Lithofacies analysis of AU scientific drill cores 09-03 and 09-04, Wetumpka impact structure, Elmore County, Alabama

Abstract

The Wetumpka impact structure is an approximately 5 km diameter crater that resulted from a shallow marine cosmic impact in the northeastern Gulf of Mexico during Late Cretaceous (latest Santonian to earliest Campanian). Presently, this impact structure is a surfically exposed feature with a broad, arcuate crystalline rim, an interior sedimentary and polymict breccia unit, and an extra-crater terrain consisting of horst and graben structures. Recent scientific core drilling within the crater-filling breccias has shown that the uppermost impact stratigraphy is well preserved. Two of those drill cores, 09-03 and 09-04, were studied by conventional lithologic logging methods and in part by using X-ray computed tomography (CT) scans in order to describe the sedimentological characteristics, specifically the Lithofacies belonging to constituent resurge and rim-collapse sediments. Qualitative results from the resurge interval of core 09-03 indicate a northerly directed debris flow occurred first, then the resurge arrived, and finally a southerly or seaward-directed, more aqueous anti-resurge occurred. Proximal field and drill core structural relationships were used to infer the preservation state of the resurge deposits, which accumulated in downthrown blocks within the crater. Lithofacies patterns and grain-size trends confirm that the core 09-04 penetrates a slumped remnant of the southern overturned flap. Lithofacies and stratigraphic patterns below the overturned flap in core 09-04 suggest that underlying impactite sands are also an interval of sheared and partially disaggregated sedimentary flap material.