Geology of Cerro de Cristo Rey
Just west of El Paso, Texas and the Franklin Mountains there is a mountain called Cerro Cristo Rey (Christ King Hill). During the Border to Beltway Field Exchange I and some fellow students from NVCC and El Paso Community College got to tour this mountain guided by UTEP graduate student, Eric Kappas. This mountain sits right on the border of Mexico and the United States along the banks of The Rio Grande. Here is an aerial photo giving you the general position of the mountain relative to El Paso:
Magma Intrusion
The stratigraphy of the Cristo Rey area consists of Cretaceous aged sedimentary rocks that have been intruded by an igneous mass called the Moleros Andesite which now stands out in high relief, forming the mountain peak. The oldest unit of these sedimentary strata consists of a black shale (Mesilla Valley Shale) overlain by the Anapra Sandstone Formation (Sarten Member of the Mojado Formation), another shale, and then a fossiliferous limestone (Muleros Formation). At one location we visited the Andesite actually has inclusions of shale which supports the assumption that the shale unit is indeed older than the Muleros Andesite. Here is an illustration showing you the supposed sequence of events that would form Cristo Rey as it looks now:
The Muleros Andesite is considered hypabyssal which means it crystallized and intruded under shallow depths below the Earth’s surface. Whenever magma came into contact with The Muleros this weak shale unit acted like pudding under the intense heat and pressure. This led to ductile structures such as ptygmatic folds (chaotic folds) not far from the contact zone. Here is a picture of some of these amazing folds in the Muleros Formation exposed along the edge of an arroyo:
Believe what you’re seeing. This is a syncline that sits on top of an anticline! The only thing I can interpret from the chaotic nature of these folds is how black organic shale must have a tendency to to deform in a ductile manner and act more like a fluid when subjected to pressure and heat even at the “shallow” depths which the Muleros Andesite intruded. This leads me to wonder whether the Mesilla Valley Shale is highly ductile due to its’ rich organic components or fineness of grain size.
Once you walk a few yards upstream from this outcrop the black shale beneath your feet abruptly disappears and turns into this dominantly white felsic rock with dark specks throughout it. This is the contact between the Mesilla Valley unit and the Muleros Andesite with a 50 million year gap in between the two:
The Andesite exposed in Cristo Rey is porphyritic in texture with Diorite xenoliths, specks of Albite, Hornblende, and dark nodules of older Andesite that recrystallized to give it a different texture.
In fact, the andesite intrusion exposed in Cristo Rey is not only found here but there is also another igneous mass with a similiar composition and date exposed roughly 3 miles to the South-East around the campus of UTEP, appropriately named “Campus Andesite”. These two formations are one in the same, both of which are thought to be caused by the Laramide Orogeny, the subduction of the Farallon Plate to the west. The age of the Muleros Andesite (~49 Ma) does fall within the time period of the Laramide Orogeny (70-40 Ma) and the long axis of the Andesite pluton is parallel to the faults of the Laramide.
Crustal Extension
The presence of normal faults around Cristo Rey indicate that other forces have played a role in shaping the region. Normal faults are typically associated with extensional forces and not plate-to-plate collisions or magma intrusions. Here is a picture of one these extensional faults found along the northern side of the laccolith:
Although the direction of movement may not be obvious with this fault I interpreted this to be normal fault where the hanging wall (on the right) has moved down a few meters relative to the footwall (on the left). I have highlighted in blue what I used as a marker bed to get an idea of the amount of displacement and direction of movement:
This outcrop appears to be evidence that Basin and Range Extension (Rio Grande Rifting) has affected this region. Regretfully I did not think to record the orientation of this fault while I was on site, however, judging by the direction of the shadow coming off this escarpment and considering the time of day (somewhere around 2-3 pm) I’m guessing this fault has a roughly east-west orientation. I was under the impression that both Basin and Range Extension and Rio Grande Rifting is typically associated with North-South trending faults so I’m not willing to say for definite that this is a product of either one.
