Refining the Conceptual Model for Flow in the Edwards Aquifer Characterizing the Role of Fractures and Conduits in the Balcones Fault Zone Segment

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2004

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The south-central section (Figure 1) of the regionally extensive Edwards aquifer is developed in 450- to 900-ft-thick Lower Cretaceous (Albian) platformal carbonates (Hovorka and others, 1996). Original sediments composed of aragonite, calcite, dolomite, and gypsum have been extensively replaced by calcite within the aquifer and are in the process of forming a highly porous and strongly heterogeneous limestone rock. Hydrologically significant heterogeneities within this rock include variable rock fabrics and structural features. Variable rock fabrics include lateral and vertical variation of depositional facies character in response to Cretaceous depositional processes, which has formed beds of varying solubility and mechanical properties. These variable rock fabrics are stacked to form regionally extensive stratigraphic intervals having distinctive rock properties that are mapped as formations and hydrostratigraphic members. Major stratigraphic units referred to in this paper include the Kainer and Person Formations of the San Marcos Platform, Devils River Formation of the San Marcos Platform margin, and West Nueces, McKnight, and Salmon Peak Formations of the Maverick Basin (Figures 1a and 2). Syndepositional karst was developed at the top of and possibly within the Edwards Group on the San Marcos Platform, and this karst has created a zone of high permeability of unknown continuity at the top of the Edwards Formation (Maclay 1995; Hovorka and others, 1998, their figure 29). Regional doming and intrusion of Upper Cretaceous mafic igneous bodies through the Edwards Group in the Uvalde Uplift created additional complexity in this part of the aquifer (Figure 1b). Extensional down-to-the-coast faulting forming the Balcones Fault system overprinted earlier-formed heterogeneity. Faulting had a critical role in aquifer evolution because it (1) increased permeability by forming fracture networks and (2) greatly increased hydrologic gradient by uplift of the base of the Edwards Group to elevations greater than 1,500 ft above sea level in the west part of the aquifer, whereas at the maximum downdip extent of the freshwater aquifer the top Edwards is at 3,400 ft below sea level (Figure 1b).

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