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Hypogene Processes in the Edwards Aquifer in South-Central Texas,
a New Conceptual Model to Explain Aquifer Dynamics*
Geary M. Schindel1, Steven Johnson1, and E. Calvin Alexander2
Search and Discovery Article #80019 (2008)
Posted October 25, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, TX, April 20-23, 2008
1Aquifer Science, Edwards Aquifer Authority, San Antonio, TX ([email protected])
2Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN.
The Balcones Fault Zone Edwards Aquifer of south-central Texas is one of the most important karst aquifers in the United States and provides water to 1.7 million people and for critical habitat for endangered species. The Edwards Aquifer extends 400 kilometers from Del Rio, east to San Antonio though Austin, and northeast to Bell County. The aquifer is from 10 to 60 kilometers wide and in places, more than 1,200 meters. The aquifer is contained within the Edwards Group limestone and associated units (Georgetown limestone). The Edwards and associated units were deposited in late Early Cretaceous time and are 150 to more than 300 meters thick. The Edwards Limestone, since deposition, has undergone subaerial exposure, burial in the middle Cretaceous, faulting in the Miocene, uplift and erosion. Faulting is mainly northeast-southwest trending, down to the gulf, en echelon normal faulting. Researchers have proposed epigene (near surface) karst processes, driven by circulating meteoric waters, formed the aquifer along paleokarst features. New interpretations suggest an additional process that contributed to the formation and structure of the Edwards Aquifer. Epigenetic karst theory assumes karst features are produced only during its downward or horizontal groundwater movement, but Klimchouk (2008) concludes that rising waters from depth are important agents of karst development. Regional flow systems, such as the Edwards Aquifer, terminate in springs where the groundwater returns to the surface. Karst processes, in general, and mixing corrosion specifically, can and do operate in the artesian zone of the aquifer. Secondary permeabilities in carbonate fabrics, formed by upward transverse speleogenesis, is an important role in water and oil reservoir formation.
· Edwards limestone has limited initial permeability
· Secondary/tertiary permeability
§ Epigenic processes –Recharge Zone
§ Hypogenic process – Artesian Zone
· Somewhat selective depending upon geography
· High yield wells very common in Artesian Zone
· Breccia zones increase permeabilities across units
· Convergent flowpaths to springs
· Analysis of Robber Baron Cave shows hypogene morphology (Paleo Spring)
· High assimilative capacity
§ Water quality will ultimately come into equilibrium with recharge waters
· Allows quality and quantity management on regional basis.
Elliott, W.R., and G. Veni, 1994, The Caves and Karst of Texas; a guidebook for the 1994 convention of the National Speleological Society with emphasis on the southwestern Edwards Plateau: 342 p.
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Klimchouk, A.B., 2008, Principal characteristics of hypogenic speleogenesis: GSA 2008 Annual Convention Program Abstracts, Houston, Texas, in press.
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