Thursday, April 23, 2009

Lone Mountain: Intro to Stratigraphy

As you may remember, a short time ago we were sitting alongside Highway 50 in central Nevada, looking north at Lone Mountain (map). The reason we were doing that - besides the fact that Lone Mountain breaks up an otherwise long stretch of Highway 50 between Austin and Eureka - is because Lone Mountain provides a good exposure of a portion of the Ordovician through Devonian stratigraphy typical of eastern facies rocks in central Nevada. “Eastern facies” refers to rocks in the lower plate of the Roberts Mountains thrust, a regional thrust that formed during the late Devonian to Mississippian Antler Orogeny (300-375 Ma or 380 Ma).

During the Antler Orogeny, western facies (eugeoclinal) Ordovician siliceous rocks (mostly cherts and black shales with minor greenstones) were thrust over eastern facies (miogeoclinal) Cambrian through Devonian carbonate rocks along the Roberts Mountains thrust (Merriam and Anderson, 1942). Mississippian paleogeography, showing the Antler highland and foredeep.

<-----The Roberts Mountains thrust is sometimes erroneously called "Roberts Mountain thrust" (after Roberts Creek Mountain??) and is also sometimes erroneously shortened to "Roberts thrust." The latter name is presumably in reference to Ralph Roberts, who worked extensively in the area mapping the formations and the thrust and naming the Antler orogeny (Roberts, 1949; Roberts, 1964). He is often known as the father of the Carlin Trend for defining the Carlin Trend and the Battle Mountain-Cortez-Eureka Trend and for telling everyone where to find gold in the region (Roberts, 1960; Roberts, 1986).

The eastern and western facies units, and the Roberts Mountains thrust itself, were then partly covered by a Mississippian through Permian overlap sequence of shallow-water carbonate and siliclastic rocks. All three facies or sequences – western, eastern, and overlap – occur at Lone Mountain; the eastern facies dominates, with western and overlap sequence rocks occuring only in low-lying pediment areas south and east of the mountain.

One summary of major geologic events in Nevada.

Geotectonic evolution of the Great Basin - Dickinson, 2006 - see Figure 2, page 355: A time-space diagram of lithic assemblages in the Great Basin and adjoining areas.

Some References:
Merriam, C. W., and Anderson, C. A., 1942, Reconnaissance survey of the Roberts Mountains, Nevada: Geol. Soc. America Bulletin, v. 53, no. 12, p. 1675-1728.

Roberts, R. J. 1949, Geology of the Antler Peak quadrangle, Nevada: U.S. Geol. Survey Open-File Rept 49-47, 108 pages (superceded).

Roberts, R. J. 1960, Alignment of mining districts in north-central Nevada: U.S. Geol. Survey Prof. Paper 400-B, p. 17-19.

Roberts, R. J., 1964, Stratigraphy and structure of the Antler Peak Quadrangle, Humboldt and Lander Counties, Nevada: U.S. Geol. Prof. Paper 459-A, 93 pages.

Roberts, R. J., 1986, The Carlin story in Tingley, J. V., and Bonham, H. F., eds., Sediment-hosted precious-metal deposits of northern Nevada: Nevada Bur. Mines and Geol. Report 40, p. 70-80.

Speed, R. C., and Sleep, N. H., 1982, Antler orogeny and foreland basin: A model: Geol. Soc. America Bulletin, v. 93, no. 9, p. 815–828.


Desert Survivor said...

Thanks for the links. I'm trying to learn more geology, particularly Great Basin geology, and your blog is terrific!

steph said...

Amazing resources for a geology project I am creating about Las Vegas. Thanks for the help!

Silver Fox said...

Steph, this particular Lone Mountain in Nevada is nowhere near Las Vegas. It's in Eureka County, near Eureka. For the Lone Mountain near Las Vegas, you will have to do your own research - this information doesn't apply.