Style of Faults and Associated Fractures in Austin Chalk: Northern Extension of Balcones Fault Zone, Central Texas

Donald F. Reaser, Edward W. Collins

Distributions, geometries, and densities of faults and associated fractures in the Cretaceous Austin Chalk were studied in outcrop within the northernmost extension of the Balcones fault zone in Ellis and northern Hill Counties, Texas. Description of the fracture systems may be applicable to hydrocarbon exploration and production from this unit and to locating the proposed Dallas-Fort Worth Area Superconducting Super Collider site in Ellis County.

Inactive normal faults with throws of less than 30 m compose this northernmost extension of the fault zone. Most of the major faults strike northeastward to east-northeastward, oblique to the regional strike of bed rock, and dip 45° to 80° northwest and southeast. Smaller faults strike northwestward. Calcite mineralization occurs along most of the fault planes, and slickensides indicate normal to slightly oblique slip. Developed within the fault zone is a series of narrow grabens that are 250-600 m wide and as much as 5.5 km long. In plan view these small grabens occur either as (1) depressed strips of bed rock bounded by subparallel hinge faults that increase in displacement to the northeast or as (2) irregularly subsided blocks bounded by double hinge faults that increase n displacement toward the central part of the trough and pinch out toward the northeast and southwest.

Areas of high fracture density within the Austin Chalk occur adjacent to the major faults and within gently warped beds located where faults terminate along strike. Narrow zones characterized by different fracture styles and densities occur subparallel to the major structures. These zones vary in width and are composed of complex minor faults (2 m wide), minor faults and joints (15 to 90 m wide), or abundant joints (60 to 140 m wide). Most minor faults have throws of less than 3 m and strike subparallel to an associated major fault or flexure, although some minor faults strike subperpendicular to major structural trends. Minor faults dip between 40° and 70° and are usually partly filled with calcite. Most systematic joints are nearly vertical and also strike subparallel to a associated major fault or flexure. Joints are only rarely filled with calcite, and abutting relationships suggest that minor faults predate joints. Average fracture spacing near faults or within flexures for 0.5 to 1 m-thick chalk beds generally ranges from 0.5 to 1.3 m. Local joint densities may be as high as 10 joints/2 m. Minor faults and many joints cut through adjacent beds of different thicknesses.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.