20 December 2013

Friday fold: Another from Cristo Rey

Posted by Callan Bentley

The laccolith of Cristo Rey, at the Chihuahua / Texas / New Mexico triple point, is host to some cool geology. It’s cored by the Campus Andesite (47 Ma, Eocene) but surrounding the intrusion are a slew of sedimentary rocks, include the Turitella-bearing limestones of the Buda Formation and the shales and sandstones of the Mesilla Valley Formation.

My colleague and friend Joshua Villalobos of El Paso Community College in El Paso, sent me this photo last week showing a sweet fold – perhaps the lower half of a recumbent fold. The pre-Eocene strata (some of which bear dinosaur footprints, indicating a Mesozoic age) were folded and faulted as they ‘slid off’ the blister of magma in the Eocene. We’ve seen other folds from this general area before. The axial plane appears to be close to horizontal immediately to the left of the person at center (for scale):


I’m not totally sure about that, though, as it looks like erosion has chewed into this fold and then, later, new strata were deposited atop the truncated strata of the fold:


At least, that’s my initial reaction, looking at the photo. Maybe I’ll get to visit this site in March, when Joshua and I bring 24 lucky “field exchange” students to Cristo Rey in person.