7 April 2014

Contact between Muleros Andesite and Mesilla Valley Formation shale at Mt. Cristo Rey

Posted by Callan Bentley

There are two rock units in this photo. One is igneous, one is sedimentary. Can you find the contact between them?


It’s somewhere along this dashed line…


The Mesilla Valley Formation is Cretaceous shale with some sandstone. The Muleros Andesite (pretty much identical to the Campus Andesite you find at UTEP) is Eocene.

Here’s a closer, more precisely-constrained, look at it:


…but that one is in the shade. It’s bolder in the direct line of the Texas sun:

IMG_2050 IMG_2050anno

NOVA and EPCC students got to check out this contact in person on our field course over spring break:



In the next photo, your perspective is orthogonal to the contact. You’re looking “through” a screen of shale into the andesite beneath:


…Same here:


We’ll be returning to these rocks again this week, to look at some additional details of the contact.