Tuesday, February 7, 2017

From the Road: Talus Stripes and Shorelines along the Truckee River

Talus, shorelines, and fall colors.
It was a late fall day, and I stopped along Route 447 to see if I could get close to some of the brilliantly colored trees along the Truckee River a few miles north of Wadsworth. I didn't end up finding a good spot for pics of fall colors—other than maybe this one taken down near the Numana Hatchery—but I did find lots of wonderful talus stripes and some Lake Lahontan shorelines.

At two stops a little farther to the north, at and near the junction of 447 with Chicken Road and at the Historic Marker 448 pullout, I grabbed a hodgepodge of photos.
Looking SSE from the historic marker pullout.
These two photos look south-southeastward along the Truckee River toward distant Fernley. A good portion of the river in this area runs parallel or sub-parallel to Walker Lane strike-slip faults. Wadsworth Amphitheater, which shows up in the Google Earth images we'll see in a minute, is not visible from any of our photo sites; it's an amphitheater-shaped topographic feature cut into Lake Lahontan sediments right along the east bank of the Truckee, beyond the dark basaltic hills on the right side of these two photos (east side of the Truckee).
I love the lone Cottonwood in this photo.
I went ahead and drew a few lines on Google Earth, then I rotated the resulting images to two different orientations so we could see a little more of the terrain. Be sure to check the north arrow in the upper right! The linears I drew are parallel or sub-parallel to the trend of the Walker Lane. You might be able to pick out a few more in the images (or go to Google Earth and looking around a bit).

The Walker Lane—on Google Earth (G.E.)and on other aerial representations such as air photos—shows up best at very small scales; that is, if we zoom way out and observe the area from way above, we can see the trend of the Walker Lane better than if we zoom way in, like we've done here. Nevertheless, individual strike-slip faults have been identified throughout the Walker Lane, and a few of these occur right in this area.
The A is centered on Wadsworth Amphitheater. The purple camera icon shows the location of the first photo.
The next G.E. image zooms in to the area we can see in the photos. I've got all three photo locations turned on, the A for Wadsworth Amphitheater, and my drawn-in linears. These linears aren't necessarily faults, although I'm pretty sure the one on the far left is. Unfortunately, because I currently have a very lousy internet setup (thanks Frontier!), I can't load very many research web pages. Google works well—and Blogger is a Google product—so I can blog even with 25 to 100% packet loss (it's 50% this very moment). And because Google Earth is a Google product, it's working for me also! (But I can't routinely access my non-Google email.)

Back to the faults and linears. I do know from personal experience that Walker Lane structures pass through the Lake Lahontan sediments in the area where the Truckee is running in a particularly linear fashion. I know this from some work I did a long time ago with J.O. Davis, who was studying Lake Lahontan and using tephra beds to date various aspects of the stratigraphy. The two linears or faults in the lower right of the image cross through the area where I've seen older Lake Lahontan sediments faulted and highly contorted by the major structures that pass through the area. I can't say that my particular linears are right on the major structures, and I can see other possible lineaments.

UPDATE 3Feb2017: I now have good internet back, and I've been able to compare my "map" to a geologic map of the area (Bell, Garside, and House, 2005). My linear passing through the hatchery location (below) is spot on for their most prominent fault, which was mapped as a dominantly extensional fault.
A zoomed in G.E. view of the area.
Not sure why, but I prefer to spell amphitheater with an "re" at the end: amphitheatre. I also have a hard time sticking the first "h" into the word!

I sure wish Frontier was a better internet service provider! (I never had this kind of semi-routine trouble with AT&T in eastern and northern Nevada; their every-day speeds were faster, and service was (generally) provided more quickly.

Back to the photos! 😊
Horizontal Lahontan shorelines and vertical basaltic talus stripes.
Some of the same talus stripes in Google Earth.
This photo shows a juxtaposition of talus stripes, faint shorelines, and layered Lahontan sediments.
On this trip, I focused mostly on trying to get good photos of the talus, because that's really what caught my eye. I later noticed that a few shots showed the Lake Lahontan sediments fairly well.
White, gray, greenish gray, and pinkish layers of the Eetza Alloformation.
While driving through the area, I'd been assuming that the prominently exposed white units were part of the Sehoo Alloformation. I was wrong! The Sehoo is barely identifiable in this zoomed in photo, and it's the older Eetza that is most conspicuous. In fact, the Sehoo here is mapped as gravelly beach deposits, so it mostly comprises the dark gray or brownish gray layered deposits above the whitish units.

Note: I usually use "formation" when talking about the subdivisions within the Lake Lahontan section, but "alloformation" is technically correct. In explanation, Bell et al (2005) say this, "Lake Lahontan and related subaerial deposits were considered lithostratigraphic units in the early studies of Morrison and were designated as formations. With the revision of the North American Stratigraphic Code in 1983, new allostratigraphic and pedostratigraphic unit definitions were added which allowed the definition of time-transgressive, lithology-independent rock units and soils."
Qsm = middle member of the Sehoo Alloformation; Qe = Eetza Alloformation.
After taking all these photos, it was time to move on, so I looked to the north, in the direction I'd be heading.
What I saw was more talus, more shorelines, and more Lahontan sediments. (And some fall colors.)
Selected Reference:
Bell, J.W., Garside, L.J., and House, P.K., 2005, Geologic map of the Wadsworth Quadrangle, Washoe County, Nevada: Nevada Bur Mines and Geology Map 153.

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