AN INTEGRATED GEOPHYSICAL STUDY OF THE SUBSURFACE STRUCTURE OF THE MARFA BASIN, TRANS-PECOS TEXAS

Trans-Pecos Texas and adjacent Mexico are part of what formerly constituted the southwestern margin of the Paleo-North American plate (Laurentia). This area has experienced several major tectonic events, extending from the Precambrian (Grenville orogeny) into the late Cenozoic (Laramide orogeny, and Basin and Range/Rio Grande rift extension). The basins in Trans-Pecos Texas are one important key to understanding these tectonic events and are also important from the standpoint of groundwater resources and petroleum potential. The Marfa basin initially formed in the late Paleozoic as a result of the Ouachita orogeny. However, the same general area is bounded by a series of large normal faults along the Sierra Vieja to the west. Abundant fault scarps and the Valentine earthquake in 1931 attest to the continuing tectonic activity. We are studying the subsurface structure in this area by integrating gravity, magnetics, remote sensing, GIS technology, and drill hole information. Thus, we are compiling a large database of geological, geophysical, and other geospatial data. We have constructed four integrated cross-sections by modeling gravity profiles. In each case, thickness variations of Phanerozoic sedimentary rocks explain a major portion of the observed gravity anomalies. However, as we proceed southward from the Van Horn, Texas area, the importance of the pre-Cenozoic strata increases. The best examples of this phenomenon are Lobo Flat on the north, which is a Cenozoic graben, and the Chalk Draw fault to the south. This fault bounds the Paleozoic Marfa basin and is associated with a steep gravity gradient. At the same time, the basement south of this fault must be denser than to the north. Another intrabasement feature is a linear gravity high that correlates spatially with the Davis Mountains. We are using the experience our group has gained by studying areas to the north to further investigate basement structures in the area.