TERRESTRIAL IMPACT BRECCIAS: SEDIMENTOLOGY AND STRATIGRAPHY

Terrestrial impact breccias (TIBs) comprise a diverse group of impact-produced and impact-related fragmental rocks or fragmentites, which mainly occur within or near (i.e., a few crater radii from) impact craters and impact structures, but may also occur in impact-affected rocks and sediments at greater distances owing to various distal impact effects (e.g., seismic shaking and tsunamis). Study of TIBs can help elucidate crater-formation processes and the sequence of crater-forming events. TIBs may form within and under the impact structure, e.g., autothonous and parauthochonous breccias, fall back into the structure (fallback breccias), slump or slide within the impact structure, or – in the instance of marine impacts - wash back in the crater (washback or surgeback breccias). Also, TIBs may be part of a proximal ejecta blanket surrounding the impact structure. TIBs may contain shocked minerals and high-pressure polymorphs (polymict breccias) or may be essentially free of these phases (monomict breccias). Or, TIBs may contain varying amounts of impact-produced glass (i.e., the fragmental breccia – suevite – impact melt rock (tagamite) continuum). Recognized stages of crater formation are related to TIB genesis. Compression, excavation, and modification stages have the potential to produce TIBs of different textures and compositions within and outside the impact structure. Early studies of TIBs treated them more like igneous and metamorphic rocks and the emphasis was on geochemical analyses and petrography. However, recent studies have also highlighted the sedimentological nature of impact breccias and their stratigraphic relations. Sedimentological tools like outcrop-based and thin-section-based grain-size analysis, surface textural analysis of large clasts, and analysis of coarse clastic arrangement have been used to help understand and elucidate formative and emplacement processes. The immense size of impact clasts requires new standard nomenclature beyond the Wentworth scale. Drilling of various impact structures and detailed field studies have shown that the stratigraphic position and relationships of TIBs to other impactites and target rocks may be complex, yet are very important in understanding crater formation, particularly where marine water plays a roles in crater modification.