PRECAMBRIAN BASEMENT PENETRATING PALEOZOIC NORMAL FAULTS IN THE LLANO INLIER AS ANALOGS FOR FAULTS REACTIVATED BY OIL AND GAS ACTIVITIES IN THE FORT WORTH BASIN

Earthquake swarms (DFW Airport, Cleburne, Irving and Venus) in the Fort Worth Basin associated with the Barnett Shale play occurred along normal faults (Quinones et al 2017) that cut the crystalline basement and the overlying Paleozoic rocks, but not the Cretaceous or the upper part of the Pennsylvanian Atokan Stage (Magnani et al 2017). These types of faults are exposed at the surface in the Llano inlier consisting of Precambrian basement rocks overlain nonconformably by Paleozoic sediments which are disconformably overlain by Cretaceous sediments. The Marble Falls 1:24000 sheet (Barnes, 1982) provides constraints on the nature of faults that cross-cut basement and Paleozoic rocks but not the overlying Cretaceous sediments. The Paleozoic tectonism consists of numerous normal faults that exhibit maximum displacements (100 meter scale) that juxtapose Atokan Stage of the Pennsylvanian carbonates (Marble Falls Limestone and clastic rocks of the Smithwick Fm) against Precambrian Town Mountain Granite. At the Hoover Point roadcut on FM 1431, cm to meter scale normal faults crosscut glauconitic cross-bedded sandstone. These faults are shear-failures with dips >70 degrees. The faults are coated with red clay, most likely derived from the overlying karst in Ellenburger Group. Meter scale faults have spacing at the decimeter scale whereas cm scale faults have meter-scale spacing. The fault-bounded hill at Hoover Point is a graben that has experienced topographic inversion. On the northwest side of the city of Marble Falls, another fault, mapped over 16 km on the Marble Falls sheet preserves remnants of upturned Pennsylvanian beds on the hanging wall of a normal fault whose footwall is Precambrian Town Mountain Granite. The km scale spacing of these 100 meter scale displacement normal faults is recorded on the large scale geologic map of Texas which clearly shows that none of the Paleozoic faults cross cuts the disconformity with the Cretaceous. The fault on the northwest side of the town of Marble Falls, is a topographically inverted lineament governed by the steeply dipping beds truncated by the footwall crystalline rocks. If this topographic inversion is a recent geomorphic feature, the Paleozoic faults have not been presently active indicating that recent earthquake activity is due to reactivation of Paleozoic faults stimulated by changes in effective stress resulting from oil and gas activities.