Sierra Madera is located ninety miles southwest of UTPB. Seventy years ago the renowned geologist P. B. King recognized that the Sierra Madera structure was a different breed of cat. The circular cluster of hills is 1.5 miles in diameter. Older strata are exposed in the central part and strata generally dip radially away from the center. Permian rocks are uplifted as much as 4000 feet in the central uplift. Cretaceous rocks dip gently away from the central uplift and form the outer rim of the structure. However, it is not a simple, structural dome because the bedding is also highly folded and faulted on the small scale.
Many domal structures to the south have igneous rocks associated with them, but Sierra Madera does not. Oil companies are attracted to structural domes, or 'four ways of dip,' and Sierra Madera was no exception. Drilling revealed an amazing phenomenon-- at deeper levels the dome did not exist!
Robert Dietz suggested that the Sierra Madera structure might be the result of a meteorite impact. Howard Wilshire studied the area and showed that shatter cones and micro-breccias indicated extremely high pressures of 200 kilobars. Wilshire's work convinced most geologists that the impact of an extraterrestrial body created a circular crater 6 miles in diameter and a central uplift caused by impact rebound.
Digital Elevation Model (DEM) of Sierra Madera impact structure showing central mountains and raised rim (red outline). Diameter of rim is 6 miles. Crest of the Central Uplift is 4583' above sea level. DEM compiled by Matthews from four 7.5' quadrangles
View northward from the rim of the Sierra Madera Crater. Strata of the central uplift were raised 4000 feet from rebound caused by the impact.
Cross section illustrating how only the upper strata were disturbed by the impact (from Wilshire, 1972).
For an illustration showing the sequence of formation of the Sierra Madera impact structure, click here.
Wilshire, H.G., Offield, T.W., Howard, K.A., and Cummings, D., 1972, Geology of the sierra madera Cryptoexplosion structure, Pecos county, Texas: U.S. Geological Survey Professional Paper 599H, 42 p.
Location map for Sierra Madera