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Opal Cement in the Eocene Castle Rock Conglomerate, Central Colorado

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Abstract

The Castle Rock Conglomerate is one of Colorado's most iconic, youngest, and coarsest grained rock units. It is also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Basin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimen-tologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bedded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and unusually large volcanic rock fragments along with some plagioclase and mica flakes. The largest volcanic rock fragments are up to ~2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age. Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and relatively proximal provenance for the unit. Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation. These opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weathering of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water super-saturated with silica. These fluids initially catalyzed precipitation of common opal (hydrous amorphous silica) and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented "con-crete" much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-made calcite-cemented concrete found in many sidewalks and roadways.
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Vol. 61, No. 1, p. 49-69. DOI: 10.31582/rmag.mg.61.1.49 THE MOUNTAIN GEOLOGIST | January 2024
ABSTRACT
e Castle Rock Conglomerate is one of Colorado’s most iconic, youngest, and coarsest grained rock units. It is
also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Ba-
sin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimen-
tologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bed-
ded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and
unusually large volcanic rock fragments along with some plagioclase and mica flakes. e largest volcanic rock frag-
ments are up to ~2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age.
Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and
relatively proximal provenance for the unit.
Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation.
ese opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial
depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast
chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weather-
ing of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water
super-saturated with silica. ese fluids initially catalyzed precipitation of common opal (hydrous amorphous silica)
and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented “con-
crete” much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-
made calcite-cemented concrete found in many sidewalks and roadways.
Opal Cement in the Eocene Castle Rock
Conglomerate, Central Colorado1
MARK LONGMAN2NIK SVIHLIK3
JOAN BURLESON4JAMES W. HAGADORN2
1. Manuscript received 8/20/2023; Accepted: 10/1/2023
2. Department of Earth Sciences, Denver Museum of Nature & Science, 2001 Colorado Boulevard, Denver, CO 80205.
3. Diversied Well Logging Services, 9780 Pozos Lane, Conroe, TX 77303.
4. Independent Geologist, Denver, CO
5. Final Draft: August 20, 2023
INTRODUCTION...............................................................................50
GEOLOGIC AND HISTORIC CONTEXT ......................................50
STUDY METHODS ............................................................................53
STRATIGRAPHY .................................................................................55
RESULTS OF THE PETROGRAPHIC WORK ................................. 56
Composition of the Detrital Grains ....................................................56
e Diagenetic Cements ....................................................................57
ROLE OF THE WALL MOUNTAIN TUFF IN
CRC DIAGENESIS ..........................................................................62
DISCUSSION .......................................................................................62
CONCLUSION ....................................................................................64
ACKNOWLEDGMENTS ....................................................................64
REFERENCES ......................................................................................66
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Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
INTRODUCTION
Two of the hardest and most erosion-resistant rock
units deposited in the Denver Basin during the past ~300
million years are also two of the youngest. ey are the
Wall Mountain Tuff, deposited during the late Eocene
about 36.7 million years ago (McIntosh and Chapin,
1994; 2004), and the slightly younger Castle Rock Con-
glomerate (CRC), which contains common clasts of the
Wall Mountain Tuff. A massive volcanic explosion during
a major caldera collapse in the area of the Sawatch Range
in Chaffee County, Colorado, about 80 miles (130 km)
west of what is today Colorado Springs is the inferred
source of the ignimbrites (aka ash-flow tuffs or pyroclastic
flows) that form the Wall Mountain Tuff (McIntosh and
Chapin, 2004). e tuff consists of rock fragments, pum-
ice, ash, and volcanic glass, and is so hard that in prehis-
toric times it was used for making stone tools (e.g., spear
points, arrowheads; Black, 2000; Gilmore, 2005). Much
later during the late 1800s and 1900s it was quarried in
the Front Range area around the town of Castle Rock for
use as a building stone in Denver and elsewhere (Wil-
liams, 1883; Richardson, 1915; Welsh, 1969).
But what is it about the CRC that makes it so resis-
tant to weathering? Named after the butte that rises above
the city of Castle Rock (Figure 1), the CRC is exposed
atop hills, mesas, and along canyon walls across the south-
western part of the Denver Basin. Like the Wall Mountain
Tuff, it defines local geomorphology because of its marked
resistance to erosion. is resistance is surprising, particu-
larly considering the formations relatively young age and
shallow burial history relative to older conglomeratic units
such as the Pennsylvanian Fountain Formation or Creta-
ceous Dakota Sandstone. e goal of this paper is to ex-
plain why the CRC is so hard that it forms these impres-
sive outcrops.
GEOLOGIC AND HISTORIC CONTEXT
e Castle Rock Conglomerate was deposited in a
southeast-trending late Eocene fluvial system (Kettle-
man, 1956; Morse, 1985; and Keller and Morgan, 2016,
2017) that is illustrated schematically in Figure 1. It is a
channel-fill deposit that is dominated by pebble- to boul-
der-sized clasts ranging in size from inches up to about 6 ft
(10 cm to 2 m; Morse, 1985). is channel complex can
be traced for more than 40 miles (60+ km) and prior to
recent erosion may have extended much farther. e main
channel ranges in width from about 2 to 6 miles (3 to 10
km) widening to the southeast toward the town of El-
bert, Colorado (Keller and Morgan, 2017; Figure 2). e
preserved conglomerate can be up to 120 ft (40 m) thick
(orson, 2011) but may be thicker locally (Richardson,
1915; Gabriel, 1933; Kettleman, 1956, Morse, 1985). Its
largest boulders are composed of the slightly older Wall
Mountain Tuff (formerly Douglas Rhyolite or Dawson
Rhyolite) that collapsed into the channel from the steep
walls of the canyon carved out by this major paleo-riv-
er system, but the majority of the clasts consist of gran-
ite and quartzite derived from the Colorado Front Range
(Gabriel, 1933; Keller and Morgan, 2016, 2017; Koch et
al., 2018).
An early comprehensive investigation of the CRC was
that of Morse (1979), parts of which were later incorpo-
rated into his 1985 publication (Morse, 1985). He iden-
tified three main fluvial facies in the CRC: 1) planar-bed-
ded sands and gravels, 2) trough cross-bedded gravels,
and 3) massive gravel beds. Morse also identified two to
three generally fining-upward gravelly sequences from 8 to
29 ft (2.5 to 9 m) thick with common cut-and-fill struc-
tures that he interpreted as having been deposited in very
high-energy, coarse-grained fluvial channel systems.
As the CRC drainages carved their way down through
the welded ash beds of the underlying Wall Mountain
Tuff, steep canyon walls formed and collapsed. us, it is
not surprising that large clasts of welded tuff form a sig-
nificant portion of the volcanic rock fragments (VRFs) in
the CRC. Based on analysis of more than 10,800 clasts in
24 widely spaced clast surveys, Keller and Morgan (2016,
2017) determined that VRFs in the channel fill form
about 23% of its clasts and range in size from a few milli-
meters to more than a meter. In contrast, the granitic and
quartzite rock fragments, which form about 60% of the
conglomerate clasts, are smaller, typically ranging in size
from 0.3 to 20 inches (1 to 50 cm). Granitic, quartzite,
and non-volcanic clasts also tend to be more rounded than
the VRFs, which are predominantly angular to sub-angu-
lar (orsen, 2011).
Because the clasts in the CRC are large and diverse in
composition, many studies have focused on their prove-
nance (e.g., Gabriel, 1933; Evanoff, 2007; Koch, 2013;
Keller and Morgan, 2016). All these studies indicate that
the Colorado Front Range was the primary source ter-
rain for clasts other than the VRFs. Koch et al. (2018)
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Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
Figure 1. Schematic reconstruction of one of the large erosional valleys carved into the Wall Mountain Tuff, here depicted
with flash-flood runoff from the Rocky Mountain Front Range. Episodic raging torrents carried boulder-sized granitic clasts from
the distant highlands where they were mixed with even larger boulders of tuff collapsing from the canyon walls. The resulting
channel complex in which the conglomerate was deposited has been mapped as being more than 40 miles (65 km) long and
2-6 miles (3-10 km) wide, spreading out to the southeast. The inset shows the butte northeast of the town of Castle Rock, a
prominent local landmark capped by this conglomerate, and after which the formation was named. Sketch by Jan Vriesen (see
Johnson and Raynolds, 2002).
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Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
Figure 2. Geologic and geographic context of the Castle Rock Conglomerate, illustrating sampling location for this paper (yellow star)
and surface exposures of the unit (orange), adapted from Keller and Morgan (2017). The interpreted trends of the main paleochannel
and associated tributary channels are labeled. Inset at upper right depicts the general Eocene stratigraphy of the Denver Basin along
the Front Range, plotted using a linear timescale, after Raynolds and Hagadorn (2016), with stages updated from v 2022/10 of the
International Chronostratigraphic Chart (stratigraphy.org).The Eocene-Oligocene White River Group is no longer present in southern
Front Range outcrops where the CRC is exposed, and is thus not shown; this unit may have originally capped the CRC and/or the basal
portion of the group may be coeval with portions of the CRC.
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Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
employed detrital zircons and petrography to show that
Precambrian quartzites and stretched-pebble conglomer-
ates of the informally named Coal Creek Canyon quartzite
(Precambrian Xq of Kellogg et al., 2008), some of which
are gold-bearing (Gabriel, 1933), came from one partic-
ular source area northwest of the city of Boulder, about
75 miles (120 km) northwest of the CRC study area. In
addition, the Pikes Peak Batholith, located a comparable
distance to the southwest, provided feldspar-rich granite
clasts to the CRC (Richardson, 1915; Kettleman, 1956;
Keller and Morgan, 2016, 2017).
Although the provenance and depositional aspects of
the CRC are now widely recognized, little about the diage-
netic history of this unit has been formally published. is
knowledge gap is due in part to the difficulty of studying
a conglomerate in thin sections, which are much small-
er in size than many of the clasts forming the conglomer-
ate. However, Kettleman (1956) mentioned that the unit
was cemented with silica, and Morse (1979) in his unpub-
lished dissertation included a useful discussion of the im-
portant silica cements in the conglomerate, but never for-
mally published his diagenetic analysis.
Although the once-continuous, extensive fluvial chan-
nel complex in which the Castle Rock Conglomerate was
deposited has now been dissected by relatively recent ero-
sion, good outcrops are common (Figure 2). One partic-
ularly accessible outcrop area spans Castlewood Canyon
State Park, just south of Franktown (Figures 2 and 3). is
park is located about 5 miles (8 km) east-southeast of the
town of Castle Rock, Colorado. Its outcrops of the CRC
contain a variety of large-scale fluvial channel crossbeds
that have been described, illustrated, and measured for
flow direction by Keller and Morgan (2017).
Except for the major differences in overall size, vertical
relief, stream gradient, stream velocity, and flow direction,
in some ways the canyon at Castlewood Canyon State
Park can be considered a miniature and walkable counter-
part to the giant canyon in which the CRC was deposited.
Instead of blocks of the Wall Mountain Tuff falling into a
river channel as happened during deposition of the CRC,
however, blocks of the conglomerate itself have collapsed
into Cherry Creek Canyon (Figure 3). is raises a ques-
tion: Why are these large blocks of conglomerate so hard
and so competent that they dont disaggregate after mil-
lennia of weathering on the floor of the canyon? Similarly,
why is the CRC so resistant that it forms prominent mesas
as resistant as other regional volcanic-capped landforms
such as North and South Table Mountain near Golden (cf.
Anderson and Haseman, 2021, p. 23)?
To test the hypothesis that cementing agents in the
CRC underpin the unit’s induration, the authors specif-
ically set out to find samples of the finer-grained sand-
stones associated with the conglomerate beds (Figure 4).
is approach allowed preparation of standard petro-
graphic thin sections containing dozens to hundreds of
grains ranging in size from ~0.2-10 mm as well as the ce-
ments between the grains. e focus of this study is to
describe, understand, and interpret the characteristics
and diagenetic sequence of these cements. Below we il-
lustrate that these cements are dominated by silicates in
the form of opal (amorphous hydrous silica) and length-
fast chalcedony.
STUDY METHODS
During an initial visit to Castlewood Canyon State
Park, the senior author was impressed with the size and
abundance of boulders not only within the CRC itself
but also forming the large debris blocks scattered across
the floor of the small, relatively narrow canyon formed by
Cherry Creek as it flows northward through the park (Fig-
ure 3). Seeking insight into the question of what was mak-
ing the conglomerate and its eroded boulders so resistant
to weathering, the park was revisited and fist-sized sam-
ples of the finer-grained lithologies between beds of con-
glomerate were collected. To avoid damaging outcrops in
this state park, an effort was made to collect float samples
from sand-dominated boulders already lying on the can-
yon floor, as well as oriented sandstone samples away from
walking paths.
Standard petrographic thin sections 30-microns thick
were prepared for each of the sandstones sampled for this
study. e billets were vacuum impregnated with blue-
dyed epoxy to reveal porosity, and the thin sections were
stained with sodium cobaltinitrite to give grains of potas-
sium feldspar a pale-yellow to dark brownish-yellow col-
or, thus making them easy to differentiate from the quartz
and plagioclase grains. e thin sections were examined
and photographed with an Olympus Vanox petrographic
microscope at magnifications ranging from 20X to 400X.
To augment the petrographic study, a subset of each
sample was powdered and analyzed by X-ray diffraction
(XRD) to quantify the mineral components including
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Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
Figure 3. A) People-sized boulders of the Castle Rock Conglomerate that have collapsed into the canyon formed by
Cherry Creek in the study area at Castlewood Canyon State Park. B and C) Closer views of some of the well-rounded
granitic boulders that comprise much of the Castle Rock Conglomerate. The large clast at the center of Photo C is about 10
inches (25 cm) in diameter.
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Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
quartz, potassium feldspar, plagioclase, clay minerals
(this includes the mica flakes so it is independent of grain
size), and other components. Because opal is amorphous
and not directly quantifiable by XRD, an experiment
was conducted to heat-treat the samples to 550ºC for 48
hours to convert the opal to quartz. Measurements ev-
ery 12 hours showed that the amount of quartz increased
during the first 24 hours at 550ºC and then flattened to
remain fairly stable after 36 hours, suggesting that most
or all opal had been converted. Analyses of the samples
were conducted on a D2 Phaser Bruker X-ray diffractom-
eter equipped with a LYNXEYE-2 (Ultrafast 1D Detec-
tor) with 2.0 mm divergent slit and 3 mm air deflector.
e XRD was operating at 30 kV and 10 mA with Co Kα
radiation (Kα=1.79026 Å). e 2θ range for the analysis
were between 5° and 55° 2θ with a step time of .2° 2θ/s. A
more detailed discussion of this new technique for quan-
tifying the amount of opal is presented elsewhere in this
volume (Svihlik, 2023).
STRATIGRAPHY
e term Castle Rock Conglomerate comes from the
prominent butte just north of the town of Castle Rock
which is capped by this conglomerate (Figure 1, inset). As
shown in the stratigraphic column in Figure 2, this con-
glomerate is a thin, relatively short-duration deposit that
thins and disappears eastward into the Denver Basin. Its
basal age constraint comes from clasts of the underly-
ing 36.7 Ma Wall Mountain Tuff (McIntosh and Chapin,
Figure 4. Slabs of the finer-grained beds of the Castle Rock Conglomerate, where sand to gravel-sized lithoclasts predominate.
Samples were photographed wet with #4 being by far the finest grained. Scale bar in centimeters.
THE MOUNTAIN GEOLOGIST | January 2024 56 Vol. 61, No. 1 | www.rmag.org
Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
1994, 2004). Its upper age constraint comes from fossils,
notably of titanotheres (brontotheres) (Cope, 1874; Rich-
ardson, 1915; Welsh, 1969) that have been correlated to
the Chadron Formation of Wyoming—a unit that defines
the Chadronian North American Land Mammal Age,
whose upper boundary is 33.9 Ma (Priabonian). us,
unless the titanothere bone fragments were reworked,
the conglomerate was likely deposited in the late Eocene
sometime between 36.7 and 33.9 million years ago.
Where the CRC channel eroded down through the
Wall Mountain Tuff, the conglomerate locally overlies the
Larkspur Conglomerate, a slightly older, thin conglomer-
ate unit lacking clasts of the Wall Mountain Tuff (or-
son, 2011). In most outcrops, however, the Larkspur
Conglomerate is absent and the CRC rests on the much
softer locally Paleocene Dawson Arkose (aka Dawson For-
mation or D2 Sequence; Figure 2). e Dawson Arkose
consists of weathered alluvial-fan and fluvial deposits that
accumulated at the foot of the growing Rocky Mountain
Front Range (orson, 2011). It is generally much finer
grained, much shalier, and much less cemented than the
Castle Rock Conglomerate. Because of these characteris-
tics, the Dawson Arkose forms gentle slopes and broad flat
areas lateral to the bluffs and cliffs of Wall Mountain Tuff
and CRC.
To better understand the paleoclimate that influenced
deposition and cementation of the CRC, one needs to
look no further than the coeval fossil-rich mudflow depos-
its at Florissant Fossil Beds National Monument about 45
miles (70 km) southwest of Castle Rock (Evanoff et al.,
2001). Among the park’s many unique fossils are petrified
stumps of giant redwood trees up to 14 ft (4 m) in diam-
eter. ese stumps are remarkably well preserved due to
replacement of the original wood by silica derived from
the associated mudstones and volcanic ash beds. e park’s
diverse flora and a similarly very well-preserved fauna, in-
cluding flies, dragonflies, and spiders, indicates a moderate
to humid climate similar to that in which Californias red-
wood forests grow today. us, California’s current climate
and some of its settings containing redwood forests may
provide a partial modern analog. ese areas have major
topographic relief, and sporadic heavy rains can produce
episodes of torrential flooding.
RESULTS OF THE
PETROGRAPHIC WORK
COMPOSITION OF THE DETRITAL GRAINS
in sections of some of the finer grained sandstones
associated with the much coarser conglomerate beds nice-
ly reveal the sizes, shapes, types, and composition of the
detrital grains present in the Castle Rock Conglomerate.
Compositionally, these sandstones are similar to the much
coarser conglomerates that have been characterized by pre-
vious authors (e.g., Gabriel, 1933; Keller and Morgan,
2017; Koch et al., 2018). e mineral components from
both petrographic study and X-ray diffraction data are
summarized in Table 1. Detrital grains are subdivided and
grouped into six types: 1) quartz (mostly monocrystalline
or “common”); 2) potassium-feldspar (K-spar, both micro-
cline and orthoclase); 3) plagioclase (twinned and mostly
quite weathered/sericitic); 4) micas (both muscovite and
biotite); 5) volcanic rock fragments (VRFs, mostly welded
tuffs with some andesites); and 6) granitic rock fragments.
In this classification scheme, for a grain to be identified as
a granitic rock fragment, it must contain both quartz and
feldspar. Polycrystalline quartz grains and quartzites with
no feldspar are grouped with the quartz.
Average grain size in the samples selected for petro-
graphic study ranges from 300-1000 µm. Most are poorly
to moderately sorted with a wide range of grain sizes—as
is typical of fluvial sands. Sorting improves as grain size
decreases. e grains range from angular shards to very
well-rounded grains, with rounding more pronounced
in the feldspars and VRFs than in the quartz grains. A
few examples of deeply embayed volcanic quartz are
also present.
Among the feldspar grains, both orthoclase and mi-
crocline are common. Grains of plagioclase tend to be
much cloudier (“dirty” or inclusion-rich) than the potas-
sium feldspar grains and are also less common (<5% vs.
20-30%). Distinct crystals of plagioclase are rare or ab-
sent in the tuff fragments and uncommon in the andesite
fragments—thus the plagioclase is inferred to have come
mainly from a granitic source. Consistent with this inter-
pretation, the grains of plagioclase are typically the same
size as the orthoclase and microcline grains.
All samples notably lack sedimentary rock frag-
ments such as chert, limestone, and dolomite clasts. Shale
clasts occur rarely and there are virtually no detrital clays
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Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
between the grains; the paucity of detrital clays is expect-
ed due to the high current energy of the fluvial system
in which deposition occurred. Also rare are well-round-
ed quartz sand grains such as can be reworked from the
Mesozoic and Paleozoic eolian sandstones that occur
along the Front Range (Blakey et al., 1988; Raynolds
and Hagadorn, 2016). ese findings are consistent with
the grain type analyses presented by Keller and Morgan
(2017) and Koch et al. (2018).
Representative photomicrographs of the finest grained
of the study samples are shown in Figure 5. ey re-
veal a heterogeneous assemblage of grain types includ-
ing monocrystalline quartz, K-feldspar, and volcanic rock
fragments along with some plagioclase and a large booklet
of muscovite. e grains are mostly angular to subround-
ed, and range in size from less than 100 µm to ~500 µm.
Sorting is poor to moderate. is is clearly a “first-cycle”
mix of grains and clasts implying a relatively short dis-
tance of grain transport, interpreted as likely less than 100
miles (160 km). us, this sample can be interpreted sim-
ply as a finer-grained mineralogic equivalent to the whole
CRC including its conglomerate beds, just with fewer
rock fragments.
THE DIAGENETIC CEMENTS
e major diagenetic components in the samples of
the Castle Rock Conglomerate are summarized in Table 1
based on petrographic study of the thin sections. Only
the intergranular cements are included in these diagenet-
ic components, but the total amount of original intergran-
ular porosity and the remaining open pore space are also
listed. Considering the heterogeneous mix of grain types,
the diagenetic components in the CRC are surprising-
ly simple. As shown in the photomicrographs in Figure 6,
there are two major cements. e first cement forms an
isopachous coating about 20-40 µm thick around all the
grains. is is amorphous silica also known as opal. e
second cement is chalcedony, a fibrous form of quartz,
which in these samples is the length-fast variety. Subtle
banding and zoning indicate that there are two or more
stages of precipitation of each of these silica cements. Dia-
genesis followed a consistent basic sequence, in which: 1)
very early opal precipitated before any grain compaction;
and 2) somewhat later, chalcedony filled most of the re-
maining porosity after opal cementation ceased. Both ce-
ments are silica polymorphs and together they effectively
bind the sand grains, creating a very hard rock that is resis-
tant to weathering.
TABLE 1
Composition of grains and cements in the Castle Rock Conglomerate study samples, based on
petrographic interpretation of thin sections (in black) and on X-ray diffraction analyses of the same
samples (in blue). Abbreviated column headers are K-spar = potassium feldspar, Plag = plagioclase,
VRFs = volcanic rock fragments, GRFs = granite rock fragments, and IG = intergranular.
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Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
Figure 5. Photomicrographs of sandstone sample CC-4 showing the mix of grain types: quartz including volcanic shards in Photos A and B,
K-feldspar (mostly orthoclase but also microcline) at upper right in Photo D (all with a yellow stain), volcanic rock fragments (darker grains,
some labeled VRF) in all photos, and a large flake of muscovite in Photos A and B. Also notable in these photos are the well-developed
isopachous rims of opal cement coating the grains; opal is followed by a clearer (whiter) pore-filling chalcedony.
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Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
Figure 6. Photomicrographs of sample CC-1, a very coarse sandstone that has grains mostly from 200 to 1500 µm in size. Grains are a mix
of “common” (monocrystalline) quartz, K-feldspar (including common microcline), granitic rock fragments, and volcanic rock fragments (VRFs
of andesite and tuff. The very loosely packed grains were cemented first with amorphous opal (faintly banded) followed by a “dust line”
of hematite and then a lighter-colored, pore-filling, length-fast chalcedony (chalcedony-2)(Photos A and B). Photos C and D show the same
two early stages of isopachous opal cement and subsequent chalcedony cement, but there is a second stage of length-fast chalcedony that
formed after a second brownish dust line, thought to be clay particles, formed in µm-sized crystals of hematite.
THE MOUNTAIN GEOLOGIST | January 2024 60 Vol. 61, No. 1 | www.rmag.org
Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
To highlight some important details, the isopachous
opal cement coating the grains in Figure 6 has a beige or
tan color when photographed with plane-polarized light
(Photos A and C), and because it is isotropic (amor-
phous), it has a uniform magenta color under crossed
polarized light when imaged with a gypsum plate in the
light path (Photos B and D). Close examination of this
isopachous cement reveals subtle growth banding. is
isopachous and isotropic cement is commonly called
opal by petrographers (e.g., Swineford and Franks, 1959;
Ulmer-Scholle et al., 2014; and many others). For this
reason, that term is used here but a more specific term
would be hydrous amorphous silica. It is not equivalent
to precious opal (e.g., Australian black opal) and lacks the
play of colors (chatoyance) seen in precious opal.
Just beneath and also coating the isopachous opal ce-
ment are very thin (microns thick) darker colored “dust
lines” interpreted to consist of micron-sized hematite
(iron oxide) and possibly clays. We hypothesize that this
hematite precipitated from oxidizing waters leaching
through the rock and absorbing the iron from the volca-
nic rock fragments or nearby Wall Mountain Tuff, proba-
bly during periods of subaerial exposure. e isopachous
nature of the opal, however, indicates that it precipitat-
ed when silica-saturated groundwaters filled the pores in
a phreatic system below the paleo-water table (vs. vadose
system above the water table, as is inferred for another
type of opal cement discussed below). Fluctuations in sil-
ica saturation, the water table, and/or rate of fluid flow
are hypothesized to have produced the subtle growth
bands in this isopachous opal. Based on the very loose
packing of the grains, this opal formed prior to any com-
paction, very soon after deposition, and at very shallow
burial depths at near-surface temperatures.
After the “dust line” coated the early isopachous opal,
a much lighter colored isopachous layer labeled “Chal-
cedony” precipitated (Photo A, Figure 6, top center). Un-
der crossed-polarized light with a gypsum plate in place
(Photo B), this same layer is quite colorful with shades of
blue, yellow, and magenta. ese colors indicate that the
mineral has first order birefringence, is not amorphous,
and is a variety of quartz composed of radiating fibers or
µm-wide parallel crystals or “hairs.” is fibrous quartz is
chalcedony and again it is the length-fast variety. Length-
slow chalcedony, an alternative form of chalcedony, com-
monly forms as a replacement of evaporite minerals (e.g.,
Folk and Pittman, 1971) or by replacing calcareous fos-
sil fragments such as brachiopods or echinoderms. e
length-fast variety in the CRC formed independently
of any evaporite minerals or fossil material and it clear-
ly post-dates the opal cement. We also hypothesize that
it precipitated from silica-rich waters during early burial,
again in a phreatic setting, but likely at a somewhat high-
er temperature or lower silica saturation than the opal.
is change in conditions gave the chalcedony a better
or perhaps longer opportunity to crystallize its hair-like,
closely packed parallel fibers.
In Figure 6, Photos B and D reveal that this length-
fast chalcedony cement takes two quite different forms.
e cement in Photos A and B is isopachous just like
the earlier opal cement but can be ~50-100 µm thick. In
contrast, the chalcedony in Photos C and D is only local-
ly isopachous (e.g., at lower center), and elsewhere, com-
pletely fills the pores and obtains a thickness of several
hundred microns (e.g., at far upper right in both photos).
Even more interesting is that in the bottom center of
Photos C and D, a thick (~50 µm), darker brown “dust
line” separates two distinct stages of nearly white chalced-
ony precipitation labeled Chalcedony 1 and Chalcedony
2 on the photos. For an unknown reason, there was more
silica-saturated water in the part of the sample shown
in Photos C and D than in the part shown in Photos A
and B; this resulted in the pores being completely filled
with chalcedony.
Another variation in the diagenesis of the samples
is shown in Figure 7. Here opal fills the majority of the
original pore space (~84%). Oddly, the common length-
fast chalcedony shown in Figure 6 forms less than 10%
of Figure 7’s cement and is completely absent in these
photos even though the sample retains about 6% open
porosity. Equally surprising is that almost all of the opal
cement is not isopachous, instead filling all the deposi-
tional pore space, estimated at 40%. However, next to
the open pores in Photos C and D, which are impreg-
nated with blue epoxy, there is a very thin (~15 µm) later
stage of clearer, lighter-colored opal cement. Despite this
heterogeneity in the character of the opal cement, the key
point here is that this sample was extensively cemented
with opal to the point that the original depositional pore
space was reduced to just 6% (and to zero in Photos A
and B) with no visible chalcedony.
Vol. 61, No. 1 | www.rmag.org 61 THE MOUNTAIN GEOLOGIST | January 2024
Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
Figure 7. Photomicrographs of sample CC-2, another coarse-grained arkosic sandstone, but this one has mainly opal cement
between the grains and relatively little preserved intergranular porosity (just 6%). The second cement of isopachous rims of length-
fast chalcedony cement so common in sample CC-1 are almost completely missing in this sample. The opal cement is mostly
inclusion-rich and cloudy.
THE MOUNTAIN GEOLOGIST | January 2024 62 Vol. 61, No. 1 | www.rmag.org
Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
A particularly interesting and profound feature of the
opal cements shown in Figure 7 is the presence of round-
ed, somewhat cloudy, botryoidal masses as seen near the
center of Photos C and D. Why these botryoidal features
formed here but not in the sample shown in Figure 6 is
unknown. It may have to do with an oversupply of silica
in the water precipitating out very rapidly. is scenario
would be consistent with the cloudy or darker centers of
the botryoids in plain light, and their milky white color
in reflected light, both of which suggest that they contain
more microinclusions (water vacuoles) than the much
clearer 10-20 µm rims lining the pores in Figure 7C.
e photomicrographs in Figure 8 show more dark
botryoids of opal but in this sample they have an unusu-
al, asymmetric distribution. e botryoids are hanging
on the bottoms of the larger grains where they form a
pendant or hanging cement. Note that no similar bo-
tryoids are present on the bottom side of these same
pores. is asymmetry suggests that the opal botryoids
formed above the water table in a vadose setting with
air occupying the remainder of the pore. Vadose pen-
dant opal cements such as these have not previously been
identified in the Castle Rock Conglomerate and they of-
fer convincing evidence that this form of opal precipi-
tated very early (inferred to be <1 million years) at very
shallow burial depths (probably <100 m where rain-wa-
ter leaching would be most common), and at ambient
earth-surface temperatures.
After the pendant opal precipitated, however, opal
continued to precipitate but by then the rock had
dropped into a phreatic setting where water complete-
ly filled the pores. is allowed the later stage(s) of opal
to form the clearer (inclusion-poor) isopachous rims of
opal and chalcedony cement visible in all four photos in
Figure 8. Similar rims of isopachous opal are much more
common in the CRC (e.g., Figure 6) than the pendant
variety, indicating that the majority of the opal precipi-
tated beneath the water table.
e photomicrographs in Figure 8 also show the fa-
miliar pattern seen in most samples of the CRC, where a
very distinct, darker “dust line,” possibly formed in part
by hematite or clays, marks the boundary between opal
precipitation and a later stage of length-fast chalcedo-
ny. In photos A and B in Figure 8, this chalcedony forms
an isopachous rim that is just 10-20 µm thick, lining
open pores (blue). In contrast, the pores in photos C and
D in Figure 8 were completely filled with much thick-
er patches of length-fast chalcedony, presumably due to
more abundant, highly saturated silica-rich waters in the
pore system.
ROLE OF THE WALL MOUNTAIN
TUFF IN CRC DIAGENESIS
To help understand the source of the common sili-
ca cements in the CRC, samples of the Wall Mountain
Tuff were collected and studied petrographically. ey re-
vealed that this is a classic welded tuff composed almost
entirely (>85%) of tiny (<100 µm) shards of isotropic
volcanic glass (Figure 9). Intermixed with the glass shards
are angular fragments of quartz including some embayed
volcanic beta quartz and a few fragments of feldspar crys-
tals. When first deposited, the volcanic debris compris-
ing the ash, which was probably transported by wind
from the volcanic center 80 miles (130 km) to the west
(McIntosh and Chapin, 2004), must have been quite po-
rous. However, the ash hardened and solidified relative-
ly quickly, probably with the help of rainwater dissolving
the fragments of amorphous silica and reprecipitating the
silica as opal cement. Compaction of the ash also likely
contributed to its lithification and expelled silica-saturat-
ed fairly fresh water upward and laterally into the slightly
younger Castle Rock Conglomerate.
Evidence of the nature of the diagenesis in these sil-
ica-rich fluids can be found in small vugs within the tuff
itself (Figures 9A and B). ese vugs commonly show
concentric banding indicative of multiple stages of ce-
mentation consisting of both opal and length-fast chal-
cedony—the same cements so common in the CRC.
us, it can be concluded that the common opal and
chalcedony cements that make the CRC so hard, were
derived, at least in part, from the Wall Mountain Tuff.
Furthermore, regional field observations suggest that
proximity to the tuff, both laterally and vertically, result-
ed in decreasing cementation of the CRC over distances
ranging from inches to many feet (cms to many tens of
cms) and increasing preservation of more of the original
intergranular pore spaces away from the tuff.
DISCUSSION
For the purpose of this petrographic study, the most
important characteristic of the opal and chalcedony
Vol. 61, No. 1 | www.rmag.org 63 THE MOUNTAIN GEOLOGIST | January 2024
Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
Figure 8. Photomicrographs of Sample CC-3, another very coarse sandstone with grains mostly 200-1000 µm in size. The sample
has common intergranular pores that are lined with localized pendant rims of inclusion-rich mammillary opal cement, followed by a
“cleaner” and clearer isopachous rim of lighter-colored opal about 50-100 µm thick (best seen in Photos C and D). The final stage of
cement lining the pores consists of length-fast chalcedony that appears nearly white in Photos A and C (taken with plane polarized light)
but is brightly colored with shades of yellow and blue in Photos B and D (taken with crossed polarized light and a gypsum plate). Darker
lines coating the isopachous opal rims and the chalcedony cement are thought to be hematite dust lines possibly precipitated during
times of subaerial exposure, when iron in the associated VRFs was being oxidized.
THE MOUNTAIN GEOLOGIST | January 2024 64 Vol. 61, No. 1 | www.rmag.org
Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
cements that are so common in the Castle Rock Con-
glomerate is their “hardness.” Both quartz and chalcedo-
ny have a hardness of 7 on the Mohs Scale, whereas cal-
cite has a hardness of 3. Diamond, the hardest naturally
occurring mineral, has a hardness of 10.
Opal, despite being dominantly silicon dioxide like
quartz, has a variable amount of bound water, making it
amorphous and giving it a variable hardness depending
on its water content. Opal generally ranges in hardness
between 5.5 and 6.5 (Hurlbut, 1971). It is thus some-
what softer than crystalline quartz, but much harder than
carbonate cements such as calcite and dolomite. When it
comes to weathering and erosion, particularly in the pres-
ence of rainwater, opal and chalcedony are both actually
far more than twice as hard as calcite. Another important
factor in determining a rock’s hardness during weather-
ing is the solubility of the cementing agent(s) in rain wa-
ter. Because rain tends to be acidic, carbonate minerals
are typically more soluble than silicate minerals. Because
of this difference, the abundant opal and chalcedony ce-
ments, which form as much as 35-45% of the CRC, cre-
ate a very durable, hard rock very resistant to erosion.
So why do opal and chalcedony cements form in
some rocks and not others? Two factors are relevant: 1)
opal-cemented sandstones are typically of Cenozoic age;
and 2) opal cement is commonly associated with volca-
nic rocks or rock fragments, which seem to have contrib-
uted the silica forming the opal (Swineford and Franks,
1959; McBride et al., 2012; Ulmer-Scholle et al., 2014).
Both these factors are true for the CRC. Moreover, opal
can form in continental sands above the water table
where volcanic ash beds are intercalated with the sands
(e.g., Stanley and Benson, 1979; McBride, 1985). Also,
Blatt (1966) observed that opal forms much more eas-
ily at low, near-surface temperatures than does quartz,
and that such opal may crystallize to chert after having
been “cooked” during deep burial. us, deep burial may
partly explain why opal is rare in Mesozoic and older
rocks. Crystallization of opal to chert has not happened
in the CRC, however, because no chert cement is present
and the rocks have experienced only very shallow burial
(<<483 m depth; Petermann et al., 2022).
As a general rule, although volcanic rock fragments
are present locally in some pre-Eocene sandstones such as
the Dawson Formation and Arapahoe Conglomerate (M.
Morgan, personal communication, 2023), glass- (opal-)
rich fragments of welded tuffs are sparse or rare in these
sandstones in the Denver Basin, and no pre-Eocene sand-
stones with opal cement have been reported anywhere in
the region. In contrast, volcanic rock fragments are pres-
ent in the Neogene Ogallala Formation in the northeast-
ern Denver Basin, where Swineford and Franks (1959)
and John Webb (personal communication, 2023) have
observed common opal cement. Although there are cer-
tainly other possible sources of amorphous silica (e.g., di-
atoms, radiolarians, silicoflagellates, etc.) that can locally
contribute silica to forming opal cement in some settings,
none of these are abundant in the Denver Basin. us,
the correlation between opal cement in sandstones and
silicic volcanics can be deemed critically important to
forming opal cements in this part of the country.
CONCLUSION
is pilot study suggests that the Castle Rock Con-
glomerate is so hard and so resistant to erosion mainly
because it contains abundant silica cement in the form of
opal and length-fast chalcedony. Additional contributing
factors are: 1) A nearly complete absence of detrital and
authigenic clays; and 2) e abundance of large clasts
of granite, quartzite, and Wall Mountain Tuff. Howev-
er, without the abundant silica cement, it would not be
possible for these hard clasts to bind together sufficient-
ly well to avoid significant erosion. In marked contrast,
the sedimentological character of most of the underly-
ing Paleocene to lower Eocene Dawson Arkose (orson,
2011) shows that, without these common siliceous ce-
ments, the CRC would be just another relatively feature-
less slope-forming Paleogene deposit. Future work on the
CRC might explore the extent of these siliceous diagenet-
ic cements and their spatial relationship to Front Range
geomorphology and/or proximity to the larger clasts of
tuff within the CRC itself.
ACKNOWLEDGMENTS
Recognizing that a relatively random hike in Castle-
wood Canyon State Park led to this petrographic study
of the CRC is surprising and shows how much remains
to be learned about certain aspects of Colorado’s diverse
geology. is study was strengthened by conversations
with and work done by Jon orson, Stephen Keller, and
Matthew Morgan of the Colorado Geological Survey.
Vol. 61, No. 1 | www.rmag.org 65 THE MOUNTAIN GEOLOGIST | January 2024
Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
Figure 9. Photomicrographs of a fresh (relatively unweathered) angular clast of the Wall Mountain Tuff about 1 ft (30 cm) in diameter
from the roadcut along Highway 83 just south of Franktown. The matrix is mainly opal with scattered phenocrystals of quartz and
feldspar. At higher magnification (Photo D taken at 100X), ghosts of former glass shards averaging about 100 µm long are visible and
mostly oriented horizontally. A few of the larger quartz phenocrysts are interpreted as beta bipyramidal) quartz and have embayments
(e.g., at right end of grain in Photo C). The homogenous magenta color of the tuff when photographed with crossed polarized light and a
gypsum plate (Photo B) suggests that the rock is more than 85% opal.
THE MOUNTAIN GEOLOGIST | January 2024 66 Vol. 61, No. 1 | www.rmag.org
Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
Our thanks to Wagner Petrographic in Lindon, Utah, for
preparing the thin sections and Brent Lounsbury (Colo-
rado Department of Natural Resources) for arranging a
sampling permit at Castlewood Canyon State Park. Dis-
cussions with John Webb, who shared his insights on
similar opal-cemented sandstones within the Oligocene
Brule (White River Group) and Ogallala formations of
northeast Colorado (Webb, 1982), also helped in writing
this paper. anks also to reviewers James P. Rogers, John
Webb, Stephen Keller, and Matthew Morgan for sugges-
tions that improved an early version of this manuscript.
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THE MOUNTAIN GEOLOGIST | January 2024 68 Vol. 61, No. 1 | www.rmag.org
Mark Longman, Nik Svihlik, Joan Burleson, James W. Hagadorn
THE AUTHORS
Mark Longman
Mark received his B.A. degree from Albion College in Michigan in 1972 followed by a Ph.D. in
Geology from the University of Texas at Austin in 1976. He then joined the research lab of Cities
Service Company in Tulsa, Oklahoma for 5 years before moving to Denver in 1981 to work for
Coastal Oil and Gas Company as an exploration geologist in the Williston Basin. From 1984 to
2006, he was a consulting geologist before joining QEP Resources, where he worked as their
“Rock Expert” until 2018. Mark then joined the Denver Museum of Nature and Science as a
Research Associate and continues to work on various projects with the Museum including his
recent work on the Castle Rock Conglomerate that is the focus of this special issue of e
Mountain Geologist. Much of his geologic work focuses on the description of cores, outcrops, and
petrographic thin sections with a goal of integrating sedimentology and petrology to interpret
depositional environments and diagenesis.
Nik Svihlik
Nik received his B.Sc. in Environmental Physical Science from Black Hills State University in
2012. Upon graduation he did a short stint in the coal industry in Gillette, Wyoming before
joining Weatherford Labs. At Weatherford labs, Nik was part of the Wellsite Geoscience Services
team that provided XRD, SRA, and XRF to clients. is is where he started his passion for
mineralogy through using X-ray diffraction. At the end of 2014 he accepted a job in Midland,
Texas being the Director of Geosciences focusing on organic geochemistry and XRD in the
Permian basin providing services to oil and gas exploration companies. In 2019 Nik moved to
Houston, Texas before joining Diversified Well Logging as their Lab Manager in 2022. In
September 2023 Nik joined Chevron as their XRD lab leader at their Briarpark lab facility. Nik
continues the passion of mineralogy as well as looking at new methods to be incorporated into
x-ray diffraction.
Joan Burleson
Joan has been associated with the Denver Museum of Nature and Science in a variety of roles for
many years, most recently in collaboration with Mark on a study of the Castle Rock Conglomer-
ate. She earned her BSc. In Geology from the University of Alberta before getting her JD from
the (now) Sturm College of Law, University of Denver, where he won the inaugural Natural
Resources Writing Competition. Always a rockhound, Joan creates glass art from her studio
Glassentricity in Denver, and has authored a memoir, “I Love You More.”
Vol. 61, No. 1 | www.rmag.org 69 THE MOUNTAIN GEOLOGIST | January 2024
Opal Cement in the eOCene Castle ROCk COnglOmeRate, CentRal COlORadO
THE AUTHORS (cont.)
James Hagadorn
James Hagadorn, Ph.D., is the Tim & Kathryn Ryan Curator of Geology at the Denver Museum
of Nature & Science. As a geologist and science communicator, he likes deciphering earth history
and making science and scientific thinking relevant. A former syndicated science columnist and
science spokesperson, Hagadorn has authored hundreds of peer-reviewed and popular scientific
articles, has a diverse digital media reach, and is known for mentoring the next generation. As a
custodian of the museum’s rock, mineral, meteorite and invertebrate fossil collections, James also
helps archive and facilitate use of collections for public benefit. He also enjoys using all five senses
to identify rocks, including salt cores!
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Article
Full-text available
We propose a new proxy that employs assemblages of fossil turtle shells to estimate the timing and depth at which fossilization and lithification occur in shallowly buried terrestrial strata. This proxy, the Turtle Compaction Index (TCI), leverages the mechanical failure properties of extant turtle shells and the material properties of sediments that encase fossil turtle shells to estimate the burial depths over which turtle shells become compacted. Because turtle shells are one of the most abundant macroscopic terrestrial fossils in late Mesozoic and younger strata, the compactional attributes of a suite of turtle shells can be paired with geochronologic and stratigraphic data to constrain burial histories of continental settings—a knowledge gap unfilled by traditional burial-depth proxies, most of which are more sensitive to deeper burial depths. Pilot TCI studies of suites of shallowly buried turtle shells from the Denver and Williston basins suggest that such assemblages are sensitive indicators of the depths (~10–500 m) at which fossils and their encasing sediment become sufficiently lithified to inhibit further shell compaction, which is when taphonomic processes correspondingly wane. This work also confirms previously hypothesized shallow Cenozoic burial histories for each of these basins. TCI from mudstone-encased turtle shells can be paired with thicknesses and ages of overlying strata to create geohistorical burial curves that indicate when such post-burial processes were active.
Technical Report
The Castle Rock Conglomerate (CRC) is a late Eocene fluvial deposit flanking the east side of the Colorado Front Range and lying within the Colorado Piedmont. It may reach about 70 m (230 ft) in thickness, is nearly flatlying, and is discontinuous, capping both high-relief mesas and gentle hills. The unit occurs in a southsoutheast- to-southeast-trending swath 63 km (39 mi) in length and about 3 to 10 km (2 to 6 mi) in width, extending from north of the city of Castle Rock to southeast of the town of Elbert. The conglomerate has an arkosic, coarse sand and granule matrix with abundant pebble to boulder-sized clasts that decrease in abundance upward. Large trough cross-beds are common in the exposed upper portion of the unit. In this interval, a comprehensive survey of paleocurrent directions from trough cross-beds yielded 2,897 measurements from outcrop exposures. At each trough, the axis azimuth (paleocurrent direction) and several other parameters were measured. Lengthazimuth rose diagrams and large-scale digital topographic maps allowed the paleocurrent data to be a really and stratigraphically grouped into 411 consolidated local paleocurrent directions. The result is a new and detailed paleocurrent map of the upper portion of the unit. The map shows the paleocurrent pattern within the main paleochannel belt, which previously has been recognized as occupying a south-southeast to southeast-trending paleovalley; and two, newly documented, northeast to east-flowing tributary paleochannnels that originated southwest of the main paleochannel belt. In the area of Castlewood Canyon State Park, the JA Ranch paleochannel widened northeastward to become an alluvial fan, now partially obscured by deposits of the main paleochannel belt. The Bucks Mountain trend joined the main paleochannel belt in an area east of Running Creek and west of Elbert. A possible third tributary system may have existed north of Castle Rock, and is suggested by a small number of measurements in three isolated areas: Castle Rock Butte, Cherokee Mountain, and the mesa north of Newlin Gulch. Clast surveys at 24 locations recorded the lithology, maximum dimension, and roundness of all clasts >2 cm (0.8 in) in maximum dimension (nearly 11,000 clasts). In order of decreasing lithologic abundance, the gravel fraction of the conglomerate consists of granitics, Wall Mountain Tuff, quartz, blue-gray quartzite, other quartzites, and probable Lower Paleozoic sedimentary rocks (including possibly the Fountain Formation). Consolidated histograms of the 15 clast surveys in the main paleochannel belt versus nine surveys in the tributaries indicate some marked differences between the two populations: notably a larger proportion of granitics and a lower proportion of tuff exist in the main paleochannel belt than in the tributaries. Histograms of local sets of main paleochannel belt clast surveys versus tributary surveys indicate other notable differences, especially in clast lithology and size distribution. Blue-gray quartzite, hypothesized by previous workers to originate from Coal Creek Canyon south of Boulder, is common in the main paleochannel belt. Well-rounded volcanic clasts of probable dacitic composition were collected from the base of the main paleochannel belt in Castlewood Canyon State Park. Sensitive High Resolution Ion Microprobe (SHRIMP)-RG U-Pb zircon age dates of these clasts range from 46 to 55 million years ago (Ma). Potential source areas for these volcanic clasts lie along a northeast trend between Leadville and Boulder. All of the clast lithologies found in the conglomerate are found along the Front Range. Absent from the clast surveys is the suite of Mesozoic sedimentary rocks now exposed along the mountain front, suggesting that today’s prominent hogbacks of these rocks were not exposed, or not within the CRC source areas during the late Eocene. The present study indicates that large quantities of granitic and volcanic material existed along the Front Range in the late Eocene. This material likely buried the Mesozoic section at the range front and left part of the Paleozoic section (mainly the Fountain Formation) exposed. Exhumation of the Mesozoic rocks occurred sometime after deposition of the CRC.
Article
The reservoir quality of sandstone is almost entirely controlled by diagenetic events. The chemical and physical processes responsible for diagenesis are complex and they influence sands during all stages of burial and, in some basins, during subsequent uplift. Petrographic studies in the past ten years by many workers provide the basis for formulating rules of sandstone diagenesis that help in predicting reservoir quality in different formations. The majority of the rules listed are empirical, and causative factors are still poorly understood.
Article
Late Paleozoic and Mesozoic eolian deposits include rock units that were deposited in ergs (eolian sand seas), erg margins and dune fields. They form an important part of Middle Pennsylvanian through Upper Jurassic sedimentary rocks across the Western Interior of the United States. These sedimentary rock units comprise approximately three dozen major eolian-bearing sequences and several smaller ones. Isopach and facies maps and accompanying cross sections indicate that most eolian units display varied geometry and complex facies relations to adjacent non-eolian rocks.
Article
Thesis--Johns Hopkins University. Vita. Includes bibliographical references (leaves 331-343). Microfilm of typescript.
Golden rocks: The geology and mining history of
  • D S Anderson
  • P B Haseman
Anderson, D.S., and Haseman, P.B., 2021, Golden rocks: The geology and mining history of Golden, Colorado: Colorado Geological Survey, MI 102, available online at https:// coloradogeologicalsurvey.org/publications/ geology-mining-history-golden-colorado/.