UTILIZING THE CONDUCTIVITY OF ROCKS TO MAP THE SUBSURFACE STRUCTURE OF THE THREE SISTERS EOCENE ANDESITE INTRUSIONS IN EL PASO, TEXAS

The westside of El Paso, Texas is dotted with Eocene andesite intrusions that made their way through the surrounding rock at locations like the Three Sisters, and contain large amounts of Cretaceous shale. The structure of this intrusion has been historically viewed as a laccolith or dome. The goal of this project is to determine whether this theory is accurate, specifically by using geophysics to examine the structure of the North Hill of the Three Sisters. It is possible the intrusions intruded the weaker layers of surrounding rock, such as shale, and along fault boundaries to make an anthill shape with over-stacking of layers of the shale and andesite below the Earth’s surface. To better determine the subsurface structure, I collected conductivity readings across suspected shale-andesite contacts using an EM31-Mk2 ground conductivity meter. This geophysical instrument penetrates to a depth of up to six meters. Data collected with the EM31-MK2 shows a clear differentiation between the conductivity of shale (25 to 40 mS/m) and andesite (3 to 5 mS/m). These large differences in conductivity allowed me to locate the contact zones between the shale and andesite, even under the Quaternary colluvium. My preliminary results reveal that the northern end of the hill is dominated by andesite, the eastern end of the hill is home to landslides, and the southern and western sides are where shale outcrops the surface.