Eocene sedimentary rocks are exposed in several small areas within the study area. In the Carlin-Pinon Range and Elko area, Eocene sedimentary rocks include a basal sequence of conglomerate, sandstone, and shale or limestone and limestone-clast conglomerate. This basal unit is overlain by conglomerate, sandstone, siltstone and limestone, cherty limestone, interlayered older intermediate volcanics, claystone, conglomerate, and oil shale that have been assigned to various unnamed units and to the Elko Formation (Smith and Ketner, 1976; Solomon and Moore, 1982a, 1982b).

The basal Tertiary unit in both the Elko East and West Quadrangles is an Eocene and Paleocene (?) conglomerate, sandstone, and shale unit that is about 2,500 feet thick in the Elko East, and 1,000 feet thick in the Elko West area (Solomon and Moore, 1982a, 1982b). This unit is composed of reddish-brown and yellowish-orange conglomerate and sandstone in medium-scale, trough-type, cross-stratified channel-form lenses. Thin bedded, horizontally stratified, light brown and gray claystone, siltstone, and pebbly sandstone, and gray limestone with ostracodes and fish bones, are present in the lower portion of the unit. Also present in this sequence is a tuff dated at 43.3 +/- 0.4 Ma (Solomon and Moore, 1982a, 1982b).

The basal Tertiary wedge in the Carlin-Pinon area and Kittridge Springs Quadrangle is composed of tan to gray, dense ostracodal limestone, and limestone conglomerate with pebble to boulder-size clasts of Mississippian Diamond Peak Formation and Pennsylvanian-Permian limestones (Smith and Ketner, 1976; Sillitonga, 1974). Clasts up to 3.5 feet across are present within the conglomerate in the Carlin-Pinon Range area. This unit overlies the conglomerate, sandstone and shale unit in the Elko West Quadrangle and is considered Eocene and possibly uppermost Paleocene (?) (Solomon and Moore 1982a, 1982b). It also overlies various Paleozoic units with angular unconformity, and in places appears to have been deposited against steep Paleozoic ridges (Smith and Ketner, 1976). The unit is about 275 feet thick in the Elko West Quadrangle, is absent in the Elko East Quadrangle, and is 635 feet thick in the Carlin-Pinon area (Solomon and Moore, 1982a, 1982b; Smith and Ketner, 1976).

A unit of conglomerate, sandstone, siltstone and limestone overlies the limestone and limestone conglomerate unit in the Carlin-Pinon area and Kittridge Springs Quadrangle, but is not present in the Elko West or East Quadrangles (Smith and Ketner, 1976; Solomon and Moore, 1982a, 1982b; Sillitonga, 1975). This yellowish to red weathering unit is dominantly composed of poorly sorted, channel conglomerates with subrounded to rounded pebbles and boulders of Paleozoic limestone, chert, and quartzite (Smith and Ketner, 1976). Pebbly, well bedded, brown to gray, quartz siltstone and sandstone are interbedded with the conglomerate, as are thin beds of brown to gray gastropod and ostracode-rich limestone. This unit is considered an alluvial and marginal lacustrine or floodplain deposit (Solomon and others, 1979). It is about 565 to 2,500 feet thick in the Carlin-Pinon area, 480 feet in the Kittridge Springs Quadrangle, and about 700 to 1,000 feet thick in the Coal Mine Basin Quadrangle (Smith and Ketner, 1976; Sillitonga, 1975; Ketner, 1973).

Overlying this unit in the Carlin-Pinon area is a unit of cherty limestone which is also present in the Elko West and East, Kittridge Springs, and Lee Quadrangles, and underlies the Elko Formation. The cherty limestone is about 200 feet in the Elko West and East Quadrangles, and about 1,000 feet in the Carlin-Pinon area (Smith and Ketner, 1976; Sillitonga, 1975; Solomon and Moore, 1982a, 1982b; Smith and Howard, 1977). This unit is dominantly gray to yellow-gray, dense, gastropod and ostracode-bearing biomicrite, with thin discontinuous layers and nodules of black and brown opaline chert. In the Elko West and East Quadrangles, the lower portion of the limestone is interbedded with red-brown, green-gray, and yellow-brown clayey siltstone and silty claystone (Solomon and Moore, 1982a, 1982b). The cherty limestone is interpreted as a transition from fluvial to marginal lacustrine deposition, with the lower siltstone and claystone representing a floodplain deposit (Solomon and others, 1979). The overlying Elko Formation is described below.