Structure of Horse Mountain anticline (southwest extension), Brewster County, Texas

The Woods Hollow Shale (Middle Ordovician), Maravillas Chert (Late Ordovician), Caballos Novaculite (Devonian?), Santiago Chert (Devonian?) and Mississippian shales of the Tesnus Formation are exposed along the Horse Mountain anticline 19 1/2 miles south of Marathon on U. S. Highway 385. Horse Mountain anticline is a narrow, northeast trending fold on the southeast flank of the complex anticlinorium which was formed by pulsatory thrusting and folding during the Mississippian and Pennsylvanian Periods. It is within the Marathon salient of the Ouachita system. All structures are related to a northwest-southeast maximum principal stress. The chert and novaculite folded concentrically above detachment planes in the Woods Hollow Shale. Subsequent failure of the shale produced a syncline along the crest of the anticline. This was followed by major thrusting over the northwest limb. The deformation culminated with underthrusting along the axis and southeast limb, right-handed and left-handed strike-slip oblique faulting and transverse faulting. Erosion has left only the nearly vertical, resistant beds on the limbs and a vestige of the former crestal syncline. Two sets of shear fractures (N 47° W, 23° SW; N 81° W, 23° NE) are perpendicular to the beds and make acute angles with the local maximum principal stress along the axis of the fold. A third set of extension fractures (N 52° W, 79° SW) is nearly perpendicular to the axis of the fold and formed when the flanks of the fold dipped 45 degrees.