THE GEOLOGIC BASEMENT IN TEXAS: RESULTS OF A NEW STATE-WIDE SYNTHESIS

Production and wastewater injection in mid-continent oil and gas provinces are both associated with increases in the rate of felt seismicity and have turned areas with historically low earthquake occurrence into earthquake-prone regions. Much of this recent seismicity has occurred along faults within the crystalline basement, well below the basin-basement interface. In an effort to better characterize the nature of the geologic basement and associated seismic risk, the Center for Integrated Seismicity Research at the Bureau of Economic Geology began a review of basement geology across Texas. We present results of this state-wide synthesis, reporting the tectonic affiliation of mapped basement, depth to basement, and distribution of superjacent strata. Basement units in central and northwestern Texas, and exposed in parts of west Texas and the Llano uplift, record Proterozoic assembly of the crust, peneplanation, and subsequent rifting. In the Fort Worth and Delaware basins, Proterozoic units are now buried to depths in excess of 15,000 and 26,000 ft subsea, respectively. The edge of the Proterozoic crust is affected and obscured by Pennsylvanian-aged thrust sheets of the Ouachita belt that extend from northeastern Texas to the Marathon uplift in the southwest. Southeast of the Ouachita belt, geologic basement is composed of Mesozoic transitional crust formed during early extension of the Gulf of Mexico, now buried to >40,000 ft subsea. Superjacent strata across much of central Texas record prolonged periods of tectonic quiescence, with intervals of local rejuvenation during the Pennsylvanian and Permian reflected in the development of younger unconformities above the Amarillo Uplift and Central Basin Platform. Mapped variations in tectonic affiliation, depth to basement, and superjacent strata help place existing patterns of seismicity into a geologic context and better inform assessment of seismic hazard below the basement-basin interface.