AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
LATE DEVONIAN - PERMIAN
Beginning in the Late Devonian, western assemblage siliceous and volcanic rocks along with portions of the outer shelf and upper slope rocks were thrust eastward against and over slope and platform margin rocks during the inital stages of Antler orogeny. Vertical crustal motion and erosion as a result of the initial plate interactions removed portions of the Lower and Middle Devonian section (Poole and others, 1977).
An elongate and asymmetric, 120 mile wide, foreland basin was created during the initial tectonic loading and downflexing of the continetal shelf by the Roberts Mountains allochthon (Johnson and Pendergast, 1981; Speed and Sleep, 1982). Differing portions of the allochthon arrived along various thrust segments at differing times. Erosion of these segments of the allochthon was also sporadic. This sporadic erosion primarily filled the Antler foreland with a thick Mississippian through Permian, shallow to deep-marine submarine fan and fan delta sequence removed from the relatively steep face of the uplifted and emergent allochthon. We have termed these rocks the Upper Paleozoic Transitional-Detrital Carbonate Assemblage.
The Late Devonian to Early Mississippian (Famennian to Kinderhookian) Pilot Formation essentially represents the first influx of terriginous sediments into the Antler foreland basin east of the locally emergent Roberts Mountains allochthon (Poole and Sandberg, 1977). The Pilot represents the first major interruption of carbonate deposition along the former platform margin since the Middle Ordovician. The details of sedimentation are poorly understood for the Pilot. Poole and Sandberg (1977) suggest that these calcareous and phosphatic siltstones and shales were slowly deposited in a relatively deep starved basin under slightly anoxic conditions. Nolan and others (1956) and Wilson and Laule (1979) have cogently argued that much of the Pilot is of shallow marine to brackish origin. This is in large part based upon the presence of abundant land plant fragments and linguloid brachiopods, and current rippled siltstones. Wilson and Laule (1979) argue that it is improbable that inital sediments within the foreland would be deposited under very deep conditions.
The Pilot Formation is missing in the southern Egan Range southward to the southern Schell Creek Range and westward in the Horse, Quinn Canyon and southern White Pine Ranges. This absence could be the result of nondeposition over a regional Late Devonian high, or post-Pilot and pre-Joana Limestone upwarp with erosion of the Pilot (Wire, 1961; Kellogg, 1963; Hyde and Huttrer, 1970; Hose and Blake, 1976). Other workers (Lumsden, 1964; Kleinhampl and Ziony, 1985) have attributed the absence of the Pilot in these ranges to Tertiary low-angle faulting, which is also prefered by this writer.
Overlying the Pilot Formation is the Mississippian (Osagean to Kinderhookian) Joana Limestone. This shallow subtidal, crinoidal, and coral-rich limestone formed low lying carbonate banks which were locally barriers to clastic deposition within the foreland (Poole and Sandberg, 1977). The roughly time equivalent Kinderhook Tripon Pass Limestone and equivalent Camp Creek Sequence represent southeast-directed limestone turbidites which were also shed into the newly formed Antler foreland basin. The clastic-free Tripon Pass and Camp Creek in the Windermere Hills, Snake Mountians-Pequop Mountains and in the southern Independence Mountains and Peko Hills are temporally equivalent with the Webb Formation in the Carlin-Pinon Range area.
The depositional environment is poorly understood for para-autochthonous and allochthonous Early Mississippian Webb and Antelope Range Formations which are composed of laminated mudstone and chert and minor amounts of siliceous limestone, siltstone and siltstone. Smith and Ketner (1975) suggested that the Webb Formation represents relatively rapid deposition of fine-grained clastic deposition from the first emerging and low lying allochthonous terrain. The lack of fossils suggests hostile bottom conditions, probably below an oxygen minimum zone, or perhaps a very muddy bottom. Hose and others (1982) felt that the Antelope Range Formation represents a deltaic or estuarine depositional environment based upon the presence of land plant fragments and nearshore and possible nonmarine spores.
During the last stages of Webb or Tripon Pass-Camp Creek and Joana Limestone deposition, the oldest beds of the clastic Eleana, Chainman, and Mountain City Formations, and the interfingering and overlying Diamond Peak, Illipah or Scotty Wash Formations were laid down. The metamorphosed quartz siltstones and chert-pebble conglomerates of the Early Mississippian Lee Canyon Argillite appear to be a locally metamorphosed and thrusted portion of the Chainman Formation, rather than a separate lithologic unit as suggested by Smith and Ketner (1975).
These Mississippian to Early Pennsylvanian rocks represent siltstones, mudstones and shales, conglomerates and minor limestones deposited within submarine fan-delta systems which spread across the foreland basin. Paleocurrent indicators suggest a southeast or east-southeast current direction for these sediments (Bloomquist, 1971; Harbaugh and Dickinson, 1981).
Harbaugh and Dickinson (1981) suggest that the Chainman-Diamond Peak depositional sequence represents a lower sequence of basin-slope and submarine-fan turbidites and an upper sequence of fan-delta slope, front, and plain sediments. In general, the sequence shallows upwards from a lower portion of graded, sole marked sediments deposited with subsidence greater than sedimentation in water depths of perhaps up to 1000 meters, and an upper sequence deposited with sedimentation in excess of subsidence in fluvial and marine waters less than 100 meters deep. Foreland basin subsidence during the Early Mississippian has been calculated at approximately 75 to 100 meters per million years (Harbaugh and Dickinson, 1981).
Sadlick (1965) suggested that the lower portions of the Chainman Formation were deposited in water depths of about 60 meters. Wilson and Laule (1979) felt that much of the Chainman was shallow marine in origin, certainly less than 1,000 meters as later proposed by Harbaugh and Dickinson (1981). They point to the abundance of shallow marine fauna such as corals, brachiopods, bryozoa, goniatites and land plant fragments, as well as rippled silts and mudcracked and burrowed muds which locally contain coals. Other geologists will argue that these inital shallow marine and locally estuarine sediments were later carried into deep water environments by turbidites.
The overlying Mississippian and Early Pennsylvanian Diamond Peak Formation represents still coarser clastic depositon of sand to boulder-size material as the source terrain continued to rise and develope considerable relief. These lenticular conglomerates, sands, and shales were deposited as stream fed fan-delta complexes draining a terrain dominated by western assemblage siliceous and volcanic rocks. The local presence of clasts from eastern carbonate rocks such as the Devils Gate Limestone suggests that local islands of Lower Paleozoic Eastern Carbonate Assemblage sediments were also exposed and eroded (Smith and Ketner, 1975). The upper portion of the Diamond Peak represents shallow marine and locally fluvially reworked older turbidites deposited as rippled and cross-bedded lenticular point bars and various meandering stream deposits (Brew, 1971; Sillitonga, 1974; Wilson and Laule, 1979).
These Mississippian units show significant variation in both coarseness and relative proportion of various lithologic constituents across the area. A complex bathymetric gradient and interfingering and overlapping fan delta systems, creates complex facies relationships within the Chainman-Diamond Peak sequence. In general the entire sequence fines to the south and east with increasing distance from the source terrain (Poole and Sandberg, 1977). In northern Nye County for instance, the Mississippian interval is about 40 percent sandstone and conlomerate in the Hot Creek Range, while in the Pancake Range to the east, less than 25 percent of the section is sandstone or conglomerate and these units are confined to the upper third of the interval. Farther to the east in the Grant, Egan, and southern Schell Creek Ranges, perhaps 5 percent of the section, confined to the upper 300 feet, is a coarse clastic sequence; the rest of the section is composed of shale and siltstone (Kleinhampl and Ziony, 1985).
In the northern portion of Elko County, the roughly time equivalent Mississippian Grossman, Banner and Nelson Formations are contact metamorphosed and poorly understood (Coats, 1969, 1985). The Grossman and Nelson appear to represent relatively shallow marine shale, limestone, siltstone and sandstone and conglomerate deposited along the margin of the foreland. The local exposures of the Nelson Formation interfinger with Banner and represent metamorphosed andesitic to basaltic flows and tuff breccias erupted from a Mississippian volcanic field.
During the Early and Middle Pennsylvanian, conditions within the foreland changed with the deposition of the shallow marine Ely Limestone and Van Duzer Limestone to the north in Elko County (Decker, 1962). Along with carbonates, local erosional influx of limestone clasts mixed with clastic detrital debris resulted in the time equivalent Moleen, Tomera, and Quilici Formations and probably the poorly understood Hammond Canyon and Poorman Peak. The chert and quartzite-pebble conglomerate and interbedded bioclastic and silty limestones and calcareous sandstones within these formations were deposited as small fan-delta systems near the Antler highland. In general the sequence shallows upward with a cessation of siliciclastic input during deposition of the bioclastic limestones within the upper portion of the Tomera and Quilici Formations (Dott, 1955; Riva, 1970; Mount, 1972).
The Late Pennsylvanian (Missourian) was marked by a gradual, westward moving marine transgression which continued into the early Permian (Leonardian). Sediments deposited during this time were also shallow marine limestones and fragmental limestone and chert-rich clastics deposited by stream fed fan delta systems. These units include the Upper Pennsylvanian and Lower Permian (Missourian to Wolfcampian) Strathearn and equivalent Sunflower Formation which overly the Diamond Peak Formation along an angular unconformity in Elko County.
In the late Middle Pennsylvanian, a period of uplift, minor deformation, and erosion is marked by a major unconformity. This Middle Pennsylvanian or Sub-Strathearn unconformity is a pronounced angular unconformity which separates Middle Pennsylvanian and older units from Late Pennsylvanian and younger rocks. Rocks below the unconformity are Desmoinesian to Atokan age and above the unconformity are Early Missourian to Wolfcampian (Steele, 1960; Dott, 1955). In some areas bedding attitudes are essentially conformable making recognition of the unconformity difficult (Smith and Ketner, 1975). The unconformity is perhaps best seen in the Carlin Canyon and Tonka station areas in the northern Carlin-Pinon Range, where nearly vertical beds of the Late Mississippian and Early Pennsylvanian Diamond Peak Formation are unconformably overlain by gently dipping beds of the Late Pennsylvanian Strathearn Formation (Dott, 1955).
In the western portion of the area, the Permian Buckskin Mountain, Beacon Flat, and Carlin Canyon Formations of the Carlin Sequence represent Wolfcampian through Guadalupian sediments that overlapped the Antler orogenic belt during a marine transgression. This transgression is first represented by deposition of the Pennsylvanian and Permian Strathearn Formation which is overlain by the Buckskin Mountain Formation. The Buckskin Mountain represents introduction of a large volume of fine-grained clastics, and deposition of shallow marine limestones and is probably a distal facies equivalent of the Carbon Ridge Formation (Fails, 1960). The clastic influx decreased during deposition of the overlying Beacon Flat Formation in the Wolfcampian which represents relatively clear and quiet shallow marine deposition (Fails, 1960). The Leonardian to Guadalupian Carlin Canyon Formation represents rejuvenation of a clastic erosional source, and the deposition of conglomerate, chert, and sandstone with interbedded limestones.
The Lower to Upper Permian conglomerate, sandstone and siltstone, shale and limestone of the Garden Valley and Carbon Ridge Formation represent delta front, delta plain, and bar-finger deposits within a small north-south, shallow marine depocenter (Bissell, 1964). This basin lay across what are now the Sulphur Springs, Diamond and Pancake Ranges.
Much of the region was locally emergent during the Lower Permian. An unconformity exists at the base of the Lower Permian (Wolfcampian) Carbon Ridge and its partial equivalent the Riepe Spring Limestone, and has removed the underlying Upper Pennsylvanian rocks across much of White Pine County and parts of western Eureka County (Hose and Blake, 1976; Merriam, 1963). Although a mild unconformity or arching in White Pine County, it is pronounced in the Diamond Mountains where the Carbon Ridge rests with angular discordance on beds as young as Early Pennsylvanian and as old as Mississippian. A related unconformity places the Permian Garden Valley Formation on the Vinini Formation, about 20 miles northwest of Eureka in the southern Sulphur Spring Range (Hose and Blake, 1976; Nolan and others, 1956). In the Monitor Range this erosional event is marked by the presence of angular limestone cobbles containing Pogonip Ordovician fossils within the Permian Garden Valley Formation (Roberts and others, 1967).
Within and to the west of the Antler highland, several local Late Pennsylvanian through Early Permian shallow marine basins developed. These basins were filled with the Antler Sequence, and the Brock Canyon and Wildcat Peak Formations. The Antler Sequence is composed of the Pennsylvanian (Atokan) Battle Formation, the Late Pennsylvanian to Early Permian (Missourian to Wolfcampian) Antler Peak Limestone, a time equivalent of the Phosphoria Formation. The pebble to boulder conglomerates, sandstones and shales of the Battle Formation represent fanglomerate and delta plain sediments deposited in two or perhaps three small basins along the western margin of the Antler belt (Roberts, 1964; Drowley, 1973; Rich, 1971, Roberts, 1964). The grain size decreases upward in the formation suggesting gradual beveling of the nearby erosional source which lay to the northeast (Roberts, 1964; Drowley, 1973).
The overlying silty and fossiliferous Antler Peak Limestone, and fine to coarse-grained clastics and limestone of the Edna Mountain Formation represent tidal flat and delta margin sedimentation (Roberts, 1964). The Late Pennsylvanian to Early Permian dolomite, conglomerate, siltstone, sandstone and shale of the Brock Canyon Sequence in the Cortez Mountains, and the sandy and conglomeratic limestone of the Wildcat Peak Formation in the Toquima Range, also represents fan delta deposition within small and locally sourced shallow marine to estuarine basins.
To the east during Strathearn time, several thousand feet of coarse, organic detrital limestones of the Early to Late Pennsylvanian Ely Formation were deposited across most of the foreland in shallow, warm and moderately turbulent marine waters (Mollazal, 1961; Sides, 1967). The Ely commonly gradationally overlies the clastics of the Diamond Peak and is overlain by various Permian carbonates across the area.
In northeastern Elko County a sequence of poorly exposed and highly faulted, lithologically variable units have been dated as Late Mississippian to Early Permian (Chesterian to Wolfcampian) in age. These rocks have been assigned to the Oquirrh Formation and are probably correlative with and deposited in the same general setting as the Strathearn, Sunflower, Ely, Chainman and Diamond Peak Formations.
In northeastern Nevada Upper-Pennsylvanian (Missourian) through Lower Permian (Guadalupian) carbonate and terriginous sediments were accumulated in a rapidly subsiding basin dominated by shallow and marginal marine sedimentation (Mercantel, 1975). Steele (1960) coined the term Butte-Deep Creek Trough for the elongate, north-south, cratonic basin within eastern White Pine and Elko Counties. The northern terminus of the basin is considered the Silver Island Range near Wendover (Bissell, 1962) and the southern margin is placed at the southern Egan Range.
Overlying the Ely Limestone, the coralline, algal, and fusulinid-bearing limestones of the Permian (Wolfcampian) Riepe Spring Formation represent shallow carbonate bank, biohermal and patch reef environments (Barosh, 1964). The overlying fine-grained calcareous sandstones and silty skeletal limestone and dolomite of the Wolfcampian Reipetown Formation probably represent shallow marine deltaic sand aprons. The upper Wolfcampian to Lower Leonardian bioclastic limestones of the Ferguson Mountain Formation represent shallow marine deposition of bioclastic limestones. The overlying bioclastic limestones, and calcareous and gypsiferous dolomites, siltstones, and sandstones of the lower Leonardian to lower Guadalupian Pequop, Arcturus, and Loray Formations represent shallow skeletal banks, and local deltaic deposition in brackish and hypersaline bays, ponds, lagoons, and estuaries (Bissell, 1964; Collinson, 1968; Mercantel, 1975).
The Park City Group was designated for Permian strata overlying the Arcturus Formation and equivalent strata in northeastern Nevada and western Utah. The Park City Group is composed of the laterally equivalent Upper Leonardian to lower Guadalupian Kaibab and Grandeur Formations, the laterally equivalent Guadalupian Plympton and Murdock Mountain Formations, and the Guadalupian Gerster Formation.
The silty and phosphatic carbonates, thin conglomerates, and phosphorite of the Grandeur Formation represent shallow, restricted interidal and lagoonal deposition (LeCompte, 1978). The laterally equivalent Kaibab Formation limestones and dolomites were deposited in shallow subtidal to supratidal environments (Roberts and others, 1965). The overlying fine-grained, cherty and gypsiferous dolomites, phosphatic cherts, mudstones and siltstones of the Plympton and Murdock Mountain Formations represent a supratidal sabkha-like environment with occasional intertidal and subtidal depositional conditions (Wardlaw, 1974; Wardlaw and others, 1979; Martindale, 1981). The skeletal-detrital micritic limestones of the Gerster Formation represent shallow subtidal deposition in low-energy tidal flat and shallow carbonate bank environments (Sides, 1967; Wardlaw, 1974; Martindale, 1981). Locally in northeastern Elko County, the upper Leonardian to lower Guadalupian Phosphoria Formation grades into and overlies the Gerster Formation. The phosphatic shales and siltstones, oolitic phosphorite, and organic and argillaceous limestone of the Phosphoria Formation represent shallow subtidal to intertidal settings including oolitic shoals and mudflats (Martindale, 1981).
To the west of the Antler highland belt, Mississippian through Permian shallow to relatively deep, tectonically interleaved, marine sediments were also deposited within the upper plate of the Permian through Triassic Golconda allochthons. These include the Late Mississippian to Early Pennsylvanian Schoonover Sequence in the northern Independance Mountains, the Pennsylvanian Mitchell Creek and Reservation Hill Formations in northern Elko County, and the Middle Pennsylvanian to Early Permian Havallah and Pumpernickle Formations in the Shoshone and Toiyabe Ranges and ranges to the west of the study area.
Several thousand feet of the Schoonover, Havallah, and Pumpernickle Formations were deposited in a subduction-related setting from the Late Mississippian through the Early Permian, and were thrust westward during the Permian through Triassic Sonoma orogeny (Roberts, 1964; Stewart and others, 1977; Miller and others, 1981). These sediments were probably derived from a distal island-arc source terrain as well as from the the Antler highland to the east (Miller and others, 1981). Sole markings at the base of coarse-grained pebbly sandstones and conglomerates suggest density current or grain flow deposition for much or all these formations. Deep marine deposition is also suggested by the abundance of radiolarian chert, convolute bedding in silty limestones, and the preservation of deep-water Nerites trace fossils (Stewart and others, 1977). Pillowed greenstones and relatively thick andesitic flows and agglomerates are common in both the Schoonover and Pumpernickle indicating a submarine volcanic source.
Roberts (1964) has suggested that the Havallah Formation shallows upward, and that much of the chert in both the Havallah and Pumpernickle are formed by spicules of sponges that lived in water depths of less than 300 feet. The presence of phosphatic minerals in Havallah sandstones, and gastropods and crusteaceans in fine-grained Pumpernickle sediments also suggests at least inital deposition in relatively shallow water.
The Pennsylvanian Mitchell Creek and Reservation Hill Formations are composed of a similar lithologic suite of dolomitic siltstone and sandstone, thin-bedded cherts, and thick sequences of meta-basalts, andesites and rhyolite flows and tuffs (Coats, 1969; 1971). These poorly studied units appear to also represent relatively deep slope and basinal subduction-related depositional settings. Crinoids and phosphatic nodules found in the Mitchell Creek suggest it may at least in part represent shallow marine sediments (Coats, 1985).