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Devonian carbonate platform of eastern Nevada: Facies, surfaces, cycles, sequences, reefs, and cataclysmic Alamo Impact Breccia

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Abstract and Figures

Devonian limestone and dolostone formations are superbly exposed in numerous mountain ranges of southeastern Nevada. The Devonian is as thick as 1500 m there and reveals continuous exposures of a classic, long-lived, shallow-water carbonate platform. This field guide provides excursions to Devonian outcrops easily reached from the settlement of Alamo, Nevada, ∼100 mi (∼160 km) north of Las Vegas. Emphasis is on carbonate-platform lithostratigraphy, but includes overviews of the conodont biochronology that is crucial for regional and global correlations. Field stops include traverses in several local ranges to study these formations and some of their equivalents, in ascending order: Lower Devonian Sevy Dolostone and cherty argillaceous unit, Lower and Middle Devonian Oxyoke Canyon Sandstone, Middle Devonian Simonson Dolostone and Fox Mountain Formation, Middle and Upper Devonian Guilmette Formation, and Upper Devonian West Range Limestone. Together, these formations are mainly composed of hundreds of partial to complete shallowing-upward Milankovitch-scale cycles and are grouped into sequences bounded by regionally significant surfaces. Dolomitization in the Sevy and Simonson appears to be linked to exposure surfaces and related underlying karst intervals. The less-altered Guilmette exhibits characteristic shallowing-upward limestone-to-dolostone cycles that contain typical carbonate-platform fossil- and ichnofossil-assemblages, displays stacked biostromes and bioherms of flourishing stromatoporoids and sparse corals, and is punctuated by channeled quartzose sandstones. The Guilmette also contains a completely exposed ∼50-m-thick buildup that is constructed mainly of stromatoporoids, with an exposed and karstified crest. This buildup exemplifies such Devonian structures known from surface and hydrocarbon-bearing subsurface locations worldwide. Of special interest is the stratigraphically anomalous Alamo Breccia that represents the middle member of the Guilmette. This spectacular cataclysmic megabreccia, produced by the Alamo Impact Event, is as thick as 100 m and may be the best exposed proven bolide impact breccia on Earth. It contains widespread intervals generated by the seismic shock, ejecta curtain, tsunami surge, and runoff generated by a major marine impact. Newly interpreted craterrim impact stratigraphy at Tempiute Mountain contains an even thicker stack of impact breccias that are interpreted as parautochthonous, injected, fallback, partial melt, resurge, and possibly post-Event crater fill.
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215
The Geological Society of America
Field Guide 11
2008
Devonian carbonate platform of eastern Nevada: Facies, surfaces,
cycles, sequences, reefs, and cataclysmic Alamo Impact Breccia
John E. Warme*
Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, USA
Jared R. Morrow*
Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA
Charles A. Sandberg*
U.S. Geological Survey, Box 25046, MS 939, Federal Center, Denver, Colorado 80225, USA
ABSTRACT
Devonian limestone and dolostone formations are superbly exposed in numerous
mountain ranges of southeastern Nevada. The Devonian is as thick as 1500 m there
and reveals continuous exposures of a classic, long-lived, shallow-water carbonate plat-
form. This fi eld guide provides excursions to Devonian outcrops easily reached from
the settlement of Alamo, Nevada, ~100 mi (~160 km) north of Las Vegas. Emphasis is
on carbonate-platform lithostratigraphy, but includes overviews of the conodont bio-
chronology that is crucial for regional and global correlations. Field stops include tra-
verses in several local ranges to study these formations and some of their equivalents,
in ascending order: Lower Devonian Sevy Dolostone and cherty argillaceous unit,
Lower and Middle Devonian Oxyoke Canyon Sandstone, Middle Devonian Simon-
son Dolostone and Fox Mountain Formation, Middle and Upper Devonian Guilmette
Formation, and Upper Devonian West Range Limestone. Together, these formations
are mainly composed of hundreds of partial to complete shallowing-upward Milanko-
vitch-scale cycles and are grouped into sequences bounded by regionally signifi cant
surfaces. Dolomitization in the Sevy and Simonson appears to be linked to exposure
surfaces and related underlying karst intervals. The less-altered Guilmette exhibits
characteristic shallowing-upward limestone-to-dolostone cycles that contain typical
carbonate-platform fossil- and ichnofossil-assemblages, displays stacked biostromes
and bioherms of fl ourishing stromatoporoids and sparse corals, and is punctuated by
channeled quartzose sandstones. The Guilmette also contains a completely exposed
~50-m-thick buildup that is constructed mainly of stromatoporoids, with an exposed
and karstifi ed crest. This buildup exemplifi es such Devonian structures known from
surface and hydrocarbon-bearing subsurface locations worldwide. Of special interest
is the stratigraphically anomalous Alamo Breccia that represents the middle member
of the Guilmette. This spectacular cataclysmic megabreccia, produced by the Alamo
*jwarme@mines.edu; jmorrow@sciences.sdsu.edu; sandberg@usgs.gov
Warme, J.E., Morrow, J.R., and Sandberg, C.A., 2008, Devonian carbonate platform of eastern Nevada: Facies, surfaces, cycles, sequences, reefs, and cataclys-
mic Alamo Impact Breccia, in Duebendorfer, E.M., and Smith, E.I., eds., Field Guide to Plutons, Volcanoes, Faults, Reefs, Dinosaurs, and Possible Glaciation in
Selected Areas of Arizona, California, and Nevada: Geological Society of America Field Guide 11, p. 215–247, doi: 10.1130/2008. d011(10). For permission to
copy, contact editing@geosociety.org. ©2008 The Geological Society of America. All rights reserved.
216 Warme et al.
INTRODUCTION
Purpose and Objectives
The main purpose of this guide is to present, in outcrop
examples, many of the common characteristics of shallow-water
carbonate-platform stratigraphic successions. Each stop exhibits
carbonate rocks that are typical of the Devonian platform facies
of southeastern Nevada and, except for the platform biota that
changes through time, are similar in structure to many carbon-
ate platforms that occur elsewhere throughout Phanerozoic and
even Proterozoic time. In addition, each stop presents objec-
tives of special stratigraphic interest that sets them apart, such
as distinctive cyclic stacking patterns, exposure surfaces, karst
intervals, organic buildups, sandstone interbeds, and evidence for
cataclysm via the Alamo Breccia.
We intend that enough time be dedicated at each stop to pro-
mote observations, descriptions, discussions, interpretations, and
possibly debates about the rocks. Each stop has been the subject
of one or more published papers, or reported in theses, but our
approach will be, as much as possible, to examine the formations
as a group of unbiased observers and interpreters and interpret
the rock properties and their signifi cances.
BACKGROUND INFORMATION
The following brief overviews are presented for the benefi t
of participants unfamiliar with the fi eld trip area and its geo-
graphic and geologic setting. Much of this information was com-
piled by Jared Morrow and presented in an unpublished fi eld trip
guidebook for participants of the September 2007 meeting of the
Subcommission on Devonian Stratigraphy (SDS) of the Interna-
tional Union of Geological Sciences (IUGS). Additional data are
excerpted from Morrow and Sandberg (2008).
Geographic Setting
The fi eld trip area lies within a southern portion of the Great
Basin (Fig. 1), which is a large part of the Basin and Range physio-
graphic province of western North America. This vast province is
characterized by hundreds of named, north-south–trending moun-
tain ranges separated by alluvium-fi lled, mainly Tertiary basins.
The ranges commonly exhibit tilted and well-exposed Paleozoic
stratigraphic sections, as shown in Figure 2, which facilitate study
and can be correlated between ranges. The Great Basin portion
includes almost all of Nevada and parts of eastern California and
Oregon, southern Idaho, and western Utah. As the name implies,
the “Great Basin” is hydrologically defi ned as the region where
streams drain into enclosed basins with no outlets to major rivers,
such as the Columbia River system to the north and the Colorado
River system to the southeast. Great Basin elevations range from
~600 m in deserts of the southern valleys to 4006 m at the summit
of Boundary Peak, the highest point in Nevada.
Geologic Overview
The oldest rocks in the Great Basin region are Paleoprotero-
zoic, 1.74 Ga metamorphic and intrusive igneous units exposed
in southern Nevada. In the east-west corridor from western Utah
to central Nevada, this crystalline basement is covered by a west-
ward-thickening prism of Neoproterozoic to Middle Devonian,
supratidal to deep-subtidal siliciclastic and carbonate sedimen-
tary rocks that were deposited along the subsiding passive craton
margin of what is now western North America (Stewart, 1980).
The sedimentary prism reaches a thickness of nearly 9000 m
in central Nevada, with Devonian strata making up more than
1800 m of this total (Fig. 3). Regional to global correlation of
Devonian rocks, sequences, and events is facilitated by conodont
biostratigraphy (Fig. 4), which provides the highest resolution
biochronology for this time interval.
Figure 5 shows the Middle Devonian to Lower Mississip-
pian formations that occur in the southeastern Nevada area of
this fi eld guide. They represent sediment accumulation in the
eastern and central longitudinal bands of the north-south sedi-
mentary prism. The carbonate-dominated formations represent
episodes of a long-lived, shallow-water, carbonate platform that
existed until the Late Devonian to Early Mississippian Antler
orogeny. Figures 6 and 7 show the relationships between eustasy
and Devonian formations that accumulated along the continen-
tal margin, including those that are the subject of this Chapter.
Figure 7 shows that guidebook stops in the Sevy, Simonson, Fox
Mountain, and Guilmette formations (Stops 1–5) lie relatively
landward on the platform, whereas the cherty argillaceous unit
and Sentinel Mountain, Bay State, and Devils Gate formations
(Stop 6) lie more seaward. As outlined below, however, Devonian
and later tectonic disturbances, fi rst mainly compressive and later
mainly extensional, severely fragmented the platform so that
paleogeographic reconstructions are diffi cult and still ongoing.
Impact Event, is as thick as 100 m and may be the best exposed proven bolide impact
breccia on Earth. It contains widespread intervals generated by the seismic shock,
ejecta curtain, tsunami surge, and runoff generated by a major marine impact. Newly
interpreted crater-rim impact stratigraphy at Tempiute Mountain contains an even
thicker stack of impact breccias that are interpreted as parautochthonous, injected,
fallback, partial melt, resurge, and possibly post-Event crater fi ll.
Keywords: carbonate platform, cyclostratigraphy, sequence stratigraphy, Devonian,
Alamo Breccia, impact deposits.
Devonian carbonate platform of eastern Nevada 217
Las Vegas
Alamo
Salt
Lake
City
Ely
Elko
Eureka
Reno
S i e r r a N e v a d a
O w y h e e
U p l a n d
C o l o r a d o R.
0100 km
Figure 1. Lambert projection digital elevation model shaded-relief image of central Basin and Range province, showing hy-
drologic boundary of the Great Basin region (dashed white line), outline of Nevada (solid white line), location of selected
Nevada cities and towns, and area of fi eld trip route shown in Figure 10 (black box). North is toward top of image. Modifi ed
from http://able1.mines.unr.edu.
Dfm
Dgysf
Dgl
Dgab
Dgu
Figure 2. View, looking north, of ~700 m of Middle and
Upper Devonian formations exposed on a fault block in the
West Pahranagat Range. Rocks of this section are almost
identical to those exposed on the traverse of Stop 1, along
“Downdropped Mountain,” 1 mi (1.6 km) to the west. De-
vonian carbonate-platform rocks include the Givetian Fox
Mountain Formation (Dfm), the Givetian yellow slope-form-
ing member of the Guilmette Formation (Dgysf), the Give-
tian to Frasnian lower member of the Guilmette Formation
(Dgl), the mid-Frasnian Alamo Breccia Member of the Guil-
mette Formation (Dgab), and the Frasnian to lower Famen-
nian upper member of the Guilmette Formation (Dgu). The
Guilmette here contains 70 documented platform carbonate
cycles, which are mixed with quartzose sandstone facies in
the upper member (Estes-Jackson, 1996).
218 Warme et al.
Beginning in the Middle Devonian and accelerating in the
Late Devonian, the effects of eastward-verging tectonic compres-
sion crossed the region. Several orogenic episodes were driven
by this compression. Most evident are the Antler orogeny (Late
Devonian–Mississippian, focused along the Roberts Mountains
thrust; Fig. 8), and the Sonoma orogeny (Late Permian–Early
Triassic). During the Devonian to Triassic interval, western
North America was fringed by a developing subduction zone
system. As shown partially in Figures 6 and 7, from east to west
across the Great Basin the sedimentary and tectonic provinces
at this time included: (1) a shallow-marine, carbonate-dominated
platform; (2) a backbulge basin (e.g., the Late Devonian–Early
Mississippian Pilot basin); (3) an eastward-migrating forebulge;
(4) a foreland basin, which received detrital sediments from both
the east and the west; (5) an emergent allochthon; (6) a western,
deep-marine basin dominated by hemipelagic sedimentation and
mafi c volcanism; and (7), at the longitude of present-day western
Nevada and eastern California, a volcanic arc terrane.
Mesozoic sedimentary rocks in the Great Basin are largely
of Triassic and Early Jurassic ages, but are not exposed in this
eld guide area. Cretaceous sedimentary rocks are represented
by continental deposits in localized tectonic basins (e.g., the
Newark Canyon Formation). By the mid-Jurassic, extensive tec-
tonic compression again spread from west to east across Nevada,
culminating in the Cretaceous to early Cenozoic Sevier orogenic
belt of easternmost Nevada and western Utah (Fig. 8).
During the middle Cenozoic, from ~34 Ma to ~17 Ma, the
region was dominated by siliceous volcanism, evidenced by
widespread ash-fl ow tuffs and rhyolites that are preserved on
most of the ranges in the fi eld trip area. From ~17 Ma to the pres-
ent, major extensional tectonism, crustal thinning, normal fault-
ing, and mafi c volcanism characterized the region, resulting in
the modern basin and range topography and basaltic fl ows that
can be young enough to follow present drainages. In the Pleis-
tocene, from ~30,000 yr B.P. to ~10,000 yr B.P., large areas of
many basins were covered by extensive pluvial lake systems,
including Lake Lahontan in western Nevada and Lake Bonne ville
in western Utah (Stewart, 1980; Hintze, 1988). Wave-cut ter-
races and fi ne-grained lacustrine deposits are a common feature
in these basins, especially north of the fi eld trip area covered in
this guide. Also during the Pleistocene, isolated alpine glaciers
formed on the highest peaks of the central Great Basin.
Figure 3. Palinspastic map of Great Basin region with isopachs (thickness in thousands of feet) and depositional provinces of Devonian rocks.
Position of Lincoln County (L.C.), Nevada, is marked. Modifi ed from Stewart and Poole (1974).
Devonian carbonate platform of eastern Nevada 219
Figure 4. Devonian conodont biochronology, scaled to numerical ages of Kaufmann (2006). Cor-
relation of zonation with German Stufen is shown for Late Devonian; Substages are those pro-
posed by Sandberg and Ziegler (1998). Subdivisions of Early rhenana and linguiformis Zones are
shown. E—Early; L—Late; semisemichatovae Subzone interval; MISS.—Mississippian. Scaling
of Famennian biozones to numerical ages is approximate and provisional. Modifi ed from Sandberg
et al. (2002), Girard et al. (2005), Kaufmann (2006), and C.A. Sandberg (unpublished).
Figure 5. Time-rock chart of Middle Devonian (Eifelian) to Lower Mississippian (Osagean) strati-
graphic units exposed at Alamo Canyon (ALA), and at Bactrian Mountain (BCT and BME) and
Silver Canyon, Mount Irish Range (MIR, Stop 3), east and north of Alamo, Nevada. Numerical
ages are from Kaufmann (2006). Time values of Alamo Breccia Member, Leatham Member, and
Mount Irish buildup are exaggerated for graphic purposes. See Sandberg et al. (1997) for Nevada
Events. Modifi ed from Sandberg et al. (1997).
220 Warme et al.
Figure 6. Devonian sea-level curve, showing relative paleotectonic and geographic settings of principal stratigraphic units of the western United
States. Roman numerals indicate transgressive-regressive (T-R) cycles of Johnson et al. (1985). F.I.—brachiopod- and conodont-based faunal
intervals of Johnson et al. (1980); T.S.—transgressive starts in the western United States; C.Z.—conodont Zones. Kobeh, Bartine, and Coils
Creek are members of McColley Canyon Formation. From Johnson and Sandberg (1989).
Devonian carbonate platform of eastern Nevada 221
Lower mbr.
Pilot Shale
Woodruff tongue
Middle
Devonian
Lower
Devonian
Givetian
Eifelian
Ems-
ian
Prag-
ian
Loch-
kovian
Continental
slope Carbonate platform
NE
SW
Upper
Devonian
Frasnian Famennian
Series Stage
Forebulge & foreland uplift
Alamo Event
Platform
margin
Platform
margin
FB
Beacon Peak Dol.
Sevy
Dol.
Lone Mountain Dol. (pt.)
Oxyoke Cnyn.
Ss.
Windmill
Ls.
Bas-
tille
Ls.
Tor-
McMon.
Ls.
Rabbit
Hill
Ls.
Kobe h
Mbr.
Coils
Crk.
Mbr.
Bartine Mbr.
Sadler Ranch Fm.
Bay State
Dolostone
Woodpecker Ls.
Sentinel Mtn. Dol.
Denay
Limestone
L.-m.
tongue
Fenstermaker Wash Fm.
Woodruff Fm.
(U. Devonian only)
U. t. Fenstermaker
Wash Fm.
Upper tongue
Woodruff
Fm.
Upper mbr.
Devils Gate Ls.
Lower mbr.
Devils Gate Ls.
Fox
Mountain
Fm.
Leatham Mbr.
Pilot Shale
Simonson
Dol.
Roberts Mtns.
Fm. (pt.)
YSF
F o r e b u l g e
T- R
Cycle
pre-Ia
Ia
Ib
Ic
Id
Ie
If
IIa
IIb
IIc
IId
IIf
IIe
Lower tongue
Woodruff
Fm.
McColley
Canyon Fm.
S l a v e n C h e r t
Barite & phosphatic black chert
To e
Guilmette Fm.
Red Hill beds
cau
Platform
margin
Upper mbr.
Lower mbr.
uam
bcm
lam
cxm
Unnamed Lower &
Middle Devonian units
(Locally present)
Figure 7. Northeast-to-southwest, Devonian time-rock transect across central and eastern Nevada, showing carbonate-platform, continental slope,
and toe of slope stratigraphic units and relative lateral shifts in platform margin through time. Transgressive-regressive (T-R) cycles Ia through IIf
(Johnson et al., 1985, 1991; Johnson and Sandberg, 1989), main intervals of turbidity current and debris-fl ow deposition (arrows), proto-Antler
forebulge initiation (FB), and timing of Alamo Impact Event are indicated. Silty dolostone and siltstone of the yellow slope-forming member
(YSF), which forms the basal unit of the Guilmette Formation and Devils Gate Limestone, is a widespread marker lithology distributed through-
out western North America (Sandberg et al., 1989, 1997, 2002). The YSF is correlated with fi sh-bearing Red Hill beds in north-central Nevada.
Four members of Simonson Dolostone are: cxm—coarse crystalline member; lam—lower alternating member; bcm—brown cliff member; and
uam—upper alternating member. Other abbreviations: cau—cherty argillaceous unit; Cnyn.—Canyon; Crk.—Creek; Dol.—Dolostone; Fm.—
Formation; L.—Lower; Ls.—Limestone; m.—middle; mbr.—member; McMon.—McMonnigal; Mtns.—Mountains; pt.—part; Ss.—Sandstone;
t.—tongue; U.—Upper. From Morrow and Sandberg (2008) and based on data from Johnson and Sandberg (1977), Johnson and Murphy (1984),
Sandberg et al. (1989, 1997, 2002, 2003), Johnson et al. (1996), and M.A. Murphy (14 September 2007, personal commun.).
222 Warme et al.
Figure 8. Index map of Great Basin region showing major thrust and strike-slip faults. Sevier thrust system, of Cretaceous to early Cenozoic
age, includes individual parts consisting of Nopah, Wheeler Pass, Keystone, Gass Peak, Wah Wah, Muddy Mountains, Blue Mountain, Nebo,
Charleston, Willard, and Paris faults. From Stewart (1980).
Devonian carbonate platform of eastern Nevada 223
DEVONIAN GEOLOGIC SUMMARY
Rocks studied on this fi eld trip represent part of the ~57-
m.y.-long Devonian geological history of the eastern and central
parts of the extensive carbonate platform that extended north-to-
south through Nevada and Utah (Figs. 6, 7). The stops in this
guide represent only the central latitudinal segment of a much
longer Devonian platform and platform-margin complex that
fringed western North America from Alaska and Arctic Canada
to Mexico (Ziegler, 1989; Poole et al., 1992). Devonian platform-
to-basin rocks in western Utah and Nevada record the complex
transition from long-term extensional to compressional tectonic
modes of the Antler orogeny, which was the fi rst in a series of
late Paleozoic to Mesozoic orogenic belts that developed over
the subduction zone fringing the western continental margin of
North America. In the middle- to outer-platform settings, Devo-
nian strata reach a thickness of over 1800 m (Fig. 3).
During the mid-1970s to early 1990s, a series of landmark
papers provided detailed stratigraphic, biostratigraphic, lithofa-
cies, isopach, structural, and coastal onlap data on the Devo-
nian continental margin in the western United States, including
the eustatic sea-level curve of Figure 6 and multiple paleogeo-
graphic and paleotectonic time-slice maps constrained by high-
resolution conodont biochronology. These important papers
include, among others: Poole (1974), Stewart and Poole (1974),
Johnson and Sandberg (1977, 1989), Matti and McKee (1977),
Murphy (1977), Poole et al. (1977, 1992), Sandberg and Poole
(1977), Johnson and Pendergast (1981), Gutschick and Sand-
berg (1983), Kendall et al. (1983), Johnson and Murphy (1984),
Murphy et al. (1984), Johnson et al. (1985, 1986, 1989, 1991),
Stevens (1986), Sandberg et al. (1988, 1989), Goebel (1991),
Johnson and Bird (1991), D.M. Miller et al. (1991), and E.L.
Miller et al. (1992).
More recent studies of the Devonian platform and platform-
to-basin transition in Utah and Nevada have focused on refi n-
ing aspects of: (1) tectonic and structural history (e.g., Oldow
et al., 1989; Giles, 1994; Giles and Dickinson, 1995; Crafford
and Grauch, 2002; Grauch et al., 2003; Sandberg et al., 2003);
(2) depositional models and facies geometry (e.g., Elrick, 1995,
1996; LaMaskin and Elrick, 1997; Cook and Corboy, 2004);
(3) conodont-based event stratigraphy and eustasy (e.g., John-
son et al., 1996; Sandberg et al., 2002, 2003; Morrow and Sand-
berg, 2003, 2008); (4) stratigraphic and structural development
in relation to synsedimentary exhalative gold mineralization
in the Carlin gold trend, northern Nevada (e.g., Emsbo et al.,
1999, 2006; Hofstra and Cline, 2000; Crafford and Grauch,
2002); and (5) relation to the early Late Devonian Alamo Impact
Event, south-central Nevada (e.g., Warme and Sandberg, 1995,
1996; Sandberg et al., 1997, 2002, 2003, 2005, 2006; Warme
and Kuehner, 1998; Warme, 2004; Morrow and Sandberg, 2005,
2006; Morrow et al., 2005; Pinto, 2006; Warme and Pinto, 2006;
Pinto and Warme, 2008).
Warme and Pinto (2006) and Pinto and Warme (2008) pre-
sented a genetic classifi cation for different facies of the Alamo
Breccia, placing known positions of the Breccia into impact
“Realms” with respect to the target zone. As shown in Figure 9,
Stops 1–5 in this chapter fall in the Ring Realm, and Stop 6 repre-
sents the only known locality in the Rim Realm, closest to the as
yet unidentifi ed central target zone. The other realms are outside
the area of this chapter, but have been described and interpreted
in references cited for the Alamo Impact Event.
Sedimentation Patterns
In western Utah and Nevada, the position of the Devonian
carbonate platform and the sedimentary facies deposited in plat-
form-to-basin settings were infl uenced by both eustasy and tecton-
ics. Since the early comprehensive work of Roberts et al. (1958),
Paleozoic sedimentary strata in western Utah and east-central
Nevada have been generally regarded to document, from east-
to-west, shallow-to-deep marine carbonate-platform, platform-
margin, slope, and basin depositional settings (Figs. 5 and 7). In
general, Devonian platform deposits are dominated by supratidal,
intertidal, and shallow-subtidal carbonate rocks such as biolami-
nated dolostone, bioturbated lime mudstone and wackestone,
bioclastic wackestone and packstone, stromatoporoid-dominated
lime mud-rich biostromes, and intraformational conglomerate
(Elrick, 1996; Poole et al., 1992; Cook et al., 1983; Cook and
Corboy, 2004). During eustatic lowstands, craton-derived quartz
sand was deposited in channels and basinward-prograding clastic
wedges across large areas of the platform (Fig. 5).
Devonian outer-platform to platform-margin rocks consist
of shallow- to deep-subtidal, bioturbated, bioclastic wacke-
stone, packstone, and grainstone. During parts of the Early and
Middle Devonian when a rimmed platform margin developed
(Elrick, 1996; Cook and Corboy, 2004), deposits included
coral- and crinoid-dominated biostromes, bioherms, and mud-
mounds. Slope and basin areas accumulated bioclastic and
sandy packstones and grainstones deposited within submarine
debris-fl ow and turbidite-fan systems, intraformational slumps
with fl at-clast conglomerates, and rhythmically deposited argil-
laceous carbonate and fi ne- to medium-grained siliciclastic
units. The most distal deposits include rhythmically bedded
radiolarian chert, very fi ne- to fi ne-grained siliciclastic units,
and minor greenstones. The Early and Middle Devonian car-
bonate platform-to-basin transition, which can be in part char-
acterized using classic carbonate platform models (e.g., Wilson,
1975; Read, 1982), was characterized by several morphologies
including homoclinal ramps, distally steepened ramps, rimmed
platform margins with intra-shelf basins, and rimmed plat-
form-margins fl anked by landward shallow-subtidal platforms
and seaward slope-aprons (Cook et al., 1983; Kendall et al.,
1983; Johnson and Murphy, 1984; Schalla and Benedetto, 1991;
Elrick, 1996; Cook and Corboy, 2004). As discussed next, how-
ever, throughout the latter half of the Devonian, proto-Antler
and Antler tectonism exerted a strong to dominant control on
the position and geometry of the carbonate- platform margin-
to-basin settings.
224 Warme et al.
Tectonic Models
Neoproterozoic to early Paleozoic time was marked by a
complex pattern of tectonic extension and rifting along western
North America as the proto-Pacifi c basin opened following the
breakup of Rodinia at ~850–650 Ma (Stewart, 1972; Stewart and
Suczek, 1977; Poole et al., 1992). Outer-platform and platform-
margin basins, which probably formed by reactivation of struc-
tures inherited from Mesoproterozoic to Neoproterozoic rifting
of underlying crystalline continental basement (Stewart, 1972;
Stewart and Poole, 1974), strongly infl uenced sedimentation pat-
terns in Nevada during the Middle Cambrian to Middle Devonian
(Johnson and Potter, 1975; Matti and McKee, 1977; Johnson and
Murphy, 1984; Miller et al., 1991; Poole et al., 1992). For the Early
and Middle Devonian interval especially, the distribution of facies
patterns, rock isopachs, strontium and lead isotope isopleths,
basement gravity signatures, and synsedimentary exhalative gold
and barite deposits all suggest that the platform margin was char-
acterized by a complex series of restricted, active, fault-bounded
sub-basins, which determined the type, extent, and thickness of
sedimentary units (Grauch, 1998; Emsbo et al., 1999, 2006; Hof-
stra and Cline, 2000; Crafford and Grauch, 2002; Grauch et al.,
2003; Emsbo and Morrow, 2005; Morrow and Sandberg, 2008).
By the Ordovician (Ross, 1977) or Silurian (Poole et al.,
1977), a volcanic island-arc system developed over subducted oce-
anic crust west of the North American continent. Localized exten-
sional or transtensional tectonics within a postulated inner-arc
basin located between the volcanic arc and the continent may have
further promoted the formation of fault-bounded platform-margin
sub-basins prior to late Middle to early Late Devonian compression
and transpression associated with the approaching Antler orogen
(Poole et al., 1977; Eisbacher, 1983; Crafford and Grauch, 2002).
1
3
4
5
6
2
Figure 9. Alamo Breccia locality map show-
ing genetic Breccia Realms, Nevada and
western Utah. Lateral Breccia Zones 1, 2,
and 3 of earlier publications (e.g., Warme and
Sandberg, 1995, 1996; Warme and Kuehner,
1998) are now equated with the Rim, Ring,
and Runup Realms (Warme and Pinto, 2006;
Pinto and Warme, 2008), respectively. Black
diamonds, offshore, deep-water Alamo chan-
nel localities; white circles, localities at Tem-
piute Mountain interpreted to be on or within
the Alamo crater rim; open triangles, carbon-
ate-platform localities with well- developed
Alamo Breccia including potentially all
Units A–D; black circles, carbonate-platform
localities with thin Alamo Breccia; black
triangle, locality with seismically disturbed
zone stratigraphically equivalent to Alamo
Breccia; white diamonds, distal, middle- to
inner-platform Alamo channel deposits;
black arrows, paleocurrent directions deter-
mined from clast imbrication; black squares,
selected towns. Field trip Stops 1–6 are indi-
cated by numbers. Modifi ed from Pinto and
Warme (2008).
Devonian carbonate platform of eastern Nevada 225
Unraveling of the complicated Devonian tectonic processes
and sedimentary responses in the Great Basin area has been
facilitated by use of conodont biostratigraphy (e.g., Figs. 4, 6,
and 7). The earliest direct stratigraphic evidence for the switch
to a convergent tectonic mode is in central Nevada, where uplift
and erosion was associated with development of the initial, shal-
low-marine to emergent, proto-Antler forebulge during the latest
Givetian to early Frasnian disparilis, falsiovalis, and transitans
conodont Zones (Figs. 6 and 7; Sandberg et al., 2003). Subse-
quent Late Devonian depositional settings and facies patterns on
the outer platform and platform margin were dominated by tec-
tonic effects of the converging Antler orogenic belt, which formed
an eastward-migrating system composed, from west to east, of an
allochthon, a foreland basin, a forebulge, and a backbulge (Pilot)
basin that developed across the carbonate-dominated platform to
the east (Poole and Sandberg, 1977; Goebel, 1991; Giles, 1994;
Giles and Dickinson, 1995).
By the late middle Famennian Early postera Zone (Fig. 6),
the carbonate platform environment was terminated by wide-
spread uplift driven by the continued eastward migration and
expansion of the Antler orogen. Erosion off the leading forebulge
formed a regional unconformity that interrupted or removed the
latest Famennian depositional record of the platform (Sandberg
et al., 1989, 2003; Poole and Sandberg, 1991). The magnitude of
this extensive unconformity was amplifi ed by a major eustatic
sea-level fall that began during the late Famennian Middle prae-
sulcata Zone and persisted into the Early Mississippian (Sand-
berg et al., 1989; 2002). At the site of the former carbonate-
platform margin, the overlying Mississippian (Kinderhookian
to Chesterian) foreland basin fan and overlap assemblage rocks
include compositionally immature, syntectonic, siliciclastic units
that were derived in large part from the Antler allochthon to
the west (Johnson and Pendergast, 1981; Poole and Sandberg,
1991). Final convergence of the Antler orogen with western
North America during the Early Mississippian thrust Devonian
and underlying lower Paleozoic basin and slope rocks as much as
145 km eastward over coeval shelf-margin and outer-shelf rocks,
forming the Roberts Mountains thrust system (Fig. 8; Stewart,
1980; Johnson and Pendergast, 1981; Poole et al., 1992). Recent
models of the Roberts Mountains thrust system characterize it
as a complex zone of intercalated, folded, thrust, and imbricated
upper Precambrian to middle Paleozoic structural-stratigraphic
units (e.g., Theodore et al., 1998; Noble and Finney, 1999; Craf-
ford and Grauch, 2002). These relationships greatly complicate
efforts to precisely reconstruct the paleogeography of the deposi-
tional systems that operated at this time.
CARBONATE PLATFORM FACIES
Hundreds of books, monographs, symposium volumes, and
journal articles have been devoted to the sedimentology, stratal
geometries, biota, petrology, diagenesis, resource potential, and
other aspects of modern and ancient shallow-water carbonate plat-
forms. The following brief overview is intended to highlight some
of the characteristics of carbonate platforms that are exhibited
at stops described in this guide. Only a few references are cited
herein, but the interested reader is encouraged to consult them and
choose from the abundant cited references that they provide.
Background
Many early reports on carbonate rocks were exhaustive
petrographic studies of the rock and fossil particles that compose
limestones and dolostones of different ages, and were published
in German, French, Italian, Russian, and other languages, as well
as in English. Much of the non-English material was ignored or
unappreciated by American geologists. By the 1970s, enough
understanding was achieved to synthesize the accumulated fi eld,
subsurface, and petrographic studies into facies models that could
help predict the distribution of rock types in carbonate depositional
systems. Research on existing shallow-water carbonate environ-
ments, such as the Bahama Banks, the Yucatan-Belize shelf, and
the Great Barrier Reef, together with regional work on classic
ancient localities such as the Devonian reef complex of northwest
Australia, the Permian reef complex of west Texas, and the Trias-
sic exhumed atolls of the Italian Dolomite Alps, all helped estab-
lish the “carbonate platform” paradigm as the basic framework for
the accumulation of most marine shallow-water carbonate rocks.
Two classic volumes are fundamental resources for carbon-
ate workers. Wilson (1975) comprehensively synthesized results
from studies worldwide and showed how carbonate platforms
functioned as similar sediment-accumulation systems through
time. He demonstrated how platforms of different ages had
similar three-dimensional forms but were generated by different
organisms that adapted and evolved over time. In addition to cor-
als and algae that dominate modern platform seaward margins,
older margins and rims were constructed by various carbonate-
xing organisms, such as bacteria, sponges, bryozoans, brachio-
pods, tube-building worms, and bivalve and gastropod mollusks.
From the stromatolitic environments in the Proterozoic to the
coral-algal (“coralgal”) reefs of today, platform carbonates accu-
mulated laterally and stacked vertically in response to relative
sea-level changes that controlled the availability of accommoda-
tion space for the in situ generation of new sediment.
The compendium edited by Scholle et al. (1983) brought
together the knowledge of varied carbonate depositional environ-
ments, from continental and lacustrine to pelagic deep-marine,
and offered several chapters on the supratidal to subtidal environ-
mental bands that occur across carbonate platforms. Many other
volumes dedicated specifi cally to carbonate platforms followed
(e.g., Crevello et al., 1989; Tucker et al., 1990; Simo et al., 1993).
Although some carbonate-dominated shorelines lead sea-
ward down gently inclined ramps, important facies faunas were
those that built and maintained a seaward shallow-platform rim
into the active wave zone. A very narrow band of reefoid facies,
along wave-infl uenced seaward platform margins, separates the
expansive and relatively quiet lagoonal environments landward of
the rims from the steep, debris-covered aprons seaward of them.
226 Warme et al.
Carbonate accumulations commonly become cemented directly
on the seafl oor, most notably strengthening the critical narrow
band of reefoid and related facies along seaward margins. Bored
hardgrounds, sharply eroded transgressive surfaces, short-term
disconformities, and other evidence show that seafl oor cementa-
tion and early burial cementation and diagenesis may completely
lithify the sediments of each short-term sediment accumulation,
or cycle, prior to initiation of the following one.
The Carbonate Platform Signature: Cyclostratigraphy
The basic building block of shallow-water carbonate, evapo-
rite, and siliciclastic sedimentary deposits is generally agreed to
be the “shallowing-upward cycle,” which has been described from
worldwide examples in rocks of Proterozoic to Recent ages (e.g.,
Ginsburg, 1975). Carbonate platforms commonly exhibit obvi-
ous stacked meter-scale cycles whose analysis has given rise to
the discipline of “cyclostratigraphy” (e.g., Elrick, 1995). A cycle
is generated when fl ooding creates accommodation space that is
invaded by carbonate-producing organisms, which establish the
“carbonate-generating factory” in situ across the platform. A fully
preserved idealized cycle is a vertically stacked package of genet-
ically related beds of subtidal, intertidal, and supratidal facies,
bounded by exposure surfaces that form between cycles (e.g.,
Hardie and Shinn, 1986; chapters in Loucks and Sarg, 1993). Of
course, each cycle has an intertidal to supratidal feather edge that
limits the internal sequence of facies, and every platform at any
moment has a particular mosaic of shoals, channels, and clusters
of carbonate-producing and sediment-trapping organisms. Thus,
cycles are expected to show internal lateral variation. Neverthe-
less, numerous examples have been documented where an indi-
vidual distinctive cycle, or bundle of cycles, was traced laterally
for many kilometers without signifi cant change in their interval
character. We will see such examples in fi eld traverses through
the Guilmette Formation during this excursion.
Debate about carbonate rock cycles, whether they are driven
mainly by external (allocyclic) forces such as climate periodicity
and global sea-level response, or internal (autocyclic) processes
such as local diastrophism and expected sediment dispersal and
accumulation patterns, has prompted numerous studies of mod-
ern and ancient platforms coincident with a range of theoretical
models. Increased computing power has allowed development
of increasingly sophisticated models. A recent example are the
models of Burgess (2006), whose paper also contains a compre-
hensive review and reference list for the history of the extrinsic/
intrinsic debate, which has not yet been fully resolved.
With the advent of “sequence stratigraphy” (e.g., Wilgus et al.,
1988), researchers tested stratal patterns for hierarchical arrange-
ments that indicated response to relative sea-level changes on
various time scales, most notably within the astronomically driven
Milankovitch time band in which climate change oscillated at four
or more different astronomically fi xed intervals, each a few tens
of thousands to hundreds of thousands of years in duration (cf.,
papers in Arthur and Garrison, 1986). The oscillations are not syn-
chronized, so that they may cancel or reinforce each other through
time. However, the concept of sedimentary cycles, driven by rela-
tive sea-level changes, regardless of cause, is widely accepted.
In sequence-stratigraphic terms, a simple system of oscillating
sea level together with continuous subsidence could account for the
generation and preservation of such cycles (Sarg, 1988). Packages
of cycles were documented whose facies prograded, retrograded,
and stacked, creating successions of shallow-water platform car-
bonates that in some cases were hundreds or even thousands of
meters in thickness, shown or presumed to be preserved behind
some form of subsiding platform rim. Cycles of shorter duration
within the Milankovitch band, the ~20,000 yr or ~40,000 yr oscil-
lations, may progress faster than carbonate sediment can accumu-
late. Some cycles exhibit evidence for transgressive drowning that
deepened the platform to below wave base, or regressive shallow-
ing that exposed the platform top to karstifi cation and pedogen-
esis. Because the sediment cycles are commonly only a few meters
thick, they may represent a compressed record of the maximum
sea levels that occurred during their genesis. In such cases, sea
level rose at a faster pace than sediment could accumulate, then
fell to meet the surface of aggrading sediments before the maxi-
mum accommodation space was fi lled. Thus, the exact magnitude
of maximum relative sea-level change over a platform during the
life of a cycle cannot be fully known, and is a product of local
subsidence, eustatic change that may have occurred, and sediment
accumulation rate. Factors that further infl uence sediment accu-
mulation in any cycle include the somewhat unpredictable mosaic
carbonate production and accumulation across any given platform,
and the results of rare events such as storms. However, together
with the preserved cycle thickness, the magnitude of bathymetric
change can be estimated using lithofacies and biofacies depth indi-
cators that may be captured and compressed within a cycle.
Within any vertical succession, several similar platform cycles
may be bundled, separated from other bundles by a surface that
represents extended exposure or an interval that indicates pro-
longed drowning. Such bundles have been interpreted, not without
controversy, to represent shorter-term Milankovitch cycles (tens of
thousands of years) captured within longer-term ones (hundreds of
thousands of years). However, platform cycles commonly appear
to be sorted into bundles of similar internal character. More signifi -
cant regional surfaces of exposure or drowning across platforms
may serve to vertically partition bundles of cycles into longer-term
sequences. Thus, platform carbonate rocks commonly exhibit a
hierarchy of shallowing-upward cycles, separated by a hierarchy of
surfaces. The life of a platform may cease by a pronounced relative
drop of sea level, exposure, and karst development, or by marine
drowning, from which the shallow-water platform environment,
with its cycle-generating mechanism, never recovers. Examples in
the fi eld trip area include the terminal exposure and karst forma-
tion at the top of the Simonson Dolostone, and the deepening and
extinction of the shallow platform at the top of the Guilmette For-
mation; both examples are exhibited on the traverse of Stop 1.
In Nevada, Devonian carbonate-platform cycles have been
studied in detail, arranged into sequences, and analyzed for the
Devonian carbonate platform of eastern Nevada 227
transgressive-regressive sea-level history that they may reveal.
North of the fi eld trip area, bed-by-bed analyses include those of
Elrick (1995, 1996) for Lower and Middle Devonian formations,
and LaMaskin and Elrick (1997) for the Guilmette Formation.
Within the fi eld trip area, Estes-Jackson (1996) provided similar
descriptive detail and interpretation of cycles in the Guilmette
exposed in the fault block (Fig. 2) adjacent to the Hancock Sum-
mit West location of Stop 1. Chamberlain and Warme (1996) and
Chamberlain (1999) developed the sequence analysis summa-
rized in the composite stratigraphic column of Figure 11 and the
accompanying Table 1.
FIELD TRIP AREA
Figure 10 shows the fi eld trip area and location of stops we
have chosen to include in this fi eld guide. Numerous other locali-
ties, in ranges within and beyond the trip area, also contain excel-
lent exposures of Devonian formations. Prior fi eld trip guides that
cover some or all of the area are those of Sandberg et al. (1997),
which was extensively drawn upon for the present guide, and
Gillespie and Foster (2004), which also contains seven reprinted
papers on the Alamo Breccia. This guide is the fi rst to focus
mainly on the Devonian platform beds and their interpretation.
Stop Locations
The stop locations described herein may be visited indepen-
dently, in any order. However, Stop 1, Hancock Summit West,
contains a thick, continuous section of Devonian formations that
are accessible, well exposed, and typical of the fi eld trip area.
Hence, Stop 1 is most useful as an initial section for comparisons
with stratigraphic units at other stops and beyond.
Starting Point
The location of each stop is described as the direction and
distance from a central point in the fi eld trip area: a wayside
rest near Crystal Springs, at the junction of Nevada State High-
ways 375 and 318, at the southern edge of the small settlement
of Hiko (Fig. 10). Global Positioning System (GPS) coordinates
at the Crystal Springs rest area are: lat 37°1357.20 N., long
115°2456.75 W., Hiko 7.5 quadrangle.
DEVONIAN FORMATIONS
Devonian formations to be studied are the Lower Devonian
Sevy Dolostone, Lower to Middle Devonian cherty argillaceous
unit and Oxyoke Canyon Sandstone, Middle Devonian Simonson
Dolostone and Fox Mountain Formation, Middle to Upper Devo-
nian Guilmette Formation, and Upper Devonian West Range
Limestone. See Figures 6 and 7 and cited papers for precise ages,
lateral equivalents, and interrelationships of these formations.
Figure 11 is a composite column of formations, nearly
5000 ft (~1500 m) thick, from the upper part of the Sevy
Dolostone to the post-platform Devonian to Mississippian Pilot
Shale and Lower Mississippian Joana Limestone. The column
represents an attempt to place the Devonian formations in the
eld trip area into a sequence-stratigraphic framework. It was
constructed by Alan Chamberlain (Chamberlain and Warme,
1996; Chamberlain, 1999) on the basis of outcrops in the cen-
tral Timpahute Range, close to Stop 3. The depicted sequence
differs in detail from sequences exposed at other stops. For
example, at Stop 3 the upper Guilmette Formation, shown in
the column of Figure 11, contains a thick carbonate buildup
(Dgb3), directly over the Alamo Breccia (Dgb2), and only
sparse, thin quartz sandstone beds. In contrast, the upper Guil-
mette at Stop 1 contains thick sandstone intervals and lacks a
buildup, whereas at Stop 4 it contains both small buildups and
sandstones over the Breccia.
The central column of Figure 11 shows basic rock types, par-
titioned vertically into sequences. The sequences were defi ned in
two ways. The fi rst method was to identify and describe individ-
ual shallowing-upward cycles in as much detail as possible from
available outcrops. This process resulted in bundles of two to as
many as 29 cycles of generally similar character. These cycles
are listed on Table 1 and shown as excursions on the graphed
line to the right of the column. The second method was to create
a companion log of the outcrops using a hand-held gamma-ray
scintillometer, shown as the graphed line to the left of the col-
umn. The two methods were combined to defi ne sequences that
contained cycles of similar lithologic character and gamma-ray
signature, separated from other sequences by distinctive but com-
monly subtle surfaces that signify exposures, signifi cant marine
transgressions, or other environmental shifts. The graphed line to
the right of the column was interpreted as a rough proxy for rela-
tive sea-level changes across the platform (for details see Cham-
berlain and Warme, 1996; Chamberlain, 1999).
Characteristics of these sequences are discussed in the
text for the stops. Note, however, that many of the sequence
boundaries tend to match the boundaries of named formations
and members, from the upper Sevy Dolostone (Dse3) at the
base to the carbonate buildup over the Alamo Breccia (Dgb3)
at the top. The overlying upper segment of the informal upper
member of the Guilmette Formation was partitioned into fi ve
thick sequences (Fig. 11), some of which could be useful for
further subdivision. This example shows an important attribute
of sequence stratigraphy, whereby rock bodies are bounded by
genetically signifi cant surfaces that mark bathymetric shifts
or other environmental events. Boundaries of many members,
lenses, and tongues, which originally may have been loosely
characterized by terms such as “transitional” or “fi rst occur-
rence” of a given lithology, can be refi ned, redefi ned, or clari-
ed if such surfaces are present and recognized.
Guilmette Formation
All stops described in this guide, except Stop 2, include
the Guilmette Formation, which is the least dolomitized and
228 Warme et al.
0
0
50 mi.
80 km
1
2
3
5
6
Alamo
Crystal Springs
37°
38°
115°
116°
Lincoln
Co.
Clark
Co.
White
Pine
Co.
Nye
Co.
NEVADA
OR ID
UT
CA
AZ
0100 km
N
Detailed
map
4
Figure 10. Index and highway map of southeastern Nevada, showing locations of fi eld trip Stops
1–6 (numbered stars) and Crystal Springs, the starting point for fi eld trip road logs. Modifi ed from:
http://www.nevadadot.com/traveler/maps/StateMaps.
Devonian carbonate platform of eastern Nevada 229
most lithologically variable formation because it contains the
Alamo Breccia Member. The Guilmette and its members at
Stop 1 are shown in Figures 11–14. The following review is a
history of studies and proposed lithostratigraphic subdivisions
of this forma tion.
Initially, Reso (1963) divided the Guilmette into informal
lower and upper members. The lower member terminated at the
top of a thick, stromatoporoid-rich breccia that was documented
at Hancock Summit West (Stop 1), along strike in Guilmette
exposures on the west side of the West Pahranagat Range, and
also near Mount Irish (Stop 3). At Mount Irish, a similar brec-
cia lies directly under a ~50-m-thick stromatoporoid-rich mound
(“Reso’s Reef”). Consequently, Reso interpreted that breccia as
mound talus, as did Dunn (1979), whose main objective was the
study of the mound proper. Because the thick breccia at Han-
cock Summit West contains abundant stromatoporoids, it was
probably also regarded as reef debris, although no large reef is
exposed there. At both localities the thick breccia is the Alamo
Breccia Member.
Estes-Jackson (1996) described and interpreted a Guilmette
stratigraphic section that is exposed on the fault block adjoin-
ing Stop 1, one mile east of Hancock Summit West (see Fig. 2).
The section is almost identical to that traversed at Stop 1. She
identifi ed 22 shallowing-upward cycles in Reso’s lower member
and 47 cycles in his upper member. She believed that the thick
breccia at the top of the lower member was a normal, but perhaps
deeper, platform facies and not a cataclysmic bed; however, it
too is the Alamo Breccia Member. She calculated that the cycles
fell within the lower part of the Milankovitch band, each of less
than 100,000 years duration. Estes-Jackson (1996) documented
the sandstone facies of the upper Guilmette, which is well repre-
sented at Stop 1 and in many other ranges of eastern Nevada, but
is largely absent from the nearby central Timpahute Range where
the stratigraphic column of Figure 11 was generated.
Kuehner (1997) subdivided Reso’s (1963) lower member of
the Guilmette Formation into three units, which are exposed in
several ranges: a basal, yellow-weathering “slope forming inter-
val,” a “ledge forming interval,” and the then newly designated
Alamo Breccia Member. He retained Reso’s upper member for
the balance of the Guilmette.
Sandberg et al. (1997) partitioned Reso’s (1963) lower
member of the Guilmette into three members (Fig. 14). Their
“yellow slope-forming” and “carbonate platform facies” mem-
bers are similar to Kuehner’s two lower “intervals.” They for-
malized the highest interval as the type Alamo Breccia Member
of the Guilmette Formation. However, elsewhere the wide-
spread cataclysmic Alamo Breccia rests not on the Guilmette,
but on an erosive surface cut into Middle Devonian formations.
At Stop 6 (Fig. 10) the Breccia takes the form of Units inter-
preted as fallback and resurge breccias associated with the Rim
Realm (Pinto and Warme, 2008), and offshore to the west as
deep-water channels of the Runout/Resurge Realm (Fig. 9) that
were described at several localities by Morrow et al. (2005) and
Sandberg et al. (2005, 2006).
Sandberg et al. (1997) also described the lower part of Reso’s
upper member, shown as the “slope facies member” on Figure 14.
The four Guilmette members shown on Figure 14 total ~220 m in
thickness. However, at Stop 1 the entire Guilmette totals ~660 m.
Our traverse will include the upper ~440 m, which is highly het-
erolithic but contains classic facies that are signatures of Devo-
nian carbonate platforms. This interval has not been described
in detail on “Downdropped Mountain,” but a description in the
adjacent fault block is available (Estes-Jackson (1996).
Chamberlain and Warme (1996) and Chamberlain (1999)
provided the composite stratigraphic column of Figure 11. The
Devonian platform formations were divided into sequences,
defi ned by bundles of similar cycles or trends in cycles. These
are briefl y described in Table 1. In general, the interval from the
base of the Guilmette to the top of the Alamo Breccia, Reso’s
lower member, is similar across several ranges in southeastern
Nevada, but the remainder of the Guilmette, Reso’s upper mem-
ber, exhibits more variation between ranges as well as within
the member.
Alamo Breccia Member of Guilmette Formation
Various facies of the Alamo Breccia Member of the Guil-
mette Formation are traversed at Stop 1 and Stops 3–6 and are
described in this guide. The Breccia has been treated in detail
in other guides and papers, so it is described herein with only
enough detail to satisfy our purposes of placing it in stratigraphic
context, showing its characteristics, and confi rming its genesis
as a cataclysmic deposit created by a bolide (i.e., a large crater-
forming projectile such as an asteroid or comet). Figure 9 shows
the general distribution of Alamo Breccia outcrops, grouped into
genetic Realms. Stop 1 and Stops 3–5 lie within the Ring Realm,
where the Breccia is expressed as four sequential units of dis-
tinctive facies, which are labeled as parts of the Alamo Breccia
Member on Figure 14.
These units were described in detail, initially by Warme
and Sandberg (1995, 1996), most recently by Pinto and Warme
(2008), and in several intervening reports. Two of the units
occur together in the Breccia. The lowest one, termed Unit D
in past reports, is a detachment monomict breccia that formed
between the undamaged carbonate platform beds below and
Unit C megaclasts composed of displaced but intact cyclic plat-
form beds above. The two remaining Alamo Breccia units are
polymict breccias of chaotically bedded Unit B, which may
extend to the base of the Breccia, as shown in Figure 14, and
sorted and graded beds of Unit A that everywhere top the Brec-
cia. For more in-depth study and understanding of the Breccia,
we suggest that this fi eld guide be augmented by the follow-
ing easily obtained references, copies of which are provided to
participants of the fi eld trip for which this guide was prepared:
Warme and Sandberg (1996), Sandberg et al. (1997, 2005),
Warme and Kuehner (1998), Morrow and Sandberg (2001),
Morrow et al. (2001, 2005), Warme et al. (2002), Warme (2004),
Warme and Pinto (2006), and Pinto and Warme (2008).
230 Warme et al.
Figure 11 (on this and following page). (A) Composite stratigraphic column of Devonian section near Silver Canyon (Stop 3), showing sequenc-
es, surface gamma-ray log, relative sea-level curve, and sequence-boundary features. (B) Legend for sequence symbols, boundary features, and
lithologic symbols used in Figure 11A. From Chamberlain and Warme (1996) and Chamberlain (1999). See Table 1 for sequence thicknesses,
numbers of cycles, and signifi cant features.
A
Devonian carbonate platform of eastern Nevada 231
TABLE 1. THICKNESSES, NUMBERS OF CYCLES, AND SIGNIFICANT FEATURES OF DEVONIAN SEQUENCES
NEAR SILVER CANYON (STOP 3), AS SHOWN IN THE STRATIGRAPHIC COLUMN OF FIGURE 11
Sequence
abbreviation
Thickness
ft (m)
No. of
cycles
Significant features; weathering profile
MDp2 115 (35) 2 Silicified stromatolites and laminated black chert; slope
MDp1 130 (39) 2 Silty limestone capped with fossil bone-bearing sandstone; slope
Dwr 153 (46) 4 Silty, burrowed limestone; partly covered slopes
Dgg 567 (172) 29 Carbonate cycles capped by thick (>3 m) quartz sandstone beds
Dgf 268 (81) 16 Slightly deeper cycles and more limestone than in adjacent sequences
Dge 235 (71) 17 Carbonate cycles capped by thin (<3 m) quartz sandstone beds
Dgd 406 (123) 23 Amphipora dolopackstone; dark-gray ledges and cliffs
Dgc 188 (57) 6 Silty limestone with abundant gastropods and burrows; slope
Dgb3 97 (29) 2 Stromatoporoid and coral reef facies; light-gray cliffs
Dgb2 179 (54) 1 Graded bed of carbonate breccia, open-marine fauna; brown-gray cliffs
Dgb1 26 (8) 2 Abundant corals, stromatoporoids, and Amphipora; limestone cliffs
Dga2 145 (44) 8 Shallowing-upward cycles that successively deepen upward, predominately
limestone, open-marine fauna; ledges and slope
Dga1 250 (76) 12 Shallowing-upward cycles that successively deepen upward, predominately
dolostone, open-marine fauna; ledges and slope
Dgys 182 (55) 10 Yellow, silty dolostone, stromatolites, and cycles capped by thin beds of very
fine-grained quartz sandstone, ostracodes; slope
Dgfm 135 (41) 4 Open-shelf fauna, brachiopod Stringocephalus; resistant cliffs
Dsiualt 285 (86) 12 Shallowing-upward cycles that successively deepen upward giving an
alternating dark and light band appearance, karst breccia; ledges
Dsibc 85 (26) 4 Open-shelf fauna with corals and stromatoporoids; dark brown-gray cliff
Dsilalt 265 (80) 12 Alternating intertidal-supratidal or dark and light bands; ledges
Dsicxln 225 (68) 4 Coarsely crystalline dolostone capped by karst surface; light-gray to light gray-
brown cliffs
Dox2 95 (29) 2 Quartz sandstone with hummocky cross-bedding at base; ledge
Dox1 100 (30) 4 Burrowed, silty dolostone with flat-pebble conglomerate at base; light-brown
slope
Dse3 240+ (73+) 12+ Light-gray, fine-grained, laminated dolostone; slopes, base concealed
TOTAL 4370+ (1324+) 188+
Note: Modified from Chamberlain and Warme (1996) and Chamberlain (1999).
Figure 11 (continued).
B
232 Warme et al.
375
Ex
t
r
a
t
err
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st
rial Hi
g
h
way
Ex
t
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rial Hi
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o
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Stop 1
Stop 1
Start traverse
Start traverse
End
End
Dsibc
Dsibc
Dsiualt
Dsiualt
Dfm
Dfm
Dgysf
Dgysf
Dgcp
Dgcp
Dgab
Dgab
Dgs
Dgs
Dgu
Dgu
MDp
MDp
Mj
Mj
Tv
Tv
Dgl
Dgl
Stop 1
Start traverse
End
Dsibc
Dsiualt
Dfm
Dgysf
Dgcp
Dgab
Dgs
Dgu
MDp
Mj
Tv
Dgl
Figure 12. Oblique Google Earth aerial
view (eye altitude: 2.2 km) to the south-
east of “Downdropped Mountain,” Stop
1 at Hancock Summit West, showing
access road, traverse route, and strati-
graphic units including brown cliff
(Dsibc) and upper alternating (Dsiualt)
members of Simonson Dolostone, Fox
Mountain Formation (Dfm) undivided,
and yellow sloping-forming member
(Dgysf), lower member (Dgl), type
Alamo Breccia Member (Dgab), and
upper member (Dgu) of Guilmette For-
mation. Dgcp and Dgs denote carbonate
platform and slope facies, respectively,
of Guilmette Formation shown in the
stratigraphic column (Fig. 14). Other
units: MDp, Pilot Shale; Mj—Joana
Limestone; Tv—Tertiary volcanic
rocks. Image modifi ed from http://earth.
google.com/.
Figure 13. West face of Hancock Sum-
mit West (Stop 1), with stratigraphic
sequence including the upper part of
Simonson Dolostone (Dsi), Fox Moun-
tain Formation (Dfm), and yellow slope-
forming member (YSF), lower member
(Dgl), type Alamo Breccia Member
(lower Units D–C and upper Units B–
A), and upper member (Dgu) of Guil-
mette Formation. The Alamo Breccia
Member is ~55 m thick. The Stop 1 tra-
verse ascends the ridge along the skyline
from right to left (Fig. 12). Stop 1 access
road is visible in foreground.
Devonian carbonate platform of eastern Nevada 233
Figure 14. Stratigraphic section, facies, conodont biostratigraphy, and conodont biofacies at Hancock Summit West (Stop 1; Figs. 12 and 13)
of upper member of Fox Mountain Formation and lower part of Guilmette Formation, showing type section of Alamo Breccia Member. Shows
position of 24 conodont samples used to constrain biostratigraphic age. Conodont biofacies abbreviations: Icr.—icriodid; Pol.-icr.—polygnathid-
icriodid; No c.—no conodonts. From Sandberg et al. (1997).
234 Warme et al.
FIELD TRIP STOPS
Stop 1. Hancock Summit West: Middle and Upper Devonian
Formations, and Type Section of the Alamo Breccia
Location
Stop 1 is at the informally named “Downdropped Moun-
tain” or “Down Dropped Block” (Fig. 12), which is a prominent
ridge south of, and parallel to, Highway 375, from 12–14 mi
(19–22 km) west of the starting point at the Crystal Springs rest
area. The entrance to Stop 1 is a gravel road at the west end of a
guardrail along the south side of Highway 375, 2.2 mi (3.5 km)
by road west of Hancock Summit. The gravel road descends into
an arroyo and highway maintenance gravel pit that bounds the
northwest side of the mountain. The road continues southwest
down the arroyo. The Stop 1 traverse begins at the lowest beds
exposed along the bank, 0.5 mi (0.8 km) from Highway 375
(Fig. 12). Coordinates at the Stop 1 parking area and base of tra-
verse: lat 37°2436.04N., long 115°2405.42W., Crescent Res-
ervoir 7.5 quadrangle.
Rock Units Exposed
Simonson Dolostone (brown cliff member, upper alternat-
ing member, informal “upper coarse crystalline member”), Fox
Mountain Formation, Guilmette Formation, and possibly West
Range Limestone (Figs. 5 and 11). The Sevy Dolostone, cherty
argillaceous unit, Oxyoke Canyon Sandstone, and lower mem-
bers of the Simonson Dolostone (Fig. 11) are not exposed along
the traverse of Stop 1; see Stop 2.
The Simonson was divided into four informal members by
Osmond (1954; Figures 5, 11): coarse member, now termed the
coarse crystalline member (Johnson et al., 1989), which appears
to be transitional from the Oxyoke Canyon Sandstone below
and becomes more coarsely crystalline upward; lower alternat-
ing member, which contains alternating light and dark bands
of beds and grades upward into the brown cliff member, which
in turn grades into the upper alternating member. The upper
member also commonly becomes coarsely crystalline upward,
exhibiting an interval that is informally referred to as the “upper
coarse crystalline member.
The Simonson Dolostone exhibits upward-shallowing cycles
that are partially exposed in each member. The more dolomitic
Simonson cycles are generally thinner and represent shallower
conditions than those that are so well preserved in the overlying
Guilmette Formation, and thus were probably situated more land-
ward. However, structureless intervals at the base of many cycles
probably represent lower intertidal to subtidal bioturbated lime-
stone that was later dolomitized along with the entire formation.
The underlying Sevy Dolostone, exposed in ranges nearby, may
represent even shallower conditions, probably accumulated
more dolomite-prone beds, and is even more strongly altered by
regional dolomitization.
The Fox Mountain interval was regarded as the upper unit of
the Simonson Dolostone in Nevada or the lower member of the
Guilmette Formation in Utah until it was recognized as a region-
ally widespread unit and given formation status by Sandberg et
al. (1997), who divided it into lower and upper members. Part of
the upper member at this locality is shown in the outcrop photo-
graph of Figure 13 and the stratigraphic column of Figure 14.
The history of studies and partitioning of the Guilmette
Formation has already been recounted under “Devonian Forma-
tions.” The lower part of the Guilmette on the Stop 1 traverse was
divided by Sandberg et al. (1997) into the four intervals shown on
Figure 14: the “yellow slope-forming” and “carbonate-platform
facies,” the type Alamo Breccia Member, and the “slope facies.
The “slope facies” is only the lower part of the upper Guilmette,
which continues upward for an additional ~440 m and contains
thick quartz sandstone intervals as well as beds that evidence
both deep and shallow carbonate-platform environments.
Objectives
Introduce several of the Devonian formations exposed in
the fi eld guide area.
Describe shallowing-upward cycles and their character in
different formations.
Document fossils and ichnofossils of platform facies.
Discover signifi cant surfaces; compare with established
formation and member limits.
Note trend upward from dolostone of Simonson to mainly
limestone of Fox Mountain and limestone/dolostone
cycles in the Guilmette and discuss probable controls.
Discuss sedimentary structures and provenance of quartz-
ose sandstone in upper member of Guilmette.
Discover varied character of Devonian exposure surfaces.
Introduce Alamo Breccia and its bolide impact signatures.
Traverse
The Stop 1 traverse begins in the brown cliff member of
the Simonson Dolostone, at the base of the stratigraphic section
exposed along the arroyo, and continues eastward up through the
stratigraphic section for ~1 mi (~1.6 km) along a ridge that forms
the drainage divide along the crest of “Downdropped Mountain”
(Fig. 12). The traverse ends at the east end of “Downdropped
Mountain,” at the base of the dip slope that marks the top of the
exposed platform facies of the Guilmette Formation, or possibly
within, or at the top of, a thin interval of West Range Limestone.
East of “Downdropped Mountain” is a strike valley of Pilot
Shale, accessible by a rough dirt road that enters from the north,
and a ridge of Joana Limestone that is cut off at the south end by
a major fault. Beyond the fault is the stratigraphic section studied
by Estes-Jackson (1996), shown in Figure 2, and a twin of the
section traversed at Stop 1.
Observations
Simonson Dolostone
Brown Cliff Member: Rich brown color; conversion to dolo-
mite of varying crystal sizes; relatively open platform evidenced
Devonian carbonate platform of eastern Nevada 235
by several forms of abundant stromatoporoids, some corals, and
other invertebrates; cycle boundaries vague; transition upward to
upper alternating member.
Upper Alternating Member: Trend upward to light-gray
color; cycles marked by structureless (bioturbated) lower intervals
and algal-laminated upper intervals; fossils less abundant, mainly
bulbous stromatoporoids; trend upward to coarser grained dolos-
tone; pockets of yellow-weathering, coarse-grained dolomite
crystals interpreted as karst fi llings; abrupt shift across bedding
upward to fi ner grained dolostone and absence of yellow crystals,
indicating exposure surface and underlying episode of diagenetic
recrystallization and solution; altered intervals with zebra rock
and coarse crystallization associated with Devonian exposure
surfaces and also with much later (Cenozoic) fault zones.
Fox Mountain Formation
The Fox Mountain was divided into lower and upper members
by Sandberg et al. (1997). The contact between those members
represents a regional shift from the underlying shallow-platform
dolostone formations to open-marine platform limestones, and
signals the termination of the long-lived very shallow platform
that had existed from the beginning of Devonian deposition. The
karst zone under the base of the lower member separates under-
lying Simonson dolostones from Fox Mountain beds interpreted
as peritidal, restricted-marine, and evaporite-solution-breccia
limestones. A second karst at the top of lower member separates
these beds from open-marine crinoidal wackestones and encrin-
ites of the upper member, which contains nautiloids, brachiopods
(including Stringocephalus), and corals. The upper member rep-
resents initiation, in the Middle varcus conodont Zone, of the
Taghanic onlap, a marine transgression recognized throughout
North America and in parts of Europe (e.g., Johnson et al., 1985;
Johnson and Sandberg, 1989). See Figure 14 for thicknesses and
details of the upper member.
Guilmette Formation
Yellow Slope-forming Member: The yellow-weathering
interval of fi ne-grained, dark-gray, silty dolostone beds forms the
topographic saddle shown in Figure 13 and is underlain by sev-
eral meters of stromatolitic dolostone that regionally represent
the basal beds of the interval (Fig. 14). Note the columnar and
branching stromatolites and yellow-weathering, silty dolostone
and dolomitic siltstone beds that are replaced upward by dark-
gray limestones of the overlying member.
Carbonate Platform Facies (Member): This member exhib-
its excellent examples of shallowing-upward cycles. They show
basal transgressive erosion surfaces and sediment lags overlain
by a lower interval of dark-gray bioturbated limestone that usu-
ally contains bulbous and other forms of stromatoporoids, and
may contain other invertebrate fossils and oncolites. The upper
part of cycles that are completely preserved becomes increas-
ingly lighter gray, dolomitic, and algal laminated.
Alamo Breccia Member: As shown on Figure 14, the
base of this Member is marked by the subtle Unit D monomict
detachment breccia, overlain by Unit C megaclasts composed
mainly of previously deposited platform cycles similar to those
in the underlying member. Unit B is a chaotic heterolithic breccia
with impact lapillistone clasts, overlain by Unit A graded beds
with shocked and hematite-studded quartz grains (Warme and
Sandberg, 1995, 1996; Morrow et al., 2005) and both local and
exotic deformed clasts. The traverse along the drainage divide
intercepts the Breccia where Unit B reaches the base of the Brec-
cia and separates two Unit C clasts that are hundreds of meters in
length. Unit B contains clasts, tens of meters in length, fl oating
in a chaotic matrix. The stacked graded beds of Unit A are well
exposed on the sloping ledge at the top of the Breccia.
Slope Facies (Member): The member over the Alamo Brec-
cia is varied. Beds are commonly mottled, fossiliferous lime-
stones of deeper-water aspect (Warme and Sandberg, 1995, 1996;
Sandberg et al., 1997), and exhibit one or more hardgrounds. The
interval is partly covered, and ends at the thick quartz sandstone
beds that continue upward into the lower part of the upper Guil-
mette (Fig. 12).
Upper Guilmette (Member): The lowest interval of Reso’s
upper Guilmette is the 35-m-thick “slope facies” member
described above, which is overlain by quartzose sandstones
that are broadly channeled, cross bedded, and exhibit biogenic
sedimentary structures. The middle part contains biostromes and
small bioherms of stromatoporoids, and beds rich in corals, gas-
tropods, and bivalves including megalodonts. Limestones of the
upper part are thin bedded and quartzose. They contain abundant
biogenic sedimentary structures and at the top may include a thin
interval of West Range Limestone.
Stop 2. Sixmile Flat: Upper Sevy Dolostone, Cherty
Argillaceous Unit(?), Oxyoke Canyon Sandstone, and
Coarse Crystalline Member and Lower Alternating
Members of Simonson Dolostone
Location
Stop 2 is situated north of Sixmile Flat, a broad valley east
of the Hiko Hills (Figs. 9 and 10). From the Crystal Springs rest
area starting point, travel 0.8 mi (1.3 km) east to Highway 93,
then east on Highway 93 for 10 mi (16 km), past the south end of
the Hiko Hills, entering Sixmile Flat and continuing to a dirt road
on the left (north) that is the entrance to a cattle pen. Travel 0.4 mi
(0.6 km) WNW to the pen, then north along a straight sectionline
fence and gravel road for 5.8 mi (9.3 km) to a fence gate and cat-
tle trail on the left (west) (Fig. 15). Stop 2 traverse begins 0.3 mi
(0.5 km) west of the gate and can be approached part way by
vehicles with high ground clearance. Coordinates at Stop 2 park-
ing area at fenceline: lat 37°4028.61N., long 115°0459.22W.,
Hiko NE 7.5 quadrangle.
Rock Units Exposed
Sevy Dolostone, cherty argillaceous unit(?), Oxyoke Can-
yon Sandstone, and coarse crystalline and lower alternating
members of the Simonson Dolostone. The traverse can be con-
236 Warme et al.
tinued up slope and into the brown cliff member, but is truncated
northward by Cenozoic layered volcanic rocks (Fig. 15).
Objectives
Overview of exposed formations.
Transition from Sevy Dolostone to local expression of
cherty argillaceous unit (Dox1? of Fig. 11) and Oxyoke
Canyon Sandstone (Dox2 of Fig. 11).
Deeper facies of Oxyoke Canyon Sandstone (Dox2 of
Fig. 11): hummocky cross bedding and trace fossils.
Transition to cyclic bedding and gradual coarsening to
top of coarse crystalline member.
Karst signatures at top of coarse crystalline member
and fi ner-crystalline dolostone at base of lower alter-
nating member.
Traverse
From dirt road proceed west on cattle trail to highest point of
rst shallow saddle, then walk south to the upper part of the Sevy
Dolostone and start of northward traverse.
Observations
Cryptic evidence for platform cycles in heavily dolomitized
Sevy Dolostone; disseminated fi ne-grained quartz in upper Sevy
(Dox1?) sequence of Figure 11; varied quartz concentrations in
upper Oxyoke Canyon (Dox2) sequence of Figure 11; identifi ca-
tion of ichnotaxa in Oxyoke Canyon; confi rmation of hummocky
cross-stratifi cation (HCS); nature of cycles, increasing coarse
crystallinity, and increasing karst features in coarse crystalline
member (Dsicxln of Fig. 11); abrupt fi ning of crystallinity at base
of lower alternating member (Dsilalt of Fig. 11).
Stop 3. Silver Canyon: Alamo Breccia and “Reso’s Reef”
Location
From Crystal Springs rest area starting point, travel north
2.5 mi (4.0 km) on Highway 318, into the settlement of Hiko, to
entrance of gravel road on left (west; unmarked Logan Canyon
Road). Travel northwest for 7 mi (11 km) to “Y” junction; turn
right (unmarked Silver Canyon Road). Note excellent views of the
white carbonate buildup at top of hill, front right; Alamo Breccia is
steep cliff below the buildup. Continue north 1.3 mi (2.1 km) to two
small, leveled parking and turnaround areas on right. Coordinates
at Stop 3 parking area and base of traverse: lat 37°3714.10N.,
long 115°2137.74W., Mount Irish SE 7.5 quadrangle.
Rock Units Exposed
Formations exposed north and east of the road are Devonian.
To west of the road, in valley, are dark orange-brown- weathering
acidic volcanic fl ows, some marked with American Native petro-
glyphs. Devonian formations in various fault blocks to east of road
can be identifi ed as Sevy Dolostone below the distinctive twin
intervals of dark-brown-weathering Oxyoke Canyon Sandstone,
and coarse crystalline member of Simonson Dolostone above
them. Thick brown beds well above the Oxyoke Canyon are in the
brown cliff member. Due east, a low ridge in the foreground has
almost continuous exposures of upper Simonson Dolostone and
Fox Mountain Formation, which are exposed below the distinc-
tive yellow slope-forming member of the Guilmette. The member
weathers to a light-yellow band across the near mountain face, and
is labeled on Figure 16. The yellow band is overlain by an inter-
val similar to the “carbonate platform facies” of Stop 1 (Fig. 14),
labeled as lower Guilmette (Dgl) on Figure 16. This interval is over-
lain by the cliff-forming Alamo Breccia Member, which extends to
the skyline on Figure 16. See description of lower three members of
the Guilmette under Stop 1 (Fig. 14). The white carbonate buildup
above the Breccia is not visible from the parking location.
Objectives
Study graded beds of Unit A, upper interval of Alamo
Breccia, containing large clasts of platform carbonates and
clasts of impact lapillistone.
Note facies directly over Alamo Breccia that shift west to
east, from dark, bedded, off-buildup intervals to massive
white carbonate buildup.
Note interfi ngering buildup and off-buildup tongues.
Document evolution of buildup from base to top.
Discuss whether the buildup is mudmound, microbial
mound, reef, or other alternatives.
Study karst developed across highest pinnacles of buildup.
Document cyclic shallow-water platform beds over buildup.
Traverse
Stop 3 traverse is ~1.75 mi (~2.8 km) each way. It begins at the
right side of parking areas, northward up a gentle ridge (Fig. 16) that
arcs eastward, crests, then drops down to a bench that separates the
graded bed Unit A at the top of the Alamo Breccia from the base
of the overlying buildup and off-buildup facies. As indicated on
Figure 16, the traverse continues eastward around reentrants along
the top of the Alamo cliff, to the center of the buildup facies over
the Breccia. The last reentrant terminates in a steep wall at the base
of the buildup, and exposes the early phases of its development.
Return westward to notches where the buildup can be traversed
upward to the crest. The traverse ends in dark carbonate beds over
the structure. Return by same route, or take shortcut westward to
intercept the ridge that leads to the parking area.
Observations
Beds at the beginning of the traverse dip westward into the
Silver Canyon fault zone and are in the upper Guilmette over
the Alamo Breccia. They contain low-spired gastropods and large
solitary rugose corals.
Alamo Breccia
The Breccia is exposed along the south-facing slope of
the traverse ridge, and the traverse follows the graded Unit A.
Devonian carbonate platform of eastern Nevada 237
Access road
Fenceline
93
A
c
c
e
s
s
tra
i
l
Stop 2
Fenceline
Access road
A
c
c
e
s
s
tra
i
l
End
Start
traverse
Dsibc
Dse Dox1? Dox2 Dsicxln Dsilalt
Figure 15. Oblique Google Earth aerial
view (eye altitude: 1.8 km) to the west
of Stop 2 at Sixmile Flat, showing ac-
cess road and trail, traverse route, and
stratigraphic units including Sevy Do-
lostone (Dse), lower and upper members
of Oxyoke Canyon Sandstone (Dox1?
and Dox2, respectively), and coarse
crystalline (Dsicxln), lower alternating
(Dsilalt), and brown cliff (Dsibc) mem-
bers of Simonson Dolostone. Image
modifi ed from http://earth.google.com/.
Dgu
Dg-buildup
Mt. Irish
Dse
Dse
Dox
Dox
Dfm
Dfm
Dsi
Dsi
Dgysf
Dgysf
Dgl
Dgl
Dgab
Dgab
Tra ver se
Traverse
Dse
Dox
Dfm
Dsi
Dgysf Dgl
Dgab
Tra ver se
Figure 16. Outcrop photograph of south-
facing Devonian units east of Silver
Canyon (Stop 3), including Simonson
Dolostone (Dsi), Fox Mountain Forma-
tion (Dfm), and Guilmette Formation
(Dgysf, yellow slope-forming member;
Dgl, lower member; Dgab, Alamo Brec-
cia Member; and Dgu, upper member,
with “Reso’s Reef” organic buildup
directly overlying Alamo Breccia).
Faulted blocks of Sevy Dolostone (Dse)
capped by dark-orange weathered cliffs
of Oxyoke Canyon Sandstone (Dox) are
visible in foreground. Stop 3 traverse
route is shown by dashed white line.
The traverse ascends ridgeline from the
parking area, out of view to left (west),
then follows the top of Alamo Breccia
cliff and base of the buildup to the center
of the buildup, then ascends the buildup
onto the platform cyclic beds over it.
238 Warme et al.
Scattered clasts of lapillistone may be encountered along the Unit
A traverse to the top of the hill. Below the crest, the west-facing
Breccia cliff exhibits a large-scale folded clast, tens of meters
long. The south-facing Breccia cliff exhibits Unit A graded beds
with sparse lapillistone clasts, and is interrupted by several bed-
parallel or tilted clasts, 10 m or more in length. One ~30-m-long
tilted clast protrudes from the graded bed at the top of Unit A.
Eastward, toward the carbonate mound, beds over the top
of the Breccia become light gray to white, fossiliferous, and
packed with stromatoporoids of both hemispherical and elongate
forms; some elongate tabular forms are 1 m or more in maxi-
mum length. The sheer cliff at the end of the traverse exhibits the
graded beds in Unit A at the top of the Breccia, is near the center
of the buildup, and displays abundant stromatoporoids and an
erect branching coral ~70 cm high, preserved in life position.
Carbonate Buildups
We use the non-genetic term “buildup” for carbonate accu-
mulations with positive relief, because they exhibit a spectrum
of compositions and frameworks and have been differently clas-
sifi ed and variously named in numerous schemes. They include
“reefs” that some workers restrict to shallow and wave-resistant
bio-accumulations, and deep-water structures that are commonly
termed “mudmounds.” A review of reef versus mudmound con-
cepts is beyond the scope of this fi eld guide. Discussion papers
continue to appear. The volume edited by Monty et al. (1995)
brings forth the controversies, and includes the contribution by
Pratt (1995) who outlined the wide variability of “mudmounds”
in time and space. They occur as early as Proterozoic and in every
Phanerozoic period, contain variable proportions of mud-sized
particles and potential frameworks, and encompass a variety of
recognizable invertebrates that change over geologic time and
that may play active or passive roles in mound formation. Most
structures contain fi ne-grained cement that, without detailed
petrographic study, is commonly diffi cult to differentiate from
originally fi ne-grained clastic calcareous particles.
Reso’s Reef
The buildup at Stop 3 three has been termed a mudmound
(e.g., Sandberg et al., 1997), but Dunn (1979) documented
potential framework of several species of stromatoporoids
and corals in the core of the structure, and used the term of
bioherm. She listed invertebrates that include gastropods, bra-
chiopods, and crinoid columnals within and adjacent to the
buildup, which appears to contain a more abundant fauna than
has been observed beyond its margins. One nautiloid has been
found. These components are diffi cult to observe in the central
core facies because of pervasive recrystallization. “Reso’s reef”
could benefi t from further investigation.
An upward traverse through the buildup offers the chance
to discuss its genesis: reef versus mudmound or other options.
The upper ~10 m of the buildup shows increasing vugs and
seams that are stained yellow, red, and orange, indicating that
the top was exposed in Devonian time. A karst cave, 1 m in
diameter, near the top exhibits a fl oor of bedded sediment that
dips at about the same angle as the regional dip, providing a
geopetal indicator and evidence that the solution occurred
before regional tilting and not after current exposure. Beds over
the buildup are cyclic and contain abundant Amphipora, indi-
cating continued platform conditions. The stratigraphic column
of Figure 11 was measured nearby. The sequence lacks thick
sandstone beds, in contrast to the abundant sandstones in the
upper Guilmette at Stops 1 and 4.
Stop 4. Hiko Hills: Alamo Breccia and Evacuation Structure
Location
Stop 4 is located at the south end of the Hiko Hills. It is accessed
by traveling east 0.8 mi (1.3 km) from the Crystal Springs rest
area starting point to the end of Highway 318, turning north, then
east on Highway 93 for 1 mi (1.6 km), and then north (left) onto
a dirt road. The road skirts the west side of a gravel pit, becomes
rough, and heads east-northeastward to the mouth of a small can-
yon at the base of the range. The canyon leads upward to outcrops
of the Alamo Breccia intercalated with the Guilmette Formation.
Coordinates at the Stop 4 parking area and base of traverse: lat
37°3251.27N., long 115°1208.38W., Hiko 7.5 quadrangle.
Rock Units Exposed
Guilmette Formation: Alamo Breccia Member; cyclic plat-
form carbonates in lower Guilmette Formation below the Brec-
cia; cyclic carbonates, carbonate buildups, and broadly channeled
sandstones above the Breccia.
Objectives
Characterize internal fabric and components of Alamo
Breccia.
Note character and distribution of lapillistone within
Breccia.
Observe post–Alamo Event recolonization of carbonate
platform: fossils, trace fossils, carbonate buildups.
View evacuation of matrix under fi nger of Unit C mega-
clast.
Traverse
From the mouth of the canyon, proceed up the main gully.
The closest Breccia section is on a down-faulted block, detached
from the mountain front. It contains excellent exposures of the
clasts and matrix of Unit B in the middle part and stacked graded
beds of Unit A in the upper part. The Guilmette above the Brec-
cia exhibits platform cycles, which are heavily dolomitized and
altered near the fault, which terminates the lower traverse.
Upward, across the fault, is a more complete second expo-
sure of the Breccia. The base of the Breccia is exposed across the
fault. The Breccia may be traversed vertically to overlying beds,
or laterally northward for several hundred meters along the west
face of the Hiko Hills to observe variations within the Breccia
and the overlying and underlying beds.
Devonian carbonate platform of eastern Nevada 239
Observations
A climb through the lower Breccia outcrop reveals the cha-
otic character of Unit B: heterolithic clasts and variable matrix.
The interval of Unit A contains three or more graded beds that
become fi ner grained and thinner upward, and shows load,
ame, and dewatering structures at bed boundaries. The top of
the Breccia merges with a silty interval that contains faint cross-
bedding and abundant ichnofossils resembling Teichichnus.
This section was discussed by Sandberg et al. (1997). Recov-
ery of the platform biota after the Alamo Event is under study
(Tapanila and Anderson, 2007).
Across the fault, the upper traverse begins with a thick sec-
tion of chaotic Unit B matrix and heterolithic clasts. As shown
in Figure 17, Unit B terminates upward at a surviving fi nger
of cyclic beds that project eastward from the top of the 80-m-
thick Unit C clast visible to the west. Pinto and Warme (2008)
described the unusual relationships between the very thick Unit
C clast, nearly in its original position, and the laterally adjoin-
ing matrix of the Alamo Breccia. The fi nger of platform beds
extends from the upper part of the thick Unit C clast, over ~60 m
of Unit B breccia. During the Alamo Event, this fi nger initially
remained intact, then fractured and parted in two places, allow-
ing well-sorted clasts from the already accumulated overlying
Unit A to cascade down the new slots. The structure is inter-
preted to have formed when some of the Unit B breccia was
evacuated from under the fi nger. This process is proposed to be
part of the ring-forming adjustments of the transient crater in
later phases of the Alamo Event (Pinto and Warme, 2008). Stop
5 provides a second example of this process.
Northward, the thick Unit C interval of intact cyclic beds
can be traversed ~0.25 mi (~400 m) to a vertical fault, beyond
which most of the Unit C clast was destroyed and replaced
by the chaotic Unit B, which displays a variety of spectacular
clasts tens of meters long. The upper 10 m of the Breccia con-
tains a train of numerous lapillistone clasts that appear to be
remnants of the same lapillistone bed. Above the Alamo Brec-
cia are discontinuous light-gray-weathering carbonate buildups
with abundant stromatoporoids and fewer colonial corals, and
stacks of broadly channelized quartzose sandstones, interbed-
ded with cyclic carbonates. Details of the Breccia in this area
were documented by Kuehner (1997).
Stop 5. Southern Delamar Mountains: Stacked Tabular
Clasts in Alamo Breccia
Location
Stop 5 is located near the southern end of the Delamar Moun-
tains, where they verge closest to Highway 93 (Fig. 18). From the
starting point at the Crystal Springs rest area, travel east 0.8 mi
(1.3 km) to the end of Highway 375, then south on Highway 93
for 35 mi (56 km), passing through the settlements of Ash Springs
and Alamo. The Alamo Breccia is the irregular interval that caps
several ridges that dip westward toward the highway. Entrances to
canyons between the ridges (Fig. 18), where the Alamo Breccia
is displayed, are ~0.6 mi (~1 km) from the highway, and can be
reached on foot. Coordinates at the Stop 5 parking area at High-
way 93 and base of traverse (approximate): lat 37°0435.01N.,
long 114°5925.24W., Pahranagat Wash 7.5 quadrangle.
Rock Units Exposed
The lower Guilmette Formation dips more steeply than
the structural dip along the western face of the southern end of
the Delamar Mountains, so that the Alamo Breccia caps sev-
eral isolated east-west–trending ridges that slope westward, the
underlying carbonate-platform facies member is exposed in the
intervening small valleys, and the yellow slope-forming member
(terminology of Sandberg et al., 1997) forms an irregular strike
valley east of the ridges. Northward, Middle Devonian beds under
the yellow slope-forming member crop out along a series of very
irregular ridges and valleys.
Objectives
Note relatively thin interval of lower Guilmette members.
Observe the scale and distribution of giant tabular clasts
that compose ~80% of the Alamo Breccia. Discuss origin
and signifi cance for genesis of Alamo Breccia.
Note unusually well-sorted matrix between clasts, and
lithologic similarity with Unit A at top of Breccia.
Inspect small stromatoporoid patch reef within Unit C
detached clast.
Note subtle expression of Unit D monomict detachment
breccia.
Time permitting, traverse eastward, down section, to the
yellow slope-forming member, and continue northeast into
the underlying Middle Devonian formation with abundant
bioherms and biostromes of pentamerid brachiopods, stro-
matoporoids, and corals.
Traverse
From Highway 93, approach the front of the Delamar Moun-
tains (Fig. 18) and traverse one or more of the isolated ridges of
Alamo Breccia and one or more intervening valleys. Time per-
mitting, walk northward in the yellow-weathering strike valley
east of the ridges, then northeastward topographically up and
stratigraphically down into the exposed Middle Devonian fossil-
rich dolostone beds.
Observations
The lower Guilmette under the Alamo Breccia is similar,
but overall thinner, to that at Stops 1, 3, 4, and other locations in
the fi eld trip area to the north. However, the fossil-rich Middle
Devonian interval, under the yellow slope-forming member,
may be either the brown cliff member of the Simonson Dolos-
tone or the upper member of the Fox Mountain Formation. This
stratigraphy suggests that the upper alternating member of the
Simonson and the lower member of the Fox Mountain were
240 Warme et al.
Figure 17. Photograph and diagram of Alamo Breccia evacuation structure at the south end of the Hiko Hills (Stop 4). Breccia interval is 100 m thick.
(A) View to northeast showing characteristic shallow-water carbonate-platform cyclic bedding of the Guilmette Formation above and below Breccia.
(B) Left: unusually thick (~80 m) preserved Unit C megaclast with thin (~10 m) Unit A graded beds over top. Center and right: cyclic carbonates
equivalent to the megaclast disintegrated down to the level of detachment (Unit D), except for an upper fi nger of beds (F) that extended over newly
formed Unit B chaotic breccia. The fi nger broke into pieces as some of the underlying Unit B was evacuated. Lapillistone clasts (L) as much as 50 m
under the fi nger represent early-precipitated beds that were broken and mixed with Unit B. One or more later lapillistone beds precipitated from the
impact plume, became partially lithifi ed, and were preserved as a discontinuous trail of broken and smeared lapillistone within a graded bed of Unit
A (black circles), which formed across the whole area in a late phase of the Alamo Event after the fi nger collapsed. Post-Event deposits in this area
include buildups, up to ~40 m high, containing stromatoporoids and corals (R). From Pinto and Warme (2008).
Devonian carbonate platform of eastern Nevada 241
either not deposited or were eroded prior to deposition of the
yellow slope-forming member.
The Alamo Breccia at Stop 5 is atypical and diffi cult to
interpret as part of the Alamo Breccia scenario. The long, tabu-
lar, stacked clasts and sparse, presorted interclast matrix (Fig. 19)
have not been observed at other Alamo Breccia localities. Stop
5 is ~85 km from the present position of Tempiute Mountain
(unrestored), Stop 6, where the beds are interpreted as a fragment
of the Alamo crater rim. At Stop 5, the Breccia is ~60 m thick
and contains stacked irregular clasts that are a hundred meters or
more in length.
The mega-fabric of the clasts suggests that they moved east-
ward, not northwestward in the direction of Tempiute Mountain
(Figs. 9 and 20) or westward toward the paleo-platform margin.
The orientation of large-scale clasts in the Alamo Breccia at other
localities, much closer to Tempiute Mountain, indicates movement
approximately westward, as if they were sliding toward the crater or
the seaward platform margin (Kuehner, 1997; Warme and Kuehner,
1998). In addition, the total thickness of the Alamo Breccia here is
~60 m (Fig. 19), whereas it is thinner at some localities closer to
the present position of Tempiute Mountain or to the trend of the
platform margin. Pinto and Warme (2008) proposed that the clasts
at Stop 5 were detached, moved eastward, and stacked over one
another during the ring-forming processes associated with adjust-
ment of the transient crater. In this scenario, listric-fault-bounded,
arcuate segments of the platform dipped outward, fault footwalls
became rings, and near surface intervals of beds detached and slid
away, perhaps collecting near the footwall of the next ring fault
outward. Figure 9 shows that both Stops 4 and 5 are located near
the outer edge of the Ring Realm of the Alamo Breccia, suggesting
that signifi cant faulting formed one or more outer crater rings and
an adjacent moat that collected Alamo Breccia.
Stop 6. Tempiute Mountain: Crater-Rim Impact
Stratigraphy
Location
From the Crystal Springs rest area travel ~33 mi (53 km) on
Highway 375 westward, past Stop 1, then northwest to the Coy-
ote Summit road marker. Continue northwest on Highway 375
for 3.0 mi (4.8 km) and turn east on a dirt road that leads to Tem-
piute Mountain (Fig. 20). Travel eastward for 1.4 mi (2.2 km) to
fork; turn left (northeast) and continue 1.7 mi (2.7 km) to canyon
mouth, where road is washed out. Continue on foot eastward up
canyon through old mining works to cliff of Alamo Breccia, Stop
6A (Fig. 20). Coordinates at the Stop 6 parking area and base
of traverse: lat 37°3658.44N., long 115°3851.84W., Tempiute
Mountain South 7.5 quadrangle.
Rock Units Exposed
The west face of Tempiute Mountain clearly exposes Paleo-
zoic formations from the Ordovician Eureka Quartzite to the
93
G
r
e
a
t
B
a
s
i
n
H
wy
G
r
e
a
t
B
a
s
i
n
H
wy
Alamo Breccia outcrops
Alamo Breccia outcrops
Tv
Tv
Alamo Breccia outcrops
Tv
Figure 18. Oblique Google Earth aerial
view (eye altitude: 1.2 km) to the south-
east of Stop 5 at the southern Delamar
Mountains, showing isolated Alamo
Breccia outcrops (labeled) capping east-
west–trending ridges. Stacked tabular
clasts within the Breccia are visible.
Highway 93 is labeled. Tv indicates
Tertiary volcanic rocks. Image modifi ed
from http://earth.google.com/.
242 Warme et al.
Figure 19. View to south-southwest of Alamo Breccia outcrop that caps the dip slope at the southern end of the Delamar Mountains (Stop 5).
Image is tilted to restore horizontal, which is indicated by trend of ridge of the Sheep Range in distance at right in (A) and slanted line in (B). (A)
Photo of northern slope of one of several east-west–trending ridges that display Alamo Breccia with unusual tabular megaclasts and very little
matrix. Note 60-m scale of total Breccia. (B) Units of the Breccia include the detachment surface (D), jumbled megaclasts that continue to the
top of the Breccia in most places (C), limited internal chaotic Breccia (B), and topmost graded beds that are now in the process of eroding away
(A). R represents stromatoporoid buildups within displaced clasts. From Pinto and Warme (2008).
A
c
c
e
s
s
A
c
c
e
s
s
r
o
a
d
r
o
a
d
Stop 6
6B
6A Traverse
Oe
Oes
Sl
Dse
Dsm
Dox
Dab
Ddg
MDp
Mj
Grants
Peak
Grants
Peak
W
i
l
d
c
a
t
C
n
y
n
.
W
i
l
d
c
a
t
C
n
y
n
.
Figure 20. Oblique Google Earth aerial
view (eye altitude: 2.1 km) to the south-
east of Stop 6 at Tempiute Mountain,
showing access road, traverse route,
Stop 6A and 6B locations, and strati-
graphic units including Eureka Quartz-
ite (Oe), Ely Springs Dolostone (Oes),
Laketown Dolostone (Sl), Sevy Dolos-
tone (Dse), Oxyoke Canyon Sandstone
(Dox), Sentinel Mountain Dolostone
(Dsm), Alamo Breccia (Dab), Dev-
ils Gate Limestone (Ddg), Pilot Shale
(MDp), and Joana Limestone (Mj). The
detailed, complex, crater-rim stratigra-
phy of the Alamo Breccia and adjacent
units is depicted in the columnar sec-
tion of Figure 21. Grants Peak (7199 ft,
2182 m) and Wildcat Canyon are also
indicated. Image modifi ed from http://
earth.google.com/.
Devonian carbonate platform of eastern Nevada 243
Mississippian Joana Limestone (Fig. 20). Devonian formations
include the Sevy Dolostone, cherty argillaceous unit, Oxyoke
Canyon Sandstone, Sentinel Mountain Dolostone (and Bay
State Dolostone, if it was locally preserved), Alamo Breccia,
Devils Gate Limestone, and Devonian to Mississippian Pilot
Shale. The Sentinel Mountain and Bay State Dolostones are
deeper platform equivalents of the Simonson Dolostone. All of
the Guilmette Formation, or its equivalent that was deposited in
this section, and probably all of the Fox Mountain Formation,
were cut out and replaced by newly defi ned Units 3–5 of the
Alamo Breccia, as shown in Figure 21 and so far discovered
only at Tempiute Mountain.
Objectives
Review characteristics of impact beds, interpreted as pre-
served inner rim of Alamo crater, as depicted in the strati-
graphic column of Figure 21.
Stop 6A: Note features of Unit 3, interpreted as large-scale
fallback breccia clasts and matrix, and base of resurge beds,
Unit 5.
Stop 6B: Discriminate between characteristics of parauto-
chthonous impact breccias (Unit 1), developed nearly in
situ in Lower to Middle Devonian Oxyoke Canyon Sand-
stone and Middle Devonian Sentinel Mountain Dolostone
(and Bay State dolostone?) and contrasting exotic clasts
characteristic of Units 3–5.
Note features of injected sedimentary sills and dikes,
Unit 2, into Unit 1.
Discuss interpretations for shatter-cone-like structures in
Unit 1 dolostones.
Note features of Unit 3, interpreted as intensely deformed
fallback breccia; Unit 4, interpreted as smeared or fl at-
tened fallback breccia; and Unit 5, interpreted as resurge
of impact debris into crater, against inner crater rim, or
across newly formed slope.
Discuss depositional environment of post-impact deep-
water limestones labeled Devils Gate (Fig. 21).
Traverse
Steeply dipping Paleozoic formations are exposed along the
washed-out track in the axis of the canyon, and are labeled on
Figure 20. Most noticeable is the Ordovician Eureka Quartzite
near the canyon mouth, and the Devonian Oxyoke Canyon Sand-
stone midway to the cliff of Alamo Breccia that terminates the
eastward segment of the road, which is Stop 6A (Fig. 20).
Stop 6B requires a signifi cant commitment of time. Return
west ~330 ft (~100 m) to intersection of steep road that leads
northward, up past mine workings and an abandoned shaft, to
drainage divide and a view northward of the stratigraphic section
on Tempiute Mountain, northwestward across Sand Spring Val-
ley, and west to the settlement of Rachel on Highway 375. This
point is the beginning of the traverse for Stop 6B, which runs
along the ridge that leads eastward to the range crest (Fig. 20).
Return via the same route.
Observations
Stop 6A
Exposures along the north side of the easternmost ~165 ft
(~50 m) of road show 20 or more lithologies of smashed and
interpenetrated Unit 3 megaclasts, some as much as several
meters across, that are interpreted as a heterolithic fallback
Alamo Breccia (Pinto and Warme, 2008). Near the cliff face
at the end of the road is a sandstone clast identifi ed as the only
megascopic fragment of probable Ordovician Eureka Quartzite
found thus far, providing evidence for crater excavation to depths
of 1.5–2.0 km. Other clasts appear milky white and marbled, and
are candidates for partial melting of target rock during the cra-
tering process (Pinto, 2006). The fallback breccia matrix is best
exposed in sparse outcrops on the steep slope that leads to the
next ridge north. The matrix contains a variety of clasts, shocked
quartz grains, and impact lapilli mixed with the heterolithic clasts
(Pinto and Warme, 2008),
Stop 6B
From the Stop B starting point (Fig. 20) the road becomes
level and continues a short distance northward, then eastward
into the next east-west drainage, and provides excellent views
of the next ridge to the north and its south-facing foreground
slope. This excursion of the traverse is shown on Figure 20.
Visible from west to east, in ascending order, are: the irregu-
larly silicifi ed upper Sevy Dolostone; the well-bedded cherty
argillaceous unit that is not well represented in localities to the
east but is a deeper-water facies that may correlate to Dox1 of
Figure 11; indurated and cross-bedded Oxyoke Canyon Sand-
stone, equivalent to Dox2 of Figure 11; and Sentinel Mountain
Dolostone and possibly Bay State Dolostone, equivalent to
the Simonson Dolostone of shallower facies on the platform.
Weathered brownish-orange patches in the Sentinel Mountain
are injected and sediment-fi lled sills and dikes of Unit 2, which
can be inspected in outcrops near the canyon axis at the end of
the road (Fig. 20). The east end of the ridge contains the Alamo
Breccia, Devils Gate Limestone, Pilot Shale, and Joana Lime-
stone, which cannot be discriminated from this viewpoint, but
can be traversed along the ridge of Stop 6B (Fig. 20). Return to
Stop 6B starting point.
Beginning at the Stop 6B starting point (Fig. 20), the traverse
eastward along the ridge axis exhibits almost continuous expo-
sures that represent impact stratigraphy, as interpreted by Pinto
(2006) and Pinto and Warme (2008). The traverse crosses brec-
cias of parautochthonous deformed bedrock overlain by exotic
fallback and resurge clasts, all interpreted to be preserved on the
inner rim of the transient Alamo crater (Fig. 21). Along the ridge,
the cross-bedded, silica-cemented, upper part of the Oxyoke Can-
yon Sandstone (S1A), together with the very dark-gray to black
beds of the Sentinel Mountain Dolostone (S1B), are designated
as the parautochthonous Unit 1 impact breccias on the strati-
graphic column of Figure 21. This 300-m-thick interval is thor-
oughly fractured, faulted, and injected with the sediment-fi lled
244 Warme et al.
Figure 21. Columnar section of De-
vonian rocks at Tempiute Mountain
(Stop 6), showing the thickness and
character of Alamo impactogenic
Units 1–5, the underlying upper part
of the Sevy Dolostone (Dse) and
cherty argillaceous unit, the overlying
deep-water Devils Gate Limestone
(Ddg), and the Pilot Shale (MDp).
Parautochthonous impact breccias of
Unit 1 are composed of the Oxyoke
Canyon Sandstone (S1A) and Senti-
nel Mountain Dolostone (S1B). Unit
2 is injected polymict breccia that
tends to be thickest where it swells,
pinches and separates S1A and S1B.
S1B contains shatter-cone–like struc-
tures, interpreted as impact-induced.
Units 3 and 4 are composed of al-
logenic limestone clasts with varied
matrices that bear shocked quartz,
lapilli, and other particles interpreted
as shock and melt indicators. Unit 5
is two thick resurge breccias. From
Pinto and Warme (2008).
Devonian carbonate platform of eastern Nevada 245
dike-and-sill system of Unit 2 and interpreted as nearly in situ
altered bedrock. Note the abundant shatter-cone–like structures
and related features in the black dolostones.
The dolostone breccias of Unit 1 are overlain by the mainly
limestone breccia Units 3 and 4. Unit 3 is a well-exposed, roughly
graded, fallback breccia with clasts as much 10 m in exposed
length. The better sorted smaller clasts of Unit 4, averaging ~5 cm
in length, are interpreted as a separate fallback event of heterolithic
clasts that were partially melted, weakened, and smeared. In con-
trast, Unit 5 is composed of two 30-m-thick graded beds of gener-
ally angular, clean-washed, heterolithic clasts that are interpreted
as resurge fl ows deposited against the inner wall of the crater rim
or across a new slope that formed by collapse of the platform mar-
gin during later stages of the Alamo Event. The upper graded bed
nes imperceptibly into the post-Breccia deep-water limestones
labeled Devils Gate on Figure 21. Of interest in the Devils Gate is
the lack of benthic invertebrates and ichnotaxa, presence of small
pelagic or nektonic species, and several debrites, as much as ~4 m
thick, of locally derived slumps and ripup clasts with varying pro-
portions of quartz grains derived from elsewhere. Hypotheses for
the depositional environment of the Devils Gate at this locality
include post-Event crater fi ll and slope veneer.
ACKNOWLEDGMENTS
We thank Wanda Taylor for benefi cial review of the manu-
script, and Gene Smith and Ernest Duebendorfer for their edito-
rial handling of the paper. Jesús Pinto provided data and assis-
tance with fi gures. Morrow’s contribution to this manuscript is
based upon work supported by the National Science Foundation
under Grant No. #0518166. Sandberg’s contribution is supported
by the U.S. Geological Survey Bradley Scholar Program.
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MANUSCRIPT ACCEPTED BY THE SOCIETY 17 JANUARY 2008
Printed in the USA
... Abbreviations: ling., linguiformis; triang., triangularis; disp., disparilis; falsi., falsiovalis; trans., transitans. by Sandberg et al. (1997), Morrow and Sandberg (2003), Casier et al. (2006), and Warme et al. (2008). Here, we summarize the key features. ...
... Here, we summarize the key features. At the HSW section, the upper Givetian to middle Frasnian Guilmette Formation is exposed along the crest of "Downdropped Mountain" (Warme et al., 2008). As the type section of the Alamo Breccia Member of the Guilmette Formation, the punctata biozone contains a 60-m-thick impact breccia that is rich in megaclasts (Warme et al., 2008;Morrow et al., 2009). ...
... At the HSW section, the upper Givetian to middle Frasnian Guilmette Formation is exposed along the crest of "Downdropped Mountain" (Warme et al., 2008). As the type section of the Alamo Breccia Member of the Guilmette Formation, the punctata biozone contains a 60-m-thick impact breccia that is rich in megaclasts (Warme et al., 2008;Morrow et al., 2009). Most clasts are locally-derived Devonian platform rocks, composed of micrite with scattered stromatoporoids, corals, and brachiopods (Warme and Sandberg, 1996;Morrow et al., 2009). ...
Article
High amplitude positive carbon isotope excursions in the Late Devonian, the punctata and Kellwasser events, reflect major perturbations in the global carbon cycle that have been attributed to increased continental weathering and subsequent ocean eutrophication. Despite the comparable carbon isotope anomalies, however, a major extinction has been reported only for the Kellwasser Events, while the punctata Event is marked by low extinction intensity. This study presents multiple sulfur isotope records of carbonate-associated sulfate (CAS) and pyrite from Late Devonian sections in the Great Basin, USA, in order to document changes in the coupled (or decoupled) geochemical cycles of carbon and sulfur during the punctata and Upper Kellwasser events. A positive sulfur isotope shift in both CAS and pyrite accompanies the onset of the punctata Event, but to a larger extent in the latter. As a result, the sulfur isotope offset between CAS and pyrite (Δ34SCAS-py) dropped to less than 10‰. In the middle of the punctata Event, a sharp negative δ34SCAS excursion and negative Δ34SCAS-py values coincide with the Alamo impact. Unlike the rapid δ34Spy and δ34SCAS oscillations associated with the punctata Event, the Upper Kellwasser was a period of relative stability, except for a brief δ34SCAS drop before the event. Paired sulfur isotope data, aided by a simple box model, suggest that the geochemical cycle of sulfur may have been partly responsible for the contrasting biological responses that define these events. High stratigraphic δ34Spy and δ34SCAS variability, coupled with strong reservoir effect, demonstrates a relatively small oceanic sulfate pool existed during the punctata Event. Further, the Alamo impact likely triggered the rapid oxidation of microbially-produced sulfide within this event. The expansion of sulfidic bottom water thus may have been impeded during the punctata Event. In contrast, the lack of a positive shift in δ34SCAS and sizable Δ34SCAS-py values ( ) throughout the Upper Kellwasser Event imply higher relative sulfate levels. A larger seawater sulfate reservoir may have promoted the development of sulfidic bottom waters in the eutrophic epicontinental seas, increasing biological stress and potentially contributing to the mass extinction.
... Deposited as part of the Taghanic onlap second-order transgression, at a time of elevated flexural loading from the Antler orogeny (Johnson et al. 1985;Johnson and Sandberg 1989;Sandberg et al. 1989Sandberg et al. , 2002Goebel 1991;Giles and Dickinson 1995), the Guilmette Formation in southern Nevada records a thick, mostly conformable succession of carbonates and rare siliciclastics throughout the Late Devonian. These circumstances were ideal for Fig. 1 Locality map of Alamo Breccia deposits in eastern Nevada (inset) through the semicircular Rim, Ring, and Runup realms of Pinto and Warme (2008). Locality numbers correspond to alphanumeric field localities in Online resource 1 Fig. 2 Stratigraphy and brief description of the Guilmette Formation, including the regional middle Alamo Breccia Member and the postimpact Upper Member. ...
... More than two decades of research reveal a wide range of features indicating that the Alamo Breccia is the result of an impact with an extraterrestrial body (see review of Warme et al. 2008). These features include a positive iridium anomaly (51-183 parts per trillion; Koeberl et al. 2003), shock metamorphism of quartz (Leroux et al. 1995), ejecta bombs (Morrow et al. 2005;Pinto and Warme 2008), and carbonate accretionary lapilli (Warme et al. 2002). ...
... Pinto and Warme (2008) developed a categorization of Alamo Breccia deposits based on breccia stratigraphy. These authors recognized a central Rim, Ring, and outermost Runup Realm. ...
Article
Eighty measured stratigraphic sections across the Late Devonian carbonate platform of Nevada (USA) document the uppermost terminal Alamo Breccia and overlying sediments, which record the waning energy at the end of an impact and the recolonization of the post-impact seafloor. Four sedimentary styles of terminal breccia, recognized by the continuity of normal grading versus reworking, and dolomitization, define patterns of sedimentary accommodation across the platform. Examined in combination with the first post-impact facies, the field area can be divided into deep subtidal, shallow subtidal, and peritidal zones with increased distance from the inferred crater center. Farthest away, peritidal outcrops have very low accommodation, and the terminal breccia is physically reworked and dolomitized. Biotic signals are rare in this zone. However, we find rare but exceptional deposits of impact fallout lapilli at or above the top of the breccia. The shallow subtidal region records reworked and pristine grading of the terminal breccia, which at several locations document burrowing directly into the top of the Alamo Breccia, confirming infaunal recolonization prior to post-impact sediment accumulation. Rare occurrences of erosional terminal breccia produced rockgrounds at the seafloor, some showing evidence of bioerosion. Deeper subtidal localities, closest to the crater center, preserve the thickest, continuously graded terminal breccias owing to high accommodation at the end of the event. Body and trace fossils are common in the first overlying lithofacies, although the abundance of oxidized firmground surfaces in deep water settings suggest these deposits were sediment-starved and fossil assemblages are strongly time-averaged.
... SPA-483) showing fibrous cave calcite precipitates (pendant cements) in dissolution cavities. Additionally, we sampled the Mid-Late Devonian transgressive slope carbonates of the Guilmette Fm. west of Handcock Summit, Nevada, in the lowermost "Yellow slope-forming member" (YSF) (Sandberg et al., 1997;Warme et al., 2008). In outcrop, several beds of columnar and branching stromatolites are evident at the base of this Late Givetian (conodont zone 20; Sandberg et al., 1997) lower member, and range from limestone to dolomite with some preservation of depositional textures, to fabric-destructive 'zebra dolomite'. ...
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... Ages (mya) from Kaufmann (2006). Chronostratigraphic placement of lithostratigraphic units after Witzke et al. (1988), Day (2006), Morrow and Sandberg (2008), Warme et al. (2008), Sandberg (2009)]. Thicknesses of each lithostratigraphic unit measured in this study are indicated in parentheses. ...
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We interpret a discrete, anomalous ~10-m-thick interval of the shallow-marine Middle to Late Devonian Valentine Member of the Sultan Formation at Frenchman Mountain, southern Nevada, to be a seismite, and that it was generated by the Alamo Impact Event. A suite of deformation structures characterize this unique interval of peritidal carbonate facies at the top of the Valentine Member; no other similar intervals have been discovered in the carbonate beds on Frenchman Mountain or in equivalent Devonian beds exposed in ranges of southern Nevada. The disrupted band extends for 5 km along the Mountain, and onto the adjoining Sunrise Mountain fault block for an additional 4+km. The interval displays a range of brittle, ductile and fluidized structures, and is divided into four informal bed-parallel units based on discrete deformation style and internal features that carry laterally across the study area. Their development is interpreted as the result of intrastratal compressional and contractional forces imposed upon the unconsolidated to fully cemented near-surface carbonate sediments at the top of the Valentine Member. The result is an assemblage of fractured, faulted, and brecciated beds, some of which were dilated, fluidized and injected to form new and complex matrix bands between beds. We interpret that the interval is an unusually thick and well displayed seismite. Because the Sultan Formation correlates northward to the Frasnian (lower Upper Devonian) carbonate rocks of the Guilmette Formation, and the Guilmette contains much thicker and more proximal exposures of the Alamo Impact Breccia, including seismites, we interpret the Frenchman Mountain seismite to be a far-field product of the Alamo Impact Event. Accompanying ground motion and deformation of the inner reaches of the Devonian carbonate platform may have resulted in a fall of relative sea level and abrupt shift to a salt-pan paleoenvironment exhibited by the post-event basal beds of the directly overlying Crystal Pass Member.
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The Devonian Alamo Breccia is a thick (<30-130 m) unit, interpreted as a bolide impact deposit, which is bracketed by marine carbonates. Samples were collected within the breccia and above/below the breccia for a contact test to determine if the breccia acted as a conduit for fluids that could have caused the widespread chemical remanent magnetizations (CRMs) present in Palaeozoic Era rocks in Nevada. The carbonates above, below and in the breccia contain a Cretaceous Period syn-tilting CRM that resides in pyrrhotite and a pre-tilting late Palaeozoic Era CRM that resides in magnetite. The contact test is negative. Despite these results, diagenetic alteration by externally derived fluids is interpreted as the most likely mechanism of remagnetization. This hypothesis is supported by 87Sr/86Sr values in the breccia and surrounding rocks that suggest alteration by fluids with a radiogenic signature. The fluids were not localized in the breccia but are interpreted to have moved pervasively through the rocks. The results differ from some other studies that found that fluids caused localized CRMs around fluid conduits.
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Shows that uprush-backwash channels generated by the megtsunami creased by the middle Frasnian punctata Zone Alamo Impact extended far northward in Nevada and into western Utah. Gives the amount of moderate iridium enrichment in the channel deposit at the well-known Devils Gate section, west of Eureka, Nevada.
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Carbonate accretionary lapilli occur in the Late Devonian Alamo Breccia of south-central Nevada. They provide evidence for the extraterrestrial impact origin of the breccia, and help unravel the complicated events that formed it. The accretionary lapilli (Alamo lapilli) are concentrated in lapilli beds, and portions of the latter occur as reworked clasts that are isolated within the upper half of the thick breccia. The Alamo lapilli resemble volcanic accretionary lapilli reported from both silicate and carbonatite volcanoes. They are variable in size and detail, but generally exhibit a nucleus interpreted to be altered target rock, an enveloping mantle of silt-and sand-sized particles, and a very fine grained peripheral crust. Spherule composition is entirely carbonate, except for sparse shocked quartz grains incorporated into the mantle and diagenetic iron oxides. Rare preservation of undeformed bed segments shows the stratigraphy of the accretionary lapilli; they were deposited in poorly size-sorted layers with varying proportions of matrix. Their preserved form is spherical, deformed, or broken, implying varying degrees of damage before or during deposition, before bed hardening, and during catastrophic reworking and dewatering of the Alamo Breccia. We propose that the carbonate accretionary lapilli were preserved when target carbonate formations were pulverized by impact pressure and calcinated by impact heat, creating quicklime. The lapilli evolved by adhesion of particles within the impact cloud; they were partially cemented in flight by hydration, and then precipitated as one or more beds over early ejectite, debris flows, and/or nearly contemporaneous tsunamites. The cementation process continued in the lapilli beds so that portions of them survived reworking and initial breccia settling and dewatering. Coherent, isolated fragments of lapilli beds and deformed bed masses are preserved as much as 25 m beneath the top of the breccia, indicating the thickness of rock reworked during the Alamo Event.