AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
In general, much of northeastern Nevada was an area of relatively subdued topography during the inception of Tertiary volcanism about 43.5 +/- 2.2 Ma (Coats, 1985). Thick ash flow tuff or ignimbrite sheets blanketed the existing topography from the Eocene periodically through the Miocene. These ignimbrites are thick near eruptive centers and in areas of generally low pre-eruptive topographic relief, and are thin or absent in areas of pre-eruptive high relief. The eruption of genetically related ash flow tuff sheets took place over about 10 m.y. intervals, with long periods of quiescence between eruptive events. K-Ar dating of ash-flows in White Pine, Nye and Lander Counties show that the minimum time between eruptions of individual but genetically related ash-flow cooling units is about 0.8 m.y. (Gromme and others, 1972).
Kleinhampl and Ziony (1985) report that ash-flow tuff eruptions ceased about 22 Ma in Nye County, and were replaced by the local eruption of intermediate composition flows, and the deposition of discontinuous lacustrine and fluvial sedimentary rocks. Silicic and basic plutonic masses were intruded throughout the Eocene, Oligocene and Miocene. Basalt and andesite flows, and tuff and tuff breccia, were periodically erupted over small areas from the Oligocene throughout the Miocene, Pliocene, and as recently as Pleistocene (5.7 +/- 0.2 Ma) in the Lunar Crater area. Oligocene and Miocene rhyolite, dacite and rhyodacite flows were also locally erupted across the study area.
Deep, structurally formed, sedimentary basins began developing in the Eocene with the inception of Tertiary extension, and were mainly blocked out during the Oligocene and Miocene with continued Basin-Range extension. Eocene conglomerate, sandstone, siltstone, cherty limestone and limestone, oil shale and intercalated volcanics have been assigned to various unnamed units as well as the Sheep Pass, Kinsey Canyon, Meadow Fork, and Elko Formations.
The Eocene Sheep Pass Formation and the Eocene Elko Formation were deposited in several large lake basins. The Sheep Pass Formation, several thousand feet of lacustrine sediments representing a single moderate size lake, unconformably overlies Paleozoic rocks in the northern Pancake, Grant and Egan Ranges (Fouch, 1977). The Late Eocene to Early Oligocene(?) Elko Formation is a lithologically diverse sequence of intertonguing fluvial channel sands, lake margin ostracode and gastropod micrites, and delta-floodplain siltstone, mudstone and shale including the Elko Oil Shale (Solomon and others, 1979).
Unlike the Sheep Pass Formation, the Kinsey Canyon Formation in the Schell Creek Range, the Elko Formation in the Elko area, and the fluvial conglomerates and sandstone of the Meadow Fork Formation in northern Elko County contain significant amounts of tuffaceous volcanic detritus and local flow rocks which record initial erosional input of Tertiary volcanics. This volcanism began about 41.4 +/- 1.4 Ma in the Kinsey Canyon (Young, 1960), with the Meadow Fork Formation confomably overlying the 39.9 Ma Dead Horse Tuff (Coats, 1985). The oldest volcanic in the Elko Formation is a tuff, dated as about 43.3 +/- 0.4 Ma (Solomon and Moore, 1982a).
Eocene andesitic to latitic flows and ash-flow tuffs occur in distinct layers which are locally incorporated within or overlie the lacustrine and fluvatile sediments. Unnamed, densely welded, rhyolitic to dacitic ignimbrites were erupted in the northwestern portion of Elko County between approximately 45.7 and 39.5 Ma. The caldera margins and eruptive centers for these tuffs are essentially unknown.
Lower to Upper Oligocene sediments deposited in structurally formed intermontane basins represent various local and thin, unnamed and named, intertonguing fluvial and lacustrine tuffaceous limestone, sandstone and siltstone, and tuff and ash. These units include the Blind Spring Formation in the Grant and Quinn Canyon Ranges (Kleinhampl and Ziony, 1985), the Gilmore Gulch Formation in the Hot Creek Range (Quinlivan and others, 1974), the thick Indian Well Formation in the Carlin-Pinon and Cortez Range area (Smith and Ketner, 1976), and unnamed units across the area. These sediments are often interfingered with or overlain by andesitic to rhyolitic flows and ash flow tuffs. Various Oligocene ash flow tuffs are widespread across the entire area and have been traced to several large caldera complexes which are in large part nested, coalescing, or overlapping (Hose and Blake, 1976; Kleinhampl and Ziony, 1985).
In eastern Lander and western Elko Counties, Oligocene volcanics about 36 Ma rest on a regional, low relief erosional surface which cuts Paleozoic and Mesozoic rocks as young as 70 Ma Cretacous plutons (Stewart and McKee, 1977, Hope and Coats, 1976). In the southern Egan and northern Schell Creek Ranges, Oligocene volcanics rest with only slight unconformity on the Eocene Sheep Pass Formation. In the northern Egan, northern Snake and Schell Creek and White Pine Ranges these Oligocene volcanics rest on Cambrian and locally Precambrian rocks (Hose and Blake, 1976). In the Maverick Springs and Butte Mountains the volcanics lie with profound unconformity on broadly and openly folded Triassic rocks. The volcanics overlap portions of low-angle normal faults in the Deep Creek, Snake, Schell Creek, Egan, White Pine and northern Pancake Ranges (Hose and Blake, 1976). A hiatus of at least 30 M.y., and up to several times that in most localities, is represented by this basal Oligocene volcanic unconformity. This unconformity dates low-angle normal faulting over portions of the area.
By late Miocene much of northeastern Nevada was a low plateau with lakes occupying intermontane basins formed by high-angle faults. Miocene ignimbrites, although widespread, are not as voluminous as the Oligocene ash-flow tuff sheets across the area. The calderas responsible for their eruption are also highly eroded and remain poorly defined.
In contrast with Eocene and Oligocene sediments which are preserved only in small local basins, Miocene sediments as well as Pliocene and Quaternary unconsolidated alluvial deposits are widespread (Stewart, 1980). The most widespread of these sediments is the Upper Miocene Humboldt Formation. The intertonguing fluviatile and tuffaceous lacustrine sediments of the Humboldt were deposited in a series of intermontain basins periodically filled with lakes, and fed by meandering stream systems that deposited and reworked sediments. Other Miocene sediments include the fanglomerates of the Slide Creek and Young America Gravels in Elko County (Coats, 1964; Bushnell, 1967), and various unnamed sequences of lacustrine limestone, ash and tuff, conglomerate, and tuffaceous and calcareous siltstone and sandstone.
Local Pliocene lacustrine and fluvial sediments of the Panaca and Muddy Creek Formations, and the White River lake beds are fresh water limestone, tuffaceous sandstone and siltstone, laminated clays, and water-lain tuffs and ash (Tschanz and Pampeyan, 1970). Perhaps the best preserved sequence, is the Middle Pliocene to Middle Pleistocene Hay Ranch Formation with vitric ash and tuff, clay, tuffaceous siltstone and sandstone, limestone and conglomerate and fanglomerate (Regnier, 1960; Smith and Ketner, 1976). The limestones in the Hay Ranch Formation are lacustrine sediments that received periodic influx of water-lain tuff and ash, and fluvial siltstone, sandstone, conglomerate, and fanglomerate.
The relative rising of the ranges and sinking of the basins continued during the Pleistocene and into the Quaternary. Glaciation left local morainal deposits in the higher portions of some ranges during the Pleistocene. Intermontane basins were filled with poorly sorted alluvial stream deposited sands and gravels and playa silts and clays, as well as local landslide deposits as drainage systems began to coalesce, and exterior drainage developed. These basins are presently being filled with stream gravels and talus and periodically contain playa lakes.