The Basin and Range physiographic province of the United States includes Nevada and parts of Arizona, Utah, and California. It consists of rugged nearly parallel northward-trending mountain ranges and broad arid trough. with interior drainage. Elevations range from 4,000 to 14,000 feet, above sea level. The valleys are filled with thick accumulations of water-laid pyroclastic rocks, silts, fanglomerates, and evaporites. Ephemeral saline lakes and playas are found in many of the valleys because of interior drainage and arid climate. Gently sloping alluvial fans extend from the mouths of canyons toward the low central portions of most of the valleys.
Evidence of alpine glaciation is apparent in the higher mountains, and shorelines of Pleistocene lakes con be observed locally along the foothills. The highly alkaline soil of many of the valleys supports little vegetation other salt grass and sage, but several types of conifers abound in the mountainous areas. The climate is semiarid to arid; therefore, the region is sparsely settled because habitable areas are dependent upon surface-water supplies.
The majority of the rocks of the Siebert Formation as evidenced relative to Seibert Mountain 2 km southwest of Tonopah, were deposited under alternating fluvatile and lacustrine conditions. The Tonopah area was probably a lake, surrounded by grassy, shrub-covered plains. The margins of the lake were shallow, and detris was supplied from the erosion of older volcanic rocks plus added pyroclastic material, most of which came from vents that were later filled by Brougher Rhyolite. The lake may have been somewhat alkaline, but was occupied by gastropods, ostracods and fish. Periods of partial desiccation occurred, and temperatures occasionally may have fallen below freezing. Algal mats were present along the shore in places and lake tufa was deposited locally.
Siebert Formation at Siebert Mountain, 2 km southwest of Tonopah. White tuffs and lapillistones overlain by a thin trachyandesite flow (dark band). A flow of Brougher Rhyolite forms the top of the mountain.
The late Tertiary was a period of increasing aridity in the Great Basin (Axelrod, 1940). The climate may be comparable to that described by Axelrod (in Barrows, 1971) in the Eastgate area, 150 km to the northwest. At that locality, the Eastgate flora indicates a rainfall of about 102 cm per year, with a mean annual temperature of 10°C and a mean annual range of 12° C. The precipitation was probably seasonal, with more moisture in the winter months. Following deposition of the sediments, a number of authigenic minerals were formed, especially in the lacustrine deposits. Zeolites (erionite, clinoptilolite, and analcime) are found as alteration products of tuffaceous material in the sediments. Also, authigenic potassium feldspar is present locally as overgrowth rims on detrital potassium feldspar grains.