The Elko Formation was named for exposures in Secs. 10 and 15, T. 31 N., R. 53 E., in the Dixie Flats Quadrangle portion of the Carlin-Pinon Range (Smith and Ketner, 1976).

The Elko Formation is considered Late Eocene or Early Oligocene in age since the oldest tuff in the formation is dated at 38.6 +/- 1.2 Ma (Smith and Ketner, 1976). The Elko Formation is unconformably overlain by volcanics and volcaniclastics of the Oligocene Indian Well Formation. The Elko is probably equivalent to the upper portion of the Sheep Pass Formation near Ely, and the Kinsey Canyon Formation near McGill (Brokaw and Shawe, 1965; Fouch, 1979).

The Elko Formation is a lithologically diverse sequence of intertonguing lacustrine sediments and minor tuffs which has been broken into various members in various areas (Smith and Ketner, 1976; Smith and Howard, 1977; Solomon and Moore, 1982a, 1982b). The lenticular nature of units makes for numerous local members. Overall the formation is characterized by thin bedded, light gray to tan, yellowish weathering limestones with local chert nodules and bands, and laminated to papery, brown and black shales which weather to white or light blue-gray. Also included and interbedded are siltstones, dolomite, tuffaceous and carbonaceous shale, tuff, and conglomerate. The 6 units broken out by Solomon and Moore (1982b) can be found in several areas, although several members have been combined by some workers, or are missing entirely in other areas. The members described in ascending order below, are a typical and relatively complete section of the Elko Formation. They are identical to those in the Elko West Quadrangle with the exception of the lower two members which are not present (Solomon and Moore, 1982a).

The lowermost oil shale and siltstone of the Burner Basin Member is about 200 feet of faintly laminated, ostracodal, reddish-yellow and yellow-brown, calcareous oil shale and siltstone with thin interbeds of light brown limestone. The overlying dolomite and oil shale member is composed of about 395 feet of brown, thinly bedded dolomite interbedded with brown, blue-gray weathering, faintly laminated and papery oil shale, and a few reddish-orange silty limestone beds. The overlying member is about 205 feet of brown and gray claystone, conglomerate with varicolored, well-rounded poorly sorted pebbles and granules, brown crossbedded sandstone, and thin brown beds of silty biosparite.

The next member is the oil shale member that is about 75 feet thick and is composed of dark-brown, blue-gray weathering siliceous and ostracodal oil shale, interbedded with minor siltstone, lignite, and tuff. This is in turn overlain by the siltstone and oil shale member which is about 610 feet of thin-bedded and laminated, light gray to reddish-orange silstone with leaf fragments, interbedded with dark-brown, laminated and papery oil shale, and minor amounts of claystone, limestone, lignite, and tuff. The upper most member in the Elko West and East Quadrangles is composed of up to 705 feet of light gray quartz-biotite tuff, light brown to gray, thinly bedded shale, and siltstone with a few thin beds of ostracodal limestone (Solomon and Moore, 1982b).

The Elko Formation is about 1,700 feet thick in the foothills of the Adobe Range (Sillitonga, 1975; Solomon and Moore, 1982a), 2,075 feet at the type section in the Dixie Flats Quadrangle (Smith and Ketner, 1976), at least 2,500 feet thick near Elko (Smith and Ketner, 1976), 2,190 feet in the Elko East and 1,005 feet in the Elko West Quadrangles (Solomon and Moore, 1982a, 1982b).

The Elko Formation is present 20 miles north, 40 miles east and at least 10 miles to the west of Elko, Nevada (Smith and Ketner, 1976). It is exposed in the Dixie Flats, Lee, Elko West and East Quadrangles, in the Adobe Range foothills in the Kittridge Springs and Coal Mine Basin Quadrangles, in the North Fork Quadrangle west of Double Mountain, and may be equivalent to oil shale reported in the Bull Run Quadrangle by Decker (1962).

The Elko Formation is a lithologically diverse sequence of intertonguing lacustrine units including fluvial channel sands and conglomerates, mudflat type ostracode and gastropod-rich micrites, and delta-floodplain siltstone, mudstone and shale such as the well-known Elko Oil Shale. Unlike the Sheep Pass Formation, the Elko Formation also contains significant amounts of tuffaceous volcanic material.

The Elko Formation represents perhaps 7 million years of relatively quiet lacustrine deposition in a lake or more probably series of lakes that covered an area of 60 miles in a north-south direction, and at least 20 miles in an east-west direction (Smith and Ketner, 1976). Solomon and others (1979) suggest that the Elko Formation records sediments which grade upward from lake margin carbonate flat, to mudflat, intradeltaic and deltaic rocks, and finally to organic rich shales deposited in an open lacustrine environment.

Oil shale from the Elko East Quadrangle was mined and retorted at the Catlin Shale Products Company plant from 1916-1924. 55 samples from the oil shale member show a maximum yield of 85.5 GOPT, with 9 samples in excess of 15 GOPT, and several intervals in excess of 15 feet thick have recoveries in excess of 17 GOPT (Solomon and Moore, 1982a, 1982b). The specific gravity of the oil ranges from 0.865 to 0.897.

Geochemical analysis performed during this evaluation suggests that the dominant kerogen type in the Elko Formation is amorphous, Type II aquatic kerogen, with minor amounts of spore and pollen related Type I kerogen. TOC averages about 3.37 percent for 6 outcrop samples within the evaluation area. The surface exposures of Elko Formation are dominantly thermally immature. The Elko Formation is a good oil-prone source rock that is capable of generating oil where thermally mature.

Farther to the north in Elko County a series of discontinuous exposures of unnamed Lower Tertiary conglomerates are assigned to the Eocene. This assignment is questionable in some areas since ages have never been determined, and Ordovician units may locally be included (Coats, 1985). These conglomerates have been found in several places in the Independence Range (Lovejoy, 1959; Churkin and Kay, 1967; Coats, 1985). The conglomerate is a massive and limonite stained lensoidal unit up to 300 feet thick near Lone Mountain, and in the Mahala Creek West, Reed Station, Lake Mountain, and North Fork Quadrangles in the central and southern portion of the Independence Mountains (Lovejoy, 1959). It contains pebbles and boulders up to a foot in diameter composed of well rounded to angular chert. In the Reed Station Quadrangle the conglomerate contains cobbles of barite. The conglomerates locally have a montmorillonitic matrix, which suggests they may in part represent volcanic mudflow or lahar deposits (Coats, 1985). East of the Reed Station Quadrangle in the southwestern portion of the Wieland Flat Quadrangle, several thousand feet of volcanic-cobble conglomerate rests on Early Tertiary ignimbrites (Coats, 1985) and appear to be related to initial basin development along steep faults.

In and near the Tuscarora Mountains, similar unnamed conglomerates are exposed in the Mount Blitzen and Sugar Loaf Butte Quadrangles. These conglomerates are coarse and very coarse grained with angular quartzitic boulders up to 4 feet across, and appear to be related to steep fault scarps (Coats, 1985). Similar units are also found in the Marys River Basin Quadrangle (Coats, 1985).

In the Mount Velma Quadrangle, reddish weathering quartzite pebble and cobble conglomerate with quartzite boulders up to 2 feet in diameter is up to 75 feet in thickness (Coash, 1967; Coats, 1985). Overlying the conglomerate are several hundred feet of coarse-grained volcanic sandstones that are included here with the Eocene section (Coats, 1985). In the Mountain City Quadrangle, boulder gravel and conglomerate are interbedded with thin hornblende-biotite ignimbrite dated at 38.0 +/- 1.1 Ma (Coats, 1985; McKee and others, 1976).

To the east of the Mountain City and Mount Velma Quadrangles, similar conglomerates have been assigned to the Eocene (?) Meadow Fork Formation by Coats (1964). This unit is described separately below.