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From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
39th Annual Meeting, South-Central Section of the Geological Society of America, San Antonio, Texas, April 1-2, 2005 Structural Characteristics of the Balcones Fault Zone and Edwards Aquifer within the Austin-San Antonio-Del Rio Region of Central Texas Abstract Maps, cross sections, and geologic data of the
Balcones Fault Zone will be presented to demonstrate similarities and
differences in the structural characteristics affecting strata composing
the Edwards aquifer segments. Between Uvalde and Austin, large normal
faults (throws of 300 to 800 ft) are main structural features that control
the aquifer’s structural position. The prolific San Antonio segment
contains a network of large and small faults and fault blocks and structural
relay ramps between en echelon fault tips. Between 1,000 and 1,900 ft
of composite structural relief exist across the San Antonio aquifer. In
contrast, the prominent structural element of the northern aquifer segment
is the large Mount Bonnell fault. The Mount Bonnell fault bounds the western
edge of the aquifer near the Colorado River, but northward this fault
cuts across the aquifer outcrop belt and marks the recharge zone’s
eastern margin. Fault intensity and the composite structural relief, 1,600
to 600 ft, decreases northward across the northern segment. South of the
Colorado River, the Mount Bonnell fault also marks the western edge of
the Barton Springs aquifer segment that contains numerous large- to small-scale
fault blocks. Balcones faults cutting Edwards Group strata within the
southern Edwards Plateau aquifer near Del Rio display relatively small
displacements and do not significantly control the aquifer’s structural
position. Throughout the study area, faults are commonly surrounded by
zones of breccia and highly fractured strata. These zones are wider for
faults with larger throws. Within some zones dissolution has caused open
fractures, vuggy areas, and small caves. However, at some locations breccia
and highly fractured strata contain clay-rich matrix and fracture-fillings.
Fault patterns throughout the Balcones Zone suggest that fault attributes,
such as intensity and geometry, differ within different strata intervals,
probably because of rock property differences. Steeply dipping clay-rich
strata adjacent to limestone fault breccia associated with a large fault
in the northern aquifer present a possible example of the potential sealing
and smearing effects of faulted clay-rich Upper Cretaceous deposits. |