Abstract

The Carta Valley Fault Zone Area of Edwards, Kinney and Val Verde Counties, Texas was recognized as a wrench fault zone in the late 1950’s. The Cretaceous Del Rio pinches out within this area of en echelon graben fault blocks, creating both stratigraphic and structural problems. The en echelon pattern can be seen on exposed Eagle Ford bedding planes, and Webster’s Geologic Quadrangle Map No. 53 (1982) depicts this en echelon pattern on a larger scale. Tchalenko (1970) pointed out that the pattern exists from very large scale on the surface to microscopic in hand specimens, all with the same patterns and angles.

Characteristics of wrench faults are: 1. Long, straight faults and/or lineaments; 2. High angle of dip on the fault plane; 3. En echelon pattern of faulting; 4. Related angles of faulting and folding; 5. Earthquakes; and 6. Offsets.

There are several regional lineaments that are distinct on a 1972 (Sheet 47) ERTS mosaic and the Geologic Map of Texas. The Devils River, Val Verde, Apache, Pecos and Texas Lineaments and the Quachita Thrust zone play important parts in explaining the left lateral wrench faulting of the region. It will be demonstrated how tensional faults exist within a compressional system.