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At Wetumpka impact structure, the surficial polymict impact breccia crops out discontinuously over a relatively small area within the intrastructure terrain. This polymict breccia unit contains a significant number of shocked quartz grains that represent a slightly lower shock pressure regime than the previously documented shocked quartz population obtained from a separate subsurface impact breccia unit. Specifically, the shocked quartz grains found in the polymict breccia unit display two distinct types of planar microstructures (named herein P1 and P2), and the host grains range in size from fine sand to pebbles and cobbles. P1 elements closely resemble planar fractures (PFs) or planar cleavage in quartz, which occur in multiple sets of open, parallel, fl at to curviplanar planes aligned with distinct crystallographic orientations. P2 elements are much shorter, much thinner, and more closely spaced than P1 planes, and they resemble in part closed, partly decorated to nondecorated, classic, longer planar deformation features (PDFs). However, these P2 elements are not PDFs. Sets of P2 elements are commonly developed off of, or are crosscut by, through-going P1 planes, which form “feather features” (FFs). P1- and P2-type planar microstructures are associated with low shock pressures (~7–10 GPa), whereas more typical planar deformation features previously described in Wetumpka's subsurface impact breccias are associated with shock pressures of 10–16 GPa. We attribute the difference in Wetumpka's shock levels (i.e., between quartz grains in the polymict impact breccia and previously described quartz grains in the deeper, subsurface impact breccias) to differences in provenance of these grains from within the impact structure's transient crater. Proximal ejecta deposits, which were derived from more shallow reaches of the target materials, are the most likely candidate for sources for the P1- and P2-bearing grains in the surficial polymict impact breccia unit. We interpret the present distribution and occurrence of the surficial impact breccia unit to be best explained by late-modification-stage slumping of proximal ejecta from the impact structure's rim.

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