The type section for the Indian Well Formation is in Sec. 30, T. 31 N., R. 55 E., in the Dixie Flats Quadrangle area of the Carlin-Pinon Range (Smith and Ketner, 1976).

The Indian Well Formation is Oligocene in age, with radiometric dates ranging from 37.6 +/- 1.3 Ma to 33.2 +/- 0.7 Ma (Smith and Ketner, 1976). The Indian Well Formation unconformably overlies the Eocene and Oligocene? Elko Formation in the Carlin-Pinon Range and Elko area. It is overlain by up to 1,000 feet of andesite or rhyolite flows. The Indian Well Formation includes rocks mapped by Regnier (1960) in the northern portion of Pine Valley as the Safford Canyon Formation.

The Indian Well Formation is a thick sequence of tuffaceous fluvial conglomerate, siltstone, and sandstone, andesitic flows and lahars, tuff and ash-flow tuff. The basal portion of the formation is characterized by water-lain tuff, tuffaceous conglomerate, gray to purple sandstone and siltstone, and minor gray, chert bearing, oolitic and algal, rippled and mudcracked limestone, and mudstone. Fragments of Devonian Oxyoke Sandstone, Mississippian Chainman and Diamond Peak Formations are present in both the sands and conglomerates (Smith and Ketner, 1976). The conglomerates commonly contain pebble and cobble-size fragments of ash-flow tuffs, and are cross-bedded with channel cut and fill structures. The finer grained sandstone and siltstone are evenly bedded with scour and fill structures indicating fluvial deposition.

The upper portion of the Indian Well consists of tuffaceous sandstones and evenly laminated lacustrine tuffs, with minor amounts of interbedded limestone and conglomerate. Rhyolitic to dacitic, welded to non-welded tuffs from several tens of feet to more than 400 feet thick, and lahars up to 30 feet thick are interstratified, and more common in the upper portion of the unit (Smith and Ketner, 1976). Non-welded, light pink to gray, quartz-latite tuffs are more common than dark-red to brown, dense welded varieties (Smith and Ketner, 1976).

The Indian Well Formation is 3,300 feet thick in the Dixie Flats Quadrangle (Smith and Ketner, 1976), and is 900 feet in the Elko East and 2,100 feet in the Elko West Quadrangles (Solomon and Moore, 1982a, 1982b).

The Indian Well Formation has been identified in the eastern Cortez Mountains, the Carlin-Pinon Range area, and near Elko in the Elko East and West Quadrangles.

The Indian Well Formation marks the end of Paleogene lacustrine sedimentation near Elko with volcaniclastic sediments derived contemporaneously with volcanism. The sediments within the Indian Well Formation represent fluvial and floodplain sediments (Solomon and others, 1979).

To the north of the Carlin-Pinon Range, undated and largely unconsolidated gravels up to 3,000 feet in thickness are present in the southern Mountain City Quadrangle, and are here included with the Oligocene sediments. Tuffaceous sediments overlying the gravels are Arikareean (Upper Oligocene) in age (Coats, 1985). Also present in the Mountain City area are poorly exposed, lignitic, arkosic, and carbonaceous tuff which contains Oligocene pollen, and thin lenses of fine-grained, locally tuffaceous gravel (Coats, 1985).

Tuffaceous sediments, tuffs, and pebble and cobble-conglomerates are also exposed in the Mount Blitzen, Mount Velma and Marys River Basin Quadrangles and Jarbidge Wilderness area. Included with these sediments are air-fall and welded tuffs of rhyolitic to dacitic composition that give K-Ar ages of 33.6 +/- 1.6 Ma in the Mount Blitzen Quadrangle (McKee and others, 1965).