ABSTRACT The Edwards (Balcones Fault Zone) Aquifer is structurally controlled by the system of normal faults following the Balcones Escarpment, with major domains, including contributing, recharge (unconfined), and artesian (confined) zones, dictated by the large-displacement (50 m to >250 m throw) normal faults and depth of erosion. Faults and extension fractures, in many cases enhanced by dissolution, localize recharge and flow within the Balcones fault zone and into the subsurface of the artesian zone. Juxtaposition of the Edwards with other aquifers provides avenues for interaquifer communication, while juxtaposition against impermeable layers and concentration of clay and mineralization along faults locally produce fault seals for compartmentalization and confinement. Fault block deformation, including small faults and extension fractures, leads to aquifer permeability anisotropy. Faults also locally provide natural pathways for groundwater discharge through springs above the confined (artesian) zone. Although the importance of joints and faults in the Edwards (Balcones Fault Zone) Aquifer system is recognized, there has not been a systematic analysis of the meter-scale structures in the Edwards and associated confining units and their influence on groundwater flow. Here, we review evidence from several key areas showing that an analysis of faults and fractures in the Edwards (Balcones Fault Zone) Aquifer and associated aquifers and confining units is needed to characterize structural fabrics and assess the permeability architecture critical for the next generation of groundwater modeling of the aquifer.

Abstract A valid structural geologic interpretation should simultaneously honor available surface and subsurface data (e.g., well and seismic) to constrain structural geometry; ideally be restorable to an original unstrained condition – taking into account the possibility of three-dimensional (3-D) movement, volume loss, or volume gain; and incorporate structural styles known or expected for the mechanical stratigraphy and deformation conditions in the region. Incorporating what is known about the mechanical stratigraphy can provide crucial constraints on viable structural styles, for example, where faults are likely to cut across stratigraphy vs. where fault displacement is likely to be accommodated by alternative mechanisms (e.g., ductile flow or folding). Conversely, the structural style can often help to understand the mechanical stratigraphy, including the recognition of dominant competent or incompetent mechanical stratigraphic units. Using this approach provides the interpreter another set of constraints toward improving interpretations, testing hypotheses, and developing valid structural interpretations. Outcrop characterization provides insights into the influence of mechanical stratigraphy and structural position on seismic- and subseismic-scale deformation in the layers. Examples of extensional deformation in Cretaceous carbonate strata in central and west Texas illustrate the utility of considering how mechanical stratigraphy influences the development of different deformation styles, even where deformation conditions are otherwise similar.