Tuesday, April 28, 2009

Lone Mountain Stratigraphy in Brief

We are back at Lone Mountain again (the Lone Mountain in central Nevada), this time looking straight down on the mountain via two slightly enhanced Google Earth images (click to enlarge).

The second image - otherwise identical to the first - shows the main geologic formations present at Lone Mountain, along with their approximate boundaries. Contacts were taken in part from Google Earth's aerial and oblique views of the area, from this regional-scale map, from my memory of E.R. Larsen's Geology of Nevada class in the late 1970's, and from my memory of exploration mapping done in the late 1980's. The stratigraphic section shown, from west to east (left to right) and older to younger is as follows:

Op: The mostly Ordovician Pogonip Group carbonates, mostly limestone, as defined by Nolan and others (1956). At Lone Mountain, only the early to middle Ordovician Antelope Valley Limestone is present.
GEOLEX: Pogonip.
GEOLEX: Antelope Valley.

Oe: The middle Ordovician Eureka Quartzite.
GEOLEX: Eureka.

SOh: The middle Ordovician to early Silurian Hanson Creek Formation, which at Lone Mountain consists of 318 feet (Merriam, 1940) of “medium- to dark-gray dolomite and dolomitic limestone” (Nolan et al, 1956).
GEOLEX: Hanson Creek.

Srm: The mostly Silurian (early Silurian to early Devonian) Roberts Mountains Formation has a total thickness of about 740 feet at Lone Mountain. It's basal black chert bed, 85 to 100 feet thick, is overlain by dark gray dolomitic limestone and dolomite.
GEOLEX: Roberts Mountains.

DSlm: The early Devonian to late Silurian Lone Mountain Dolomite, which has its type locality at Lone Mountain. It's a light gray to medium gray dolomite, is about 1570 feet thick, and has gradational contacts at its base and top.
GEOLEX: Lone Mountain.

Dn: The Devonian Nevada Formation. This is a formation name that was first used in 1876; the formation was revised many times (see Nolan and others, 1956, and GEOLEX) and was abandonded in 1982 (Hose and others). The name does not have current usage, but it can be found on county maps and is sometimes used for regional mapping. Lithologically, it is composed mostly of dolomite (Nolan et al, 1956). It consists of or is equivalent to several other formations including, in places, the Simonson Dolomite.
GEOLEX: Nevada.

Dd: The middle to late Devonian Devils Gate Limestone - a limestone that used to be part of the Nevada Formation. About 1100 feet of Devils Gate Limestone is exposed on the east flank of Lone Mountain.
GEOLEX: Devils Gate.

Some References:
Hose, R. K, Armstrong, A. K., Harris, A. G., and Mamet, B. L., 1982, Devonian and Mississippian rocks of the northern Antelope Range, Eureka County, Nevada: U.S. Geol. Survery Prof. Paper 1182, 19 pages.

Merriam, C. W., 1940, Devonian stratigraphy and paleontology of the Roberts Mountains region, Nevada: Geol. Soc. America Special Paper 25.

Nolan, T. B., Merriam, C. W., and Williams, J. S., 1956, The Stratigraphic Section in the Vicinity of Eureka, Nevada: Revision of the pre- Tertiary stratigraphy of east-central Nevada: U.S. Geol. Survey Prof. Paper 276, 77 pages.

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