New Mapping of Late Paleozoic Faults and Local Igneous Aquifers,Llano Uplift
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Abstract Recent geologic Mapping of Backbone Ridge, which includes the well-known Hoover Point road cut, within the Kingsland, Texas Quadrangle establishes a link between late Paleozoic faults and local igneous aquifers found within the Llano uplift. The Llano uplift exposes Proterozoic regional metamorphic rocks and granite batholiths. Late Paleozoic NE-striking normal faults segmenting the uplift were caused by the Marathon-Ouachita thrust loading. Backbone Ridge has previously been mapped by Barnes (1976, 1:24,000-scale). Mapping for this project was at 1:8000 scale. Backbone Ridge and Hoover Point expose Precambrian Town Mountain Granite overlain by Middle Cambrian through Lower Ordovican sandstones and carbonate rocks of the Hickory Sandstone Member of the Riley Formation, Moore Hollow Group through the Tanyard and Gorman Formations of the Ellenburger Group. Faults at the Hoover Point and Backbone Ridge strike NE and dip 55 – 80 NW and SE. Fault breccia 1 – 20 m thick consists of Town Mountain Granite and Tanyard Formation fragments. Drag folds well exposed at Hoover Point indicate normal slip. Major range-front faults show ≤2,000 ft offset. Faults control several aspects of the Central Texas aquifer system: a) all springs and most oak trees within the mapping area are located along faults, b) fault-related fractures increase permeability in paleovalleys of highly weathered granite, the local aquifer of the Llano uplift, and c) geochemical water analyses allow water flow to be traced. This fractured and weathered igneous aquifer may be analogous to granite wash hydrocarbon plays.