A 1:24,000-scale, full-color geologic map of the Lahontan Mountains 7.5-minute quadrangle, in Churchill County, Nevada, with descriptions of 30 geologic units. Accompanying text includes full unit descriptions and references. Map 168 (second edition) and Map 173 share the same text.

The Lahontan Mountains and Grimes Point quadrangles are located just east of Fallon, Nevada at the southeastern margin of the Carson Sink (fig. 1). Principal physiographic features include the Lahontan Mountains, collectively consisting of Sehoo, Eetza, Salt Wells, and Rainbow mountains. These quadrangles are within the greater Lake Lahontan basin that occupied the southern Carson Desert, previously mapped by Roger B. Morrison between 1947 and 1950. The products of Morrison's studies are contained in the 1964 U.S. Geological Survey Professional Paper 401, Lake Lahontan: Geology of Southern Carson Desert, Nevada, which stands as a seminal study on late Pleistocene Lake Lahontan in western Nevada and provides the fundamental basis for all subsequent studies of Lake Lahontan geology, including this one. The subtitle to Professional Paper 401 accurately describes the significance of Morrison's contribution to the field of Quaternary geology in the Great Basin:

"A stratigraphic study of the Cenozoic geology of part of the basin of Lake Lahontan, one of the great late Pleistocene lakes of Western United States."

Morrison‘s study built on the pioneering stratigraphic observations of Russell (1885) who spent considerable time in the Carson Desert and identified and described key Lake Lahontan localities. These localities together with those described by Morrison are classical sites that are still visited by Quaternary geologists interested in better understanding this great late Pleistocene lake.

We undertook this study of the Lahontan Mountains and Grimes Point quadrangles for several reasons. First, these quadrangles provide much of the record for late Pleistocene and Holocene alluvial and lacustrine deposits in the region. They contain several of the key or type localities for the formations of the Lahontan stratigraphy defined by Morrison, in particular, the Eetza, Sehoo, and Wyemaha Formations and the Churchill soil—stratigraphic units that are ubiquitous throughout the 21,000 km2 area occupied by Lake Lahontan. Initially defined as time stratigraphic units by Morrison (1964), he later redefined their formational status as allostratigraphic units (Morrison, 1991) based on stratigraphic and geomorphic relations developed in this area. The original mapping of the Lahontan Mountains area by Morrison (1964) was at a scale of 1:31,680, and we believed that larger-scale mapping could enhance and complement Morrison's maps. Second, Morrison developed most of the Lake Lahontan stratigraphic relations prior to the development of radiocarbon dating and tephrochronology, and thus his professional paper contained no numerical age data. The first radiometric dating of Lake Lahontan deposits in this area was reported in an article by Broecker and Kaufman (1965) which contained about a dozen 14C dates on middle Sehoo samples collected by Morrison in the Lahontan Mountains. We believed that a systematic remapping of the Lahontan Mountains type localities combined with new radiocarbon dating and new tephrochronologic age control could help improve numerical age control in this sparsely dated, but important, stratigraphic locality. Lastly, Morrison‘s mapping was done prior to the Rainbow Mountain–Stillwater earthquakes in 1954, and thus did not delineate the fault rupture traces. We previously conducted a detailed study of the 1954 surface faulting in the vicinity of Rainbow Mountain (Caskey et al., 2004), and we have incorporated some of those results into this study.

Probable Miocene through late Holocene-age deposits comprise the late Cenozoic stratigraphy in the Lahontan Mountains (fig. 2). Probable Miocene basaltic, dacitic, and rhyolitic flows and intrusives occur at Rainbow Mountain (fig. 3) and at Eagles House. These igneous rocks are unconformably overlain by Tertiary sediments. All of these probable Miocene rocks and sediments are strongly faulted and tilted locally, and are capped unconformably by flat-lying or gently dipping basalt flows of probable late Miocene or Pliocene age. The Tertiary volcanic-sedimentary section is unconformably overlain in the Lahontan Mountains by middle to late Pleistocene deposits of the Lake Lahontan sequence, with the most recent deposits of Holocene age associated with alternating cycles of shallow-lake and fluvial (Carson River) deposition.