The Edwards Limestone in the Balcones fault zone, south-central Texas




Abbott, Patrick L.

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The Edwards Limestone (Albian) is a mosaic of shallow water, back-reef, carbonate lithofacies averaging about 450 feet thick, that have been dolomitized, chertified and dedolomitized. Intermittent subaerial exposure during and shortly after deposition of the Edwards resulted in secondary solution-enlargement of some primary voids. Slow upwarping of the northwestern margin of the subsiding Gulf of Mexico basin elevated the Edwards Group above sea level late in the Cretaceous. Down-to-the-coast, "en echelon," normal faulting along the Balcones system during the Early Miocene accentuated the topographic position of the Edwards above sea level. Rejuvenated Gulfward flowing streams cut into the upthrown fault block and exposed the top of the Edwards Limestone in deep canyon bottoms. This created discharge sites that initiated a continuously circulating ground-water system in the Edwards Limestone. Early porosity systems have increased in size through the self-ramifying cavern solution process that occurs in carbonate rocks. The resultant cavern system presently supplies water for most of south-central Texas. Some faults have acted as barriers which have caused a preferential channelization of ground-water flow into separate but parallel systems. Ground water moving toward low discharge points, e.g. Comal Springs, has created widespread cavern systems. Removal of much Upper Cretaceous overburden during the Neogene and declines of the water table due to stream incision have changed the Edwards Group in the eastern Edwards plateau into a lower yield unconfined aquifer. The prolific artesian aquifer system beneath the western margin of the Gulf Coastal plain is still increasing in size.