Petrography and field relationships of impact lithologies at the Gardnos impact structure, Norway

New mapping and laboratory studies of the impactites at the Gardnos impact structure (Norway) show a variety of impact-deformed rocks. Our mapping and petrographic analyses have subdivided these breccias into three distinct categories: (a) melt-bearing sueivitic breccias, melt-bearing polymict breccias; (b) melt-free, polymict lithic impact breccias; and (c) monomict lithic impact breccias. This illustrates the dynamic nature of crater floor processes where mixing occurs in multiple ways. Feldspar grains exhibit evidence of intense shear, micro-faults, and alternate twin deformation in feldspar. We also observe the development of additional, amphibole-like planar elements (or cleavage) in biotite. Melt-bearing breccias contain carbon concentrations up to an order of magnitude higher than the target rocks. Unusual textures of carbon petrographically associated with shock and post-shock features in feldspars suggest significant postimpact hydrothermal mobilization of carbon within these rocks. Gardnos, therefore, represents an important terrestrial analog for understanding a suite of impact- and postimpact geologic processes.