Abstract: Abstract: Sheep Pass (Cretaceous? to Eocene) and Associated Closed-Basin Deposits (Eocene and Oligocene?) in East-Central Nevada--Implications for Petroleum Exploration

Thomas D. Fouch

Rocks interpreted as representing deposition in widespread Cretaceous(?) and early Tertiary internal-drainage systems are present within an area of more than 8,060 sq km in east-central Nevada. The units may represent at least two periods of closed-basin sedimentation, separated in the area of the Egan Range by an unconformity. The entire Sheep Pass Formation previously has been considered Eocene in age on the basis of mollusk and ostracod biostratigraphy. New identification of several well-preserved specimens of the ostracod Cypridea (Bisulcocypridea) bicostata (Cretaceous) from the type section and of early Paleocene palynomorphs from cores at Eagle Springs field probably indicates a Cretaceous(?) through Eocene age for the first phase of lacustrine sedimentation. The p esence of the vertebrate Hyrachyus in beds of the second lacustrine phase in the northern Egan Range indicates that deposition of the sediments occurred in the Eocene, although lacustrine deposition probably continued into Oligocene time.

Sediments deposited during the first period of sedimentation may be free of volcanic detritus and are represented by the Sheep Pass Formation at its type section. The formation can be subdivided into a basal sequence consisting of 240 m of conglomerate, which is chaotic in the lower 168 m but occurs in graded cycles in the upper part of the member. The basal conglomerate interfingers with and underlies as much as 230 m of mud-supported carbonate (B member) containing ostracods, charophytes, mollusks, polygonal cracks, plant fossils, burrows, vertical branching-tube structures (roots), and bird's-eye structures, which collectively indicate freshwater, shallow-lacustrine sedimentation. The beds have a petroliferous odor on freshly broken surfaces; however, laboratory analysis of represe tative samples indicates that the units are not rich in organic matter at surface exposures. A sequence of units (C member) as much as 220 m thick consists of sandstone and minor conglomerate, claystone, and carbonate rock. Beds of the C member intertongue with and overlie beds of the B member. The rocks represent deposition in fluvial-alluvial, fluvial-lacustrine, and lacustrine settings. A poorly exposed complex of buff and white, siliceous and dolomitic, aphanitic mud-supported carbonate (D member), containing nodular and bedded chert, overlies the C member. Some of the chert beds represent Magadi-type cherts, and rare silica-filled salt-crystal cavities are present in some units. The rocks contain vertical branching-tube structures (roots) and some ostracod and mollusk species; howev r, much of the D member probably represents a more saline phase of the lake history. A return to fresher water deposition is indicated by abundant ostracods and mollusks in the upper 90 m of the formation, which is composed largely of mud-supported carbonate units and calcareous claystone (E and F members). The freshwater carbonate beds are interbedded with and overlie the white aphanitic carbonate units (D member).

The second phase of closed-basin sedimentation, which lies above the unconformity, is represented by lacustrine and alluvial units that include rocks designated as "Sheep Pass(?) Formation" near Ely, and by ostracod- and mollusk-bearing carbonate beds at Kinsey Canyon in the Duck Creek Range. In addition, at Sheep Pass Canyon, the basal part of the Stinking Spring Conglomerate unconformably overlies the type Sheep Pass Formation and may be equivalent to part of the sequence of beds near Ely and in Kinsey Canyon. Some rocks representing the second period of closed-basin deposition contain abundant volcaniclastic detritus and, in many areas, the second period of lacustrine deposition may have been terminated by Eocene(?) and Oligocene volcanic activity.

Reconstruction of lithofacies of the two periods of close-basin sedimentation seems to indicate that some of the lakes had open-lacustrine cores rich in organic matter, which subsequently has been oxidized on surface exposures. Beds assigned to the Sheep Pass Formation at Eagle Springs oil field in Railroad Valley serve as reservoir rocks, and analysis of available cores of the unit indicates that a significant thickness of potential source rock, containing in excess of 4 gal of oil per ton (17 l/t), is present in the subsurface. This new reconstruction of Cretaceous(?) and early Tertiary lake systems indicates that lacustrine and alluvial units are present in a band greater than 48 km wide that is oriented northeast for approximately 168 km. Rocks representing these nonmarine deposit onal systems are present in outcrop exposures of mountain blocks and in the subsurface of some of the valleys, where they may contain potential hydrocarbon source and reservoir beds in addition to those at Eagle Springs field.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado