Geology on the Wing Wednesdays #12 – Fault in Pennsylvanian carbonates, El Paso, Texas

@Neotectonics and I flew over this fault on our way to El Paso a few weeks ago. It is an excellent exposure of a reverse fault in the Magdalena Formation, a mixed clastic-carbonate succession in the Franklin Mountains near El Paso, Texas (hint – its in the left-center of the image, just to the left of the larger arroyo). Coordinates are 31.9591601917, -106.517622624 if you want to copy-paste them into Google Earth. Pretty beautiful alluvial fan too!

fault-in-el-paso-1

I include a zoom-in to show the details – because the beds are steeply dipping to the right (and upside down in this view) and cutting across topography, there is apparent strike-slip offset of the beds. However, this appears to be a reverse fault, with the lefthand side as the hanging wall. This fault must have formed prior to the tilting of these beds during Laramide deformation. The exposure is kind of a mind-bender, so twist it around in Google Earth and see if you agree with me (I might be wrong!).

fault-in-el-paso-1-zoom

I include a very basic line drawing (remember, I am not a structural geologist) of the fault for you to critique. We are looking almost right down the fault plane in this view, so the fault looks fairly straight. Scroll down to see the map view of this fault to see how it is influenced by modern erosion and topography.

 

fault-in-el-paso-1-zoom-draw

Below is a Google Earth screengrab with a line drawing below for good measure. What you see is that the fault plane must be dipping south due to the rule of V’s.

fault-in-el-paso-2

fault-in-el-paso-2-draw

Finally, here is a geologic map, with the fault shown in the red circle. There is another fault, which can be seen in the image above (right at the bend in the arroyo), but is not as well exposed as the northern one. Good mapping by Harbour!

geologic map of franklin mountains

By the way, if you want to download this map or any other geologic map in the U.S., go to this fantastic USGS website!

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