Shock‐metamorphic microstructures in quartz grains from Albian sandstones from the Tin Bider impact structure, Algeria

Abstract

AbstractTin Bider is a 6‐km‐diameter complex impact structure, the largest one recognized in Algeria. The crater was excavated in Cretaceous sedimentary rocks composed of, from the base to the top, Albian sandstones, Cenomanian clays, Cenomanian‐Turonian limestones, undifferentiated Coniacian to Maastrichtian clays and limestones. The age of the impact event is poorly constrained to <66 Ma by stratigraphy, the youngest geological unit affected by the event being the ~66 Myr old Maastrichtian limestones. Albian sandstones outcrop in the central sector of the structure and represent the only occurrence at outcrop of this geological unit in the structure. Here we report on a detailed petrographic analysis of eight Albian sandstone samples that were searched for shock‐metamorphic features. We confirm the presence of rare shocked quartz grains with planar deformation features (PDFs) and report on their crystallographic orientations as determined using the universal stage microscope. PDFs oriented parallel to the π{} and ω{} orientations are the most abundant ones. For the first time in impactites from Tin Bider, PDFs with basal (0001) orientation, corresponding to amorphized mechanical Brazil twins, are reported. Our results indicate that locally the peak shock pressure was of at least 20 GPa, but much lower in average for the investigated samples.