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SEQUENCE STRATIGRAPHIC FRAMEWORK FOR THE SILURIAN-DEVONIAN
BOOTSTRAP LIMESTONE, ROBERTS MOUNTAINS, AND DEVONIAN
POPOVICH FORMATIONS, NORTHERN CARLIN TREND, ELKO AND EUREKA
COUNTIES, NEVADA
By
Roger A. Furley
ii
A thesis submitted to the Faculty and Board of Trustees of the Colorado School of
Mines in partial fulfillment of the requirement for the degree of Master of Science
(Geology).
Golden, Colorado
Date _____________
Signed: ________________________ Roger A. Furley
Approved: ________________________ Dr. John D. Humphrey Thesis Advisor Golden, Colorado Date _____________
________________________ Dr. Murray W. Hitzman Professor and Interim Head, Department of Geology and Geological Engineering
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ABSTRACT
Sediment-hosted (“Carlin-type”) gold deposits are present in Siluro-Devonian
rocks of the Carlin trend. Although the major stratigraphic relations have been
previously investigated and are well known, the objective of this study is to define a
finer-scaled stratal and sequence stratigraphic framework, and reconstruct the
paleogeography of the study area. The objectives of this study were met through detailed
facies analysis of recently acquired diamond drill-hole cores consisting of non- to weakly
hydrothermally altered rocks
The study area is located 27 miles northwest of Carlin, NV, in the northern
portion of the Carlin trend, from Barrick Goldstrike north past the Dee-Rossi property.
The local stratigraphy consists of Ordovician Hanson Creek, Silurian-Devonian Roberts
Mountains, Silurian-Devonian Bootstrap Limestone, Devonian Popovich, Devonian
Rodeo Creek, Ordovician Vinini, and Tertiary Carlin Formations. This study described
the Roberts Mountains Laminated Micritic Limestone and Apron (from this study);
Bootstrap Limestone reef and shoal (from this study); and Popovich Wispy, Planar, Soft-
Sediment Deformation, and Micritic facies in detail and modeled them using a systems
tracts analysis.
The sequence framework constructed from this study shows that during a
highstand systems tract (HST), a massive Bootstrap Limestone platform facies was
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deposited adjacent to the Roberts Mountains Laminated Micritic Limestone member,
representing slope and basinal facies. A topographic high developed, resulting in
deposition of an apron facies at the base-of-slope. The subsequent fall in relative sea
level resulted in a sequence boundary and deposition of the overlying lowstand systems
tract (LST) Popovich Wispy basinal facies. The overlying Popovich Planar facies
signifies another change in relative sea level and the beginning of the trangressive
systems tract (TST). Rapid rise in relative sea level during the TST led to submergence
of the platform and starvation into the basin. A Monograptus sp. and dendroidal variety
graptolite zone found in the upper portion of the Planar facies represents a condensed
section of the maximum flooding surface. During the subsequent HST, reactivation of
the platform carbonate factory resulted in deposition of additional Bootstrap Limestone
platform facies. The Popovich Soft-Sediment Deformation slope facies represents
instability of carbonate muds rapidly deposited onto the slope during the early HST. Sea
level continued to rise, eventually drowning the system, resulting in retrogradation of the
shoreline and deposition of the overlying Popovich Micritic basinal facies member. The
contact between the Micritic member and the overlying Rodeo Creek Formation
represents another sequence boundary and a change from a carbonate to more of a clastic
influence.
The chronostratigraphic framework constructed from this study shows that initial
deposition of the Bootstrap Limestone, Roberts Mountains LL, and Roberts Mountains
Apron facies began in late Llandoverian to early Wenlockian time and abruptly ended in
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the early-middle Lochkovian. The overlying Popovich WS facies was deposited during
middle Lochkovian time. During the middle Lochkovian, Bootstrap Limestone facies
was rejuvenated and was emplaced laterally adjacent to the Popovich PL facies, with
continued deposition through to early-middle Emsian time. Deposition of the overlying
Popovich SSD followed and continued through the Pragian. During the Emsian,
deposition of the Popovich UM facies began and continued through to early Frasnian
time. A 6-7 m.y. hiatus spanned from early Frasnian through late-middle Famennian
time, with deposition of the overlying Rodeo Creek not occurring until Frasnian to late
Famennian time.
Past depositional models for the Bootstrap, Roberts Mountains and Popovich
facies only provided static representations of the carbonate system for the northern Carlin
trend based on an instant in geologic time. This study successfully used systems tracts to
overcome the static problem by integrating time and relative sea-level changes to track
migration of facies.
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TABLE OF CONTENTS
ABSTRACT........................................................................................................................ iii
LIST OF FIGURES .............................................................................................................x
ACKNOWLEDGMENTS .................................................................................................xiii
INTRODUCTION ...............................................................................................................1
Location..................................................................................................................... 2
Research Objectives .................................................................................................. 2
Research Contributions ............................................................................................. 4
REGIONAL GEOLOGY.....................................................................................................6
Paleozoic Geology..................................................................................................... 6
Mesozoic Geology..................................................................................................... 8
Cenozoic Geology ..................................................................................................... 9
STRATIGRAPHY .............................................................................................................13
Introduction ............................................................................................................. 13
Lower Plate Rocks................................................................................................... 13
Ordovician Hanson Creek Formation.............................................................13 Silurian-Devonian Roberts Mountains Formation..........................................15 Devonian Popovich Formation.......................................................................16 Devonian Rodeo Creek Formation.................................................................19
Upper Plate Rocks ................................................................................................... 20
Ordovician Vinini Formation .........................................................................20 Tertiary Carlin Formation...............................................................................21 Quaternary Alluvium ......................................................................................21
STRUCTURE ....................................................................................................................22
Introduction ............................................................................................................. 22
Faults ....................................................................................................................... 22
vii
Folds ........................................................................................................................ 24
CARLIN TREND MINERALIZATION...........................................................................25
RESEARCH METHODS ..................................................................................................27
Field Portion............................................................................................................ 27
Laboratory Portion................................................................................................... 28
RESEARCH RESULTS ....................................................................................................30
Facies Identification ................................................................................................ 30
Bootstrap Limestone Formation.....................................................................30 Roberts Mountains Formation........................................................................33 Popovich Formation........................................................................................44
Facies Distributions ................................................................................................. 62
Bootstrap Limestone Formation.....................................................................62 Roberts Mountains Formation........................................................................67 Popovich Formation........................................................................................69
Ichnology................................................................................................................. 75
Biostratrigraphy....................................................................................................... 77
Graptolites.......................................................................................................77 Conodonts .......................................................................................................78 Ostracods ........................................................................................................81
Diagenesis ................................................................................................................ 81
Dolomitization................................................................................................81 Stable sotopes .................................................................................................82
CARBONATE DEPOSITIONAL ENVIRONMENTS.....................................................89
Carbonate Platform Margin ..................................................................................... 89
General............................................................................................................89 Study Area ......................................................................................................91
Slope ........................................................................................................................ 94
General............................................................................................................94 Study Area ......................................................................................................95
Basin ........................................................................................................................ 96
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General............................................................................................................96 Study Area ......................................................................................................97
DEPOSTIONAL MODELS...............................................................................................99
Carbonate Apron Models ........................................................................................ 99
Model..............................................................................................................99 Study Area ....................................................................................................102
The Armstrong and Others (1997) Model............................................................. 103
Model............................................................................................................103 Study Area ....................................................................................................105
SEQUENCE STRATIGRAPHIC FRAMEWORK .........................................................106
General Background .............................................................................................. 106
Significant Surfaces ............................................................................................... 107
Maximum Flooding Surface .........................................................................107 Sequence Boundary ......................................................................................109
Systems Tracts ....................................................................................................... 109
Lowstand Systems Tract ...............................................................................109 Transgressive Systems Tract ........................................................................111 Highstand Systems Tract ..............................................................................111
Sequences .............................................................................................................. 112
Depositional History.............................................................................................. 112
Devonian Sea Level Curve .................................................................................... 116
CONCLUSIONS..............................................................................................................119
Summary................................................................................................................ 119
Significance ........................................................................................................... 120
Recommendations ................................................................................................. 121
REFERENCES CITED....................................................................................................123
APPENDIX A – Base Map ..............................................................................................133
APPENDIX B – Data.......................................................................................................135
ix
APPENDIX C – Core Logs .............................................................................................158
APPENDIX D – Conodont Reports.................................................................................168
APPENDIX E – Stable Isotope Data ...............................................................................191
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LIST OF FIGURES
Page Figure 1: Location of the study area along the northern Carlin trend ..............................3
Figure 2: Paleozoic tectonic provinces.............................................................................7
Figure 3: Late Triassic to Eocene deformation..............................................................10
Figure 4: Miocene extensional tectonism.......................................................................11
Figure 5: Generalized tectono-stratigraphic column for Goldstrike ..............................14
Figure 6: Preliminary distribution map of Popovich Formation for Goldstrike ............18
Figure 7: Location of gold deposits, faults, and intrusive bodies that define the
NW-trending Carlin trend ...............................................................................23
Figure 8: Generalized sequence-stratigraphic column for northern Carlin trend ...........31
Figure 9: Core example of the Bootstrap Limestone shoal facies..................................32
Figure 10: Photomicrographs of Bootstrap Limestone shoal facies ................................34
Figure 11: Core example of the Bootstrap Limestone reef facies ....................................35
Figure 12: Photomicrographs of Bootstrap Limestone reef facies...................................36
Figure 13: Core example of the Roberts Mountains Laminated Micritic
Limestone facies .............................................................................................37
Figure 14: Outcrop of the Roberts Mountains Laminated Micritic Limestone
facies ...............................................................................................................38
Figure 15: Photomicrographs of Roberts Mountains Laminated Micritic
Limestone facies .............................................................................................39
Figure 16: Core example of the Roberts Mountains Apron Laminated Micritic
Limestone/Debris Flow facies ........................................................................41
Figure 17: Core example of the Roberts Mountains Apron Laminated Micritic
Limestone/Wispy/Debris Flow facies.............................................................42
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Figure 18: Core example of the Roberts Mountains Apron Wispy/Debris
Flow facies ......................................................................................................43
Figure 19: Photomicrographs of the Roberts Mountains Apron Laminated
Micritic facies .................................................................................................45
Figure 20: Photomicrographs of the Roberts Mountains Apron Debris Flow
facies ...............................................................................................................46
Figure 21: Photomicrographs of the Roberts Mountains Apron Wispy facies ................47
Figure 22: Core of the Popovich Wispy facies ................................................................48
Figure 23: Photomicrographs of the Popovich Wispy facies ...........................................49
Figure 24: Core comparison of the Roberts Mountains and Popovich Wispy
facies ...............................................................................................................51
Figure 25: Core example of the Popovich Planar facies ..................................................52
Figure 26: Photomicrographs of the Popovich Planar facies ...........................................53
Figure 27: Core example of the Popovich Soft-Sediment Deformation facies ................55
Figure 28: Outcrop of a Popovich Soft-Sediment Deformation boulder .........................56
Figure 29: Photomicrographs of the Popovich Soft-Sediment Deformation
facies ...............................................................................................................57
Figure 30: Photomicrographs of the Popovich Soft-Sediment Deformation reef-
and shoal-derived clasts ..................................................................................58
Figure 31: Core example of the Popovich Micritic facies ...............................................59
Figure 32: Outcrop contact between the Popovich Soft-Sediment-Deformation
and Micritic facies ..........................................................................................60
Figure 33: Outcrop of the Popovich Micritic “chert lenses”............................................61
Figure 34: Photomicrographs of the Popovich Micritic facies ........................................63
Figure 35: Data base map with core and cross section locations .....................................64
Figure 36: Cross section A-A’ through Bootstrap Limestone reef facies ........................65
Figure 37: Cross section B-B’ through Bootstrap Limestone shoal facies ......................66
Figure 38: Roberts Mountains Apron facies distribution map .........................................68
xii
Figure 39: Popovich facies distribution map....................................................................70
Figure 40: Popovich Wispy facies isopach map ..............................................................72
Figure 41: Popovich Planar facies isopach map...............................................................73
Figure 42: Popovich Soft-Sediment Deformation facies isopach map ............................74
Figure 43: Popovich Micritic facies isopach map ............................................................76
Figure 44: Siluro-Devonian conodont zonation...............................................................79
Figure 45: Chronostratigraphic Chart ..............................................................................80
Figure 46: Idealized vadose and phreatic δ13C and δ18O isotope signatures....................83
Figure 47: RU8 core interval 3484 to 3511 ft ..................................................................85
Figure 48: Isotope plot for RU8 interval 3475 to 3498 ft ................................................86
Figure 49: RU8 core interval 3250 to 3314 ft ..................................................................87
Figure 50: Isotope plot for RU8 interval 3232 to 3312 ft ................................................88
Figure 51: Generalized carbonate platform-slope-basin profile ......................................90
Figure 52: Generalized stratigraphic columns for the northern Carlin trend
carbonate platform, slope, and basin ..............................................................93
Figure 53: Slope apron models.......................................................................................101
Figure 54: The Armstrong et al. (1997) model ..............................................................104
Figure 55: Relative sea level curve with systems tracts .................................................108
Figure 56: Idealized depositional sequence and systems tracts model ..........................110
Figure 57: Cross section A-A’ with sequence systems tracts ........................................114
Figure 58: Cross section B-B’ with sequence systems tracts .........................................115
Figure 59: Comparison of a Devonian sea-level curve for the western United
States and a relative sea-level curve developed for the northern
Carlin trend ...................................................................................................117
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ACKNOWLEDGMENTS
I would like to express my gratitude to the following individuals and companies
who contributed to the completion of this thesis and helped me during my graduate
studies:
• Dr. John D. Humphrey, my advisor, for his endless help, enthusiasm, and
guidance throughout my studies at CSM. My experience in graduate school and
this thesis were greatly enhanced by his presence for which I am forever grateful.
• Drs. Murray W. Hitzman and Samuel B. Romberger for serving on my
committee. Thanks for your time in the field as well as your time and suggestions
during the completion of this thesis.
• Greg L. Griffin for getting this project going, serving on my committee and as a
mentor during the duration of this project. In addition, I appreciate his friendship
during my time spent in Carlin and at the mine.
• Barrick Goldstrike Mines, Inc. for funding the majority of this study, employment
at the mine during two summer field sessions, and providing the bulk of the data
used in this study. In particular, thanks are due to Keith Bettles, Eric Lauha, Jeff
Volk, Francois Robert, Jeff Borhauer, Dave Park, Gary Allan, Kent Thompson,
Jane Zimmerman, Gary Baschuk, Charlie Sulfrian, Pam Zohar, Al Lander, Steven
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Kulinski, Robert Malloy, Bruce Templeton, Ellen Rochefort, and Nancy
Bodenhamer.
• Barrick Gold Exploration, Inc. for providing additional funding and data. In
particular, thanks are due to Paul Doback, Richard Hipsley, Dave Arbonies, and
John Katseanes.
• Meridan Gold, Inc., Cameco Gold, Inc., and Newmont Mining Co. for providing
data. In particular, thanks are due to Michael Visher, Vance Spalding, and Steve
Grusing.
• Society of Economic Geologists, Harry C. Kent Foundation, and BP Amoco Co.
for providing additional financial support.
• The Department of Geology and Geological Engineering for the financial
assistance; and to all the staff, including Marilyn Schwinger, Debbie Cockburn,
Charlie Rourke, and John Skok; and fellow graduate students.
• Dr. Anita Harris for providing conodont analysis of numerous samples.
• Dr. Stan Finney and Matt Zimmerman for conodont sample preparation.
• And especially my wife, Melinda Furley, for her endless support, patience, and
love she provided me during my time at CSM and summers away at the mine.
1
INTRODUCTION
Sediment-hosted disseminated (“Carlin-type”) gold deposits in northeastern
Nevada form the largest and most prolific accumulation of gold in North America. More
than 40 separate deposits have been delineated since disseminated gold in carbonate
rocks was discovered in 1961. To date, more than 25 million ounces of gold have been
mined from 26 separate operating or past-producing mines along the Carlin trend (Teal
and Jackson, 1997). The major stratigraphic relations of the Siluro-Devonian host rocks
have been previously investigated and are well known. Recent developments by local
mine geologists have defined a need for a more detailed understanding of stratal relations,
facies variability, and time correlations because gold grade is commonly tied to particular
facies.
By using methodology originally developed for clastic seismic sequence
stratigraphy by Vail et al. (1977), this study integrates time and relative sea-level changes
in order to track migration of facies. This project developed finer-scaled stratal relations
and a coherent facies architectural depositional model based on sequence stratigraphy and
systems tracts for the Bootstrap Limestone, Roberts Mountains, and Popovich Formations
of the northern Carlin trend.
2
Location
The Carlin trend is a forty mile-long, north-northwest-trending alignment of gold
deposits located in northeastern Nevada. The study area is approximately 28 mi2 and is
located 27 miles northwest of the town of Carlin, Nevada, in the northern section of the
trend from the Betze-Post deposit northwest past the Dee-Rossi deposit (Figure 1).
Research Objectives
The objective of this research project was to derive an understanding of the
sequence stratigraphic framework for the Silurian-Devonian Bootstrap Limestone,
Roberts Mountains, and Devonian Popovich Formations of the northern Carlin trend.
This study did not concentrate on diagenesis or later hydrothermal alteration of these
rocks but focused on the following:
• Recognition and logging of facies/sub-units of the members of the Popovich and
Roberts Mountains Formations by describing and characterizing the various
sedimentary facies and structures;
• Identification of lateral changes in character and thickness of the various members;
• Identification of significant surfaces, such as condensed sections and unconformities
and their correlative conformable surfaces;
3
4
• Chronostratigraphic framework reconstruction by utilizing conodont biostratigraphic
dating of various samples;
• Reconstruction of a relative sea-level curve and comparison to published Paleozoic
eustatic sea-level curves;
• Construction of a 3-D paleogeographic analog model;
• Final compilation in order to construct a sequence stratigraphic and systems tracts
framework for the various members.
Research Contributions
Sedimentary rocks in the Carlin trend serve as hosts for the bulk of the “Carlin-
type” gold deposits. Even though the major stratigraphic relations of the Siluro-
Devonian rocks are well known, a spatial distribution of the stratigraphic units was
essential for targeting and delineating the best potential horizons of mineralization. Past
depositional models utilized a static stratigraphic representation that did not account for
lateral and vertical distribution patterns of Roberts Mountains and Popovich facies. The
implementation of sequence stratigraphy and systems tracts has allowed for the dynamic
integration of relative sea-level changes, resulting in a more detailed description of the
depositional and erosional history of the area. Knowledge of the depositional history and
a recognizable spatial distribution of facies will allow for enhanced lateral and vertical
predictability across the trend. Furthermore, from an improved understanding of the
5
stratigraphy, structural complexities, as well as controls on ore emplacement, can be
better deciphered.
6
REGIONAL GEOLOGY
The Carlin trend is located centrally within the larger Great Basin physiographic
province of the western United States. Gold deposits in the trend are hosted in
Ordovician to Mississippian age sedimentary rocks, Jurassic-Cretaceous granodiorites,
and Tertiary intrusion and volcanic units, as well as along regional and localized
structures.
Paleozoic Geology
During the early Paleozoic, the Carlin trend was located along the western edge of
a passive margin that formed during late Precambrian rifting (Stewart and Poole, 1974;
Poole et al., 1977). Deposition along the passive margin is thought to have occurred on a
broad continental shelf, where predominantly shallow-water carbonates were deposited.
To the west, deeper-water carbonate and siliceous sediments were deposited from a
continental slope to ocean basin setting. A westward-thickening wedge resulted, grading
from eastern miogeoclinal shallow carbonates to a western eugeoclinal siliceous clastic
and cherty sequence (Figure 2; Christensen, 1996; Teal and Jackson, 1997).
The late Devonian to early Mississippian Antler orogeny terminated widespread
marine conditions and resulted in large-scale thrusting. Thrusting displaced the western
7
8
eugeoclinal allochthonous siliceous rocks eastward over the miogeoclinal autochthonous
carbonate rocks along the Roberts Mountain thrust (Roberts et al., 1958). The leading
edge of the overriding thrust plate resulted in an accretionary wedge that formed the
emergent Antler highlands. During the middle Mississippian to early Pennsylvanian,
coarse, siliceous clastic sediments were eroded from the Antler highlands and shed
eastward into the adjacent foreland basin, which extended from southern Nevada to
central Idaho (Roberts et al., 1958; Smith and Ketner, 1975; Poole et al., 1977). Locally,
Pennsylvanian and Permian age carbonate and clastic rocks overlie Mississippian rocks
along an angular unconformity (Christensen, 1993).
To the west during the Permo-Triassic Sonoma orogeny, the Golconda allochthon
was thrust eastward onto miogeoclinal carbonates and the Roberts Mountains allochthon
assemblages. Miller et al. (1992) stated the rocks of the Golconda allochthon were
originally deposited west of the Antler orogenic belt in the Havallah basin. They
suggested the composition of the Golconda allochthon sediments, consisting of volcanic
and clastic sedimentary rocks, cherts, volcanic tuffs and flows, resulted from an arc-
continent collision.
Mesozoic Geology
Subduction along the western margin of the North American plate during the
Triassic through the Oligocene resulted in the formation of the middle to late Jurassic
9
Elko and Eureka fold-and-thrust belt in eastern Nevada (Figure 3; Thorman et al., 1992)
and the late Cretaceous and early Tertiary Sevier fold-and-thrust belt in western Utah
(Armstrong, 1968). Mesozoic compression resulted in broad doming and folding of the
Paleozoic section and development of the north-northwest-trending, north-plunging
Tuscarora antiform (Roberts, 1960; Christensen, 1996). Local structural highs formed
traps for hydrocarbons at some time in this history, and may have influenced the later
deposition of gold as well (Christensen, 1996). Several igneous bodies were
subsequently focused along this zone, producing the Goldstrike stock, which has been
dated at approximately 158 Ma (Arehart, 1992). Emsbo (1999) described an auriferous
mineralization event, Jurassic in age, characterized by small quartz and base-metal
sulfide veins hosted in the Goldstrike stock. Felsic and mafic dikes of Jurassic age occur
at the Meikle mine and elsewhere in the area (Evans, 2000).
Cenozoic Geology
Development of the current physiography of the region began with the inception
of pre-Basin and Range extension during the mid-Tertiary (40 to 20 Ma; trending NW-
SE) and Miocene (20 to 10 Ma; trending WSW-ESE) (Figure 4; Christiansen and Yeats,
1992; Christensen, 1996). These extensional events were accompanied by discrete
intervals of igneous activity (Thorman et al., 1992). The current Basin and Range
10
11
12
topography resulted from additional extensional faulting initiated during the early
Miocene and continuing through the present (Evans, 1980).
Most of the gold deposits are found marginal to exposed windows of
autochthonous eastern-assemblage carbonates of the lower plate. These windows are
located where overthrusted western assemblage rocks were uplifted and eroded. From
north to south, these are the Bootstrap, Lynn, Carlin, and Rain windows (Christensen,
1996). This study covers the area to the north and south of the Bootstrap Limestone
window.
13
STRATIGRAPHY
Introduction
The Roberts Mountains thrust separates the predominately gold-hosting Silurian-
Devonian lower plate rocks from deeper water, clastic rocks of the upper plate (Figure 5).
The lower plate of the Carlin trend consists of the autochthonous Hanson Creek,
Bootstrap Limestone, Roberts Mountains, Popovich, and Rodeo Creek Formations. The
upper plate is composed of the allochthonous Vinini Formation overlain by the Tertiary
Carlin Formation and Quaternary alluvium.
Lower Plate Rocks
Ordovician Hanson Creek Formation
The Hanson Creek Formation, first described by Merriam (1940) from exposures
in the Roberts Mountains, is a dark gray to black, massive dolomite to dolomitic
limestone. The upper part of the formation is often marked by brown sandy dolomite or
sandstone. The contact between the Hanson Creek Formation and the overlying Roberts
Mountains Formations represents a regional disconformity.
14
15
Silurian-Devonian Roberts Mountains Formation
The Roberts Mountains Formation was first locally described by Evans (1980) in
the Tuscarora Mountains. The formation unconformably overlies the Ordovician Hanson
Creek Formation regionally and ranges in thickness from 1100 to 1500 ft. Along the
northern Carlin trend, the formation is comprised of two distinct facies: massive
fossiliferous limestone and laminated micritic limestone.
At Meikle and Dee-Rossi, the Roberts Mountains Formation is primarily a
massive, light gray, fossiliferous to oolitic wackestone to packstone. In general, the
massive limestone is a well-cemented carbonate rock with little or no inherent
permeability or porosity (Armstrong et al., 1997). In the literature, this facies has been
informally named the Fossiliferous Limestone member (Volk et al., 1996) and also the
Bootstrap Limestone (Evans and Mullens, 1976; Mullens, 1980; Armstrong et al., 1987;
Armstrong et al., 1997).
South of Dee-Rossi and east of Meikle, a second facies is composed of alternating
light and dark gray, thin-bedded to laminated, fine-grained mudstone. The individual
beds commonly are graded with interbedded debris flow beds. This facies has been
informally named the Laminated Micritic Limestone member (Volk et al., 1996). The
remainder of this thesis will informally refer to the massive limestone facies as the
Bootstrap Limestone Formation and the laminated mudstone as the Roberts Mountains
Laminated Micritic Limestone member.
16
The Roberts Mountains Formation has been interpreted as having been deposited
along a series of northwest-trending silled basins representing shallow shelf and lower
slope-basin environments (Ettner, 1989; Bettles and Lauha, 1991; Volk and Zimmerman,
1991; Armstrong et al., 1997). The Roberts Mountains Formation is in depositional
contact with the overlying Popovich Formation.
Along portions of the trend, the Roberts Mountains Formation is the primary gold
host. Locally, the formation hosts minor ore-grade mineralization, except in areas where
structurally complex and broken. The Meikle deposit, for example, hosts 6+ million
ounces of high-grade ore in a brecciated zone of massive limestone associated with the
Post fault (Evans, 2000).
Devonian Popovich Formation
The Popovich Formation was first defined by Roberts (1958) at Popovich Hill
located in the Carlin mine. Hardie (1966) re-defined the formation as interbedded limey
siltstones and dolomitic siltstones with intercalated limestone, overlain by gray
fossiliferous medium- to thin-bedded limestone and sandy limestone with bioclastic units.
Radtke (1985) further divided the formation into three members: lower (composed of
fine-grained limestone), middle (composed of thin-bedded, fine-grained, gray silty
dolomitic limestone), and upper (composed of thick-bedded, fine-grained, dark gray
limestone).
17
Along the northern Carlin trend, the formation is an approximately 700 to 900 ft
thick sequence divided (by Barrick Goldstrike mine geologist and this study) into four
mappable informal members: Wispy, Planar, Soft-Sediment Deformation, and Micritic
members. Griffin (1999; 2000), based on detailed mapping on Goldstrike property,
locally recognized that the lower Popovich members laterally pinch out against the
Bootstrap Limestone to the NNE (Figure 6).
The Wispy member is the lowest stratigraphic member, consisting of bioturbated,
light and dark gray, laminated calcareous mudstone with interbedded bioclastic debris
flows near the base. The overlying Planar member consists of laminated allodapic
calcareous mudstone beds, with interbedded thin fossiliferous debris flows, locally
termed “fossil hash.” The contact with the overlying Soft-Sediment Deformation
member is abrupt, marked with a thin layer containing Monograptus sp. and dendroidal
variety graptolites (Armstrong et al., 1997). The Soft-Sediment Deformation member
consists of laminated to medium-bedded, gray to black calcareous mudstone, that became
convoluted during syndepositional slumping. The overlying Micritic member consists of
dark gray to black, finely laminated mudstone-to-siltstone graded beds. Thin, concordant
layers of pyrite, locally termed “pinstripe pyrite” at the mine, are common along the
laminations. In addition, the Micritic member is commonly interbedded with thin
argillaceous chert, thin fossil hash beds, and black lenticular lenses, locally termed “chert
lenses.”
18
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The Popovich Formation has been interpreted as slope to toe-of-slope facies
having been deposited in a progressively deepening basin that developed over the
drowned Bootstrap Limestone shallow shelf (Ettner, 1989; Armstrong et al., 1997).
The contact between the upper Popovich Formation and the overlying Rodeo
Creek Formation appears gradational and depositional, but, based on conodont and
radiolarian ages, it is actually a hiatus (Armstrong et al., 1997; Griffin, personal
communication, 2001).
The Popovich Formation is the major host for mineralization along the trend.
Approximately 85% of the reserves at Betze-Post and Meikle are hosted by this
formation (Volk et al., 1996).
Devonian Rodeo Creek Formation
Along the northern Carlin trend, the Rodeo Creek Formation is an approximately
800 to 1000 ft thick informal stratigraphic sequence consisting of a distinctive unit of
rhythmically thin-bedded gray siltstone, mudstone, chert and argillite that grade upward
into an intercalated siliceous mudstone, thin-bedded siltstone, and calcareous siltstone
(Ettner, 1989; Teal and Jackson, 1997). The cherts are gray to black and enriched in
carbon and pyrite. The formation is essentially devoid of fossils except for radiolarians.
Sedimentary structures include slump structures, abundant siliceous mud lumps, and
pervasive thin laminations. Armstrong et al. (1997) interpreted the depositional
20
environment as a deep anoxic silled basin. The Rodeo Creek Formation is in thrust
contact with the overlying upper plate rocks along the Roberts Mountains thrust.
In general, the Rodeo Creek Formation is a poor ore host, even though over two
million ounces have been recovered primarily from oxidized calcareous siltstone and
highly fractured argillites (Volk et al., 1996).
Upper Plate Rocks
Ordovician Vinini Formation
The Vinini Formation ranges from 0 to 3000 ft thick. It consists of interlayered
sedimentary chert, sandstone, siltstone, and argillite, with minor interbedded limestone
units in the middle portion of the formation (Roberts et al., 1967; Stewart, 1980; Volk et
al., 1996). In general, carbonate content and grain size increase up-section in the Carlin
area (Christensen, 1996). The Vinini Formation is unconformably overlain in places by
either the Carlin Formation or Quaternary alluvium.
The Vinini Formation is generally a poor ore host, except where structurally
broken. At Capstone, Big Six, Fence, and Antimony Hill areas, the formation is host to
smaller high-angle, fault-controlled and vein deposits (Figure 7; Teal and Jackson, 1997).
Minor low-grade oxide gold mineralization is also present peripheral to the Betze-Post
deposit (Volk et al., 1996).
21
Tertiary Carlin Formation
The Carlin Formation ranges from 0 to 2000+ ft thick, consisting of fluvial and
lacustrine tuffaceous sediments deposited with varying amounts of interbedded
conglomerates with clasts derived from the surrounding Paleozoic exposures (Volk et al.,
1996). Also common are ash-fall tuffs of various compositions, and rhyolite to
rhyodacite flows of varied thickness developed penecontemporaneously with
development of the Basin and Range (Griffin and Borhauer, personal communication,
2001). The Carlin Formation may or may not be found unconformably overlain by
Quaternary alluvium.
Quaternary Alluvium
The Quaternary alluvium consists of reworked tuffaceous sediments and gravels,
and caliche deposits, and do not include topsoil and present day stream channel fill
(Griffin and Borhauer, personal communication, 2001).
22
STRUCTURE
Introduction
The Carlin trend lies along a north-northwest-trending belt subjected to a number
of regional deformational events and reactivation of earlier structures from Paleozoic to
Cenozoic age. Gold deposits found along the northern portion of the trend are typically
associated with three dominant high-angle structural sets that served as conduits for ore
fluids. These three sets strike NNW (340o – 350o), NE (35o – 60o), and NW (315o – 340o)
with the NNW-striking faults being the most prominent (Volk et al., 1996). In addition,
three sets of mesoscopic folds have been documented along the district (Volk et al.,
1996). These three sets trend NW-SE, N-S, and SW-NE, with the NW-SE-trending folds
being the most prominent (Teal and Jackson, 1997).
Faults
The NNW-striking faults dip 50o to 75o both east and west (Teal and Jackson,
1997). The Post, Genesis, Castle Reef, Leeville, and Good Hope fault systems are
examples of NNW-trending fault types (Figure 7: Volk et al., 1996) that control as well
as offset ore zones (Teal and Jackson, 1997). These faults constitute the most continuous
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24
mappable high-angle structures in the district, often displaying evidence of oblique-slip,
dip-slip, and strike-slip movement (Volk et al., 1996).
The NE-striking faults dip 40o to 70o to the NW and SE. They play an important
role for focusing gold mineralization at the deposit scale (Volk et al., 1996). The NW-
striking faults are the least common of the three and are often cut by the NNW and NE
fault systems (Teal and Jackson, 1997).
Folds
The NW-SE-trending folds are NE-verging, tight to isoclinal, strongly
asymmetric, mesoscopic folds well developed within the Rodeo Creek and Vinini
Formations. The fold hinges are intensely fractured and veined, and deformation is
characteristically brittle-ductile (Teal and Jackson, 1997). On a regional scale, the NW-
SE mesoscopic folds are commonly associated with NW-trending anticlines and
synclines that play an important role as structural traps to fluid migration (Volk et al.,
1996).
A less common set of N-S-trending folds, commonly associated with thrust
faulting, is tight and asymmetric. A third set of folds trending SW-NE, has been
documented at the Meikle mine (Teal and Jackson, 1997).
25
CARLIN TREND MINERALIZATION
The Carlin trend is one of the largest gold producing gold mining district in North
America. Sediment-hosted disseminated (“Carlin-type”) gold deposits mainly occur
within altered calcareous sedimentary rocks of the Roberts Mountains and Popovich
Formations, with minor contributions from igneous and other sedimentary rocks units
(Radtke, 1985; Kuehn, 1989; Bakken, 1990). The extent of alteration (carbonate
dissolution, argillization, silicification, and sulfidation) is controlled partially by the
composition of the original host rock. In deposits hosted within fossiliferous limestone,
decalcification tends to be restricted around high-angle fluid conduits and major lithology
contacts. In deposits hosted in silty limestones, decalcification is more pervasive and
intense due to the original porosity and permeability of the host rock (Teal and Jackson,
1997).
In addition, mineralization is associated with complex structural and lithologic
controls. These include major faults that provide fluid conduits, permeability and
reactivity of the host rocks, and increased permeability created by faulting, fracturing,
and brecciation (Bakken, 1990; Christensen, 1993; Leonardson and Rahn, 1995; Volk et
al., 1996; Teal and Jackson, 1997).
Timing and genesis of the gold deposits along the Carlin trend appear to range
from Cretaceous to mid-Tertiary. Arehart et al (1993) and Drews et al. (1996)
26
determined that sericitic hydrothermal alteration occurred at 117 Ma and 95 Ma,
respectively. Geochemical evidence by Ilchik (1995), Emsbo et al. (1996), and Embso
(1999) suggests a mid-Tertiary timing by arguing that a biotite-feldspar-porphyry dike,
which dated at approximately 39 Ma (Arehart et al., 1993), is cut by the main Carlin-type
alteration and mineralization event.
27
RESEARCH METHODS
The research objectives were fulfilled by integrating data collected from field and
laboratory studies by the author. The field portion involved two consecutive summer
sessions (1999, 2000) based at Barrick Goldstrike mine. The laboratory portion was
completed during the spring and fall, 2000.
Field Portion
The majority of the two field sessions were spent logging core and collecting core
photographs and logs (Appendix A). Thirty-two continuous diamond-drill cores were
logged from the Goldstrike and Dee-Rossi properties. The logging focused on detailed
descriptions of the cores using Dunham’s (1962) classification of the carbonate facies and
depositional environments for the Roberts Mountains and Popovich Formations. Several
5 to 10 ft core intervals were collected from selected non-transported (when possible)
material for later conodont study. In addition, hand samples of various lithologies,
lithologic contacts, clast types, and textures were collected for later petrographic analysis.
Finally, photographs and logs from 261 cores were collected from Goldstrike (Barrick
Goldstrike), Tara (Newmont Mining), Ren (Cameco Gold), and Dee-Rossi (Barrick Gold
Exploration and Meridian Gold) properties for later photographic interpretation.
28
Laboratory Portion
Core photographic interpretation was conducted on the photographs collected
during the field portion. The interpretations were used in conjunction with logged cores
for the construction of cross sections, isopach maps, and facies distribution maps.
One-hundred-twenty-five thin sections were prepared for reconnaissance
petrographic examination, and 35 sections were described in detail using both Folk
(1959) and Dunham (1962) classifications. Petrographic analysis provided additional
support for core-based observations and allowed for further identification of debris flow
compositions and sources.
Initial crushing of 57 carbonate samples for conodont analysis (Appendix D) was
conducted in the lab at the Colorado School of Mines. The crushed samples were
processed by either Dr. Stan Finney, California State University at Long Beach, or Matt
Zimmerman, University of Nevada at Reno, by soaking them in 10% hydrochloric acid.
Final analysis of conodont samples was conducted by Dr. Anita Harris, U. S. Geological
Survey. Dr. Harris successfully dated 28 conodont-bearing samples and provided an
additional 14 dates previously analyzed for other individuals. Both the processing and
analysis of the conodont samples were undertaken on a contract basis through Barrick
Goldstrike.
An isotopic study was conducted on two cores, DC9501 (Dee-Rossi) and RU8
(Ren). The study was undertaken to try to recognize sequence boundaries within the
29
Bootstrap Limestone by identifying shifts and trends in δ18O and δ13C isotopic signatures
associated with meteoric conditions. The samples were processed and analyzed by Dr.
John Humphrey at the Colorado School of Mines, Golden, CO. Samples were reacted
off-line with 100% phosphoric acid at 90oC. Samples were run on a VG Micromass 903
stable isotope ratio mass spectrometer. Data are reported as a per mil difference from the
PDB Standard (Appendix E).
30
RESEARCH RESULTS
Facies Identification
This study developed a new sequence stratigraphic column for the northern Carlin
trend (Figure 8) modified from the Volk et al. (1996) tectono-stratigraphic column
(Figure 5) originally constructed for the Goldstrike property. The informal Bootstrap
Limestone Formation, previously identified as a member of the Roberts Mountains
Formation, is a shallow-water, platform-derived limestone facies. The informal Roberts
Mountains Formation is a thick interval of dark, micritic limestone divided into two
members: Laminated Micritic Limestone and Apron Facies (from this study) members.
The informal Popovich Formation is a dark, laminated, micritic limestone divided into
four members: Wispy, Planar, Soft-Sediment Deformation, and Micritic members. The
following sections are detailed descriptions for these facies.
Bootstrap Limestone Formation
The Bootstrap Limestone Formation is a massive, shallow-water limestone
composed of two facies types deposited on a platform margin. In core, the first facies is a
massive light gray limestone with visible crinoids and ooids (Figure 9). In thin section,
the facies is a massive, coarse-grained, moderately to well sorted crinoidal-oolitic
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grainstone (Figure 10). In core, the second facies is a massive, light to dark grayish-blue
limestone with visible crinoids, branching and rugose coral, bryozoans, mollusks,
gastropods, and algae (Figure 11). In thin section, the facies is massive, coarse-grained,
poorly to moderately sorted, oolitic grainstone to fossiliferous boundstone (Figure 12).
Both facies were deposited in a shallow-water platform environment with the first type
representing a shoal facies and the second a reef facies.
Roberts Mountains Formation
Laminated Micritic Limestone Member (LL).— In core and outcrop, the LL is a thick
sequence of alternating light and dark, laminated silty limestone to calcareous siltstone
(Figures 13 and 14). The individual laminations range in thickness from less than an inch
to several inches and are graded, which may not be detected by the naked eye. In thin
section, the LL facies is a well to moderately sorted mudstone to wackestone consisting
of fine-grained, shallow-water, reef- and shoal-derived carbonate grains (Figure 15). The
LL facies is interpreted to represent turbidites deposited in an anoxic, basin environment.
Apron Facies Member.— Three distinct units make up the Apron facies: Laminated
Micritic/Debris Flow (LL/DF), Laminated Micritic/Wispy/Debris Flow (LL/WS/DF), and
Wispy/Debris Flow (WS/DF) units.
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35
36
37
38
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40
In core, the LL/DF facies is similar to the previously mentioned Roberts
Mountains LL facies with the addition of interbedded debris flows (Figure 16). The
individual debris flows range from less than an inch to several feet thick, consisting of
reef-, shoal-, and slope-derived sediment. The overall thickness of the unit ranges from 0
to 100 ft.
In core, the LL/WS/DF facies is similar to the LL/DF facies with the addition of
interbedded “wispy” texture, a local term used to describe bioturbation. Presence of both
the LL and WS facies signifies the transitional change from an anoxic to aerobic
environment (Figure 17). The interbedded debris flows range from less than an inch to
few feet thick, consisting of reef-, shoal-, and slope-derived sediment. In addition to the
debris flows, large platform-derived, sub-angular boulders, ranging from an inch to a few
feet in diameter, are common. The overall thickness of the unit ranges from 0 to 200 ft.
In core, the WS/DF facies is similar to the LL/WS/DF facies minus the LL
portion, signifying a complete transitional change from an anoxic to aerobic environment
(Figure 18). The interbedded debris flows range from less than an inch to several feet
thick, consisting of shallow-water, reef- and shoal-derived sediment. In addition to the
debris flows, large platform-derived, sub-angular to angular boulders, ranging from a few
inches to several feet in diameter, are common. The overall thickness of the unit ranges
from 0 to 150 ft.
In thin section, the Apron facies is quite variable because of the three intermixed
types. The LL portion is a well to moderately sorted, graded mudstone to wackestone
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consisting of fine-grained, reef- and shoal-derived carbonate grains (Figure 19). The DF
portion is a poorly sorted packstone to grainstone consisting of coarse-grained, shallow-
water, reef- and shoal-derived carbonate grains (Figure 20). The WS portion is a
moderately sorted, bioturbated mudstone to wackestone consisting of fine-grained,
shallow-water, reef- and shoal-derived carbonate grains (Figure 21).
The Apron facies, identified from this study, represents slope to base-of-slope
deposition with a transitional change from an anoxic to aerobic environment. The LL/DF
facies represents the distal portion of the apron deposited in an anoxic environment. The
LL/WS/DF facies represents the mid-portion of the apron deposited in a transitional
environment. The WS/DF facies represents the proximal portion of the apron deposited
in an aerobic environment.
Popovich Formation
Wispy Member (WS).— In core, the WS facies is a thick sequence of “wispy” textured,
light to dark gray, finely laminated, graded mudstone (Figure 22). Wispy texture resulted
from churning and stirring by burrowers and sediment eaters during deposition. In thin
section, the WS facies is a well sorted, bioturbated mudstone consisting of very fine-
grained, shallow-water, reef-derived skeletal grains (Figure 23). The WS facies
represents distal turbidites deposited in a deep, aerobic, basin environment and
subsequently bioturbated.
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A distinction is made in core between the wispy texture of the Popovich and
Roberts Mountains Formation (Figure 24). Wispy texture of the Roberts Mountains
Apron facies is coarse and inconsistent. Wispy texture of the Popovich WS facies is
finer-scaled and more continuous. Since living organisms need time to completely
disturb all the underlying sediment, the different types of wispy texture reflect different
rates of sedimentation for each facies. Slope Apron facies experienced a fluctuation in
high and low relative rates of sedimentation as compared to the basinal Wispy facies,
which experienced more constant, lower relative rates of sedimentation.
Planar Member (PL).— In core, the PL facies consists of a black, finely laminated,
graded mudstone, commonly found interbedded with less than an inch to few inch thick
debris flow beds consisting of reef-derived skeletal grains, locally termed “fossil hash”
(Figure 25). In addition, brassy pyrite is commonly found oriented along bedding planes.
An upper portion (top 10 ft) of the member is marked by a Monograptus sp. and
dendroidal variety graptolite condensed section. In thin section, the PL facies is quite
variable. The laminations are well to moderately sorted mudstone to wackestone
consisting of fine carbonate grains (Figure 26a). The fossil hash beds are poorly sorted
packstones consisting of coarse, reef-derived skeletal grains (Figure 26b). The PL facies
represents distal turbidites deposited in a deep, anoxic, basin environment.
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Soft-Sediment Deformation Member (SSD).— In core, the SSD facies is an alternating
light to dark gray, convoluted, and slumped calcareous mudstone (Figure 27). Floating
light gray limestone clasts, ranging from less than an inch to several feet in diameter, are
common, and shed from the shallow-water platform margin (Figure 28). In addition, 1 to
3 ft, light, wispy textured intervals sporadically occur throughout the member. In thin
section, the SSD facies is quite variable. Convoluted beds are moderately sorted,
deformed calcareous wackestone (Figure 29). Floating clasts are massive, angular to
subangular, poorly to moderately sorted oolitic grainstones to fossiliferous boundstones
(Figure 30). The SSD facies represent syndepositional gradient-induced slump and slide
sediments deposited in a disaerobic, slope to base-of-slope environment.
Micritic Member (UM).— In core and outcrop, the UM facies is a dark gray to black,
finely laminated, graded mudstone to siltstone (Figures 31 and 32). Thin, concordant
layers of pyrite, locally termed “pinstripe pyrite” by Barrick Goldstrike geologists, are
common along the laminations. In addition, UM member is commonly interbedded with
argillaceous chert and/or thin fossil hash beds. A marker bed, consisting of locally
termed “chert lenses” (1 to 7 inch black lenticular bodies that easily scratch, oriented
along bedding planes; also locally called “CTL” because its known the lenses are not
chert) by Barrick Goldstrike geologists, occurs near the base of the member (Figure 33).
In thin section, the UM facies is quite variable. Black laminations are well sorted
mudstones to wackestones consisting of fine carbonate grains (Figure 34a). Chert lenses
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are opaque, probably due to included disseminated fine-grained pyrite (Figure 34b).
Fossil hash beds are poorly sorted packstones with reef-derived skeletal grains (Figure
34c). The UM facies represents distal turbidites deposited in an anoxic, deep-water
environment associated with drowning of the local carbonate platform (Bootstrap facies).
Facies Distributions
The following section is a description of the lateral and vertical facies
distributions for the Bootstrap Limestone, Roberts Mountains, and Popovich Formations
in the study area (Figure 35; Appendix B). Descriptions are based on distribution maps
for the Apron and Popovich facies, cross sections through the reef and shoal facies, and
isopach maps for the Popovich facies constructed for this study. The following maps and
cross sections have been corrected for displacement caused by fault movement. Dikes
and sills were removed from the core data to better illustrate facies distributions.
Bootstrap Limestone Formation
The Bootstrap Limestone facies ranges from 0 to 2000 ft thick and, where present,
unconformably overlies the Ordovician Hanson Creek Formation. The Bootstrap
Limestone facies is located in the NNE section of the study area with a lateral facies
change into the Roberts Mountains LL facies to the SSW (Figures 36 and 37) defining
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the NW-SE trending platform margin. Distribution of the reef and shoal facies along the
platform margin has been interpreted to reflect slope gradient (Figure 38). Based on a
model by Cook (1983), and Mullins and Cook (1986), the Bootstrap Limestone reef
facies is interpreted as being associated with a steep gradient (> 4o) and shoal facies
associated with a shallow gradient (< 4o). Limited data prohibited identification of the
exact location of the lateral transitional change between the reef and shoal facies.
Roberts Mountains Formation
Laminated Micritic Limestone Member (LL).— The LL facies ranges from 1100 to 1500
ft thick and, where present, unconformably overlies the Ordovician Hanson Creek
Formation. The LL facies is located in the SSW section of the study area, with a lateral
facies change into the Bootstrap Limestone facies to the NNE (Figures 36 and 37).
Minimal deep core penetrations into the LL and Bootstrap Limestone facies prohibited
the identification of the exact location and profile of the lateral transitional change
between them.
Apron Facies Member.— The Apron facies ranges from 0 to 300 ft thick and
conformably downlaps onto the LL facies to the SSW and onlaps onto the Bootstrap
Limestone facies to the NNE. The Apron facies lateral extent and vertical-stacking
patterns are dependent on the slope gradient defined by the distribution of the Bootstrap
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Limestone reef and shoal facies (Figure 38). Facies associated with a steep slope are
relatively less areally extensive and are focused at the base-of-slope (Figure 36). The
general shape is concave upward and thicker in the mid-section, with all three Apron
units stacked in place. The facies associated with a shallow slope are relatively more
areally extensive, ranging from the base-of-slope up to the platform margin (Figure 37).
The general shape is lenticular and relatively constant, with a maximum of two units
stacked in one place along the apron.
Popovich Formation
The Popovich Formation represents a sequentially deepening succession of facies
from an aerobic WS to anoxic UM facies. Popovich facies are quite variable across the
study area (Figure 39). To the SSW, all four facies are present and to the NNW, the
Popovich facies progressively pinch out, beginning with the WS facies and ending with
the SSD facies. The UM facies is relatively uniform and regionally extensive throughout
the study area. The lateral distribution is further complicated when associated with either
a steep (Figure 36) or shallow (Figure 37) slope.
Wispy Member (WS).— The WS facies ranges in thickness from 0 to 300 ft and
unconformably overlies LL facies to the SSW and unconformably onlaps the Apron
facies to the NNE. In general, thickness increases to the SSW into the basin, with the
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exception of pinching onto the Apron facies (Figure 40). Thick and thin areas represent
topographic relief present prior to deposition of the WS facies.
Planar Member (PL).— The PL facies ranges in thickness from 0 to 250 ft and
conformably overlies the WS facies when present. When the WS facies is not present,
the PL facies either unconformably onlaps the Bootstrap Limestone (steep slope) or
Apron (shallow slope) facies to the NNE. In general, the relative thickness is uniform
with the exception of pinching onto the Apron and Bootstrap Limestone facies (Figure
41). Thick and thin areas probably represent remnants of uncorrected structural
complications and not topographic relief prior to deposition.
Soft-Sediment Deformation Member (SSD).— The SSD facies ranges in thickness from 0
to 350 ft and conformably overlies the PL facies when present. When the PL facies is not
present, the SSD facies either conformably overlies the Bootstrap Limestone (steep slope)
or unconformably onlaps the Apron (shallow slope) facies to the NNE. In general,
relative thickness increases to the SSW into the basin with the exception of pinching onto
the Apron and Bootstrap Limestone facies to the NNE (Figure 42). Thick and thin areas
represent base-of-slope sediment accumulations resulting from syndepositional slumping,
and slope failure associated with a steep gradient.
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Micritic Member (UM).— The UM facies ranges in thickness from 120 to 300 ft and
conformably overlies the SSD facies when present. When the SSD facies is not present,
the UM facies unconformably overlies the Bootstrap Limestone facies in both steep and
shallow slope environments. This facies is relatively uniform and laterally continuous,
differing from the other Popovich members that depositionally pinch out to the NNE
(Figure 39). In general, the relative thickness slightly decreases to the WSW into the
basin (Figure 43). Thick and thin areas represent topographic relief formed during the
deposition of the SSD facies.
Ichnology
Trace fossils are useful when interpreting depositional environments and relative
bathymetry. They record the behavior of benthic organisms, as dictated or modified by
environmental constraints, and do not refer to the type of organism that made the trace
fossil (Pemberton et al., 1992).
Six trace fossils commonly found along bedding planes of the Roberts Mountains
Apron and Popovich Wispy facies are Chondrites, Planolites, Zoophycus,
Rhizocorallium, Paleodictyon, and Cosmorhaphe. These ichnofacies represent
adaptation of trace-making organisms to bathyal environmental conditions (Frey and
Pemberton, 1984; Frey et al., 1990). Therefore, the presence of the trace fossils indicate
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deposition in a disaerobic to aerobic environment related to water depths greater than 650
ft for both the Roberts Mountains Apron and Popovich WS facies
Biostratrigraphy
Graptolites
Two common graptolites found along the same bedding-plane parting surfaces of
the Roberts Mountains and Popovich Formations are Monograptus sp. (Silurian to late
Devonian) and dendroidal (middle Cambrian to Carboniferous) graptolites. Berry and
Murphy (1975) and Griffin (personal communication, 2000) identified Monograptus
hercynicus in the graptolite horizon found at/near the top of the Popovich Planar member.
The Monograptus hercynicus fossils have been dated as late Lochkovian (Ancyro. delta
to Ped. p. pesavis zones) in age (Figure 45; Berry and Murphy, 1975; Springer and
Murphy, 1994).
The presence of pelagic Monograptus sp. indicates an anoxic environment in
close proximity to a shallow, oxygenated environment from where the neritic dendroidal
graptolites were transported (Clarkson, 1998; Griffin, personal communication, 2000).
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Conodonts
Conodonts are microscopic apatitic hard parts of extinct primitive marine
vertebrates widely distributed from very late Cambrian to late Triassic. They have been
intensely studied by many individuals (Aldridge et al., 1986; Aldridge, 1987; Aldridge
and Theron, 1993; Donoghue et al., 1998; Donoghue et al., 2000, among others) as
premier biostratigraphic indicators. In the past 35 years, at least 200 subdivisions
throughout their geologic range have been identified (Harris, personal communication,
2001). This study utilized Silurian-Devonian conodont zonations (Figure 44) to construct
a chronostratigraphic chart based on data from this study and previous work compiled by
for the Bootstrap Limestone, Roberts Mountains, Popovich, and Rodeo Creek Formations
(Figure 45; Appendix D).
Initial deposition of the Bootstrap Limestone, Roberts Mountains LL, and Roberts
Mountains Apron facies began in late Llandoverian (Pt. Amorphognathoides zone) to
early Wenlockian (Oz. bohemica bohemica zone) time and abruptly ended in the early-
middle Lochkovian (I. w. hesperius/I. woshmidti zones). The Popovich WS facies was
deposited during middle Lochkovian (Oz. eurekaensi zones) time. During the middle
Lochkovian (Ancyro. delta zone), the Bootstrap Limestone facies was rejuvenated and
laterally deposited adjacent to the Popovich PL facies and continued through to early-
middle Emsian (Po. kitabicus zone). Deposition of the overlying Popovich SSD followed
and continued through Pragian time. During the Emsian, deposition of the Popovich UM
facies began and continued through to early Frasnian time (Pa. punctata zone). A 6-7
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m.y. hiatus spanned from early Frasnian (Pa. hassi zone) through late-middle the
Famennian (Pa. rhomboidea zone), with deposition of the overlying Rodeo Creek
beginning in late Famennian (Pa. m. marginifera zone) time.
Ostracods
At Devils Gate Pass, NV, Casier et al. (1996) and Casier and Lethier (1998)
documented an ostracod fauna mass extinction event that marked the Frasnian—
Famennian boundary (FFB). They concluded that the extinction of numerous ostracod
species was in response to either a global sea-level fall or possibly to reorganization of
oceanic circulation induced by plate tectonics. The FFB extinction they recognized
corresponds with the hiatus documented in this study using conodonts (Figure 45).
Diagenesis
Dolomitization
Embso (1999), Evans (2000), and this study identified at least three
dolomitization events that affected the Bootstrap Limestone, Roberts Mountain, and
Popovich formations. A penecontemperaneous dolomitization event affected each
formation that included micritization and pervasive marine cementation. Some time after
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early diagenesis, hydrothermal fluids were channeled along NNW-trending, high-angle
faults resulting in pervasive ferroan dolomitization that produced a zebra texture in the
Bootstrap Limestone. Finally, a minor dolomitization replacement event occurred in
each formation during burial metamorphism.
Stable sotopes
Allan and Matthews (1982) developed a model for stratigraphic variation in the in
δ18O and δ13C isotopic signatures that have undergone recrystallization in meteoric
waters (Figure 46). The model indicates δ18O enrichment within a few feet of a subaerial
exposure surface due to evaporation, and overall depletion in δ18O for the vadose and
phreatic zone. Progressive enrichment to marine values occurs in the phreatic mixing
zone. The δ13C signature becomes progressively enriched with increasing depth through
the vadose zone. The vadose/phreatic boundary is marked by a sharp downward increase
in δ13C and δ18O, progressively converging on normal marine values with
recrystallization in successively more saline waters.
This study used the Allan and Matthews (1982) model to attempt to identify
sequence boundaries based on relative shifts in δ18O and δ13C signatures analyzed for
significant surfaces that were previously identified in core. Isotopic signatures for
DC9501 showed inconclusive evidence related to potential sequence boundaries
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identified in core (Appendix E). Isotopic signatures for RU8 showed evidence related to
two potential sequence boundaries logged at a depth of 3493 ft and 3270 ft.
The first sequence boundary (3493 ft) identified in core is a break between
karsted shallow-water carbonates and additional overlying shallow-water carbonates
(Figure 47). The karsted shallow-water carbonates are depleted in δ18O and
progressively (with depth) enriched in δ13C relative to the normal marine signatures in
the overlying shallow-water carbonates (Figure 48). The narrow enrichment in δ18O
found near the boundary is probably due to surface evaporation.
The second sequence boundary (3270 ft) identified in core is a break between a
relative deepening succession of carbonates and overlying shallow-water carbonates
(Figure 49). The deepening succession of carbonates are depleted in δ13C and
progressively (with depth) enriched in δ13C relative to the normal marine signatures in
the overlying shallow-water carbonates (Figure 50). The fluctuation in δ13C signature
may be due to variations in carbon content in the deeper-water carbonate muds found
near the sequence boundary.
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CARBONATE DEPOSITIONAL ENVIRONMENTS
The interpretation of depositional environments in outcrops and cores is important
for developing a sequence stratigraphic framework. Depositional environments are
interpreted by observing and describing lithology, physical and biogenic sedimentary
structures, stratigraphic position, and position in a shelf-slope-basin setting. This section
includes a general summary of the major facies and depositional processes that occur in
carbonate platform margin, slope, and basin environments (Figure 51).
Carbonate Platform Margin
General
Carbonate platforms are dynamic depositional systems influenced by a variety of
natural processes, including fluctuations in relative sea level. Changes in climate,
circulation patterns, salinity, water temperature, and other processes affect carbonate
environments, resulting in changes in shallow-water facies (Tucker and Wright, 1990).
Fluctuations in relative sea level, in response to varying rates and styles of subsidence or
eustasy, cause changing environmental conditions. If relative sea level falls below the
shelf-edge, carbonate production terminates and the platform is subjected to meteoric
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diagenesis (Jones and Desrochers, 1992). If carbonate production maintains pace with
relative sea-level rise, thick accumulations of shallow-water carbonate facies are
deposited (Tucker and Wright, 1990). If relative sea-level rise outpaces the rate of
vertical sediment accumulation, the platform is drowned and characterized by a
deepening upward facies succession (Tucker and Wright, 1990; Schlager, 1991; Jones
and Desrochers, 1992).
Platform margins can assume two different morphologic profiles: rimmed and
unrimmed. Rimmed carbonate platforms are shallow-water shelves with pronounced
slope-breaks into deep water, and may be formed by rapid carbonate sediment
aggradation. They are high-energy zones characterized by the development of shallow-
water boundstone reefs and oolitic grainstone shoal facies acting as barriers (Wilson,
1975; Tucker and Wright, 1990). Unrimmed carbonate margins are shallow-water
shelves with no clear shelf-slope breaks. These shelves range from ramps, with relatively
uniform slopes (Read 1982; 1985), to open shelves, with distally increasing slopes
(Ginsburg and James, 1974). Unrimmed platforms are low-energy zones characterized
by mud-supported facies with no grain-supported facies acting as barriers.
Study Area
The high-energy Bootstrap Limestone reef and shoal facies located to the NNE is
interpreted as being deposited on a shallow-water, rimmed carbonate platform with a
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pronounced slope-break into deep-water. During the late Silurian to early Devonian, the
rimmed carbonate platform was influenced by series of fluctuations in relative sea level.
The platform was submerged during the Llandoverian to early-middle Lochkovian and
the carbonate factory maintained pace with relative sea level, resulting in thick
accumulations of shallow-water reef and shoal facies. During the early Lochkovian,
relative sea level fell below the shelf edge, terminating carbonate production and
subjecting the platform to meteoric conditions as indicated by karsting observed in RU8
(Figure 47). By mid-Lochkovian time, relative sea level was rapidly rising and
submerged the platform. The carbonate factory was rejuvenated, but the relative rise in
sea level eventually outpaced carbonate production resulting in the drowning of the
platform and deposition of the overlying Popovich UM deep-water facies.
This depositional pattern is most easily observed in cores taken to the NNE, in the
platform environment, that will show a vertical stacking-pattern consisting of Bootstrap
Limestone overlain by Popovich UM facies (Figure 52). The section will not contain
Roberts Mountains (LL and Apron) or lower Popovich (WS, PL, and SS) facies.
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Slope
General
The slope environment is located between the carbonate rimmed platform and the
slope-basin break (Figure 51). Most carbonate slopes are concave upward due to the
presence of framebuilding organisms and early cementation near the platform-slope break
(Kenter and Schlager, 1989). The sediment budget and/or locus of deposition can
influence the type of slope formed (Coniglio and Dix, 1992). Three types of slopes have
been recognized in modern carbonate depositional environments: accretionary, bypass,
and erosional slopes. Accretionary slopes are low-angle slopes constructed of
sedimentary gravity flow deposits. Bypass slopes are relatively steep slopes that allow
sediment to bypass the upper portion and deposit the majority of the sediment at the base-
of-slope. Erosional slopes are very steep, resulting in a net sediment loss on the slope
(Coniglio and Dix, 1992; Wright and Burchette, 1996).
Many physical processes act on these carbonate slopes (Figure 51). Processes
include pelagic settling, bottom currents, resedimentation, erosion, and bypass (Cook and
Mullins, 1983; Tucker and Wright, 1990; Coniglio and Dix, 1992). The sediments
deposited by these processes are highly variable. Suspended fine-grained carbonates
shed from the shallow platform and pelagic material are uniformly deposited as
periplatform oozes over the slope and into the basin (Mullins and Neumann, 1979).
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Bottom (contour) currents are responsible for winnowing, mobilizing, and redistributing
large volumes of carbonate sediment (Cook and Mullins, 1983). Resedimentation
processes from gravity flows transport large volumes of coarse, shallow-water carbonate
sediment onto the slope and into the basin. Sediment gravity-flows are further divided
into turbidity currents, fluidized flows, liquefied flows, grain flows, and debris flows
(Lowe, 1976). Erosional processes including rock falls, slides, and slumps, result in
platform- and slope-derived thick accumulations of base-of-slope talus, to distally
developed thin, basinal debris flows and turbidites (Varnes, 1978; Tucker and Wright,
1990). Bypass results in non-deposition up-slope and thick accumulation of sediments,
via gullies and canyons, down-slope and into the basin by gravity flows (Wright and
Burchette, 1996).
Study Area
Steep and shallow slopes have been interpreted along the northern Carlin trend
during late Wenlockian to early Lochkovian time that were responsible for deposition of
the Roberts Mountains Apron facies. Steep slopes developed adjacent to pronounced
slope-breaks along the platform margin associated with shallow-water reef facies.
Bypassing the upper portion of the slope, erosional processes and sediment gravity-flows
deposited relatively thick accumulations of sediment at the slope-basin break as a base-
of-slope apron. Shallow slopes developed adjacent to slope-breaks associated with
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shallow-water shoal facies. Sediment gravity-flow processes, including turbidity currents
and debris flows, emplaced uniformly thick accumulations of sediment that extended
from the slope-basin break up to the shelf-slope break as a slope apron.
During the late-late Lochkovian through Pragian time, the slope was subjected to
a rapid rise in relative sea level over the platform. The rapid rise caused major instability
along the slope, resulting in catastrophic slope failure and deposition of the Popovich
SSD facies.
A core taken on the slope environment will be highly variable. The section will
reflect the type of gradient (steep or shallow) and relative location along the slope. On a
steep slope (Figure 52a), the Roberts Mountains Apron facies is focused at the base-of-
slope with the overlying Popovich facies sequentially onlapping it and the Bootstrap
Limestone facies. On a shallow slope (Figure 52b), the Roberts Mountains Apron facies
extend up to the platform margin with the Popovich facies sequentially onlapping it.
Basin
General
The basinal environment is located beyond the slope-basin break (Figure 51).
Slope facies are easily distinguished from shallow platform facies; however, the same
statement is not true when talking about slope and basin facies. Slope facies gradually
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merge with basin facies, and both slope and basin facies accumulate large volumes of
pelagic material (Cook et al., 1972; Wilson, 1975; McIlreath and James, 1978; Enos and
Moore, 1983), making the distinction between them difficult. Some major differences
between them do exist; for example, slope facies are usually associated with large-scale
deformation (Cook and Mullins, 1983) and coarser debris flows, in contrast with basinal
facies consisting of finer-grained, laminated gravity flows (Cook and Egbert, 1981).
Many physical processes acting on slopes act in basins as well. These include
pelagic settling, bottom currents, resedimentation, erosion, and bypass (Cook and
Mullins, 1983; Tucker and Wright, 1990; Coniglio and Dix, 1992). The sediments
deposited by these processes are variable. A discussion of these processes is included in
the slope portion of this section.
Study Area
The Roberts Mountains LL, Popovich WS, and Popovich PL basinal facies to the
SSW consist of thick accumulations of laminated distal turbidites and thin debris flows
derived from the platform and slope environments. Additional suspended fine-grained
and pelagic sediment was deposited into the basin as periplatform oozes.
A core taken to the SSW in the basin environment will have a distinct vertical
facies stacking-pattern, and will not be affected by the different slopes created by the
Bootstrap Limestone reef and shoal facies to the NNE. The vertical section will consist
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of the Roberts Mountains LL facies unconformably overlain by a complete section of the
Popovich facies (Figure 52). The section will not contain Roberts Mountains Apron or
Bootstrap Limestone facies.
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DEPOSTIONAL MODELS
Carbonate Apron Models
Model
Sediment gravity-flows, found depositionally along the deep-water flanks of
carbonate platforms, typically produce wedge-shaped carbonate aprons parallel to their
adjacent shelf/slope breaks, instead of submarine fans (Mullins and Cook, 1986).
Line sources produce slope deposits in broad aprons arrayed parallel to the shelf
margin (Cook, 1983; Cook and Mullins, 1983; Mullins, 1983; Mullins et al., 1984;
Mullins and Cook, 1986). The aprons commonly are composed of grain flows, debris
flows, turbidites, and periplatform ooze (Sheehan et al., 1993). In contrast, non-
carbonate slopes are characterized by point sources; fans of sediment radiate from
canyons that are fed clastic material by rivers and longshore currents (Mutti and Ricci
Lucchi, 1978; Normark, 1978). Point-source deposits, such as siliciclastics fans, are
uncommon in carbonate systems (Cook and Egbert, 1981; Cook, 1983; Ruiz-Ortiz, 1983;
Wright and Wilson, 1984). Thus, the submarine fan model (Nelson et al., 1970; Mutti
and Ricci Lucchi, 1978; Normark, 1978; Walker, 1978) cannot be unequivocally applied
to carbonate sediment gravity-flow deposits.
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In review and expansion of the carbonate apron model (Cook, 1983), Mullins and
Cook (1986) recognized two end-member types of carbonate aprons: slope apron and
base-of-slope apron. The major difference between the two models is that redeposited
carbonate sediments of the first model developed on a relatively shallow slope that
extended up to the shelf-edge, whereas the latter developed on relatively steep slopes that
involved an upper-slope bypass zone.
Slope Apron Model.— Slope aprons (Figure 53a) develop immediately adjacent to shoal-
water sediments (Mullins and Cook, 1986) along carbonate platform margins that have a
relatively gentle gradient (< 4o) into the adjacent basin (Cook, 1983). Carbonate gravity
flow deposits extend up to the adjacent shelf-slope break without an upper slope bypass
zone (Mullins and Cook, 1986). A random vertical succession of turbidity current and
debris-flow deposits are abundant in slope-aprons (Cook, 1983). Rockfalls, slides and
slumps have minimal sedimentation contribution due to lower gradients.
Base-of-Slope Apron Model.— Base-of-slope aprons (Figure 53b) develop along
relatively steep (> 4o), high-relief platform margin slopes and may exhibit thickening-
upward cycles (Mullins and Cook, 1986). Shoal-water derived sediment gravity-flow
deposits will be more likely to traverse down the slope (sediment bypass) and deposit
most of their debris at or near the base-of-slope (Cook, 1983). In addition, a multitude of
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small canyons and gullies and/or numerous submarine slides and slumps may form on the
slope (Cook, 1983), thus bypassing additional sediment to the base-of-slope.
Study Area
The previous sections described two Roberts Mountains Apron facies. The first
facies was associated with a shallow slope that developed adjacent to the Bootstrap
Limestone shoal facies and the second associated with a steep slope that developed
adjacent to the Bootstrap Limestone reef facies. The first facies resembles the Slope
Apron end member proposed by Cook (1983) and Mullins and Cook (1986). The facies
uniformally extended from the base-of-slope up to the platform-slope break. Laminated
turbidites and thin debris flows deposits dominated this facies. The second facies
resembles the Base-of-Slope Apron end member proposed by Mullins and Cook (1986).
The facies bypassed the upper slope, focusing deposition and a thick accumulation of
sediment at the base-of-slope. Rockfalls, slides, slumps, and thick sediment gravity-flow
deposits dominated this facies.
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The Armstrong and Others (1997) Model
Model
Armstrong et al. (1997) proposed a depositional model (Figure 54) for the
Bootstrap Limestone, Popovich and Rodeo Creek Formations based on a series of cores
drilled into the Bootstrap Limestone.
Armstrong et al. (1997) interpreted the Bootstrap Limestone, consisting of ooid
packstone, grainstone, wackestone, and lime mudstones, as being deposited on a wide,
shallow shelf margin adjacent to a basin in which the Popovich Formation was deposited.
They concluded that the base of the Popovich Formation was deposited in a progressively
deepening basin that developed above the drowned Bootstrap Limestone shelf due to
downwarping and/or faulting. The three upper members of the Popovich Formation were
interpreted as foreslope deposits in an oxygenated to anoxic environment located at the
edge of the Bootstrap Limestone ooid shoals. The deposits represented slide- and slump-
transported ooid-shelf sediments that were mixed and interbedded with basinal silty
dolostones and lime mud. Finally, they interpreted that the Rodeo Creek Formation as
being deposited in a continued deepening, silled starved basin, anoxic environment.
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Study Area
The depositional model proposed by Armstrong et al. (1997) was based on a basic
concept proposed by Wilson (1975). Their model suggested the basinal Rodeo Creek and
Popovich fore-slope-to-basin margin facies were deposited in a progressively deepening
sequence that drowned the Bootstrap Limestone due to downwarping and/or faulting.
This would have resulted in a deepening succession found laterally uniform throughout
the entire area. In fact, preliminary work by Griffin (1999; 2000) and this study
recognized that the Popovich WS, PL, and SSD facies laterally pinch out to the NNE
onto the Bootstrap Limestone and Roberts Mountains Apron facies, with no evidence of
an overall deepening sequence encompassing the entire area.
Ultimately, the Armstrong et al. (1997) model only provided a static
representation of the carbonate system by depicting an idealized distribution pattern of
facies and paleoenvironments based on an instant in time and in the absence of relative
sea-level changes. Now, by using methodology originally developed for clastic seismic
sequence stratigraphy by Vail et al. (1977), we can utilize sequence stratigraphy for
carbonates to overcome the static problem inherent in previous models, and integrate
time and relative sea-level changes in a new model.
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SEQUENCE STRATIGRAPHIC FRAMEWORK
General Background
Over the past several decades, geologists have routinely used carbonate facies
models to describe and interpret lateral facies relationships in ancient carbonate
platforms. This model only provides a static representation of the carbonate platform by
depicting an idealized distribution of facies and paleoenvironments. This results in an
interpretation based on an instant in time and in the absence of relative sea-level changes.
This approach is unrealistic, because carbonate platforms appear, migrate, disappear, and
reappear in response to depositional and erosional processes associated with marine
transgressions and regressions imposed by relative sea-level changes (Handford and
Loucks, 1993). This study successfully utilized a new approach by integrating sequence
stratigraphy and the use of systems tracts to overcome the static problem by incorporating
time and relative sea-level changes to track migration of facies.
Fundamental carbonate depositional principles and geologic-based observations
are used to construct depositional sequence and systems tracts responding to lowstand,
transgressive, and highstand conditions. This section does not attempt to summarize all
of the history, definitions, and concepts of sequence stratigraphy. However, it will
briefly define some sequence stratigraphic terms and methods used in sequence analysis.
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Loucks and Sarg (1993) published more detailed summaries of the history and terms of
carbonate sequence stratigraphy.
Significant Surfaces
Significant surfaces comprise maximum flooding surfaces and sequence
boundaries. These significant surfaces are bounding surfaces between systems tracts and
their associated parts.
Maximum Flooding Surface
By definition, the Maximum Flooding Surface (MFS) is the inflection point that
signifies a change from a continuous increase to gradual slowing of the rate of relative
sea-level rise with respect to subsidence (Figure 55). The surface is located just above
the condensed section (Loutit et al., 1988; Wendt, 1988). These sediments, composed of
pelagic or hemipelagic fauna and flora, are typically thin due to very low rates of
sedimentation and/or periods of non-deposition (Handford and Loucks, 1993), but
represent a long period of geologic time.
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Sequence Boundary
Sequence boundaries (SB) are unconformities or their correlative conformities
bounded above and below by genetically related strata (Vail et al., 1977). Sequence
boundaries are time transgressive, separating the underlying highstand and overlying
lowstand deposits (Figure 55).
Systems Tracts
A systems tract is a linkage of contemporaneous depositional systems (Brown and
Fisher, 1977). Three systems tracts developed in response to varying relative sea-level
responses are recognized: the lowstand, transgressive, and highstand systems tracts (Van
Wagoner, 1995).
Lowstand Systems Tract
Lowstand Systems Tract (LST) represents the mid- to late-part of a relative sea-
level fall, stillstand, and the early part of a relative sea-level rise (Figure 55; Van
Wagoner et al., 1988). The platform is exposed and carbonate production is limited to
the area seaward of the platform margin, resulting in minimal shelf-edge progradation
(Figure 56). Karstification is common on the platform when relative sea level falls below
the shelf edge, exposing the platform to meteoric conditions. LST sediment gravity-flow
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deposits contain skeletal grains and clasts shed penecontemporaneously from shelf-edge
environments and clasts derived from the older, subaerially exposed shelf edge. Ooid
and peloid production is minimal and these grain types are rarely found in LST gravity
flows. (Schlager, 1991).
Transgressive Systems Tract
Trangressive Systems Tract (TST) represents the maximum transgression with
respect to relative sea level (Figure 55; Van Wagoner et al., 1988). Maximum
transgression commonly leads to sediment starvation and the deposition of hemipelagic
and pelagic sediments in the basin (Figure 56; Handford and Loucks, 1993), resulting in
the formation of the MFS. In most cases, a rapid rise in relative sea level will submerge
the platform. Under most conditions, initiation of the carbonate factory lags behind
platform submergence in response to the rapid rise, and eventually catches up (Kendall
and Schlager, 1981). In some cases, however, carbonate production is outpaced by sea-
level rise and the platform is drowned.
Highstand Systems Tract
Highstand Systems Tract (HST) represents the late part of a relative sea-level rise,
a still stand, and early part of relative sea-level fall (Figure 55; Van Wagoner et al.,
1988). During the HST, shallow-water platforms produce large quantities of fine-grained
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sediment, ultimately shedding a large portion of carbonate sediment to the adjacent
slopes and basins (Figure 56; Neumann and Land, 1975; Mullins, 1983). This results in
deposition of aggradational to progradational shelf, shelf-edge, and slope facies (Sarg,
1988). Progradating HST shelf-edges and slopes commonly oversteepen and
catastrophically fail by way of rockfalls, sediment slides, and sediment gravity-flows,
forming base-of-slope aprons (Handford and Loucks, 1993). Ooid production is common
during the HST and corresponding sediment gravity-flows will contain oolitic grains,
making it possible to distinguish between LST and HST sediment gravity-flow deposits.
Sequences
A sequence is a stratigraphic unit composed of a relatively conformable
succession of genetically related strata bounded above and below by unconformities or
their correlative conformities (Mitchum et al., 1977). Depending on the location in the
carbonate system, a complete sequence may or may not contain all three systems tracts.
Depositional History
During the HST from late Llandoverian to early-middle Lochkovian, a thick
accumulation of Bootstrap Limestone facies developed on a shallow-water, rimmed
carbonate platform laterally adjacent to the Roberts Mountains LL anoxic, slope and
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basinal facies (Figures 57 and 58). By early Lochkovian time, a significant accumulation
of shallow-water sediment on the platform developed a pronounced platform-slope break
and slope where Roberts Mountains Apron facies was deposited. The subsequent fall in
relative sea level during the late-early Lochkovian resulted in a sequence boundary.
During the LST, the platform was exposed to meteoric conditions and deposition
of the Popovich WS facies was limited to the aerobic, slope and basin environment. The
overlying Popovich PL facies signifies another change in relative sea level during middle
Lochkovian time and the beginning of the TST.
Rapid rise in relative sea level during the TST led to drowning of the system,
including the platform, and a change to an anoxic environment in the basin. The lag-time
of carbonate production, in response to the rapid rise, resulted in minimal carbonate
platform sediment accumulation and starvation into the basin. The Monograptus sp. and
dendroidal variety graptolites found in the upper portion of the Popovich Planar facies
represent the condensed section of a MFS.
During the subsequent HST from Pragian to middle Emsian, reactivation of the
platform carbonate factory resulted in deposition of additional Bootstrap Limestone
facies on the platform. The Popovich SSD slope facies represents instability of carbonate
muds rapidly deposited onto the slope during the Pragian time. Sea level continued to
rise, eventually drowning the platform during the early-middle Emsian, resulting in
termination of the Bootstrap Limestone facies and deposition of the overlying Popovich
UM basinal facies.
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The Popovich UM abruptly ended in early-middle Frasnian time with a significant
time gap through the late-middle Famennian. The unconformable contact between the
Popovich UM facies and the overlying Rodeo Creek Formation represents a major
sequence boundary and a change from carbonate to dominantly clastic deposition.
Devonian Sea Level Curve
Johnson and Sandberg (1988) proposed a Devonian eustatic sea-level curve for
the western United States based on recognition of major biotic responses to eustatic
events (Figure 59). This study developed a relative sea-level curve for the northern
Carlin trend based on the sequence stratigraphic framework and chronostratigraphy
previously discussed.
The relative sea-level curve for the northern Carlin trend shows a relatively
consistent rise, then an abrupt fall, during early Lochkovian time. The abrupt change is
marked by a sequence boundary that separates the Roberts Mountains and Popovich
Formations. Another relative sea-level rise continued from early-middle Lochkovian
through early Frasnian time. This relative rise is marked by a second abrupt change
identified as a sequence boundary separating the Popovich and Rodeo Creek Formations.
A hiatus separating the formations extended from early Frasnian to middle Framennian
time, followed by another rise in relative sea level.
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In comparison, the Popovich portion of the relative sea-level curve mimics the
eustatic sea-level curve of Johnson and Sandberg (1988). The sequence boundary and
hiatus separating Popovich and Rodeo Creek Formations is consistent with a fall
identified near the Frasnian—Famennian boundary on the eustatic curve. This suggests
that the Popovich and overlying Rodeo Creek Formations were largely influenced by
eustatic controls during deposition instead of local tectonism.
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CONCLUSIONS
Summary
The highstand Bootstrap Limestone reef and shoal facies was deposited on a
shallow-water platform that developed pronounced slope-breaks at the platform margin.
A steep slope developed adjacent to the reef facies that allowed for sediment bypass and
formation of a base-of-slope apron. A shallow slope developed adjacent to the shoal
facies and formed a slope-apron. A subsequent relative fall in sea level resulted in a
sequence boundary and exposed the platform to meteoric conditions. The lowstand
Popovich Wispy facies was deposited above the sequence boundary in the basin. The
overlying trangressive Popovich Planar facies represents another change in relative sea
level that eventually submerged the platform. The relative rise in sea level began to slow
during another highstand and carbonate production was rejuvenated on the platform.
Thick accumulations of sediment were deposited onto the slope that became unstable and
failed, forming the Popovich Soft-Sediment-Deformation facies. Relative rise in sea
level eventually outpaced carbonate production and deposited the Popovich UM deep-
water facies uniformly over the area.
The relative changes in sea level resulted in a variable lateral and vertical facies
distributions along the northern Carlin trend. The platform, located to the NNE, consists
of the Bootstrap Limestone overlain by the Popovich Micritic facies. The slope, located
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between the platform and basin, reflects the type of gradient (steep or shallow) and
relative location along the slope. On a steep slope, the Roberts Mountains Apron facies
is focused at the base-of-slope with the overlying Popovich facies sequentially onlapping
both it and the Bootstrap Limestone facies. On a shallow slope, the Roberts Mountains
Apron facies extended up to the platform margin, with the Popovich facies sequentially
onlapping it. The basin, located to the SSE, consists of the Roberts Mountains Laminated
Micritic Limestone facies unconformably overlain by a complete section of the Popovich
facies.
Significance
• Detailed stratigraphic analysis of the study area identified an apron facies that is
time equivalent to the Roberts Mountains Laminated Micritic Limestone and
Bootstrap Limestone members.
• The Popovich Wispy, Planar, and Soft-Sediment Deformation facies
depositionally pinch out onto a previously deposited Bootstrap Limestone and
Roberts Mountains Apron facies.
• The sequence stratigraphic framework model developed in this study better
explains the lateral and vertical distribution of Siluro-Devonian succession of
facies by using multiple time lines.
121
• Recognition of spatial change in stratigraphy must be considered when
reconstructing structural complexities.
• Geometry of the carbonate platform likely reflects a structural control, which led
to two different kinds of slopes that affected the type of aprons present.
• Eustatic control on sedimentation appears to be pronounced during the Popovich
and Rodeo Creek Formation times based on comparison of sea-level curves for
the northern Carlin trend derived from this study with a published eustatic sea-
level curve for the western United States.
Recommendations
• A follow-on study, utilizing additional stratigraphic control and focusing on
diagenesis and hydrothermal alteration of specific facies types, could help to
better understand the relationship between lateral facies changes and
mineralization.
• A more detailed sequence stratigraphic analysis of the Silurian-Devonian Roberts
Mountains and Silurian-Devonian Bootstrap Limestone Formations should seek to
identify the presence of additional sequence boundaries that may be associated
with other potential gold-hosting apron and slope/basin facies.
• A sequence stratigraphic study of the Rodeo Creek Formation should be
undertaken. The Rodeo Creek Formation likely represents another stratigraphic
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sequence and a change from carbonate-apron facies to a clastic-dominated
submarine-fan facies.
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REFERENCES CITED
Aldridge, R. J., 1987, Conodont Paleobiology – a historical review, in Aldridge, R. J. ed., Paleobiology of Conodonts: British Micropaleontological Society, European Conodont Symposium 4, p. 11-34.
Aldridge, R. J. Briggs, D. E., Clarkson, E. N., and Smith, M. P., 1986, The affinities of
conodonts – new evidence from the Carboniferous of Edinburgh, Scotland: Lethaia, v. 19, p. 279-291.
Aldridge, R. J., and Theron, J. N., 1993, Conodonts with preserved tissue from a new
Upper Ordovician Konservat-Lagerstate: Journal of Micropaleontology, v. 12, p. 113-117.
Allan, J. R., and Mathews, R. K., 1982, Isotope signatures associated with early meteoric
diagenesis: Sedimentology, v. 29, p. 797-817. Arehart, G. B., 1992, Age and fluid chemistry of the sediment-hosted gold deposits in the
Great Basin, Nevada: Unpublished PhD dissertation, University of Michigan, Ann Arbor, Michigan, 163 p.
Arehart, G. B., Foland, K. A, Naeser, C. W., and Kesler, S. E., 1993, 40Ar/39Ar and
fission track geochronology of sediment-hosted disseminated gold deposits at Betze-Post, Carlin trend, northeastern Nevada: Economic Geology, v. 88, p. 622-646.
Armstrong, R. L., 1968, Sevier orogenic belt in Nevada and Utah: Geological Society of
America Bulletin, v. 79, p. 429-458. Armstrong, A. K., Bagby, W. C., Ekburg, C., and Repetski, J., 1987, Petrographic and
scanning electro microscope studies of samples from the Roberts Mountains and Popovich Formations, Carlin Mine area, Eureka County, Nevada: U.S. Geological Survey Bulletin 1684, 23 p.
Armstrong, A. K., Theodore T. G., Kotlyar, B. B., Lauha, E. C., Griffin, G. L., Lorge, D.
L., and Abbott, E. W., 1997, Preliminary facies analysis of Devonian autochthonous rocks that host gold along the Carlin trend, Nevada, in Virkre, P., Thompson, T. B., Bettles, K., Christensen, O., and Parratt, R., eds., Carlin-Type
124
Gold Deposits Field Conference: Society of Economic Geologists, Guidebook Series, v. 28, p. 53-74.
Bakken, B. M., 1990, Gold mineralization, wall rock alteration, and the geochemical
evolution of the hydrothermal systems in the main ore body, Carlin Mine, Nevada: Unpublished PhD Dissertation, Stanford University, Stanford, CA, 200 p.
Bettles, K. H., and Lauha, E. A., 1991, Gold deposits of the Carlin trend, Nevada: World
Gold ’91, Gold Forum on Technology and Practice, Second AusIMM-SME Joint Conference, 21-26 April, Cairns, Australia, p. 251-257.
Brown, L. F., and Fisher, W. L., 1977, Seismic-stratigraphic interpretation of depositional
systems – examples from the Brazilian rift and pull-apart basins, in Payton, C. E., ed., Seismic Stratigraphy – Applications to Hydrocarbon Exploration: American Association of Petroleum Geologists Memoir 26, p. 213-248.
Casier, J. G., and Lethiers, F., and Claeys, P., 1996, Ostracod evidence for an abrupt mass
extinction at the Frasnian/Famennian boundary, Devils Gate, Nevada, USA: Comptes Rendus de l'Academie des Sciences, v. 322, p. 415-422.
Casier, J. G., and Lethiers, F., 1998, The recovery of the ostracod fauna after the late
Devonian mass extinction – the Devils Gate Pass section example, Nevada, USA: Comptes Rendus de l'Academie des Sciences, v. 327, p. 501-507.
Christensen, O. D., 1993, Carlin trend geologic overview – gold deposits of the Carlin
trend, Nevada, in Christensen, O. D., ed., Society of Economic Geologists Field Trip Guidebook, v. 18, p. 12-16.
Christensen, O. D., 1996, Carlin trend geologic overview, in Steven, M. G., and
Struhsacker, E., eds., Geology and Ore Deposits of the American Cordillera: Geological Society of Nevada, Field Trip Guidebook Compendium, p. 147-156.
Christiansen, R. L., and Yeats, R. S., 1992, Post Laramide geology of the U. S.
Cordilleran Region, in Burchfield, B. C., Lipman, P. W., and Zoback, M. L., eds., The Geology of North America – The Cordilleran Orogen: Geological Society of America, v. G-3, p. 261-406.
Clarkson, E. N., 1998, Invertebrate Paleontology and Evolution, Cambridge, Blackwell
Scientific Publications, 452 p.
125
Conigilio, M., and Dix, G. R., 1992, Carbonate slopes, in Walker, R. G., and James, N. P., eds., Facies Models – Response to Sea Level Change: Geological Association of Canada, Geoscience Canada Reprint Series 4, p. 349-373.
Cook, H. E., 1983, Ancient carbonate platform margins, slopes, and basins, in Cook, H.
E., Hine, A. C., and Mullins, H. T., eds., Platform Margins and Deep-Water Carbonates: Society of Economic Paleontologists and Mineralogists Short Course 12, p. 1-189.
Cook, H, E., McDaniel, P. N. Mountjoy, E. W., and Pray, L. C., 1972, Allochthonous carbonate debris flows and Devonian bank (reef) margins, Alberta: Bulletin of Canadian Petroleum Geology, v. 20, p. 439-497.
Cook, H. E., and Egbert, R. M., 1981, Late Cambrian-Early Ordovician continental
margin sedimentation, central Nevada, in Taylor, M. E., ed., 2nd International Symposium on the Cambrian System Proceedings: U.S. Geological Survey, Open-File Report 81-743, p. 50-56.
Cook, H. E., and Mullins, H. T., 1983, Basin Margin Environment, in Scholle, P. A.,
Bebout, D. G., and Moore, C. H., eds., Carbonate Depositional Environments: American Association of Petroleum Geologist Memoir 33, p. 539-617.
Donoghue, P. C., Purnell, M. A., and Aldridge, R. J., 1998, Conodont anatomy, chordate
phylogeny and vertebrate classification: Lethaia, v. 31, p. 211-219. Donoghue, P. C., Forey, P. L., and Aldridge, R. J., 2000, Conodont affinity and chordate
phylogeny: Biological Review, v. 75, p. 191-251. Drews, A. S., Romberger, S. B., and Whitney, C. G., 1996, Clay alteration and gold
deposition in the Genesis and Blue Star deposits, Eureka County, Nevada: Economic Geology, v. 91, p. 1383-1393.
Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture,
in Ham, W. E., ed., Classification of Carbonate Rocks: American Association of Petroleum Geologists Memoir 1, p. 508-537.
Emsbo, P., 1999, Origins of the Meikle high-grade gold deposit from the superposition of
late Devonian Sedex and mid-Tertiary Carlin-type gold mineralization: Unpublished PhD Dissertation, Colorado School of Mines, Golden, CO, 379 p.
Emsbo, P., Hofstra, A., Zimmerman, J. M., and Snee, L., 1996, A mid-Tertiary age
constraint on alteration and mineralization in igneous dikes on the Goldstrike
126
property, Carlin trend, Nevada: Geological Society of Nevada Abstract with Programs.
Enos, P. and Moore, C. H., 1983, Fore-reef slope, in Scholle, P. A., Bebout, D. G., and
Moore, C. H., eds., Carbonate Depositional Environments: American Association of Petroleum Geologists Memoir 33, p. 507-538.
Ettner, D. C., 1989, Stratigraphy and structure of the Devonian autochthonous rocks,
north-central Carlin trend of the southern Tuscarora Mountains, northern Eureka County, Nevada: Unpublished MS Thesis, Idaho State University, Pocatello, Idaho, 177 p.
Evans, D. C, 2000, The physical and mineralogical characteristics, morphology,
distribution, and genesis of carbonate-hosted breccias in the Meikle Mine, Nevada and their relationship with gold mineralization: Unpublished MS Thesis, Colorado School of Mines, Golden, Colorado, 279 p.
Evans, J. G., 1980, Geology of the Rodeo Creek and Welches Canyon quadrangles,
Eureka County, Nevada: U. S. Geological Survey Bulletin 1473, 81 p. Evans, J. G., and Mullens, T. E., 1976, Bootstrap Limestone window, Elko and Eureka
Counties: U.S. Geological Survey Journal of Research, v. 4, p. 119-125. Folk, R. L., 1959, Practical petrographic classification of limestone: American
Association of Petroleum Geologist Bulletin, v. 43, p. 1-38. Frey, R. W., and Pemberton, S. G., 1984, Trace fossil facies model, in Walker, R. G., ed.,
Facies Models: Geological Association of Canada, Geoscience Canada Reprint Series 1, p. 189-207.
Frey, R. W., Pemberton, S. G., and Saunders, T. D., 1990, Ichnofacies and bathymetry –
a passive relationship: Journal of Paleontology, v. 64, p. 155-158. Ginsburg, R. N., and James, N. P., 1974, Holocene carbonate sediments of continental
margins, in Burke, C. A., and Drake, C. L., eds., The Geology of Continental Margins, New York, Springer – Verlag, p. 137-155.
Griffin, G., 1999, The stratigraphy of a “Carlin-type” deposit – who needs it?: Geological
Society of America Cordilleran Section Meeting, Abstracts with Programs, v. 31, p. 50.
127
Griffin, G., 2000, Paleogeography of late Silurian-Devonian autochthonous carbonates – implications for old faults and intrusive distribution, Goldstrike property, Nevada: Geological Society of Nevada Symposium 2000, Abstracts with Programs, p. 52.
Hanford, R., and Loucks, R. G., 1993, Carbonate depositional sequences and systems
tracts – responses of carbonate platforms to relative sea-level changes, in Loucks, R. G., and Sarg, J. F., eds., Carbonate Sequence Stratigraphy – Recent Developments and Applications: American Association of Petroleum Geologists Memoir 57, p. 3-42.
Hardie, B. S., 1966, Carlin Gold Mine, Lynn District, Nevada: Nevada Bureau of Mines
Report 13 part A, p. 73-83. Ilchik, R. P., 1995, 40Ar/39Ar, K/Ar, and fission track geochronology of sediment-hosted
disseminated gold deposits at Post-Betze, Carlin trend, northeastern Nevada: Economic Geology, v. 90, p. 208-210.
Johnson, J. G., and Sandberg, C. A., 1988, Devonian eustatic events in the western
United States and their biostratigraphic responses, in McMillian, N. J., Embry, A. F., and Glass, D. J., eds., Devonian of the World – Proceedings of the Second International Symposium of the Devonian Systems: Canadian Society of Petroleum Geologists Memoir 14, p. 171-178.
Jones, B., and Desrochers, A., 1992, Shallow Platform Carbonates, in Walker, R. G., and
James, N. P., eds., Facies Models – Response to Sea Level Change: Geological Association of Canada, Geoscience Canada Reprint Series 4, p. 277-301
Kendall, C. G., and Schlager, W., 1981, Carbonate and relative changes in sea-level:
Marine Geology, v. 44, p. 181-212. Kenter, J. A., and Schlager, W., 1989, A comparison of shear strength in calcareous and
siliciclastics sediments: Marine Geology, v. 88, p. 145-152. Kuehn, C. A., 1989, Studies of disseminated gold deposits near Carlin, Nevada –
evidence for a deep geologic setting of ore formation: Unpublished PhD Dissertation, Pennsylvania State University, University Park, PA, 395 p.
Leonardson, R. W., and Rahn, J. E. 1995, Geology of the Betze-Post gold deposits,
Eureka County, Nevada, in Coyner, A. R., and Fahey, P. L., eds., Geology and Ore Deposits of the American Cordillera: Geological Society of Nevada Symposium Proceedings, v. 1, p. 61-94.
128
Loucks, R. G. and Sarg, J. F., eds., 1993, Carbonate Sequence Stratigraphy – Recent Developments and Applications: American Association of Petroleum Geologists Memoir 57, 545 p.
Loutit, T. S., Hardenbol, J., Vail, P. R., and Baum, G. R., 1988, Condense sections – the
key to age determination and correlation of continental margin sequences, in Wilgus, C. K., Hastings, B. S., Kendall, C. G., Posamentier, H. W., Ross, C. A., and Van Wagoner, J. C., eds., Sea-Level Changes – An Integrated Approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, p. 183-213.
Lowe, D. R., 1976, Subaqueous liquefied and fluidized sediment flows and their deposits:
Sedimentology, v. 23, p. 285-308. McIlreah, I. A., and James, N. P., 1978, Carbonate slopes, in Walker, R. G., ed., Facies
Models: Geological Association of Canada, Geoscience Canada Reprint Series 1, p. 133-143.
Merriam, C. W., 1940, Devonian stratigraphy and paleontology of the Roberts Mountains
region, Nevada: Geological Society of America Special Paper 25, 114 p. Miller, E. L., Miller, M. M., Stevens, C. H., Wright, J. E., and Madrid R. J., 1992, Later
Paleozoic paleogeographic and tectonic evolution of the western U.S. Cordillera, in Burchfield, B. C., Lipman, P. W., and Zoback, M. L., eds., The Geology of North America – The Cordilleran Orogen: Geological Society of America, v. G-3, p. 57-106.
Mitchum, R. M., Vail, P. R., and Thompson, S., 1977, Seismic stratigraphy and global
changes of sea-level – the depositional sequence as a basic unit for stratigraphic analysis, in Payton, C. E., ed., Seismic Stratigraphy – Applications to Hydrocarbon Exploration: American Association of Petroleum Geologists Memoir 26, p. 53-62.
Mullens, T. E., 1980, Stratigraphy, petrology, and some fossil data of the Roberts
Mountains Formation, north-central Nevada: U.S. Geological Survey Professional Paper 1063, 67 p.
Mullins, H. T., 1983, Base-of-slope carbonate aprons – an alternative to the submarine
fan model: American Association of Petroleum Geologist Bulletin, v. 67, p. 521.
129
Mullins, H. T., and Neumann, A. C., 1979, Deep carbonate bank margin structure and sedimentation in the northern Bahamas, in Doyle, L. J., and Pilkey, O. H., eds., Geology of Continental Slopes: Society of Economic Paleontologists and Mineralogists Special Publication 27, p. 165-192.
Mullins, H. T., Heath, K. C., Van Buren, H. M., and Newton, C. R., 1984, Anatomy of a
modern open-ocean carbonate slope – northern Little Bahamas Bank: Sedimentology, v. 31, p. 141-168.
Mullins, H. T., and Cook, H. E., 1986, Carbonate apron models – alternatives to the
submarine fan model for paleoenvironmental analysis and hydrocarbon exploration: Sedimentary Geology, v. 48, p. 37-79.
Murphy, M. A., and Berry, W. B., 1975, Silurian and Devonian graptolites of central
Nevada: California University Publications in Geological Sciences, v. 110, 109 p. Mutti, E., and Ricci Lucchi, F., 1978, Turbidities of northern Appennines – Introduction
to facies analysis: International Geological Review, v. 20, p. 987-1035. Nelson, C. H., Carlson, P. R., Byrne, J. V., and Alpha, T. R., 1970, Development of the
Astroia Canyon-fan physiography and comparison with similar systems: Marine Geology, v. 8, p. 259-291.
Neuman, A. C., and Land, L. S., 1975, Lime mud deposition and calcareous algae in the
Bight of Abaco, Bahamas – A budget: Journal of Sedimentary Petrology, v. 45, p. 763-786.
Normark, W. R., 1978, Fan valleys, channels, and depositional lobes on modern
submarine fans – characters for recognition of sandy turbidite environments: American Association of Petroleum Geologist Bulletin, v. 62, p. 912-931.
Pemberton, S. G., MacEachern, J. A., and Frey, R. W., 1992, Trace fossil facies models –
environmental and allostratigraphic significance, in Walker, R. G., and James, N. P., eds., Facies Models – Response to Sea Level Change: Geological Association of Canada, Geoscience Canada Reprint Series 4, p. 47-42.
Poole, F. G., Sandberg, C. A., and Boucot, A. J., 1977, Silurian and Devonian
paleogeography of the western United States, in Stewart J. H., Stevens, C. H., and Fritsche, A. E., eds., Paleozoic Paleography of the Western United States: Society of Economic Paleontologists and Mineralogists, Pacific Coast Paleogeography Symposium 1, p. 39-65.
130
Radtke, A. S., 1985, Geology of the Carlin gold deposit, Nevada: U.S. Geological Society Professional Paper 1267, 124 p.
Read, J. F., 1982, Carbonate platforms of passive (extensional) continental margins –
types, characteristics, and evolution: Tectonophysics, v. 81, p. 195-212. Read, J. F., 1985, Carbonate platform facies models: American Association of Petroleum
Geologists Bulletin, v. 66, p. 860-878. Roberts, R. J., 1960, Alignment of mining districts in north-central Nevada: U. S.
Geological Survey Research 1960 Professional Paper 400-B, p. B17-B19. Roberts, R. J., Hotz, P. E., Gilluly, J., and Ferguson, H. G., 1958, Paleozoic rocks of
north-central Nevada: American Association of Petroleum Geologist Bulletin, v. 42, p. 2813-2857.
Roberts, R. J., Montgomery, K. M., and Lehner, R. E., 1967, Geology and mineral
resources of Eureka County, Nevada: Nevada Bureau of Mines and Geology Bulletin 64, 152 p.
Ruiz-Ortiz, P. A., 1983, A carbonate submarine fan in a fault controlled basin of the
Upper Jurassic, Beltic Cordillera, southern Spain: Sedimentology, v. 30, p. 33-48. Sarg, J. F., 1988, Carbonate sequence stratigraphy, in Wilgus, C. K., Hasting, B. S.,
Kendall, C. G., Posamentier, H. W., Ross, C. A., and Van Wagoner, J. C., eds., Sea Level Changes – An Integrated Approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, p. 155-181.
Schlager, W., 1991, Depositional bias and environmental change – important factors in
sequence stratigraphy: Sedimentary Geology, v. 70, p. 109-130. Sheehan, P. M., Pandolfi, J. M., and Ketner, K. B., 1993, Isolated carbonate bodies
composed of stacked debris-flow deposits on a fine-grained carbonate lower slope of Devonian age, Antelope Peak, Elko County, Nevada: U.S. Geological Survey Bulletin 1988-E, p. E1-E12.
Smith, J. F. Jr., and Ketner, K. B., 1975, Stratigraphy of Paleozoic rocks in the Carlin –
Pinon Range area, Nevada: U. S. Geological Survey Professional Paper 867-A, p. A1-A87.
131
Springer, K. B., and Murphy, M. A., 1994, Punctuated stasis and collateral evolution in the Devonian lineage of Monograptus hercynicus: Lethaia, v. 27, p. 119-128.
Stewart, J. H., 1980, Geology of Nevada: Nevada Bureau of Mines and Geology, Special
Publication 4, 136 p. Stewart, J. H., and Poole, F. G., 1974, Lower Paleozoic and uppermost Precambrian
Cordilleran miogeocline, Great Basin, western United States, in Dickerson, W. R., ed., Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists, Special Publication 22, p. 28-57.
Teal, L., and Jackson, M., 1997, Geologic overview of the Carlin trend gold deposits and
description of recent deep discoveries, in Virkre, P., Thompson, T. B., Bettles, K., Christensen, O., and Parratt, R., eds., Carlin-Type Gold Deposits Field Conference: Society of Economic Geologists, Guidebook Series, v. 28, p. 3-38.
Thorman, C. H., Ketner, K. B., and Peterson, F. B., 1992, The middle to late Jurassic
Elko orogeny in eastern Nevada and western Utah: Geological Society of America, Abstracts with Programs, v. 24, p. 66.
Tucker, M. E. and Wright, V. P., 1990, Carbonate Sedimentology, Oxford, Blackwell
Scientific Publications, 482 p. Vail, P. R., Mitchum, R. M., Todd, R. G., Widmier, J. M., Thompson, S., Sangree, J. B.,
Bubb, J. N., and Hatlelid, W. G., 1977, Seismic stratigraphy and global changes of sea level, in Payton, C. E., ed., Seismic Stratigraphy – Applications to Hydrocarbon Exploration: American Association of Petroleum Geologists Memoir 26, p. 49-212.
Van Wagoner, J. C., 1995, Overview of sequence stratigraphy of foreland basin deposits
– terminology, summary of papers, and glossary of sequence stratigraphy, in Van Wagoner, J. C., Bertram, G. T., eds., Sequence Stratigraphy of Foreland Basin Deposits – Outcrop and Subsurface Examples from the Cretaceous of North America: American Association of Petroleum Geologists Memoir 64, p. ix-xxi.
Van Wagoner, J. C., Posamentier, H. W., and Mitchum, R. M., 1988, An overview of the
fundamentals of sequence stratigraphy and key definitions, in Wilgus, C. K., Hastings, B. S., Kendall, C. G., Posamentier, C. K., Ross., C. A., and Van Wagoner, J. C., eds., Sea Level Changes: An Integrated Approach: Society of Economic Paleontologists and Mineralogists Special Publication 42, p. 39-45.
132
Varnes, D. J., 1978, Slope movement types and processes, in Schuster, R. L., and Krizek, R. J., eds., Landslides – Analysis and Control: Transportation Research Board, National Academy Science Special Report 176, p. 11-33.
Volk, J. A., and Zimmerman, J. M., 1991, Stratigraphic framework of Ordovician-
Devonian rocks at the Goldstrike Mine are, Eureka and Elko Counties, Nevada – Roberts Mountains thrust revisited: Geological Society of America Abstract with Programs, v. 23, p. 106.
Volk, J. A., Lauha, E., Leonardson, R. W., and Rahn, J. E., 1996, Structural geology of
the Betze-Post and Meikle deposits, Elko and Eureka Counties, Nevada, in Green, M., Stuhsacker, E., eds., Geology and Ore Deposits of the American Cordillera: Geological Society of Nevada, Field Trip Guidebook Compendium, p. 180-194.
Walker, R. G., 1978, Deep-water sandstone facies and ancient submarine fans – models
for exploration for stratigraphic traps: American Association of Petroleum Geologists Bulletin, v. 62, p. 932-966.
Wendt, J., 1988, Condensed carbonate sedimentation in the late Devonian of the eastern Anti-Atlas (Morocco): Eclogae Geologica Helvetica, v. 81, p. 155-173.
Wilson, J. L., 1975, Carbonate facies in geologic history, Berlin, Springer – Verlag, 471
p. Wright, V. P., and Wilson, R. C., 1984, A carbonate submarine fan sequence from the
Jurassic of Portugal: Journal of Sedimentary Petrology, v. 54, p. 394-412. Wright, V. P., and Burchette, T. P., 1996, Shallow-water carbonate environments, in
Reading, H. G., ed., Sedimentary Environments – Processes, Facies, and Stratigraphy, Oxford, Blackwell Scientific Publications, p. 325-394.
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APPENDIX A – BASE MAP
Appendix A is a base map consisting of all the core (red), core photograph
(black), and conodont (blue) sample locations and core labels. Both BGMI mine (red)
and longitude/latitude (green) coordinates are included on the map. The BGMI mine
coordinates for each sample are listed in the data spread sheet in Appendix B.
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APPENDIX B – DATA
Appendix B is a spread sheet of all the data utilized in this study for the
construction of isopachs, cross sections, and distribution maps. The spread sheet includes
the sample number; BGMI mine coordinates; and Bootstrap Limestone, Roberts
Mountains, and Popovich facies picks.
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APPENDIX C – CORE LOGS
Appendix C is a representative example of both core and core photograph logs
constructed for this study. The logs include detailed descriptions of the carbonate facies
and depositional environments.
Key:
RC Rodeo Creek
UM Popovich Micritic member
SSD Popovich Soft-Sediment-Deformation member
PL Popovich Planar member
WS Popovich Wispy member
WS/DF Roberts Mountains Apron Wispy/Debris Flow unit
LL/WS/DF Roberts Mountains Apron Laminated Micritic Limestone/
Wispy/ Debris Flow unit
LL/DF Roberts Mountains Apron Laminated Mictritic Limestone/
Debris Flow unit
LL Roberts Mountains Laminated Micritic Limestone member
Boot Bootstrap Limestone Formation
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APPENDIX D – CONODONT REPORTS
Appendix D consists of a spread sheet reporting all the barren and conodont-
bearing samples analyzed from this study and additional dates provided by Dr. Anita
Harris from previously analyzed samples conducted for others in the same general
location. Also included is a detailed chronostratigraphic chart for the Bootstrap
Limestone, Roberts Mountains, Popovich, and Rodeo Creek Formations constructed for
this study.
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APPENDIX E – STABLE ISOTOPE DATA
Appendix E consists of a spread sheet reporting the δ13C and δ18O isotope data
(reported in PDB) for RU8 and DC9501 and two corresponding excel plots for DC9501.
The excel plots for RU8 are included in the text (Figures 48 and 50).
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