Apache Wrench Fault Zone of West Texas

G. Pat Bolden

The Apache Fracture Zone is present in West Texas from the Apache Mountains southeast to San Felipe Springs at Del Rio, Texas. Detailed geological surface mapping and checking of high altitude photographs have documented the presence of faults with 28^prime-2,720^prime of horizontal slip, translatory faults and a fracture pattern characteristic of left-lateral faulting in the fracture zone. Collapse features are associated with the faulting.

A 65-foot deep cut for U.S. Highway 285, 8 miles north of Sanderson, TerreIl County, Texas, displays left-lateral slip directions of 90°T and 320°T, right-lateral slip direction of 54°T and a complex system of joints trending 350°T which predates the faulting. Isostatic rebound of 27^Prime has apparently offset the 1965 drill holes for the highway along three bedding surfaces.

Nine miles west of Dryden, Texas, a deep railroad cut exposes a near vertical east-west, left-lateral fault surface with horizontal slickensides. The fault displaces the 350°T joint system as well as the 54°T conjugate Riedel fracture. All fault surfaces of the left- lateral wrench system show horizontal slickensides but none have been found to date in the joint system.

Karst topography in the area exposes fractures, faults and caves, many of which are calcite filled whereas others contain a secondary bedded cave fill. A 6" to 3^prime thick caliche zone covers parts of the area and appears to contribute to the difficulty of mapping the faults and fractures on the surface.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California