Structural Controls on the Edwards Aquifer/Trinity Aquifer Interface in the Camp Bullis Quadrangle, Texas

Author DA Ferrill, DW Sims, AP Morris, DJ Waiting, and N Franklin
Year 2003
Description Structural analyses for geologic framework model of the area around the intersection of the Edwards and Trinity Aquifers
Publisher CNWRA, Southwest Research Institute®
Location Edwards Aquifer, Balcones Fault Zone, San Antonio Segment, Trinity Aquifer, Bexar County
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The purpose of this study was to generate a three-dimensional computer model and predictions of localized fault related deformation in the Edwards Aquifer and the Trinity Aquifer in the study area. The study also provided an analysis of the potential for hydraulic communication across the interface between the two aquifers, taking into account fault-related deformation and juxtaposition of the aquifers across key faults. The study was completed in December 2003 by Southwest Research Institute, San Antonio, Texas.

[From the Summary]
The purpose of the project reported here is to characterize the structural architecture of the Edwards and Trinity Aquifers for the area of the Camp Bullis 7 ½ minute quadrangle, and extending north to include Cibolo Creek in the southern part of the Bergheim 7 ½ minute quadrangle. Included in this analysis are tasks to generate a three-dimensional computer model of the Trinity and Edwards Aquifer, and perform field investigations to characterize the

mechanisms and products of localized fault-related deformation in the Edwards and Trinity Aquifers in and near the study area. An important objective is to analyze the potential for communication between the Edwards Aquifer and the Trinity Aquifers, taking into account fault-related deformation and juxtaposition of the aquifers across key faults.

Results of the project show the aquifer architecture throughout the study area, the location and interpreted geometry of the most important mapped faults in the study area, and the deformation
mechanisms and deformation style in fault zones in the rocks of both the Edwards and Trinity Aquifers. The three-dimensional geologic framework model of the Camp Bullis area reveals
(i) juxtaposition of permeable and relatively impermeable hydrogeologic units, (ii) structural thinning of the Edwards Aquifer and Trinity Aquifers, (iii) potential for cross-fault communication between the Trinity and Edwards Aquifers, (iv) faults expressed on the surface as potential infiltration pathways, and (v) maximum offset concentrated along a small number (two or three) fault systems. This information, along with an understanding of fault zone
deformation mechanisms and the role of fault zones as barriers or conduits, can assist in locating environmentally sensitive areas. It is useful for aquifer water flow path studies and contributes to
the identification of areas where communication between the Trinity and the Edwards Aquifers is suspected.

In this geologic framework model, the Edwards and Trinity Aquifers are subdivided into seven stratigraphic horizons which are offset by a network of 40 faults. Vertical offset (fault throw) ranges from near zero to an approximate maximum of 110 meters (361 ft). Displacement sense is normal, commonly down to the southeast, and lateral displacement gradients are small. In map view, fault blocks are elongate, with the long axis oriented NE-SW. Maximum offset is concentrated along three fault systems, the southernmost of which forms the northern boundary of the aquifer recharge zone, where rocks of the Edwards Group are in faulted juxtaposition with
rocks of the Glen Rose Formation.

Fault displacements within the Camp Bullis study area are too small to place the base of the Edwards Aquifer (Basal Nodular layer) against the permeable Lower Glen Rose layer. However, each fault decreases the effective aquifer thickness. This structural thinning of aquifer layers can cause flow constrictions, which in turn diverts flow and causes fluctuations in the local water table from fault block to fault block. Areas of such flow constrictions can be identified using a
map of fault throw distribution; constriction is greatest where fault throw is greatest.

In the Castle Hills quadrangle, immediately south of the Camp Bullis quadrangle, several faults offset the Edwards Aquifer by distances equal to or greater than its full thickness. Smaller fault displacements in the Camp Bullis area and the northern part of the Castle Hills quadrangle reduce the amount of direct juxtaposition of Kainer against Lower Glen Rose in these areas to a minimum. The lack of fault juxtaposition of the recognized highly permeable units of the Edwards Group and Glen Rose Formation in the Camp Bullis and Castle Hills areas suggests that simple juxtaposition is not likely to be a major source of aquifer communication in this area.

3DStress™ analysis of measured faults, and regional stratigraphic thicknesses based on published maps yield a stress system during faulting of: vertical effective stress = 15 MPa; minimum horizontal effective stress (F3′) = 4 MPa with an azimuth of 150°; and an intermediate principal effective stress = 9.5 MPa. When applied to the fault surfaces exported from the three-dimensional
geologic framework model, this stress tensor indicates that the dominant, NE-SW striking faults experience high slip tendencies and are well oriented to have accommodated regional strains developed within the inferred stress system. A few NW-SE trending faults
experience low slip tendencies and probably formed in response to local stress perturbations, indicating that local perturbations resulting from such effects as displacement-gradient-driven fault block deformation were not widely developed in this area. In addition to experiencing high slip tendencies in the inferred stress system, the predominant faults are also subject to high dilation tendencies. This combination of high slip and dilation tendencies implies that the major faults could have been effective fluid transmission pathways at the time of faulting. If a similar stress system were extant today, the faults would be in favorable orientations for fluid transmissivity.

Fault block deformation calculated using cutoff lines generated from the three-dimensional geologic framework model results in cutoff line elongations that rarely exceed 2% (positive or negative). These small cutoff elongations reflect the low displacement gradients on faults within the Camp Bullis study area. At the scale of the three-dimensional model, competent units exhibit gentle dips, which is consistent with relatively rapid lateral and vertical fault propagation, until intersection with other faults occurs (laterally) or intersection with a weaker mechanical layer occurs. This lack of steep lateral displacement gradients suggests rapid fault propagation with
respect to the rate of displacement accumulation on the faults.

Field work reveals interesting contrasts between faults in the Edwards and Trinity Aquifers. Faults with displacements of 5 m (16 ft) to tens of meters in the Glen Rose Formation (Trinity Aquifer) commonly have damage zones with widths on the order of meters, within which small faults and rotated fault blocks are common. Although faults with displacements of 5 m (16 ft) to tens of meters in the Edwards Group limestones typically have numerous associated small faults,
block rotation and bed tilting is not common. This characteristic difference in structural style between the Edwards Group limestones and the Glen Rose Formation appears to be related to lithologic differences and the resulting differences in mechanical behavior of the two stratigraphic sections. The Glen Rose Formation contains both competent massive limestone beds and incompetent argillaceous limestone and shale beds. Incompetent beds tend to arrest fault propagation during fault growth. Consequently, with increasing fault displacement, fault tips (terminations) episodically propagate then arrest. Continued displacement on a fault with an
arrested fault tipline will produce fault tipline folding and associated local deformation such as  intense small scale faulting. Resulting fault damage zones can be quite complex and variable along a fault, related to the structural position (including displacement magnitude) and the associated mechanical stratigraphy. Permeability in fault zones and fault blocks is likely to be strongly influenced by the different deformation styles in mechanical layers, and the deformation progression with increasing fault displacement.

The large fault surfaces that cut multiple layers depicted in the Camp Bullis geologic framework model provide potential pathways for both vertical and lateral movement of water and hydraulic
communication between aquifers. These fault surfaces along with localized zones of relatively intense small scale faulting and extension fracturing, and limestone solution (karst conduit formation) provide likely communication pathways between the Trinity and Edwards Aquifers.

The structural analyses presented in this report provide the framework for more detailed investigations of groundwater levels, multiwell pumping (drawdown) tests, tracer studies, and geochemical investigations to further investigate potential groundwater communication between the Trinity and Edwards Aquifers in the Camp Bullis and Castle Hills Quadrangles.