SEDIMENTOLOGY AND MICROFOSSIL ANALYSIS OF MYSTERY STRATA FOUND AT THE THREE SISTERS ANDESITE INTRUSIONS IN EL PASO, TEXAS

Trans-Pecos Texas and southern New Mexico contain several volcanics and intrusives of late Eocene age. The Three Sisters andesite is a porphyritic sill or laccolith exposed near the Rio Grande river in El Paso, Texas. The southernmost Three Sisters intrusion contains an unknown layer of yellowish-brown strata, with a total thickness of about 2 meters. It is thought to be Cretaceous in age, however, its color and texture distinguishes it from other exposed strata nearby. The outcrop of mystery strata has been folded, and has been rotated to a near vertical position. Originally, this rock was thought to be a shale, however, microscopic analysis shows it to be a limestone. Thrust faulting in this area has displaced and repeated Cretaceous strata and identification of this limestone will help reconstruct largely buried structure. This limestone may also provide insight into the stratigraphic and structural factors controlling the Eocene andesite intrusions. Because of the rarity of macrofossils in the formation, I used a microscope and looked for microfossils to better determine its age. However, no microfossils were present except for unidentifiable crushed shells. Therefore, the bedding, lithology and sedimentology were compared to potential matches for the limestone. I analyzed the sedimentology by taking various measurements of thickness and bedding, and making observations of grain size, color, hardness, and sedimentary structures. A detailed stratigraphic column proved to be an almost exact match to the Late Albian Muleros Formation, the nearest exposure of Cretaceous strata. Other candidate formations include the Cenomanian Del Rio and Buda Limestones, However these are much thicker bedded and contain more fossils.