Abstract: Tertiary Geology and Oil Shale Resources of South Elko Basin, Nevada

Barry J. Solomon, Edwin H. McKee, Charles A. Brook, J. Ward Smith

As much as 470 m of middle Cenozoic petroliferous lacustrine strata is exposed in the south Elko basin, unconformably overlying the Mississippian and Pennsylvanian Diamond Peak Formation. The lower 95 m consists of lean oil shale, mudstone, and limestone, with minor sandstone, conglomerate, and air-fall tuff. The tuff has been K-Ar dated at 43.3 ± 0.4 m.y. These informally named rocks are conformably overlain by, and appear to be closely related to, strata of the Elko Formation. The Elko Formation is 375 m thick and is divided into five informal members. In ascending order they are: (1) chert-pebble conglomerate with interbedded lean oil shale and mudstone; (2) rich oil shale with interbedded siltstone and minor lignite; (3) lean oil shale, mudstone, and minor thin b ds of calcareous siltstone and limestone (the lateral equivalent of members 1 and 2); (4) tuffaceous siltstone overlain by lean oil shale, mudstone, siltstone, and minor tuff and lignite; and (5) air-fall tuff overlain by shale, siltstone, tuff, and ash beds. A K-Ar age from near the base of the fifth member is 38.8 ± 0.3 m.y. The Indian Well Formation overlies the Elko Formation and consists of approximately 600 m of fluvial strata and air-fall tuff.

Analyses of oil shale indicate a yield of up to 360 l/ton of 24 to 34° API oil. The oil share is noncoking. Preliminary estimates indicate resources of approximately 150 million bbl of oil.

The Indian Well Formation and older rocks generally dip eastward and are cut by normal faults. They are overlain unconformably by unnamed subhorizontal beds of air-fall tuff and andesite dated at 37.1 + 1.0 and 31.0 + 1.0 m.y., respectively. Local remnants of nonpetroliferous rocks of the Miocene Humboldt Formation (restricted) unconformably overlie older rocks along the basin margins.

The radiometric dates (about 43 to 38 m.y. for tilted and faulted strata and 37 to 31 m.y. for subhorizontal rocks) indicate that this part of the Elko basin was strongly deformed during a short interval in the latest Eocene and earliest Oligocene. Later block faulting has influenced the present topography of the region.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California