Friday, January 14, 2011

Geology off the Road: a bit of Tuff, Fossil Leaves, and EarthScope

It was a gray, early December day, but we continued our exploration on backroads near Middlegate Station. From the the exposures of flow-banded and brecciated rhyolite near Highway 50 — this was before The Shoe Tree had been cut down and could be used as an excellent landmark — we headed north on the main dirt road into the valley between the two southern branches of the Clan Alpine Mountains, the valley of Bench Creek. We took a sharp right at the first opportunity, within about a half mile of the pavement (the turn can be seen in the next-to-last picture of the previous post in this series).
The side road took us up the main wash, where we were tasked with looking for "little white flakes" of the middle Miocene Middlegate Formation, a formation composed of coarse- to fine-grained, often tuffaceous, sometimes zeolitized lacustrine sediments with interbeds of tuff, all lying unconformably on the older volcanic rocks of the Clan Alpine and Desatoya Mountains. A short distance up the wash road, we came to this exposure of white rocks... and though the weathering pattern looked blocky rather than flaky to me, we stopped to check it out.

Darn, we'd pulled over for a poorly welded lithic-rich ash-flow tuff, no doubt older than the rocks we were looking for. We moved on quickly, continuing up the wash.
Following the main wash road as we had been instructed by a hand-drawn map given to us at Middlegate Station, we finally came to a saddle overlooking this sedimentary section. These are the same tuffaceous sediments that can be seen north of Highway 50 when driving between Cold Springs and Middlegate gap, where the Shoe Tree used to live. The sediments were deposited in a small Miocene basin known as Middlegate Basin. Leaf fossils and petrified wood are reported in these rocks, and the zeolite erionite has been mined, at least in a small way, near the junction of S.R. 722 (old Highway 50, former Route 2) and Highway 50.
We trekked over to the white patches we could see from the saddle, and wandered around the overly muddy area looking for leaf fossils, breaking open numerous small slabs of white tuffaceous shale. We were unsuccessful for some time, and that's because we were in the wrong place. Close, but not quite.
Part of the Eastgate 1:24,000 topo map via MSRMaps courtesy USGS; X marks the saddle, not the fossil locations; sections are a mile on each side.
The saddle roads form a crude X in the center of this Google Earth image.

In order to find the fossils, you should not stop at the saddle (we had turned left and had parked on the narrow muddy ridge northwest of the saddle) but should instead keep driving down a fairly steep over-the-saddle road to the ENE; then, near the bottom of the hill, you should turn and drive southeast down another wash to a main wash heading SSE to south. In fact, while wandering around before driving over to the one fossil site we found, we could see a small hill with small diggings. That hill, located near the bottom of the GE image, is where we found the fossils.
It looked like the area had been pretty well picked over — we banged around quite a bit and only took these meager specimens. I suspect there are better localities, possibly shown here. I haven't the vaguest idea which of the many possible Middlegate flora leaf types (or stems, seeds) are represented by our bits and pieces. See these three lists (from Table 1 in Axlerod, 1985).
This plant growing near the saddle caught our eye as an unusual bush, then after our eyes became attuned to it, we saw it everywhere.

After fossil hunting, we tried to exit southward out of the wash. It was ultimately too muddy, and I don't think it's easy to find an Eastgate Wash crossing without a good map or current air photo. So we drove back to the saddle, up the steep road, and back down to the main wash road.
After trying for some unknown reason to drive south toward The Shoe Tree, and then stopping just barely in time before entering an impassable field of mud, we drove back to the main wash road and saw a fence on a little hill. We drove up to it and found this EarthScope installation.
This particular EarthScope observatory, is one of several plate boundary observatories strung across central Nevada near Highway 50.
This is Station P068's GPS receiver, with Fairview Peak in the background.
As we leave the wash, near our first tuff stop, Fairview Peak looms in the distance.

Some References:
Axlerod, D.I., 1956, Mio-Pliocene floras from west-central Nevada: University of California Press, 54 pp.

Axlerod, D.I., 1957, Late Tertiary floras and the Sierra Nevada uplift: Geological Society of America Bulletin, vol. 68, p. 19-46 [abstract].

Axlerod, D.I., 1985, Miocene floras from the Middlegate Basin, west-central Nevada: University of California Press, 279 pp.

Barrows, K.J., 1980, Zeolitization of Miocene volcaniclastic rocks, southern Desatoya Mountains, Nevada: Geological Society of America Bulletin, v. 91, no. 4, p. 199-210 [abstract].

Burke, D.B., and McKee, E.H., 1979, Mid-Cenozoic volcano-tectonic troughs in central Nevada: Geological Society of America Bulletin, v. 90; no. 2, p. 181-184 [abstract].

Eakin, T.E., 1962, Ground-water appraisal of Gabbs Valley, Mineral and Nye Counties, Nevada: Nevada Department of Conservation and Natural Resources, in Ground-water Resources - Reconnaissance Series Report 9, 54 pp.

Papke, K.G., 1972, Erionite and other associated zeolites in Nevada: Nevada Bureau of Mines and Geology, Bulletin 79, 36 pp [link to purchase].


tony edger said...

In my (east coast) mind, a fossil hunt in that amazingly beautiful landscape has to be considered a success regardless of what you might find.

Silver Fox said...

It was a great excuse to get out and do some exploring. And after not finding anything for a while, what we did find was exciting.

Anonymous said...

I am very disappointed that you gave directions to this scientifically invaluable fossil plant locality. I recommend that edit this blog entry to remove all directions to the site. Please consider this recommendation.

Silver Fox said...

Fossil locations are already published, for example by Axlerod and by the Nevada Bur Mines and Geology. Also, locations via Google. This post is only on the second page.