Tuesday, December 6, 2016

More From the Road: Tilted Every Which Way?

I stopped while driving through the Virgin River Gorge in northwest Arizona to take this picture of some of the dipping sed rocks that are common through the greater Colorado Plateau area. (This area, lying barely within the Basin-and-Range province, is just outside the Colorado Plateau proper, but the rocks here are correlative to those throughout the plateau region, and they have been subjected to some of the same tectonic forces.)
It looks like the sedimentary layers are dipping every which way!
This second photo zooms in just a bit, focusing on the far hill, which shows a slight bend in some of the layers.
I've drawn in a few of the beds, as before, and point out a dip slope formed on the top of the reddish layer.
This Google Earth image of the area shows the photo location in purple.
I decided to see whether the beds were really dipping every which way. I used beds I could identify in Google Earth, picked out two points along the beds that were at the same elevations, and drew strike and dip symbols from these two points (method described here). The dip slope labeled above is the small lens-shaped hill beneath the central strike-dip symbol. It became apparent from looking at Google Earth that the primary reason the dips in the photos look cattywampus is because the rocks in the foreground are dipping toward the photographer (to the WNW), and most of the other beds are dipping in a more northerly direction (NW, N, or NE).
The same Google Earth image with some hypothetical strike-dip symbols.
So, how did I do on the strikes and dips? And is there anything else going on in the area? Well, okay (ish) and yes.
Map I-2165 (Bohannon et al, 1991) courtesy USGS, overlain on Google Earth.
The Cedar Wash high-angle reverse fault cuts right through the area, separating the strata dipping toward the west (the foreground strata in our photos) from strata dipping in a northerly direction. You can see, by clicking on the several images and going back and forth between them, that I did well on the strikes on the west and east, and not so well (in general) on the strikes in the center. Either that, or the strikes of beds in the center varies more than shown on the map (I'm pretty sure I did pretty well on the labeled dip slope, but I will never climb that hill to check it out!)

Read a little about the Cedar Wash fault and the general geology of the region here.

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