STRATIGRAPHIC AND STRUCTURAL CONTROLS ON THE EMPLACEMENT OF EOCENE INTRUSIONS IN THE EL PASO, TEXAS REGION

Through the UTEP-ROCCS (University of Texas at El Paso-Research Opportunities for Community College Students) program, we are using surface mapping techniques to identify geologic structural features and correlate Cretaceous shale and limestone strata between the three hills of Three Sisters andesite outcrop site to further understand the geological processes that appear to dictate emplacement of Eocene intrusions. Although previous researchers assumed nearly all the andesite bodies had laccolith (lense) shapes, this may only be true for the larger intrusions (>1 km in diameter). It appears the smaller sized intrusions’ shapes are more strongly controlled by local structure and stratigraphy and may resemble an 'ant-hill' shape rather than laccolith. The smaller andesite outcrops appear to be a series of stacked sills that are separated by weak shale layers as well as connected through dikes that may follow the trends of strike-slip faults. The methods being used to determine the validity of the 'ant-hill' shape model include scouting all of Three Sisters outcrops to search for signs of deformation (faults and folds) as well as correlating stratigraphic beds with known formations to determine if there has been any overthrusting or if any beds are missing. The current data collected suggests that compressional stress at different rates from the East (faster) and West (slower) created warped anticlines that line up with mini overthrusts and several strike-slip faults that are seen at the South Sister with andesite running along them. This suggests a cross hatching of andesite intrusions in three planes like an 'ant-hill'.