Geologic mapping of the Edwards Group resulted in the identification of seven informal hydrostratigraphic units (Kainer and Person Formations) that exhibited lateral continuity through the San Marcos platform facies. The Kainer Formation comprises, in ascending order, the basal nodular member, the dolomitic member, the Kirschberg evaporite member, and the grainstone member. Ascending units of the overlying Person Formation comprise the regional dense member, the leached and collapsed member (see below), and the cyclic and marine member. The disconformably overlying Georgetown Formation is included as an eighth hydrostratigraphic unit because of its hydrologic connection with the Edwards Group.
Three-dimensional (3-D) geologic modeling of the aquifer can quantitatively evaluate the connectedness of hydrostratigraphic units across fault and fracture zones to determine the distribution of geologic units, structural features, and other parameters, such as porosity and permeability. These parameters are complex variables that reflect original depositional conditions, alteration and dissolution, and dislocation. Geologic 3-D framework modeling is also useful for visualizing the geologic structures within fault zones and the interactions of en echelon fault strands and related flexed, relay ramps. This type of modeling has allowed the USGS to evaluate the controls on aquifer permeability and to provide insight about aquifer processes to researchers and water managers alike.
A newly-released, 3-D EarthVision (EV) model (USGS SIR 2004-5226) reveals the subsurface geology and ground-water flow units of the Edwards and Trinity aquifers where water wells are 200-1000 feet or more in depth. This digital, three-dimensional faulted hydrostratigraphic model was constructed to illustrate the geologic framework of the Edwards aquifer system in the northern San Antonio area of Bexar County, Texas. The model is based on mapped geologic relationships that reflect the: (1) complex structures of the Balcones fault zone, (2) detailed lithologic descriptions and interpretations of about 40 principal wells (and qualified data from numerous other wells), and (3) gross geometry of the Edwards Group hydrostratigraphic units derived from prior interpretations of depositional environments and paleogeography.
This digital model depicts water well data, the complicated intersections of numerous major and minor faults in the subsurface, as well as their individual and collective impacts on the continuity of the aquifer-forming units in the Edwards Group and the Georgetown Formation. The model also aids in visualizing the lateral connections between hydrostratigraphic units of relatively high and low permeability across the fault strands and allows for detailed examination of the extent of fault dislocation from place to place, and thus, the extent to which the effective cross-sectional area of the aquifer is reduced by faulting (see below).
The Seco Creek model shows the complex intersections of both major and minor faults in the subsurface and their impacts on the continuity of the aquifer-forming units of the Trinity and Edwards Groups. This model is being designed to explore the extent of faults from place to place and the effects these subsurface structures have on water communication between fault boundaries. Our main goal is to combine data and information sources to accurately represent the faulted lithologic units comprising the hydrogeology of the area. This approach is self checking in the sense that drill hole information confirms HEM and surface geology, the HEM information confirms drill hole and surface geology, etc. The resultant model is more accurate because of the diversity and self-referencing nature of the data and information. The interactive model is also an educational aid to understand the many complexities of the aquifer as defined by the local geology and structure.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://esp.cr.usgs.gov/info/edwards/modeling.html
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Page Last Modified: Tue 4-Oct-2005 14:53:57 MDT
