Deformational features and impact-generated breccia from the Sierra Madera impact structure, west Texas

Abstract

Deformational features from the Sierra Madera impact structure, west Texas, were studied to estimate shock temperature and shock pressure values involved during its formation. Sandstone, limestone, and polymict impact-generated breccia samples occur within the central uplift. The sandstones host several shock-related deformational features including toasted quartz grains and planar microstructures (planar fractures and planar deformation features) in quartz. Zircon mineral grains separated from the sandstone samples provide evidence of shock deformation in the form of planar fractures and granular surface textures. Both sandstone and carbonate within the central uplift contain shatter cones, whereas polymict impact-generated breccia samples contain shatter cone fragments and clasts of mixed lithology. Most high-temperature features (e.g., high-temperature mineral phases and a melt sheet) typically associated with less eroded structures of similar size are lacking; however, some small (millimeter-size) possible devitrified glass clasts within polymict breccias occur at several locations. Deformational features within zircon mineral grains, shatter cones, and quartz grains with multiple planar microstructures record pressure and temperature conditions during the impact of ∼3–20 GPa and postshock temperatures between 350 and >1000 °C. Deformational conditions during the impact event were almost certainly higher, given that erosion has presumably removed most of the more severely shocked rocks.