Quantitative Analysis of the Deformational History and Timing of the Sierra Madera Impact Structure, West Texas

Sarah Huson
Washington State University, Department of Geology
Pullman, Washington
sahuson@hotmail.com

The Sierra Madera structure in west Texas is a well-exposed eroded remnant of a complex impact crater. Previous deformation studies of impact craters relied almost exclusively on analyses of quartz grains. However, since not all impacts contain quartz-bearing rocks, these results are not applicable to the ~50 meteorite craters occurring in carbonate rocks. Since the Sierra Madera structure is hosted in carbonate and siliciclastic rocks and was last studied in the 1960’s and early 1970’s, it is an excellent site for deformation and timing studies using modern research techniques. Two specific questions remain to be addressed to better understand the Sierra Madera structure: 1) what is the pressure and temperature history during the formation of this crater; and 2) when did this impact event occur?

Field observations supplemented by standard and luminescent petrography and X-ray diffraction (XRD) analysis will determine the amount of shock deformation that has occurred in Sierra Madera. Analytical measurements involving XRD analyses of quartz and calcite grains should provide a quantitative pressure history of the region.

The project will determine the age of the impact event through fission-track and (U-Th)/He thermochronology of apatite grains in Permian and Cretaceous age sandstone samples. The concentration of fission tracks in the apatite samples from Sierra Madera will indicate when the sample cooled through ~120°C and record the impact age. Similarly, (U-Th)/He thermochronology of apatite from the Permian and Cretaceous sandstone samples will determine when the apatite crystals cooled below ~65°C, providing a second quantitative age from this impact event.