Geology on the Road: Highway 50 #1

Austin Summit, Nevada:
These photos are from just east of Austin Summit, Nevada, just barely on the eastern, downhill side of the summit, between Milepost 27 and Milepost 28. Two exposures, this being the second on the way downhill, show welded ash-flow tuff. In this exposure, the red-gray to reddish gray ash-flow tuff is lying on a whitish unit of poorly welded tuff, air-fall tuff, or base surge tuff. The white unit lies on a gray, blocky-weathering ash-flow tuff, which can be seen to the left in the stitched photo. The compaction foliation and jointing in the red-gray and reddish ash-flow tuff to the right is roughly parallel to jointing or foliation in the tuff on the left, and roughly parallel to the contact.

The tuff and tuff-tuff contact is dipping roughly northeastward to eastward, away from the summit. I tried to find a reference giving the name or origination of these particular tuff formations, but so far haven't found one. It would fall into the 17 to 34 m.y. old category of central Nevada ash-flow tuffs (part of the Mid-Tertiary ignimbrite flare-up), and is probably in the 22 to 25 m.y. old range (that's my guess based on work in the region quite a while ago - the rocks haven't aged too much since then).

I thought while looking at this outcrop the last time I stopped, that there might be some faulting or shearing between the whitish layer and the upper, densest part of the welded tuff (the reddish gray, bouldery, hard-looking stuff) with possible shearing taken up in part in the brighter reddish zone, in which you can still see some compaction foliation.

Andrew has quite a lot of links about Nevada Geology, inlcuding one to this map, which shows a pinkish blob of Tv just a bit to the left of the label, next to some reddish "TMzi" - the granite of Jurassic or Cretaceous age. The map is from the Nevada Bureau of Mines and Geology (NBMG).

Also see:

Stewart, J.H. and Carlson, J.E., 1976, Distribution and Lithologic Character of 34- to 17-Million-Year-Old Sedimentary and Igneous Rocks of Nevada, Showing Centers of Volcanism, Nevada Bureau of Mines and Geology Map 52, Sheet 2, Cenozoic Rocks of Nevada.

UPDATED 10July2010