Lower Paleozoic

AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA

REGIONAL PALEOGEOGRAPHY

LOWER PALEOZOIC

Moving from east to west, three tectono-stratigraphic or paleogeographic elements controlled and maintained sedimentation essentially uninterrupted from the Lower Cambrian through the Lower Devonian. These are 1) an eastern limestone and dolomite assemblage deposited on a broad shallow carbonate platform under shallow-subtidal to intertidal and supratidal conditions; 2) a narrow transitional limestone-clastic suite deposited west or oceanward of the shelf margin under shallow to deep subtidal, basinal, and basin-slope conditions; 3) a coeval basinal bathyal to locally neritic subduction related carbonate and volcaniclastic suite, also west or oceanward of the limestone-clastic suite with a cryptic original boundary complicated by the Roberts Mountains thrust zone (Matti and others, 1974; Poole and others, 1977).

Depositional belts were irregular and gradational, and fluctuated in position over a few tens of miles from the Cambrian through the Late Devonian. The nearly 50 mile wide transitional zone was centered at the approximate present latitude of the Toquima Range during the Cambrian and Ordovician, and at the Roberts Mountains during the Silurian and Devonian (Johnson and Potter, 1975; Smith and Ketner, 1975). The original sites of deposition for the now allochthonous and para-autochthonous sequences of the transitional and western assemblage cannot be exactly established, making the relative position between time-equivalent units problematic.

CAMBRIAN - LOWER DEVONIAN

Eastern Platform

Overlying the terrigineous-detrital Upper Precambrian rocks is a thick sequence of Cambrian and Ordovician sediments which have been included with the Lower Paleozoic Eastern Carbonate Assemblage.

Along the broad shallow water carbonate platform, Middle and Upper Cambrian carbonates and minor amounts of shale, chert, and intraformational conglomerates represent a complex of environments form peritidal to tidal, lagoonal, carbonate bank, and shallow and deep sublittoral with local basinal settings (Taylor and Cook, 1976; Stewart, 1980). These Cambrian units roughly correspond with the the Cambrian shallow-water middle carbonate belt of other workers (Palmer, 1971; Stewart and Suczek, 1977).

The oldest exposed Cambrian units, the Gold Hill Formation and Prospect Mountain Quartzite, contain archeocyathids, Girvanella algae and trilobites, worm burrows, and various sedimentary structures such as ripples, planar and festoon cross-bedding, and mud cracks. The Gold Hill and Prospect Mountain represent shallow nearshore strandline and shelf sediments. The Prospect Mountain was at least locally emergent, and may in part contain fluvial lithologies as suggested by Clark and others (1985). The Girvanella and trilobite-bearing Pioche Shale is commonly the overlying unit suggesting relatively shallow and quiet water conditions along the broad platform.

Middle and Upper Cambrian units overlying the Pioche are unnecessarily complicated by local geographic names given to various lithologies which are in large part lateral facies equivalents of one another. These units can be described within four general time slices.

The lower Middle Cambrian interval is represented by the Eldorado Dolomite and overlying Geddes Limestone, which are roughly equivalent to the Pole Canyon Limestone, and to the Lyndon Limestone and overlying Chisholm Shale, and the lower Highland Peak Formation. The oolitic, Girvanella-bearing limestones of the Eldorado Dolomite and laminated carbonaceous Geddes Limestone represent shallow, relatively quiet, and perhaps locally restricted shelf sedimentation. The oolitic limestones of the Pole Canyon intertongue with the generally shallower water Pioche Shale with Amphipora, Girvanella algae, and trilobites also suggesting shallow shelf and perhaps bank-type deposition. The Lyndon Limestone, and Girvanella, trilobite, and brachiopod- rich Chisholm Shale represent shallow and perhaps locally restricted shelf deposition.

The upper Middle Cambrian interval is represented by the Secret Canyon Shale and the roughly equivalent Patterson Pass Shale, the upper portion of the Highland Peak Formation, and the Lincoln Peak Formation. The Secret Canyon Shale represents quiet water, outer shelf sedimentation much like the inner shelf, relatively quiet, local basinal or restricted shelf sediments of the Patterson Pass Shale. The Highland Peak is composed of Girvanella, brachiopod, and trilobite bearing, cross-bedded carbonates, calcareous siltstones and sandstones, and intraformational conglomerates representing lagoonal to local strandline environments. The Lincoln Peak Formation appears to shallow upwards from shales and shaley limestones to shallower water, oolitic, shelf limestones with Girvanella, worm burrows, and intraformational conglomerates.

The lower Upper Cambrian interval is represented by the Hamburg Dolomite and overlying Dunderberg Shale which are roughly equivalent with the Johns Wash Limestone and Emigrant Springs Formation and overlying Corset Spring Shale, and the lower portion of the Mendha Formation. The Hamburg Dolomite is oolitic, contains trilobites and intraformational conglomerate beds, and is locally cross-beded suggesting shallow shelf deposition in wave agitated waters. The overlying Dunderberg Shale contains thinly laminated shales and interbedded oolitic, brachiopod and trilobite-bearing limestone beds perhaps suggesting a somewhat deeper and quieter shelf position than the underlying Hamburg. The bioclastic, oolitic, pelmatazoan-bearing carbonates of the Johns Wash Limestone suggest subtidal shelf deposition. The Emigrant Springs Formation contains echinoid and crinoid debris suggesting shallow shelf deposition. The overlying Corset Springs is very similar lithologically to the Dunderberg and may also suggest a somewhat deeper and quieter environment than the underlying Emmigrant Springs Formation. The lower portion of the Mendha Formation with abundant algal balls, gastropods, brachiopods, trilobites and intraformational limestone conglomerate suggests shallow restricted shelf deposition.

The upper Upper Cambrian interval is represented by the Windfall Formation and the roughly equivalent Whipple Cave Formation, upper portion of the Mendha Formation, and the Notch Peak Limestone. The algal and trilobite bearing, thin-bedded and cherty carbonates of the Windfall Formation represent shallow and probably subtidal and intertidal shelf sedimentation. The siltstones and quartzites within the Windfall may represent beach and barrier sediments. Similar environments are suggested for the Notch Peak which is very similar lithologically to the Windfall Formation. The abundance of algal stromatolites and flat pebble conglomerates suggests a shoal-water depositional setting for the Whipple Cave Formation (Cook and Taylor, 1975). The upper portion of the Mendha Formation, like the lower portion, represents shallow restricted shelf deposition.

Sedimentation continued essentially uninterrupted along the carbonate platform into the Ordovician. Water depths probably ranged form 50 to perhaps 250 feet across the entire area. The Lower Ordovician Pogonip Group represents shallow-water carbonate bank and lagoonal muds and marls. The Antelope Valley, Ninemile and Goodwin Formations represent subtidal, intertidal and supratidal deposition of carbonate, shale and sandstone and siltstone along the shelf and as far west as the upper portion of the slope. The Goodwin shows upward progradation and shallowing in several sections (Ross, 1977). The Ninemile appears to have been deposited in quiet embayed portions of the Lower Ordovician shelf. Shallow water brachiopod and trilobite-rich carbonate mud mounds were also deposited in the Coal Mine and Narrows Quadrangles in Elko County during Pogonip time (Ross, 1977). Regional uplift and warping took place in the Ordovician at the close of Pogonip time, and probably during the deposition of the overlying Eureka Quartzite (Merriam, 1963) which is missing locally as in the Ruby Mountains (Willden and Kistler, 1967).

The Pogonip is unconformably overlain by the Middle Ordovician Copenhagen Formation along the western margin of the platform in what are now the Antelope, Monitor, Toiyabe, northern Toquima, and Hot Creek Ranges. The Copenhagen Formation contains coral, gastropod, crinoid, brachiopod and bryozoan bearing carbonates, as well as cross-laminated siltstones and sandstones suggesting near shore subtital and intertidal shelf conditions. Across the rest of the broad platform, the Cambrian carbonates are unconformably overlapped by the Middle Ordovician (post-Whiterockian) Eureka Quartzite. Fine-grained, extremely pure and texturally mature quartz sands of the Eureka spread across and smothered most of the carbonate platform as near shore, intertidal and beach sands to more open marine sand sheets. The Copenhagen Formation from Antelope Valley to the Toquima Range essentially represents the only sand-free embayment on the shelf during Eureka time (Ross, 1977). The sources of Eureka sand, perhaps to the northeast of the study area (Ketner, 1966) were submerged in the Upper Ordovician (Cincinnatian) and vast carbonate mud flats were again created.

In the Eureka area, the Cambrian Secret Canyon Shale (Shwin Formation equivalent), Hamburg Dolomite, Dunderberg Shale, Windfall Formation, and the Ordovician Pogonip Group lie beneath the Eureka Quartzite. To the west in the Shoshone Range, no pre-Eureka rocks are present so that more than 3,000 feet of Late Cambrian and Early Ordovician strata have been removed along an unconformity (Gilluly and Gates, 1965). The Eureka Quartzite rests upon progressively older rocks northwestward from the Eureka area (see discussion of the Eureka Quartzite) with the base of the Eureka representing a regional unconformity. The relative amounts of non-deposition as a result of pre-Eureka relief, versus actual erosional removal is difficult to determine. The unconformity surface itself does not appear to be notably angular.

The Eureka locally grades upward, and is regionally overlain unconformably by the carbonate muds and sands of the coral and brachiopod-rich Hanson Creek Formation. The thin to thick-bedded, silty, fossiliferous limestones of the Hanson Creek Formation represent units deposited on high energy shoals, in open marine waters, in low energy subtidal shelf lagoons, and within peritidal mudflats along the outer shelf (Dunham, 1977). The facies equivalent dark dolomites of the Ely Springs and Fish Haven to the east in northeastern Nevada and western Utah, were deposited in restricted subtidal to intertidal, lagoonal and shoal environments, probabbly in water depths of 100 feet or less (Ross, 1977).

A largely subaerial paleosurface was developed during a marine regression during the latest Ordovician and earliest Silurian. This event exposed different portions of the outer shelf at differing times, and developed a complex surface with local topographic depressions that became the outer shelf during an Early Silurian transgression (Matti and McKee, 1977).

The western edge of the platform margin was localized a few miles east of the lattitude of Copenhagen Canyon in the Monitor Range throughout the Silurian and Early Devonian (Matti and others, 1975). Overlying the Upper Ordovician Hanson Creek - Ely Springs - Fish Haven sequence are Middle to late Silurian and Lower Devonian dolomites of the Laketown Dolomite, and western equivalent Lone Mountain Dolomite which interfinger with formations within the lower portion of the Nevada Group. These units accumulated in supratidal, intertidal and shallow subtidal environments time equivalent to transitional outer shelf and upper slope environments to the west (Matti and others, 1975). Both the reefal outer platform Lone Mountain and inner platform Laketown Dolomite show evidence of shallowing upward deposition (Johnson and Murphy, 1984; McGoveney, 1977).

Above the Laketown and Lone Mountain are the Early Devonian Sevy and equivalent Beacon Peak Dolomites and the overlying Middle Devonian Simonson Dolomite and roughly equivalent Lower to Middle Devonian Oxyoke Canyon and Sadler Ranch Formations. The Sevy carbonates were deposited on a broad sabkha. Quartz sands in the upper portion of the formation were deposited by streams, or were blown across the periodically emergent mudflats by winds (Osmond, 1962; Kendall, 1975). To the west, the Oxyoke Canyon Formation represents a beach and barrier bar sequence which separates the roughly time equivalent intertidal to supratidal Sevy Dolomite from the quiet subtidal carbonates of the Sadler Ranch Formation (Kendall, 1975). The overlying Simonson was deposited in subtidal to intertidal, and local supratidal environments along the shelf.

The Middle and Upper Devonian Guilmette Formation and western equivalent Devils Gate Limestone represent lagoonal and local stromatoporoid bank-type carbonate buildups. Limestones of the Devils Gate interfinger with the dominantly dolomite facies of the Nevada Group in the Carlin-Pinon Range. Smith and Ketner (1975) have shown the distribution of the boundary between dolomite and sandstone to the east, and limestone on the west, in the Nevada Group along the western margin of the Carlin-Pinon Range.

Devonian uplift is suggested by local unconformities in the Simonson Dolomite, Guilmette Formation, and the overlying Pilot Formation within southern Lincoln County (Tschanz and Pampeyan, 1970), as well as abundant sandstone in various Devonian carbonates including the top of the Sevy Formation (Osmond, 1954), and the top of the Guilmette Formation (Osmond, 1954; Tschanz and Pampeyan, 1970). Anomolously thin Sevy and Simonson have been attributed to a local paleogeographic high in the Snake Range area by Osmond (1954). Most, if not all of this thinning, however, appears to be related to tectonic elimination along low-angle normal faults.

The Middle and Upper Devonian sequence in the northern Antelope Range is quite unique and may represent a local basin. This sequence includes the Late Devonian Davis Spring, Middle to Late Devonian Fenstermaker Wash, and Late Devonian Davis Spring Formations. The Fenstermaker Wash Formation is an upward shallowing sequence of local basinal debris flow units and warm intertidal and subtidal units while the laminated carbonates and cherts of the Davis Spring appear to represent relatively quiet basinal shelf to outer slope conditions. Devonian units farther to the west have been erosionally removed in the Toquima and Toiyabe Ranges within northern Nye County (Kleinhampl and Ziony, 1985).

Transitional Outer Shelf and Inner Slope

Cambrian units included in the Lower Paleozoic Transitional Assemblage have been generally assigned to the western deeper-water outer detrital belt by other workers (Palmer, 1971; Stewart and Suczek, 1977). The details of lithology and therefore depositional setting are generally poorly understood for these Cambrian units. In general, they represent a somewhat limited upper slope and outer shelf sequence which received fine-grained siliciclastics, carbonates and volcanics.

The oldest transitional unit exposed along the western margin of the study area is the Middle Cambrian Schwin Formation, which has been correlated with the Secret Canyon Shale along the platform (Gilluly and Gates, 1965). The Shwin is composed of a lower unit of interlayered, metamorphosed diabasic volcanics, laminated and calcareous shale and mudstone, and an upper unit of shaly limestone and shale-pebble conglomerate. It appears to represent an upward shallowing slope to outer shelf sequence.

The Middle to early Late Cambrian (pre-latest Dresbachian) Swarbrick Formation forms the base of the Paleozoic section in the Hot Creek Range. The presence of sponge-spicule cherts, laminated shale, and laminated silty limestone suggests a quiet and relatively deep, probably anoxic outer slope environment.

The basal unit in the Toiyabe and southern Shoshone Ranges is the Middle and Late Cambrian Crane Canyon Sequence (Means, 1962). The Crane Canyon is equivalent in age to the Dunderberg-Windfall platform interval to the east in the the Eureka area. The laminated shales, cherts, and limestone of the Crane Canyon were deposited below wave base in quiet water environments along the outer shelf and upper slope.

In Elko County, the Lower Cambrian Edgemont and overlying Upper Cambrian Porter Peak Formations are both moderately contact metamorphosed along Mesozoic and Cenozoic intrusives making sedimentalogic reconstruction difficult. The unfossiliferous Edgemont appears to represent moderate water deposition along the outer shelf to upper slope. The overlying coralline and algal Porter Peak with its intraformational conglomerates appears to represent nearshore shallow warm water shelf deposition (Decker, 1962). The overlying Cambrian to Lower Ordovician Tennessee Mountain Formation exposed in northern Elko County, also appears to represent shallow shelf deposition, given the presence of algal stromatolites in the formation. Both the Porter Peak and Tennessee Mountain are essentially "transitional" between the platform and true transitional assemblage. The Ordovician Aura Formation represents a slope equivalent of the Eureka Quartzite and is composed of metamorphosed shale, chert, and limestone and vitreous quartzite (Decker, 1962).

The Late Ordovician to Early Silurian(?) Diana Formation exposed in the northern Toquima Range contains corals and brachiopods. It appears to represent shallow marine shelf deposition and is here included with the outer shlef and inner slope facies.

The outer Silurian - Devonian shelf was somewhat irregular bathymetrically, containing local silled basins, as well as a major western basin sill, the Toiyabe Ridge, which formed the outer margin (Matti and McKee, 1977). Water depths were probably between 300 and 800 feet along the outer shelf and inner slope (Matti and McKee, 1977; Mullens, 1980).

The Upper Silurian Gatecliff Formation and overlying Upper Devonian to Middle Silurian Roberts Mountains Formation or Masket Shale, Lower Devonian McMonnigal and Windmill Limestones, and Lower Devonian Rabbit Hill Limestone, are low-energy basin and basin-slope facies of time-equivalent platform and platform margin facies represented by the Lone Mountain Dolomite and the burrowed subtidal lagoonal McColley Canyon Formation (Johnson and Murphy, 1969; Matti and others, 1974). Local uplift and emergence along the outer shelf is recorded by the local (pre-Oriskany) disconformity at the close of the Silurian present at the base of the Rabbit Hill Limestone within the Monitor Range.

Bioclastic, graded carbonates within the Roberts Mountains, Windmill and Rabbit Hill Formations were deposited by turbidity current and/or grain flow and debris flow mechanisms. Calcareous organic material was primarily derived from the platform carbonate buildups within the Lone Mountain Formation to the east and to the west from the Upper Devonian to Middle Silurian Tor Limestone (Matti and others, 1975). The Tor Limestone represents a linear north-south-trending carbonate buildup (the Toiyabe Ridge) which supplied turbidite sheets to the McMonnigal Limestone, and Masket Shale or Roberts Mountains Formation. Laminated, organic-rich mudstones within the Roberts Mountains, McMonningal, Windmill and Rabbit Hill represent quiet water, anoxic, basinal conditions rather than an algal or tidal origin. This is suggested by the absence of laminated dolomite or evaporites, vertical burrows, scour and fill or ripple cross-laminae, or an indigenous shelly fauna in these rocks (Matti and others, 1975).

Mullens (1980) suggests that arcs in the facies trends and distribution of the Roberts Mountains Formation are the result of the influence of the Cortez-Uinta Arch which was active both during the Devonian-Silurian and again in the Pennsylvanian and Permian. The influence of an irregular margin which was structurally dismembered by the Antler orogeny also plays a large role in the offset or bent facies trend of the Roberts Mountains Formation.

The upper portion of the outer shelf facies is represented by the lower Middle Devonian Denay Limestone and the Upper Devonian Wenban Limestone. The lower portion of the Denay Formation probably represents quiet water outer shelf or inner slope deposition, the middle portion represents debris flows and the upper portion appears to represent shallow slope deposition similar to the Roberts Mountains Formation (Potter, 1976). The richly fossiliferous Wenban also represents debris or turbidity flow carbonates and laminated mudrock facies suggestive of quiet anoxic basinal settings along the outer shelf or inner slope (Matti and McKee, 1977).

Western Subduction Zone

Initial deposition of western assemblage rocks within the island arc setting is represented by the Cambrian(?)-Ordovician Palmetto Formation and Ordovician Valmy and Vinini Formations. The details of sedimentation are poorly known for the several thousand feet of now allochthonous western assemblage rocks. Current thinking suggests that the western assemblage rocks represent a subduction-related depositional suite which was exotic with respect to the passive margin of North America. This siliciclastic sequence of rocks was probably deposited in both shallow and relatively deep environments in various portions of the basin.

The oldest of these western assemblage rocks, the Cambrian(?)- Middle Ordovician Palmetto and Middle Ordovician Valmy Formations are dominantly composed of moderately to poorly sorted, well rounded, fine to coarse-grained sandstones which show both sole markings at the base of some beds, and abundant current-type cross-bedding, and bedded cherts and greenstones. The Valmy and Palmetto contain very few fossils, suggesting that depositional conditions were generally inhospitable for organisms. The pillowed Valmy greenstones, however, locally contain trilobite, gastropod, and shelly faunas (Ross, 1958). The Valmy grades laterally eastward into the finer-grained siliceous mudstones, cherts and limestones of the Middle and Upper Ordovician Vinini Formation which contains much less quartzite and greenstone than the Valmy (Stewart and Poole, 1974). Siltstones, limestones, and sandstones in the Vinini are commonly horizontally laminated, contain climbing ripple cross-lamination and are current cross-bedded. Sarniak (1979) suggests that much of the Vinini is composed of repetitive, upward coarsening sequences of shale, climbing rippled and current cross-bedded or laminated siltstones, and cherts that are overlain by orthoquartzite beds. Lensoidal Vinini limestones are locally brachiopod, trilobite and gastropod coquinas.

Stanley and others (1977) suggest that the Valmy represents a deep outer slope bathyal to abyssal-plain sequence with the Vinini and Palmetto Formations being upper continental slope units. Sarniak (1979) suggests the Valmy represents distal delta front subduction complex sediments, with the eastern facies Vinini representing delta front and local delta plain sediments.

It seems clear from the foregoing discussion that the paleontologic, and most of the sedimentalogical evidence, does not support bathyal-abyssal deposition for much of the Valmy, Palmetto and Vinini Formations (Ross, 1961; Gilluly and Gates, 1965; Smith and Ketner, 1975). Worm tracks and trails, small-scale cross-bedding and lamination abundant in the quartzites indicate relatively shallow water sandstone deposition. The high maturity and sphericity of these quartzites also suggests high-energy shelf conditions. Lensoidal limestones may represent shallow water bank deposits. Certainly, finely laminated mudstones, shales and cherts were deposited in relatively deep and anoxic settings. The presence of grading and local sole markings at the base of some sandstone beds may suggest that although originally deposited under relatively shallow marine conditions, portions of both the Vinini and Valmy may have been redeposited in deeper environents through turbidity currents. Bathymetry probably fluctuated from dominantly shallow to local moderately deep marine conditions.

Silurian and Devonian age western assemblage rocks are locally interlayered with rocks mapped as the Vinini and Valmy Formations. Lithologically these rocks are identical to the Ordovician units and can only be differentiated by very sparse graptolite faunal control.

The Lower Silurian (Wenlockian to Ludlovian) Noh Formation has been mapped in the northern portion of the area within the HD Range and Windermere Hills (Riva, 1962, 1970; Oversby, 1972). The Noh is dominantly composed of fine grained, calcareous and tuffaceous, quartz siltite and arenite, laminated chert and chert pebble conglomerate. Jordan (1981) felt that the presence of small-scale crossbeds, grading, flow and load casts, and convolute bedding in the Noh sediments indicated deposition by turbidity currents within a submarine fan system. The Early Silurian Fourmile Canyon Formation and Elder Sandstone in the Shoshone and Cortez Ranges represent a structurally dismembered sequence that is a time equivalent of the Roberts Mountains Formation (Gilluly and Gates, 1965; Gilluly and Masursky, 1965). These units are composed of interbedded chert, tuffaceous graptolitic shale, and cross-bedded and graded feldspathic and tuffaceous sandstones and siltstones which probably represent turbidites.

Devonian western assemblage units include the middle to early Late Devonian Woodruff Formation and the Middle to Late Devonian Slaven Chert. Details of sedimentation and bathymetry are poorly understood for these units as well. The presence of arthropods and crinoids in Woodruff shales limestones, and siltstones, as well as phosphate nodules, and secondary and primary dolomite suggest relatively shallow water deposition. Radiolaria abundant in the thinly laminated Woodruff cherts are suggestive of relatively deep water deposition (Smith and Ketner, 1975). Farther to the west in the Shoshone Range and Cortez Mountains, the Slaven Chert is composed of chert, siltstone, sandstone and limestone. The limestones contain corals, brachiopods and bryozoans suggesting at least initial shallow marine sedimentation. It is certainly possible that these limestones, as well as clastics within the Slaven, were re-deposited in deeper conditions under the influence of turbidity currents.