamprophyre dikes (not
mapped, but well exposed in Clear Creek Canyon) are among
some of the late Precambrian intrusives. A north-northwest-
striking gabbro dike (“Iron dike,” Ygb), dated at 1,316±50
Ma (Braddock and Peterman, 1989), can be traced north to the
Wyoming border from the northern part of the Denver West
quadrangle just west of Boulder. Emplacement of the anoro-
genic, peralkalic Pikes Peak batholith at ≈1,080 Ma (Unruh
and others, 1995) in the southern Front Range marks the final
major Proterozoic rock-forming event.
The Proterozoic rocks of the Front Range are transected
by a number of northeast- to east-trending, discontinuous,
en echelon shear zones consisting of steeply dipping fabric
exposed in mylonitic and nonmylonitic rocks. Three of these
major shear zones, the Idaho Springs–Ralston shear zone
(Sheridan and others, 1967; Wells, 1967), the Saint Louis Lake
shear zone in the Byers Peak quadrangle (Shaw and others,
2002), and the Montezuma shear zone in the Montezuma
quadrangle, transect the rocks of the Denver West quadrangle.
Recent study of the Idaho Springs–Ralston shear zone
and another major shear zone (Homestake shear zone) west
of the quadrangle shows that a zone of steeply dipping, highly
strained but nonmylonitic rock formed about 1,720 Ma.
Mylonites and ultramylonites formed locally along the same
shear zones during or slightly after the ≈1,400 Ma plutonic
event (Shaw and others, 2001, 2002; McCoy and others,
2005). Youngest shear-sense indicators in mylonites within
the shear zones suggest southeast-up reverse movement (Brad-
dock and Cole, 1979; Selverstone and others, 2000). The
shear zones are interpreted by Shaw and others (2001) to have
initially developed as a system of diffuse, high-strain zones
related to continental assembly of terranes to form the Early
Proterozoic crust of the region. Braddock and Cole (1979)
conversely suggested that shearing was localized by gravita-
tional buoyancy of major ≈1,400 Ma granitic plutons.
Northeast of the Mount Evans batholith, the Idaho
Springs–Ralston shear zone marks a discontinuity in the trends
of the major folds in the metamorphic rocks, although there
is no major lithologic contrast across the zone. North of the
zone, folds trend north-northeast, whereas south of the zone
they generally trend northwest.
A third period of plutonism at ≈1,080 Ma (Unruh and
others, 1995) is marked by emplacement of the anorogenic
Pikes Peak Granite batholith, the northern lobe of which occu-
pies the southern part of the quadrangle.
Paleozoic and Pre-Laramide Mesozoic History
During the early Paleozoic, thin continental-shelf
sequences of quartz-rich sands and carbonates were depos-
ited in shallow seas over the region. In the late Paleozoic,
however, northwest- and north-northwest-trending mountain
ranges and basins formed during the Ancestral Rocky Moun-
tain orogeny. Erosion during uplift of the Ancestral Front
Range removed the lower Paleozoic sedimentary cover and no
sedimentary strata older than Pennsylvanian are preserved in
the quadrangle. About 500 m of mostly red and pink, arkosic
sandstone and conglomerate of the Fountain Formation (Phf),
exposed along the east flank of the Front Range, were depos-
ited adjacent to the east flank of the Ancestral Front Range.
Permian eolian deposits of the Lyons Formation (Pl) and
Permian and Triassic fluvial and near-shore deposits of the
Lykins Formation (dPl) overlie the thick clastic sequences of
the Fountain Formation. By the Middle Jurassic, the Ancestral
Front Range had been eroded to low relief and was mostly

Geologic History of the Denver West Quadrangle 27
covered by fluvial and lacustrine deposits of the Morrison
Formation (Jm) and, on the east side of the range, the Ralston
Creek Formation. Near the end of the Early Cretaceous, major
subsidence coeval with a rise in sea level caused transgression
of the western interior seaway over the entire Front Range,
which commenced with deposition of shoreline deposits of
the Dakota Group (Kd), followed by deposition of more than
2 km of marine shale and minor amounts of sandstone and
limestone [Benton Group (Kb), Niobrara Formation (Kn), and
Pierre Shale (Kp)].
The Laramide Orogeny
The Laramide orogeny was a 20-m.y. period of crustal
contraction, uplift, faulting, and igneous activity that initi-
ated the building of the present Rocky Mountains. Its early
stirrings were marked by renewed uplift of the Front Range
region in the Late Cretaceous. The western interior seaway
began to withdraw from the quadrangle area after 69 Ma,
the age of the youngest ammonite zone in the Pierre Shale
(Scott and Cobban, 1965; Cobban, 1993). This age is based
on 40Ar/39Ar dating of tuffs outside the quadrangle but in
the same ammonite zone (Obradovich, 1993). The Upper
Cretaceous–lower Tertiary rocks overlying the Pierre Shale
record the uplift history, starting with the regressive shoreline
deposits of the Fox Hills Sandstone, followed by coastal-
plain deposits that formed the sandstones and coal beds of the
Laramie Formation (the namesake for the Laramide orogeny),
in turn overlain by the fluvial conglomerates, sandstones, and
claystones of the Arapahoe Formation and Denver Formation
(Raynolds, 1997, 2002). Uplift in this area was geologically
rapid; only a few million years separate the ages of the Upper
Cretaceous marine deposits of the Pierre Shale and the earliest
conglomerates of the terrestrial Upper Cretaceous Arapahoe
Formation (Ka), which contains clasts derived from Protero-
zoic basement rocks. During this short period, the newly
formed Rocky Mountains rose from the sea, and more than 2
km of upper Paleozoic and Mesozoic sedimentary rocks were
eroded from the core of the range.
Debris of roughly andesitic composition derived from
volcanoes somewhere west of the present mountain front
forms a major part of the uppermost-Cretaceous–lowest
Paleocene Denver Formation (bKd). Mostly alkali-calcic
and alkalic dikes and stocks intruded the central Front Range
region beginning about 68 Ma and continuing until about 27
Ma. The only Upper Cretaceous or lower Tertiary extrusive
equivalent of the intrusions in this region are the lahars and
minor andesitic flows in the Windy Gap Volcanic Member of
the Middle Park Formation, which crops out in the northwest-
ern part of the quadrangle (Izett, 1968; Taylor, 1975).
In the Golden area, the upper part of the Denver Forma-
tion contains ≈65 Ma potassic basalt (shoshonite) flows (bdb)
that almost certainly erupted from a source a few kilometers to
the north (Ralston Buttes intrusive). On South Table Moun-
tain, the K-T (Cretaceous-Tertiary) boundary layer (65.4 Ma;
Obradovich, 1993) is 71 m below the lowest of these basalts.
Paleomagnetic directions from the Ralston Buttes intrusive
(bbi) are rotated, indicating that the body was emplaced
before major movement on the Golden fault (Hoblitt and
Larson, 1975) and, by inference, before uplift of the Front
Range. Near the summit of Green Mountain, about 240 m
stratigraphically above beds that are laterally equivalent to the
basalts, the Green Mountain Conglomerate (bg), which over-
lies the Denver Formation, contains a 64 Ma tuff (Obradovich,
2002). The similarity of all these ages within a relatively thick
sedimentary sequence attests to rapid sedimentation, which,
in turn, was due to the rapid erosion of the uplifting Front
Range, during the close of the Cretaceous and the opening of
the Tertiary.
The northeast-trending Boulder-Weld fault zone (Davis
and Weimer, 1976) in the northeastern part of the quadrangle
has been interpreted as a series of predominantly high-angle
normal and reverse faults (“horst-and-graben” structures) that
offset coal beds in the upper Laramie Formation (Kl) (Spen-
cer, 1961; Lawrie, 1966). These faults are remarkably on
strike with the Proterozoic Idaho Springs–Ralston shear zone,
and have been interpreted as the result of renewed right-lat-
eral strike-slip movement along this trend (Spencer, 1961),
although intervening east-dipping beds of Dakota Group have
not been offset or deformed relative to beds north and south
of the trend. More recently, these faults have been interpreted
as resulting from southeast-directed, low-angle decollement
faulting on weak beds in the Pierre Shale, producing a series
of southeast-directed reverse faults at the surface (Kittleson,
1989). Many of the faults of the Boulder-Weld fault zone have
northwest-side-down offset, which could be explained in the
decollement model as back thrusts. If the Boulder-Weld fault
zone is due to decollement faulting, alignment with the Idaho
Springs–Ralston shear zone is probably fortuitous.
At the margin of the Laramide Middle Park basin in the
Bottle Pass 7.5-minute quadrangle west of Fraser, Paleocene
and Upper Cretaceous(?) Middle Park Formation unconform-
ably overlies rocks as old as Proterozoic biotite gneiss and as
young as the Late Cretaceous Pierre Shale. At the base of the
Windy Gap Volcanic Member of the Middle Park Formation
(Kmw), which palynomorph data suggest is Late Cretaceous
in age (Izett, 1968), these relationships indicate that deforma-
tion and erosion started at this locality before the end of the
Cretaceous. Trachyandesite lahars, conglomerates, and minor
lava flows are interbedded with well-bedded fluvial conglom-
erate and sandstone containing debris from both Cretaceous
volcanic rocks and Precambrian rocks. Volcanic detritus
decreases up section from the Windy Gap Volcanic Member,
and 150 m above its top, clasts in the upper member of the
Middle Park Formation (bKmu) are mainly derived from
Precambrian basement rocks.
The principal Laramide structure on the east side of the
Front Range is the Golden fault. Seismic sections and a few
well data indicate that the Golden fault dips about 50°–70º to
the west and has as much as 3 km of eastward thrust dis-
placement (Weimer and Ray, 1997). On the west side of the

28 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado
Front Range, the Williams Range thrust defines the western
structural boundary of the Front Range, and, in contrast to
the Golden fault, is a low-angle thrust with a minimum lateral
displacement of 9 km, a distance known due to a thrust win-
dow related to uplift by the 38 Ma Montezuma stock (Ulrich,
1963; Kellogg and others, 2004). Movement on the Williams
Range thrust is thought to be as young as late Paleocene and
early Eocene, by analogy with the timing of movement on the
probable southern extension of the Williams Range thrust, the
Elkhorn thrust in South Park (Bryant and others, 1981a). The
connection between the Williams Range and Elkhorn thrusts is
obscured by intrusive rocks and surficial deposits.
In the Bottle Pass quadrangle west of Fraser (Taylor,
1975), Proterozoic rocks are faulted against rocks of the Paleo-
cene and Upper Cretaceous Middle Park Formation along a
high-angle reverse fault. Exploratory drilling on the crest of
an anticline in the Middle Park Formation west of that fault
revealed that Cretaceous rocks as old as the Dakota Group
were thrust at least 2 km over the Middle Park Formation. The
age of this thrusting is not closely constrained but is probably
Paleocene or early Eocene. These reverse faults are overlain
by the Oligocene and Miocene Troublesome Formation, which
covers most of the Fraser basin.
In the western part of the quadrangle, the northeast-
trending zone of closely spaced faults is called the Loveland
Pass–Berthoud fault zone (Bryant and others, 1981b); perva-
sive fracturing of rocks in this zone is widespread. Fracturing
may have been caused during Laramide movement along the
low-angle Williams Range thrust, in which brittle hanging-
wall Proterozoic rocks accommodated a bend in the thrust
surface, from steep at depth to gentle near the surface (Kel-
logg, 2001; Kellogg and others, 2004).
The Colorado Mineral Belt
The Colorado mineral belt is a northeast-trending irregu-
lar zone of Late Cretaceous and Tertiary (68–27 Ma) mostly
alkali-calcic and alkalic stocks and dikes, some of which are
associated with several world-class ore deposits. The mineral
belt crosses a large part of the area of Precambrian outcrop in
the Denver West quadrangle. It extends northeastward across
the mountainous part of Colorado from the western San Juan
Mountains in southwestern Colorado to the east flank of the
Front Range north of Boulder. The mineral belt contains
most of the major metallic-mining districts in Colorado and
seems to be related to a zone of crustal weakness marked by
the northeast-trending ductile shear zones in the Precambrian
rocks (Tweto and Sims, 1963). The belt of intrusives has been
interpreted as the expression of a large subjacent batholith or
series of batholiths, a suggestion that is consistent with the fact
that the mineral belt is nearly coincident with a major grav-
ity low, one of the lowest Bouguer gravity minimums in the
United States (Tweto, 1975).
The abundant Laramide and post-Laramide stocks and
dikes of the Colorado mineral belt have a wide range of
composition and a long and complicated history of emplace-
ment. The earliest intrusions in the Front Range region are
chiefly monzonites and granodiorites, followed by more
alkalic magmas, all emplaced during the interval from 68 to
about 54 Ma (Rice and others, 1982). Ore deposits at Central
City and Idaho Springs may have developed from a short-
lived (≈1 m.y.), complex hydrothermal system that developed
about 62 Ma (Nelson and others, 2003) at the end of the period
of alkalic intrusive activity. The deposits consist of a zoned
hydrothermal system having a core of gold-bearing pyrite-
quartz veins, an intermediate zone of pyrite veins carrying
copper, lead, and zinc sulfide minerals, and a peripheral zone
of galena-sphalerite-quartz-carbonate veins (Sims and others,
1963; Moench and Drake, 1966; Sims, 1988; Rice and others,
1982).
Farther to the southwest, large granitic plutons emplaced
at about 40±5 Ma (late Eocene) include the Montezuma
stock in the Denver West quadrangle. Hydrothermal sys-
tems that formed the base- and precious-metal deposits at
the Georgetown, Silver Plume, Montezuma, and (just west
of the quadrangle) Breckenridge districts are associated
with these younger granitic intrusives. The ≈37 Ma Silver
Plume–Georgetown district, west of the Central City district,
has silver-lead-zinc-bearing quartz-carbonate veins and some
gold-silver veins trending east to northeast and controlled by
several sets of steeply dipping fractures (Bookstrom, 1988;
Bookstrom and others, 1987).
Some of the largest ore bodies in the mineral belt are
associated with the youngest intrusives (30–26 Ma) in the
Front Range region, many of which are high-silica alkali
granite or rhyolite porphyries (rhyolite-A suite) (Geissman and
others, 1992; White and others, 1981). One of these ore bod-
ies is the world-class molybdenum deposit at the Henderson
mine on Red Mountain (Wallace and others, 1978).
Post-Laramide Cenozoic History of the
Front Range
Paleogene
By the close of the Laramide orogeny, during the early
Eocene (about 50 Ma), erosional debris derived from the
Laramide Front Range uplift formed a sedimentary apron, now
largely eroded, that lapped onto the flanks of the range. Much
of the topographic relief now visible along the eastern margin
of the Front Range is due to post-Eocene erosion (Leonard and
Langford, 1994), which removed most (as much as about 400
m) of the sedimentary apron (Kelley and Chapin, 1995). By
the end of the Eocene (about 34 Ma), a widespread erosion
surface cut on Precambrian rock had formed across the Front
Range (Epis and Chapin, 1975), due in part to the relatively
stable base level, unusually warm equable climate, and deep
weathering during the Eocene (Chapin and Kelly, 1997). The
erosion surface was a mature landscape of low relief, locally

Geologic History of the Denver West Quadrangle 29
bordered by subdued ridges and summits, some as much as a
few hundred meters high (Epis and others, 1980). The relict
surface is visible from Denver and consists of accordant ridges
and low-relief terrain at elevations of about 2,200–2,750 m
(Scott and Taylor, 1986). High peaks, similar in form to those
along the present-day Continental Divide, rose west of the
erosion surface.
Extensive gently sloping alpine regions are locally pres-
ent in the map area above modern timberline at elevations of
about 3,550–3,800 m along and near the Continental Divide
and about 3,770–3,850 m near Mount Evans. These areas
are composed chiefly of scattered granitic bedrock tors sur-
rounded by thin (probably about 1–2 m) regolith that consists
of rock debris produced and transported by periglacial pro-
cesses and, locally, residuum formed by in-place disintegration
of granitic bedrock. Geologists have debated the origin of
alpine regions for nearly 90 years (Bradley, 1987); they may
be the alpine equivalent of the montane erosion surface (Epis
and others, 1980) east of Mount Evans at elevations of about
2,200–2,750 m (Scott and Taylor, 1986).
The Fraser basin began to form at the end of the Oli-
gocene, at about the same time that the graben beneath the
Blue River valley formed, beginning about 29 Ma in response
to tectonic activity along the Rio Grande rift (Tweto, 1978).
Basal deposits of the sedimentary fill (Troublesome Forma-
tion) in both areas are of late Oligocene age (Izett, 1974; Kron,
1988; Naeser and others, 2002).
Neogene
Renewed uplift of the western part of the Front Range
and probable eastward tilting of the range during the Miocene
(Raynolds, 1997; Steven and others, 1997; Naeser and others,
2002) were accompanied by active faulting in the western
part of the range (Geissman and others, 1992). Uplift of the
range and the west side of the Denver Basin promoted ero-
sion, which removed large volumes of the upper Paleozoic
and Mesozoic sedimentary rocks. Erosional detritus was
deposited in the Ogallala Group on the eastern plains. During
the Miocene, ancestral Clear Creek cut a paleovalley 1.5 km
or more wide and about 240 m deep (Epis and others, 1980).
High-level gravel deposits (Ng) preserved on some of the
ridges in the Front Range east of the Continental Divide are
probably coarse-grained proximal equivalents of the Ogallala
Group that filled or partly filled Miocene paleovalleys (Scott
and Taylor, 1986; Steven and others, 1997). These east-trend-
ing, gravel-capped ridges, once valley bottoms, are resistant
to erosion and, therefore, now form an inverted topography.
Some of the gravel deposits have sinuous patterns and are not
aligned with bedrock structure, suggesting deposition by low-
gradient streams. Other deposits, such as those near Tungsten
Mountain southeast of Nederland, contain boulders 1–2 m
long in a granule-rich, poorly sorted matrix and may have been
deposited by debris flows.
Following the deposition of the Ogallala Group during
the Miocene (about 18–5 Ma; Swinehart and others, 1985),
the Pliocene Epoch appears to have been chiefly a time of
widespread erosion in upland areas, major canyon cutting in
Precambrian crystalline rock along the east flank of the Front
Range, and incision by major streams of Tertiary and under-
lying Upper Cretaceous sedimentary rocks in the adjacent
piedmont (Steven and others, 1997). The South Platte River
and its major tributaries eroded the Ogallala Group and a
substantial amount of the underlying sedimentary rocks from
the eastern part of the map area, but extensive deposits of the
Ogallala Group are present north and east of Denver (Leon-
ard, 2002), outside the map area. During Pliocene time, Clear
Creek Canyon was incised about 405–435 m below the level
of the montane erosion surface (Epis and others, 1980).
Accelerated stream erosion, canyon cutting, and steep-
ened stream profiles were probably fostered by greater stream
power, due in part to wetter climate as well as tectonic and
isostatic uplift during the Pliocene. Marine isotopic records
since about 15 Ma (middle Miocene) show a long-term trend
toward global cooling, except for a warming trend during the
early part of the Pliocene (about 5–3 Ma; Zachos and oth-
ers, 2001, and references cited therein). The Pliocene Epoch
in the western United States was characterized by climatic
conditions significantly wetter (more effective precipitation)
and stormier than those of today, particularly about 4.5–3.5
Ma (Forester, 1991) and 3.2–2.8 Ma (Smith and others,
1993). Globally averaged temperatures during the early part
of the epoch (5–3 Ma) were substantially higher than they
are today (Fedorov and others, 2006). At 3 Ma there was a
major change in ocean circulation (Federov and others, 2006)
accompanied by renewed global cooling (Zachos and others,
2001, and references cited therein), significant increases in the
magnitude of temperature ranges during cold (glacial)/warm
(interglacial) climatic cycles (Morrison, 1991), and the devel-
opment of major northern hemisphere ice sheets after 2.5 Ma
(Shackleton and others, 1984; Prell, 1984; Thompson, 1991;
Ravelo and others, 2006).
Some of the sediments produced by erosion and stream
incision during the Pliocene are preserved in the Nussbaum
Alluvium (about 3 Ma) along the South Platte River in north-
eastern Colorado (Scott, 1978, 1982). Post-Ogallala tectonic
and erosion-induced isostatic uplift are inferred from post-
depositional increases in tilt of the Ogallala Group in south-
ern Wyoming (McMillan and others, 2002) as well as from
warping of the base of the Ogallala in the Colorado Piedmont
(Leonard, 2002).
Erosion rates in Front Range canyons and on upland sur-
faces are linked to, and were enhanced by, erosional processes
that were intensified by global cooling during the past 3 m.y.
Measurements of cosmogenic isotopes 10Be and 26Al suggest
maximum rates of surface lowering of 5–9 m/m.y. for gran-
ite tors in alpine upland areas in the Front Range (Small and
others, 1997; Anderson and others, 2006). These long-term
rates are compatible with short-term rates of surface lowering
of 10 mm/k.y. (10 m/m.y.) in other alpine areas, such as the
crest of Niwot Ridge and in the Green Lakes valley west of
the town of Ward (Caine, 1974, 1984), about 5 km north of

30 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado
the map area. Small, unglaciated, upland drainage basins that
formed on Precambrian granite and gneiss in montane areas of
the northern part of the Front Range and adjacent Laramie and
Medicine Bow Mountains to the north have long-term erosion
rates of 18–30 m/m.y. (Dethier and others, 2002); those near
Boulder Canyon have rates of about 22 m/m.y. (Dethier and
Lazarus, 2006). Soil-profile development at stable sites on the
upland surface in and near the northern part of the map area
suggests erosion rates much less than 0.01 m/k.y. (10 m/m.y.)
and perhaps no more than 0.001 m/k.y. (1 m/m.y.) (Birke-
land and others, 2003). Long-term erosion rates in Front
Range canyons are considerably greater than those deter-
mined in nearby alpine upland areas (Small and Anderson,
1998; Anderson and others, 2006), but are lower than the late
Pleistocene rate of roughly 0.15 m/k.y. (150 m/m.y.) at a major
knickpoint in Boulder Canyon (Schildgen and others, 2002).
Quaternary
Quaternary deposits and landforms within the map area
reflect the influence of earth-surface processes in concert with
global climate-driven glacial-interglacial cycles during the past
1.8 m.y. Global climatic cooling of the later part of the Plio-
cene continued into the Pleistocene and intensified after 900
ka (Clark and Pollard, 1998). The stratigraphic record sug-
gests that there were at least 12 glaciations in the mountains of
the western United States during the Quaternary (Richmond
and Fullerton, 1986b, Chart 1). Pleistocene glacial deposits in
Colorado and elsewhere in the Rocky Mountains are com-
monly correlated with Pinedale, Bull Lake, and pre-Bull Lake
glaciations (Meierding and Birkeland, 1980; Pierce, 2004).
Blackwelder (1915) named the Pinedale (last major) and Bull
Lake (penultimate) glaciations for younger and older sets of
moraines on the eastern and western flanks of the Wind River
Range, Wyo. Glacial deposits that predated the Bull Lake gla-
ciation typically lack morainal form and commonly are locally
preserved just beyond the lower limits of Pinedale and Bull
Lake ice. These deposits have been identified in a few areas
near the map area (Meierding and Birkeland, 1980). Sharp-
crested moraines produced by post-Pinedale glacial advances
of Holocene age and latest Pleistocene age are locally present
within cirques in the map area (Davis, 1988).
There are no known glacial deposits of pre-Bull Lake age
exposed within the map area. The absence of these depos-
its is probably due chiefly to erosion and subsequent burial
by deposits of younger and more extensive glacial advances
(Meierding and Birkeland, 1980). Glacial deposits of early
Pleistocene age are especially likely to be buried by younger
glacial deposits because cold (glacial)/warm (interglacial)
climatic cycles prior to 900 ka [marine oxygen isotope stage
(MIS) 22] were of lower amplitude (lower global ice volume)
and of much shorter duration (about 40 percent as long) than
those after 900 ka (Clark and Pollard, 1998). Also, marine
oxygen isotope records show only two glacial episodes (MIS
12 and 16; about 475–424 ka and 675–621 ka, respectively;
Lisiecki and Raymo, 2005) as severe (in terms of temperature
and global ice volume) as those during the Bull Lake and Pine-
dale glaciations. This suggests that all pre-Bull Lake glacia-
tions prior to MIS 16 were likely to be less extensive than the
Bull Lake and Pinedale glaciations.
The oldest glacial deposits exposed in the map area are
till and, locally, stratified drift, which form subdued moraines
of the Bull Lake glaciation. Bull Lake deposits in their type
area in Wyoming are about 170–120 ka (Sharp and others,
2003; Pierce, 2004). These deposits accumulated during one
or more major cold climatic episodes during MIS 6 (190–130
ka, Lisiecki and Raymo, 2005) and probably during the early
part of MIS 5 (130–70 ka, Lisiecki and Raymo, 2005; Pierce,
2004). Mapped deposits of the Bull Lake glaciation may
locally include small, unrecognized deposits of pre-Bull Lake
glaciations.
Till and minor amounts of stratified drift of the Pinedale
glaciation form well-preserved moraines that are widespread
in the upper part of glaciated valleys of the Front Range at the
lower limit of glaciation. Pinedale glacial deposits in Colo-
rado are about 30–12 ka (Nelson and others, 1979; Madole,
1986; Schildgen and Dethier, 2000; Benson and others,
2004, 2005). They accumulated during a major cold climatic
episode of MIS 2 (35–14 ka; Lisiecki and Raymo, 2005). Till
and other ice-contact deposits of the Pinedale glaciation may
locally include or overlie glacial deposits of early Wisconsin
age (MIS 4, 70–55 ka; Lisiecki and Raymo, 2005), which are
identified in a few areas of the western United States (Pierce,
2004). For example, till near Mary Jane Creek, about 9 km
southeast of Fraser, is older than 30,480 and 30,050 14C yr B.P.
(Nelson and others, 1979) and may be early Wisconsin in age
(Richmond, 1986; Richmond and Fullerton, 1986b); or it may
have been deposited during an early advance of Pinedale ice
(Sturchio and others, 1994). This till is overlain by three thin
(30–50 cm) till units or debris-flow deposits that accumulated
during the Pinedale glaciation and by interstratified silty lake
sediments, all of which are older than 13,740 14C yr B.P. (Short
and Elias, 1987).
Glaciers in the map area during the Bull Lake and
Pinedale glaciations were as much as 7–19 km in length, and
typically descended to elevations of about 2,500–2,850 m. In
the valley of Clear Creek, however, glaciers were much larger
because of the large area of ice accumulation in the upper part
of the valley. The lowermost ice-contact deposits of the Bull
Lake glaciation in the Clear Creek drainage are about 39 km
down-valley of the Continental Divide at an elevation of about
2,400 m. In the Boulder Creek drainage just west of Neder-
land, glaciers during the Bull Lake glaciation reached about
2,500 m and were about 300 m thick (Porter and others, 1983).
In comparison, glaciers during the Bull Lake and Pinedale
glaciations throughout the Front Range typically attained
lengths of 10–20 km, terminated at elevations between 2,500
and 2,700 m, and were 180–350 m thick (Benson and others,
2004).
The youngest glacial deposits in the map area are tills
of Holocene age (Benedict, 1985) and latest Pleistocene age
(about 12–10 ka; Davis, 1988). These tills lie above present

Geologic History of the Denver West Quadrangle 31
treeline within roughly 1 km of cirque head walls, typically
above an elevation of about 3,350 m. They were deposited
during minor glacial advances after the Pinedale glaciation,
chiefly during MIS 1 (14–0 ka; Lisiecki and Raymo, 2005).
Snow lines during the Pleistocene in the western United
States were roughly 1,000 m lower than present (Porter and
others, 1983). Fossil beetles near Denver (1,731 m), dated
at 14,500 14C yr B.P., suggest that during full- or late-glacial
climatic conditions, mean July temperatures were 10°–11°C
colder than present, and mean January temperatures were
26°–30°C colder than present (Elias, 1996). Relict permafrost
features in Wyoming suggest that temperatures could have
been 10°–13°C colder than they are at present (Mears, 1981).
More effective precipitation and vigorous freeze-thaw action
likely accompanied expanded periglacial environments during
glacial episodes, and would have promoted slope instability
and intensified mass-movement processes in the Front Range
and adjacent Colorado Piedmont. Much of the coarse debris,
which forms features such as block fields and block streams
on interfluves above and beyond the limit of glacial ice in the
Front Range, and some of the large landslide deposits along
the hogback belt probably formed chiefly under periglacial
conditions. Some of the landslide deposits in glaciated valleys
formed after glaciers retreated and glacial ice no longer pro-
vided lateral support to weakly consolidated material on steep,
unstable slopes. Increased infiltration of precipitation may
have locally promoted deep-seated rock creep on steep slopes
in periglacial environments.
Streams draining from glaciers within the map area
produced broad, gravelly glacial outwash deposits in moun-
tain valleys, particularly during times of significantly greater
sediment yield during deglaciation (Church and Ryder,
1972). Some of the glacial outwash likely is slightly younger
than corresponding tills, because fluvial deposition lagged
(perhaps by a few to several thousand years) the onset of the
climatic change that affected glaciation, such as the transi-
tion from glacial to interglacial climates (Church and Ryder,
1972; Hancock and Anderson, 2002). Large glacial outwash
deposits within the map area locally extend about 16 km or
more downstream of former glacier fronts, such as those in the
valley of Clear Creek. Maximum grain size and the surface
slope of glacial outwash deposits decrease significantly within
a distance of about 10 km downstream of former glacier fronts
(Church and Ryder, 1972; Ritter, 1987).
Glacial outwash deposits and till of the Pinedale glacia-
tion in the Front Range are correlated with the Broadway
Alluvium in the Colorado Piedmont east of the mountain
front; deposits of the Bull Lake glaciation are correlated with
the Louviers Alluvium (Scott, 1975; Madole, 1991a). These
correlations (table 1) are based chiefly on the (1) morphology
of surface soils formed in these deposits (Birkeland and others,
2003) and (2) height of glacial outwash and piedmont alluvial
deposits above present streams. Recent cosmogenic dating
supports these correlations (Schildgen and Dethier, 2000).
10Be and 26Al ages of Pinedale (32–10 ka) and Bull Lake (130
ka) glacial outwash deposits in Boulder Canyon east of
Nederland (Schildgen and others, 2002) and their heights
above stream level support the concept that Pinedale outwash
(8–12 m above present stream level) is equivalent in age to
Broadway Alluvium (8–12 m), and Bull Lake outwash (15–20
m) is equivalent in age to Louviers Alluvium (12–24 m).
Deposition of alluvium of Holocene age in present flood plains
and as terrace deposits less than 5 m above present streams in
the Colorado Piedmont are difficult to correlate with climatic
episodes, but some may reflect past climatic conditions that
were colder than at present, and possibly more moist (or due
to more effective precipitation) than at present. These latter
climatic episodes promoted glacial and (or) periglacial activity
in the Front Range during the Holocene (Scott, 1975).
The depositional chronology, as well as the climatic
(glacial or interglacial) and fluvial conditions that prevailed
during the deposition of most of the pre-Louviers (pre-Bull
Lake) alluvial deposits in the Colorado Piedmont near Denver,
can only be inferred. The following interpretations and age
estimates are based on tentative correlation with marine oxy-
gen isotope stages and a nonlinear rate of stream incision in
the Denver area since the deposition of the 640 ka Lava Creek
B tephra. Stream incision is likely to be nonlinear, because
numerical modeling suggests that the formation of strath ter-
races (and probably pediments) can span tens of thousands of
years of stream stability between shorter episodes of stream
incision (Hancock and Anderson, 2002), and heights of terrace
deposits of known or inferred ages suggest increased rate of
stream incision after about 400 ka (Dethier, 2001; Hancock
and Anderson, 2002). Pre-Louviers alluvial deposits were
transported and deposited by streams that headed in glaciated
as well as nonglaciated drainage basins in the Front Range.
Many of these deposits probably accumulated chiefly during
glacial episodes, particularly during maximum glaciation or
deglacial phases of glacial episodes, when climatic (glacial
and periglacial) and fluvial conditions, as well as abundant
sediment supply, promoted increased stream discharge and
sediment load (Church and Ryder, 1972; Sinnock, 1981;
Ritter, 1987; Madole, 1991c). However, events unrelated to
glaciation, such as stream capture and minor periods of cutting
and filling unrelated to major climatic events (such as lateral
migration of the South Platte River and its influence on tribu-
tary streams), likely accounted for deposition of some of these
deposits (Ritter, 1987; Reheis and others, 1991).
Comparison of the isotopic ages for deposits of the
Pinedale glaciation (30–12 ka) and Bull Lake glaciation
(170–120 ka) with cold and warm climatic episodes of the
marine oxygen isotope record (for example, Shackleton and
Opdyke, 1973, 1976; Lisiecki and Raymo, 2005) suggest that
outwash deposits of the Pinedale and Bull Lake glaciations
and the temporally correlative fluvial deposits in the Colo-
rado Piedmont (Broadway Alluvium and Louviers Alluvium,
respectively) were deposited chiefly during major cold cli-
matic episodes, and in part during succeeding warm climatic
episodes. These relations suggest that pre-Louviers alluvial
deposits in the Colorado Piedmont may have accumulated
under somewhat similar climatic conditions.

32 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado
Younger (topographically lower) and older (higher)
deposits of Slocum Alluvium are locally present within the
map area. Near the hogback belt, these deposits are as much
as about 30 and 45 m, respectively, above present streams.
Tentative correlation with marine oxygen isotope stages and a
nonlinear rate of stream incision in the Denver area since the
deposition of the 640 ka Lava Creek B tephra suggest that the
younger deposits may have accumulated between about 300
and 220 ka and older deposits between about 390 and 320 ka
(table 1).
Younger (topographically lower) and older (higher)
deposits of Verdos Alluvium are also locally present within
the eastern part of the map area. Near the hogback belt, these
deposits are as much as about 60 and 75 m, respectively,
Table 1. Estimated age ranges and correlation of alluvial deposits younger than the Rocky Flats Alluvium in the Colorado Piedmont near
Golden and Morrison, Colorado.
Deposit Height Age Correlative Marine oxygen isotope
(m)a or age glaciation stage and age (ka)b
estimate in Rocky
(ka) Mountains
Post-Piney Creek <5 0–4c Minor 1 (0–14)
alluvium and Piney post-Pinedale
Creek Alluvium. ice advances.
Broadway Alluvium 12 12–30d Pinedale 2 (14–35)
3 (35–55)
No known deposits -- -- Early 4 (55–70)
Wisconsin.
Louviers Alluvium 20 120–170d Bull Lake 5 (70–130)
6 (130–190)
Slocum Alluvium 30 220–300e -- 7 (190–243)
younger deposits. 8 (243–300)
Slocum Alluvium 45 320–390e -- 9 (300–337)
older deposits. 10 (337–390)
11 (390–424)
Verdos Alluvium 60 410–475e, f -- 12 (424–475)
younger deposits.
No known deposits -- -- -- 13 (475–533)
14 (533–570)
15 (570–621)
Verdos Alluvium 75 610–675g Sacagawea 16 (621–675)
older deposits. Ridge.
aApproximate height of deposit above present stream level near the east side of the hogback belt.
bAges for marine oxygen isotope stages are those of Lisiecki and Raymo (2005, fig. 4 and table 3), and are based on benthic δ18O records from 57 globally
distributed sites. Even-numbered stages represent cold (glacial) climatic episodes; odd-numbered stages represent warm (interglacial) climatic episodes (for
example, Shackleton and Opdyke, 1973, 1976).
cAge of alluvial deposits based on limited radiocarbon analyses (Scott, 1962, 1963a; Madole, 1976; Lindsey and others, 1998; Madole and others, 2005).
dAge of till and outwash of the Pinedale glaciation is based on radiocarbon and cosmogenic isotopic ages (Nelson and others, 1979; Madole, 1986; Schildgen
and Dethier, 2000; Schildgen and others, 2002; Benson and others, 2004, 2005). Age of till and outwash of the Bull Lake glaciation is based on cosmogenic
ages (Schildgen and Dethier, 2000; Schildgen and others, 2002; Sharp and others, 2003; Pierce, 2004).
eAge estimate for alluvial deposits based on tentative correlation with marine oxygen isotope stages and a nonlinear rate of stream incision in the Denver area
since the deposition of 640 ka Lava Creek B tephra.
fYounger deposits of Verdos Alluvium may locally include alluvium deposited during marine oxygen isotope stage 14 (about 533–570 ka).
gAge estimate for alluvial deposits based on stratigraphic position of 640 ka Lava Creek B tephra and tentative correlation with marine oxygen isotope stages.

Geologic History of the Denver West Quadrangle 33
above present streams. The age of older deposits of Verdos
Alluvium is fairly well constrained, because these deposits
locally overlie, contain, or are overlain by water-laid deposits
of Lava Creek B tephra (Machette, 1975; Machette and others,
1976; Van Horn, 1976), which was deposited when glaciers
were retreating, during the transition from glacial to intergla-
cial climatic conditions (Dethier, 2001; D.S. Fullerton, oral
commun., 2006). The age of the tephra (640 ka) indicates that
older deposits of Verdos Alluvium were deposited near the end
of a major cold climatic episode during MIS 16 (675–621 ka;
Lisiecki and Raymo, 2005), and possibly in part during the
early part of MIS 15. These deposits are similar in age to till
and outwash of the Sacagawea Ridge glaciation near the type
area for this glaciation on the eastern flank of the Wind River
Range, Wyo., where water-laid Lava Creek B tephra is present
in the top of the outwash (Jaworowski, 1992; Chadwick and
others, 1997). The stratigraphic position of the water-laid
tephra with respect to older pediment and fluvial deposits of
the Verdos Alluvium suggests that the older pediment deposits
are younger than the older fluvial deposits (Machette, 1975).
Near the hogback belt, younger deposits of the Verdos
Alluvium are as much as about 60 m above present streams.
Tentative correlation with marine oxygen isotope stages and a
nonlinear rate of stream incision in the Denver area since the
deposition of the 640 ka Lava Creek B tephra suggest that the
younger deposits may have accumulated between about 475
and 410 ka (table 1). Younger deposits of the Verdos Allu-
vium may be temporally correlative in part with shoreline and
near-shore deposits that formed during a high stand of Lake
Alamosa at about 450 ka in the San Luis basin in southern Col-
orado (Machette, 2006). If the age estimate for younger depos-
its of Verdos Alluvium is correct, it suggests limited deposition
of terrace and pediment deposits during cold (glacial) climatic
conditions of MIS 14 (about 570–533 ka) in the Colorado
Piedmont near Denver. Alternatively, younger deposits of Ver-
dos Alluvium may locally include alluvium deposited during
MIS 14 as well as during MIS 12 (about 475–424 ka).
Cosmogenic dating (on 10Be and 26Al) of alluvial-fan
deposits (typically ≤5 m thick; Knepper, 2005) that constitute
the upper part of the Rocky Flats Alluvium at Rocky Flats
indicates that they date from about 1.5 Ma (Dethier and others,
2001) or about 2–1 Ma (Riihimaki and others, 2006). How-
ever, the buried soil formed in the upper part of the unit at
the type section (Scott, 1960) for the Rocky Flats Alluvium
(Birkeland and others, 1996) suggests that the underlying
valley-fill deposits (≤25 m thick; Knepper, 2005) that form
the lower part of the unit may be much older. Morphologic
and paleomagnetic properties of buried soils at a site about
3 km south of Rocky Flats geomorphic surface support the
cosmogenic ages and suggest that the Rocky Flats Alluvium
dates from at least 1.6–1.4 Ma (Birkeland and others, 1996),
and possibly is about 2 m.y. old (Birkeland and others, 2003).
Although the Coal Creek drainage basin (the source of the
Rocky Flats Alluvium on Rocky Flats) was never glaciated,
the Rocky Flats Alluvium may have been deposited during one
or more glacial episodes. Considering the age constraints for
the Rocky Flats Alluvium and the fact that no evidence has
been found for a major glaciation in the United States between
1.5 Ma and 900 ka (Fullerton and Richmond, 1986; Richmond
and Fullerton, 1986b, Chart 1), the Rocky Flats Alluvium may
have been deposited after 2.5 Ma but before 1.5 Ma, the onset
of major northern hemisphere glaciations (Shackleton and
others, 1984; Prell, 1984; Thompson, 1991; Ravelo and others,
2006).
Eolian deposits are widespread east of the mountain front
in the Colorado Piedmont. Age assignments, spatial distribu-
tion, and downwind fining of the particle size of eolian sand
and loess in the Colorado Piedmont suggest a close relation
between the genesis of these deposits and active aggradation
of sparsely vegetated flood plains, chiefly during the Pinedale
and Bull Lake glaciations. Some of the loess, and possibly
some of the eolian sand, in and northeast of the map area
were derived from bedrock sources (Scott, 1962; Madole,
1995; Aleinikoff and others, 1999). Much of the loess within
the map area probably was deposited during two episodes
during the Pinedale glaciation (about 20–14 ka and 13–10
ka; Muhs and others, 1999). Additional accumulation may
have occurred during one or more episodes of the Bull Lake
glaciation (about 150 ka; Forman and others, 1995). Much of
the eolian sand within the map area probably was deposited
during one or two episodes of flood-plain aggradation and
eolian transport during the Pinedale glaciation (about 27–11
ka; Muhs and others, 1996) and possibly during a later episode
following the Pinedale glaciation between 11 and 4 ka (Muhs
and others, 1996) or 4.5 ka (Scott and Lindvall, 1970), prior to
deposition of the Piney Creek Alluvium (about 4–1 ka; Mad-
ole, 1976; Madole and others, 2005).
Potential Geologic Hazards
Potential geologic hazards in the mountains, hogback
belt, and piedmont of the Denver West quadrangle merit con-
sideration, because urban growth and development in recent
years commonly have occurred in parts of the landscape where
geologic, topographic, and hydrologic conditions favor such
hazards. Potential geologic hazards in the Denver West quad-
rangle include (1) mass movement, (2) expansive soils and
bedrock and heaving bedrock, (3) compactable and compress-
ible soils, (4) floods, (5) abandoned mines, (6) seismicity, (7)
elevated radon, and (8) snow avalanches.
Mass Movement
Rock units and surficial deposits on moderate to steep
slopes are prone to various forms of mass movement, includ-
ing rock fall, debris flow, landsliding, and creep. Landslide
deposits are common in areas of the Denver West quadrangle,
particularly on steep slopes underlain by weak rocks such
as Pierre Shale, Denver Formation, and locally on highly
fractured and altered basement rocks. Landslides include
material displaced chiefly by rotational rock slides, rotational

34 Geologic Map of the Denver West 30’ x 60’ Quadrangle, Colorado
earth slides, debris slides, earth flows, and earth slide–earth
flows (as defined by Cruden and Varnes, 1996). Conditions
and processes that promote sliding and other types of mass
movement locally include (1) toe slopes over-steepened by
erosion or human excavation; (2) gravitational spreading of
mountain flanks (Varnes and others, 1989); (3) bedding, folia-
tion, or other planes of weakness oriented parallel to slope; (4)
deforestation resulting from logging, wildfires, and (or) con-
struction of roads, buildings, and associated infrastructure; (5)
high water content and elevated pore pressure due to intense or
prolonged rainfall or rapid snow melt; (6) contrasts in mate-
rial properties (such as dense, stiff material over plastic, easily
deformed material); and (7) shrink-and-swell processes and
low shear strength of clayey material (Cruden and Varnes,
1996). In addition to sliding, mass-movement deposits within
the map area are also produced by rock fall, debris flows, and
downslope creep. Debris flows are locally generated on steep
(>30°) hillsides in small catchments during intense summer
rainstorms (for example, Godt and Coe, 2007). They are par-
ticularly common in areas recently deforested and destabilized
by wild fires (for example, Cannon and others, 1995; Soule,
1999; Cannon, 2001) and locally damage roads and structures.
Expansive Soils and Bedrock and
Heaving Bedrock
Expansive soils and bedrock and heaving bedrock pose
potential problems for roads, building foundations, and
other man-made structures built on them. These problems
are particularly serious at and near the western margin of
the Colorado Piedmont where clayey sedimentary rocks and
surficial deposits derived from these rocks have a high content
of expansive clays (typically smectite and mixed-layer illite-
smectite).
Expansive bedrock occurs in level to gently inclined,
layered, clayey sedimentary rocks and commonly swells
evenly and produces uniform heave at the ground surface;
by contrast, heaving bedrock occurs in moderately to steeply
inclined, layered, clayey rock and commonly swells unevenly
and produces linear-heave features at the ground surface (Noe
and others, 1997). Expansive clays in clayey rock units have
the capacity to adsorb a substantial amount of water, which
causes them to swell. During dry periods, clays in these
units release adsorbed water and shrink. East of the Front
Range, the Pierre Shale and Denver Formation are particularly
susceptible to marked volume change, related to the gain and
loss of adsorbed water, especially in layers of altered volcanic
ash (Hart, 1974; Shroba, 1982). However, in the Blue River
valley west of the Williams Range thrust, the Pierre Shale is
apparently more indurated than Pierre east of the Front Range,
possibly due in part to low-grade thermal metamorphism
by Tertiary intrusive activity (W.A. Cobban, oral commun.,
2000), and is likely to be less expansive.
Differential heaving of inclined bedrock is locally a
problem in the outcrop belt of the Pierre Shale southeast of
Green Mountain (Noe and others, 1999). In this area, the
effects of heaving bedrock are manifest by the vertical growth
of somewhat parallel ridges underlain by beds of expansive
bedrock separated by swales underlain by beds of less expan-
sive bedrock. Differential movement in excess of 0.6 m has
occurred in a few areas within a few years after unloading
of the bedrock due to excavation and subsequent wetting by
natural processes and human activity. This differential move-
ment has locally resulted in considerable damage to structures,
roads, and utilities. Appropriate construction procedures help
to stabilize and provide suitable drainage for roads, structures,
and utilities built on expansive rock units and sediments.
These procedures commonly include the installation of drilled-
pier foundations, floating-slab floors, and over-excavation and
replacement of expansive bedrock with at least 3 m of nonex-
pansive fill material in areas of heaving bedrock (Noe, 1997;
Noe and Dodson, 1997; Noe and others, 1997).
Compactable and Compressible Soils
Silty and slightly silty eolian sediments (loess and eolian
sand), organic-rich, silty sediments, such as Piney Creek
Alluvium, and poorly compacted deposits of artificial fill are
common in the eastern part of the Denver West quadrangle.
When placed under a heavy load, these deposits commonly
decrease in total volume due to particle compaction. Organic-
rich mineral deposits, such as Piney Creek Alluvium and
some artificial fill, also undergo compression under load due
to reduction of the open structure of organic material. Both
compaction and compression can result in excessive and
non-uniform decrease in volume, which results in differential
settlement at the ground surface, especially when these depos-
its become water saturated by natural processes or human
activity (Simpson, 1973; Shroba, 1982). The lower the density
and the higher the content of the organic material, the more
likely deposits are to compact and (or) compress, and undergo
differential settlement. Other factors, such as moisture content
and possibly the amount and type of clay-size material and
shape and orientation of the silt- and clay-size particles, influ-
ence the amount of consolidation of loess (Shroba, 1982). If
either compactable or compressible material is present at a
site, foundations can be designed to transmit the load to more
competent material at depth, or the material can be removed or
replaced with satisfactory fill material (Simpson, 1973).
Floods
Intense summer downpours or rapid melting of thick
snowpack during unusually warm spring thaws may cause
localized stream flooding in and near stream channels, and
localized sheet erosion and sheet flooding on slopes. Floods
commonly are restricted to low-lying areas. Roads, struc-
tures, and utilities built on historical flood plains and low (<5
m) terraces east of the mountain front may be susceptible to
stream flooding during periods of high runoff. Construction

of buildings and roads decreases the area available for infiltra-
tion of rainfall and increases the magnitude of periodic flash
floods, due chiefly to increased runoff. In mountainous areas,
unusual intense summer thunderstorms, such as those that
formed above the upper part of the Big Thompson Canyon
(about 40 km north of the map area) and other nearby canyons
during July 31–August 1, 1976, can produce 25 cm of precipi-
tation in a few hours (McCain and others, 1979) in areas that
normally receive about 30–35 cm for an entire year (Hansen
and others, 1978). In the 1976 Big Thompson instance, the
ensuing devastating flash floods resulted in significant loss of
life, destruction of property, and geomorphic change (Shroba
and others, 1979). Heavy rainfall from intense summer
thunderstorms generated by orographic uplift and convective
instability commonly falls at elevations of about 2,100–2,750
m (Cole, 2004). As a result, low-lying areas on the east flank
of the Front Range and in the adjacent piedmont at or below
an elevation of about 2,750 m are susceptible to local summer
thunderstorm-induced stream and sheet flooding. In addi-
tion to flooding, these storms locally initiate sheet erosion,
landslides, debris flows, and rockfall, any or all of which
can locally cause considerable damage to roads, buildings,
and other structures in mountain valleys and adjacent slopes
(Shroba and others, 1979).
Abandoned Mines
Collapse and subsidence of abandoned mine portals and
stopes pose a potential hazard in areas mined during the latter
part of the 19th and early part of the 20th centuries. Precious
metals, copper, lead, zinc, and tungsten were mined in the
Colorado mineral belt near the towns of Central City, Idaho
Springs, Georgetown, Empire, and Nederland; and coal was
mined in the Colorado Piedmont near Marshall and Louisville.
Drainage from mines in the Colorado mineral belt tends to be
acidic, due to oxidation of sulfide minerals (mostly pyrite),
and commonly contains toxic levels of heavy elements such
as zinc, cadmium, lead, and arsenic in areas near the sources
(Ficklin and Smith, 1994; Emerick and others, 1994). Soil
contaminated by acidic mine drainage may be corrosive
to untreated metal and concrete, pose a hazard to building
foundations, and locally render nearby surface and subsurface
water unsuitable for human use and consumption. Differen-
tial subsidence is locally a problem for structures built on fill
material in reclaimed clay pits near Golden (Noe and others,
1999).
Seismicity
Large-magnitude historical earthquakes have rarely
occurred in and near the Front Range. An earthquake of
inferred magnitude 6.5 was widely felt in parts of Colorado,
Wyoming, Utah, and Kansas in 1882 and caused local struc-
tural damage in the northern part of the Front Range (Kirkham
and Rogers, 1986, 2000). Scattered earthquakes of smaller
magnitude periodically shake the region, such as the magni-
tude 2.8 earthquake near Conifer in 1981 (Butler and Nichols,
1986).
Inferred seismogenic faults in and near the map area
locally displace Quaternary deposits. Quaternary sediments
and faults exposed in an exploratory trench about 200 m east
of the Golden fault near the town of Golden document 5.5 m
of vertical displacement since the deposition of Verdos Allu-
vium and the overlying Lava Creek B tephra (about 640 ka),
but prior to the development of a soil formed in colluvium that
is similar to soils formed in Slocum Alluvium (Scott, 1970;
Kirkham, 1977). However, interpretation of this structure is
equivocal and the displacement may be due to gravitational
processes. Another possible seismogenic fault is located near
Valmont Butte, just north of the map area (Scott and Cobban,
1965), where Slocum Alluvium is displaced (Scott, 1970).
Historical records and geologic investigations (Scott,
1970; Kirkham, 1977) suggest that the probability for damag-
ing earthquakes in the Denver West quadrangle is low com-
pared to more seismically active areas of the country (http://
nationalatlas.gov/natlas/Natlasstart.asp).
Radon
Radon is a naturally occurring gas that can damage lung
tissue and may increase the risk of lung cancer if it is inhaled
at a certain level over a period of time. Radon is radioactive
and is one of the byproducts of radioactive decay. Elevated
radon is of concern, especially in confined areas such as
homes and other structures. Most of Colorado, including the
area of the Denver West quadrangle, has elevated radon values
compared to those in other parts of the country due to ura-
nium- and thorium-rich crystalline rocks and sediments. One
out of three homes in Colorado have radon values greater than
4 picocuries per liter (pCi/L), the cutoff value for allowable
household radon determined by the U.S. Environmental Pro-
tection Agency. Mitigating action is recommended for homes
with values greater than 4 pCi/L (Environmental Protection
Agency, 1993). Granite and felsic gneiss are relatively radio-
genic compared to most other rocks, and surficial deposits,
such as alluvium and till derived from Proterozoic bedrock,
may have elevated radon values (Otton and others, 1993).
The hazard increases with increased permeability; therefore,
weathered rock and loose sediment may have a higher radon
risk than fresh rock and well-consolidated sediment. Shale
(particularly black shale, as in the lower part of the Pierre
Shale) can also have elevated radon values (Dubiel, 1993).
Testing for radon is relatively easy, and inexpensive remedial
procedures are available to mitigate the hazard (Environmental
Protection Agency, 1993).
Snow Avalanches
Ever since gold was first discovered in the Front Range
west of Denver during the late 1850s, snow avalanches have
Geologic History of the Denver West Quadrangle 35

posed a serious hazard to prospectors, miners, and mine
structures. In recent decades, snow avalanches in the moun-
tains have posed an ever-increasing hazard to human life and
property due primarily to accelerated growth in both winter-
sport activities and the construction of mountain homes (Ives
and others, 1976). Snow avalanches can occur anywhere
that (1) slopes are steeper than about 25° (90 percent of snow
avalanches develop on slopes of 30°–45°), (2) snow accumu-
lates to a sufficient depth, (3) a weak layer or layers develop at
depth within the snowpack, and (or) (4) triggering mechanisms
initiate snowslides (Colorado Avalanche Information Center,
2000). The prevailing winds in the region are from the west
and northwest and most slides start on the lee (downwind) or
east side of ridges where snow commonly accumulates. Large
avalanches can flow across a valley and move hundreds of
meters up the opposite valley side. Triggers for snow ava-
lanches might be a skier, an animal, or a sonic boom, but most
avalanches are caused simply by the weight of accumulated
snow; avalanches commonly occur when shear stress exceeds
shear strength along a weak layer within the snow pack.
Established avalanche tracks are commonly devoid of large
living trees; broken tree trunks and limbs litter the paths of
recurrent avalanches. Snow avalanches locally contain frag-
ments of trees and rocks.
For additional information on geologic hazards, as well
as on the environmental geology of the region, refer to reports
by Rogers and others (1974), Hansen and Crosby (1982), and
other reports cited in this section.
Acknowledgments
This report was much improved by comments by James
C. Cole, Jeremy B. Workman, and Ed Dewitt, U.S. Geological
Survey, and Peter W. Birkeland, Professor Emeritus, Univer-
sity of Colorado. Digital editing and GIS compilation were
ably performed by Scott R. Snyders and Paco Van Sistine.
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