St. Thomas Gap, St. Thomas District, Clark Co., Nevada, USAi

Regional Level Types
St. Thomas Gap	- not defined -
St. Thomas District	Mining District
Clark Co.	County
Nevada	State
USA	Country

Age: Cenozoic (0 - 23.03 Ma)

Lithology: Sedimentary rocks

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Age: Cenozoic (11.62 - 28.1 Ma)

Stratigraphic Name: Horse Spring Formation

Description: Consists of the Horse Spring Formation in Clark and southern Nye Counties. This unit corresponds to unit Ths from the 1978 State map, and likely represents a composite of units Ts3 and Ts2. It is poorly known and may include rocks of other ages including Cretaceous.

Comments: Original map source: Crafford, A.E.J., 2007, Geologic Map of Nevada: U.S. Geological Survey Data Series 249, 1 CD-ROM, 46 p., 1 plate; Scale 1:250,000.

Lithology: Major:{sedimentary}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Age: Cenozoic (15.97 - 28.1 Ma)

Stratigraphic Name: Thumb Member

Description: Dominant lithofacies of Thumb exposed from east side of Frenchman Mountain, through the Gale Hills to Echo Bay, and in northeast part of quadrangle. Mostly brown, fine-grained, well-sorted sandstone or siltstone in thin, parallel continuous beds, which are commonly calcareous, sometimes ripple-laminated, sometimes with thin granule or pebbly layers. Elsewhere, clastic rocks are brown to red, fine- to coarse-grained sandstone and siltstone, commonly cross-stratified, with parallel to lenticular bedding and thin, channel-filling conglomeratic sandstone beds. Locally includes medium to thick beds of crudely-stratified, poorly-sorted sandstone with floating pebbles and granules. Also includes pale-green zeolitized to white or gray, fine-grained, airfall tuff with rare to common phenocrysts of biotite, hornblende, sanidine and plagioclase; tuff is commonly reworked at base and top and sometimes contains lithic fragments. Tuff beds are massive and structureless or channel-filling and cross-bedded; occur as thin to thick beds throughout most of the section. Bohannon (1984) reported a fission track age of 13.2 ± 0.9 Ma from an airfall tuff south of Lava Butte. 40Ar/39Ar dates from the Thtf facies in the Black Wash area, South Virgin Mountains, is 15.1 ± 0.08 Ma (M. Kunk, written communication, 2000) which matches well with an age of 15.13 ± 0.03 Ma from the lower of two tuffs on the west side of Pakoon Ridge (Beard, 1996). Unit thickness unknown but exceeds 1200 m in thickest part of sections. Outcrop at north end of Wilson Ridge is tentatively correlated to Thumb Member. Described by Naumann (1987) as dominantly red-brown cross-bedded fine-grained quartz sandstone that is thin bedded, with abundant ripple laminations; lesser amounts of light-gray shale and yellowish siltstone increase in abundance up section. Intruded by Wilson Ridge pluton

Comments: The Horse Spring Formation is a pre- to syn-extension basin deposit defined by Longwell (1921, 1922) and divided into four members by Bohannon (1984). The four members are, in descending order, the Lovell Wash, Bitter Ridge Limestone, Thumb, and Rainbow Gardens Members. They range in age from as young as 13 Ma to about 24 Ma or older. Geochronologic data published since Bohannon (1984) originally defined the Horse Spring Formation members indicates that the stratigraphic relations between the members are quite complex, because deposition was mostly syntectonic with basin evolution. Although Bohannon originally defined the Lovell Wash Member as about 13 to 12 Ma and Bitter Ridge Member as 13.5 to 13 Ma, more recent geochronologic data have suggested that the upper part of the Bitter Ridge Member may be time correlative to the Lovell Wash Member, and the lower part of the Bitter Ridge Member may be time correlative to upper parts of the Thumb Member (Castor and others, 2000, Lamb and others, 2005). We consider the revised range of the two upper members to be from 14.3 to 13.0 Ma. Bohannon (1984) bracketed the Thumb Member to be between 17.2 and 13.5 Ma in age. In the south Virgin Mountain area this unit is more tightly constrained between ~16.2 and 14.2 Ma (Beard, 1996). New 40Ar/39Ar dates on sanidine and biotite (Donatelle and others, 2005; Martin, 2005) from the Thumb Member within the Echo Wash area range from 16.4 (biotite) to 14.6 Ma (sanidine). We consider the Thumb Member age to be most likely between about 14.2 to 16.4 Ma. Beard (1996) recognized a disconformity to angular unconformity between the Thumb and underlying Rainbow Gardens Members. Although Bohannon considered the Rainbow Gardens Member to be no older than 20 Ma, Beard (1996) reported 40Ar/39Ar ages ranging from ~26.0 to less than 18.8 Ma in the South Virgin Mountains. Carpenter and others (1989) also reported a K/Ar age on biotite of 24.3 ± 1.0 Ma from a vitric tuff in the Rainbow Gardens Member in the northern Grand Wash trough

Reference: Beard, L.S., R.E. Anderson, D.L. Block, R.G. Bohannon, R.J. Brady, S.B. Castor, E.M. Duebendorfer, J.E. Faulds, T.J. Felger, K.A. Howard, M.A. Kuntz, and V.S. Williams. Preliminary Geologic Map of the Lake Mead 30' X 60' Quadrangle, Clark County, Nevada, and Mohave County, Arizona. U.S. Geological Survey Open-File Report 2007-1010 version 1.0. [151]