CRATER-FILLING MATERIALS, WETUMPKA IMPACT STRUCTURE, ALABAMA

The Late Cretaceous Wetumpka impact structure is a marine-target crater located in central Alabama. The target region was comprised of weathered crystalline rock of the Piedmont metamorphic terrain, which was overlain by several tens of meters of poorly consolidated sediments, specifically the Upper Cretaceous Tuscaloosa Group and Eutaw Formation. The water depth is interpreted to have been approximately in the range of 35 to 100 m. Wetumpka averages about 5 km in diameter but it reaches a maximum NE-SW diameter of 7.6 km. Wetumpka’s surficial geology consists of a deformed, semi-circular, crystalline-rim, and a relatively lower relief area, composed by deformed sediments and mega-blocks from sedimentary and crystalline target rocks, as well as resurge chalk deposits. Wetumpka impact structure has no evident central uplift, despite a diameter where such a feature is generally thought likely to develop. Wetumpka impact structure’s crater-filling materials have been investigated during field campaigns (1997-date) and core-drilling campaigns (1998 and 2009). Outcrops and cores drilled so far reveal some details of the upper ~ 200 m of the Wetumpka crater-filling materials, yet the crater bowl of Wetumpka is likely to have as much as 1 km of material within it. A gravity model, based a single gravimeter-based, west-east, trans-crater profile, shows an interpreted cross section of the crater fill. We suggest that the lower unit of higher density (2.6 g/cm³; not yet drilled) is likely composed of crystalline bedrock blocks or that it contains a much higher proportion of crystalline blocks than the upper unit. The implications of field, drill core, and geophysical analysis is that the sequence of events in the development of Wetumpka impact crater involved collapse of crystalline materials from the transient crater rim, which was followed in turn by collapse of a substantial volume of sedimentary rim materials (2.1 g/cm³), and the return of displaced sea water.