Characterization of Fractures in Limestones--Northern Segment of Edwards Aquifer and Balcones Fault Zone, Central Texas

Edward W. Collins

Fracture distributions, orientations, and densities in Comanche Peak, Edwards, and Georgetown limestones (Edwards aquifer strata) were determined in conjunction with geologic mapping near the San Gabriel River from Lake Georgetown to Weir, Texas, to increase the understanding of the geology of the Balcones fault zone and to provide data useful in identification of potential recharge areas and assessment of local ground-water flow.

Cretaceous Comanche Peak, Edwards, Georgetown, Del Rio, Buda, Eagle Ford, and Austin strata dip gently (1°) eastward, and are overlain in places by terrace deposits and alluvium. Several major normal faults, downthrown to the east, strike northward across the area. Gentle flexures, possibly related to faulting, parallel the faults.

Minor normal faults and joints are most abundant in areas adjacent to major faults and flexures. These fractured-strata zones probably parallel the length of the faults or flexure axes and may be as wide as 1.6 km. Most minor faults strike 340°-040°, have displacements less than 2 m, and dip 40°-80° both eastward and westward. Slickensides on minor fault plants indicate slight oblique slip that could be related to rotation of fault blocks. Most joints strike 340°-020° and 260°-300°. Fracture densities range from four joints/1 m to one joint/5 m in beds 1-2 m thick.

Fractures in the Comanche Peak, Edwards, and Georgetown limestones exhibit both similar and dissimilar characteristics important to ground-water flow. Apertures of fractures in the Comanche Peak and Georgetown limestones are generally less than 1 mm, whereas apertures in Edwards limestones can be several centimeters wide. Nearly vertical joints and minor faults in Comanche Peak and Georgetown strata appear to be common only near major faults or flexures. Major joint sets in all three units have similar strikes, and many of the minor fault planes are filled with calcite. Joints do not have mineral fillings, and abutting relationships suggest minor faults formed before joints.

Faults and joints in limestones of the northern part of the Edwards aquifer probably influence ground water in the same way they do in souther parts of the aquifer. The nonuniform distribution of fractures suggests that the hydrologic characteristics of the aquifer also are nonuniform. Highly fractured areas adjacent to faults should be more permeable than areas farther from faults. For example, springs in Georgetown City Park discharge Edwards aquifer water that may migrate upward along fractures associated with a major fault 1 km west of the springs.

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.