Rocky Mountain Section - 57th Annual Meeting (May 23–25, 2005)

Paper No. 11
Presentation Time: 11:15 AM


HUSON, Sarah A. and POPE, Mike C., Geology, Washington State Univ, Pullman, WA 99164-2812,

The ~12 km diameter Sierra Madera structure in west Texas is a well-exposed eroded remnant of a complex impact crater. Past studies documented shock deformational feature such as impact generated breccias, shatter cones, planar deformational features (PDFs) in quartz, grain fracturing, and deformed quartz and carbonate minerals from this crater.

We are using modern research techniques to analyze deformational features produced during the impact event. Deformational features were studied using thin section observations and x-ray diffraction analysis (XRD). Identifying shock features through thin section observations leads to a better understanding of temperature and pressure conditions at the time of impact. Thin section analysis of impact breccias from the central uplift reveals subtle flow textures. Sandstone samples contain fractured quartz and PDFs indicate shock pressures were at least 10-20 GPa. To better quantify shock pressures generated across this crater, calcite-rich samples (impact breccias, limestone shatter cones, and coarse calcite hand samples) are being analyzed with XRD to determine the occurrence, extent and distribution of peak broadening (e.g. Skalá and Jakes (1999)).

Since the Sierra Madera crater lacks high-temperature minerals and was not deformed subsequent to its formation, low temperature dating techniques are being used to determine the age of this crater. Two complimentary techniques, apatite fission track dating and (U-Th)/He geochronology should provide an overlapping estimate of the age of the crater. However, fission track work has yielded problematic results; the date is Jurassic in age when we know from stratigraphic evidence the crater formed sometime in the Late Cretaceous. (U-Th)/He work is currently being conducted.