MICRO-SHOCK DEFORMATION ADJACENT TO THE SURFACE OF SHATTER CONES FROMTHE BEAVERHEAD IMPACT STRUCTURE, MONTANA

Petrographic and transmission electron microscopy (TEM) study reveals that planar fractures, pseudotachylite (PST) veinlets, and planar defo rmation features (PDF) in quartz and K-feldspar grains are developed o nly within similar to 2 mm of the surface of shatter cones in Proteroz oic arkosic sandstones from the Beaverhead (Montana) impact structure. This unique association reinforces the impact-shock origin attibuted to all these shock-metamorphic features. No high-pressure SiO2 polymor phs were found in the PST, but possibly this is because these cones fo rmed far from the center of the impact structure, in a shock-pressure zone below the stability field of the high-pressure polymorphs.