MODES OF OCCURRENCE AND TYPES OF SHOCK FEATURES IN BIMODAL QUARTZ GRAIN POPULATIONS – WETUMPKA IMPACT STRUCTURE, ALABAMA

Wetumpka impact structure, a 5-km diameter, Late Cretaceous, marine-target impact feature in central Alabama, contains shocked materials in two main modes of occurrence: subsurface polymict impact breccias and surficial impact-breccia resurge deposits. Wetumpka’s shocked materials have an approximately bimodal size distribution: (1) silt and fine sand and (2) cobbles and pebbles. Silt- and fine-sand-sized quartz grains from both modes of occurrence display planar fractures (PFs) and planar deformation features (PDFs), whereas quartz cobbles and pebbles from the resurge deposits show a variety of shock features including PFs, PDFs, and feather features (FFs). In most instances, the FFs resemble previously reported FFs from other impact structures; however, there are notable differences in FFs at Wetumpka. For example, the distribution and number of FFs that occur within individual grains are highly variable and not uniform. FF morphology ranges from straight and planar to strongly curved. Further, the association of FFs with PFs or PDFs is also non-uniform. In many instances, FFs occur without PFs and appear to be originating from grain boundaries or sub-grain domain boundaries. We interpret the variation in FFs to be largely controlled by pre-shock regional metamorphic effects within host quartz grains. Pre-impact strain on the quartz grains likely perturbed the shock wave thereby causing localized changes in deformation response as seen by the very sharp origination or termination of FFs along grain boundaries and sub-grain domain boundaries. Quartz veins within the target Emuckfaw Schist of the Appalachian piedmont, the candidate source for the shocked cobbles and pebbles, were heavily strained long before impact. Extinction patterns delineate sub-grain domains in the quartz cobbles and pebbles, which are found in impact-breccia resurge deposits. In the latter, FFs may originate from these sub-grain domain boundaries. Our observations illustrate the importance of pre-shock target properties influencing shock effects, particularly in work with structures formed within complex geologic terrains such as the southern Appalachians.