MAPPING OF EOCENE INTRUSIONS WITHIN CRETACEOUS STRATA SUGGESTS LATERAL EMPLACEMENT

Multiple Eocene andesite intrusions (48-47 Ma) outcrop in the western El Paso, Texas region. Previous thoughts on the andesite were that these intrusions formed as separate laccolith structures. In this study we propose the andesite intrusions followed lateral emplacement via local Cretaceous aged shale strata. Mapping was conducted at three sites with exposed andesite and strata; Coronado, Three Sisters and Thunderbird. Our methods to accomplish this study include; field paper mapping, strike and dip measurements, digital mapping on Qgis software, and cross-section diagrams derived from these maps. Strike measurements along the Cretaceous aged strata range from 112° SE to 190° SW, with shallow dips ranging from 8° to 19°. Mapping geometry reveals lateral andesite distribution at multiple emplacement levels within Cretaceous strata dominated by shale and interbedded with limestone. Shallow dip angles of the intruded strata reveal minimal deformation at all three sites, while general strike and dip directions indicate singularly aligned strata orientations, as opposed to multiple cardinal directions typical of laccolith-type intrusion geometry. These factors indicate that the andesite was laterally emplaced as sills rather than multiple laccolith structures.