X-RAY DIFFRACTION STUDY AT SIERRA MADERA IMPACT STRUCTURE, WEST TEXAS

X-ray powder diffraction (XRD) analysis of carbonate and siliciclastic samples from the Sierra Madera structure indicates moderate shock pressures (8 to 30 GPa) were generated during the formation of this crater. Three sets of samples are being analyzed (quartz-rich, carbonate-rich and impact generated) using a method developed by Skála and Jakeš (1999).

Samples of the sandstone members of the Word Formation and Gilliam Limestone, Cathedral Mountain Formation and the Basal Cretaceous sandstone were collected from the central uplift of the structure. The Basal Cretaceous sandstone also was sampled on the crater rim.

Carbonate-rich rocks were sampled from the Permian Hess and Word Formations, Gilliam and Tessey Limestones. All carbonate-rich samples were collected from within the central uplift of the structure.

Samples containing impact generated features (breccias, shattercones, and centimeter scale dikes) were also analyzed. Shattercone samples were collected from the sandstone member of the Word Formation and the Tessey Limestone.

In all cases, the XRD patterns show slight to pronounced broadening indicating the presence of low to moderate shock pressures. Variable shock results were expected for the central uplift due to complex interactions during the formation of the structure. Generally, samples showing more deformation in hand sample have a broader and wider XRD pattern when compared to undeformed samples.

The assignment of moderate shock will be assessed using Rietveld crystal refinement methods however, the current classification agrees well with previously documented shock features from the Sierra Madera structure such as impact generated breccias, shattercones, planar deformation features (PDFs) in quartz, grain fracturing, and deformed quartz and carbonate minerals.