Lower Cretaceous Geology, Northwestern Karnes County, Texas

Abstract

Northwestern Karnes County, Texas, is underlain by about 5,000 ft of Lower Cretaceous sediments. Most of this sequence consists of two dolomitic limestone bodies which were deposited in the backreef provinces of two barrier-reef complexes. Each carbonate body underlies a blanket-type deposit of impermeable shale and shaly limestone; hydrocarbon accumulations are in the carbonates below the upper impermeable sequence but have not been located below the lower. The sediments were deposited on the nearly level San Marcos platform and in the more rapidly subsiding Karnes trough which crosses the platform to connect the East Texas and Rio Grande basins. Post-Early Cretaceous regional tilting on the southeast added to depositionally produced dips on the northwestern side of the trough and subtracted from them on the southeastern side. Hydrocarbons probably accumulated in the structural closure formed along the trough's southeastern hingeline before the directions of depositional dips on the southeastern side were reversed. Up-to-the-coast faulting occurred to trap part of these hydrocarbons before further regional tilting emptied the closures. Isopachous maps of key-bed-bounded units reveal the position of the critical southeastern hingeline as ell as the overall configurations of the Karnes trough and surrounding San Marcos platform. Histograms and scatter diagrams of various production, structural, and stratigraphic quantities verify and dramatize the relations established by the conventional isopachous and structural contour maps. Normal faults of the Sample fault system follow the trend of the Karnes trough through the study area. Two down-to-the-coast normal faults with a total vertical displacement of about 1,400 ft exist on the northwestern side of the trough and at least 12 up-to-the-coast normal faults with a total vertical displacement of about 1,400 ft occur on the other side. The total horizontal displacement of all faults is about 2,000 ft. The nearly equal vertical displacements, the opposing fault pattern, and the geometric configuration of the strata closely fit a gravitational slide-block model. The down-to-the-coast faults are thought to flatten at depth to represent the major slip planes along which the slide block moved downdip toward the axis of the Gulf Coast geosyncline. A potential v id 2,000 ft wide was generated at the head of the slide and up-to-the-coast faulting occurred to fill the potential void. The potential void concept can be extended to provide a philosophical basis for a quantitative approach to many extensional tectonics problems, especially those connected with normal faults. The three basic tectonic mechanisms which create potential voids and the resulting normal faults are punch or sag type stresses, extensional flexing, and lateral separation. The spatial characteristics and historical development of the faults associated with the several types of potential voids are distinctive and predictable.