Abstract

At the southeastern corner of the Marathon Uplift in Brewster County, Texas is an unusual fish-hook shaped structural “tail” that swings around Horseshoe Mesa. At first look the structure appears to be the continuation of a Marathon fault bending around from an ENE trend to a southwestern trend. On closer examination of the structure, using Landsat imagery and aerial photographs, however, it appears to be a composite of four intersecting structural trends, as follows:

N40E

This is the predominant trend of the “Fish Hook”, formed by a high-angle normal fault zone, down to the southeast. This trend roughly parallels the Dagger Flat Anticlinorium.

N15E

A subtle anticlinal fold runs NNE from the core of the Fish Hook for about seven miles. A few linears within the Marathon Fold Belt have this orientation.

N70E

This trend roughly parallels the Hell’s Half Acre Thrust Fault and other prominent faults and folds in the Marathon Fold Belt. It runs from the Fish Hook southwestward just south of the Cretaceous/Paleozoic contact to apparently intersect the Santiago Mountains.

NW-SE

Folds, faults, and joints with this orientation sweep across the area northeast and east of the Fish Hook. They are approximately parallel to the Laramide-age Santiago Mts., which lie southwest of the Marathon Uplift, and they may have a similar origin.

The shape of the Fish Hook Structure is outlined at the surface by the Cretaceous/Paleozoic contact. The oldest rocks exposed in the core of the structure are Ordovician. Most of the rest of the area of study is covered by Lower Cretaceous predominantly limestone, Tertiary volcanics, and Quaternary alluvium.

The practical result of the study is the possibility that two of these trends, the N40E and N15E trends, may project northward beneath the Cretaceous cover into the Val Verde Basin. There they may reflect basement faults similar to those that control traps for Lower Paleozoic reservoirs at the giant Puckett, Grey Ranch, and Brown-Bassett fields.