STRATIGRAPHY DRILLED AT WETUMPKA IMPACT STRUCTURE, ALABAMA

During summer 1998, two core holes were drilled near the geographic center of the Wetumpka impact structure (32°, 31.2 min N; 86°, 10.4 min W), a 7.6 km diameter, Late Cretaceous complex impact structure located in Elmore County, Alabama. The core holes were located ~ 200 m apart and both penetrated ~ 200 m of unconsolidated material and rock. The structure-filling stratigraphy consists of two units. A ~ 60 m-thick, unconsolidated, cross-stratified, red gravelly sand forms an upper (surficial) unit that comprises nearly all mappable terrain within Wetumpka's crystalline rim. (In some locales, this surficial unit contains mega-blocks of target rock several 100 m² in area -- but not at the drill sites). Also, there is a ~ 140 m-thick lower unit of inter-bedded, impact-related lithologies, which are not exposed well at the surface. The order of lithologies in the lower unit differed between wells; and there were three main lithologies: (1) sandy breccia and sand units (most common); (2) impact breccias (containing shocked quartz in matrix); and (3) target rock blocks. The latter consisted of 1 to 10 m blocks of (a) schist or gneiss (i.e., deep crystalline basement) or (b) sedimentary strata (i.e., clayey sands and sandy clays of the Upper Cretaceous target layers -- Tuscaloosa Group, Eutaw Formation, and Mooreville Chalk). Upward facies-transition analysis applied to the lower unit's interbedded lithologies indicates a preferred upward transition from (i) impact breccias to (ii) target rock blocks to (iii) sandy breccia and sand units. Wetumpka has been previously shown to be a marine-target crater, which formed upon the Gulf of Mexico shelf in ~ 35 m to perhaps at most ~ 100 m water depth. With this environment of cratering, it is feasible for the preferred upward-transition sequence to represent: (i) fall-back deposition followed by (ii) early rim collapse of target rock blocks, which was succeeded by (iii) centripetal flows of sediment and water within the early crater. The 60-m surficial layer is best explained as a later re-surge unit deposited by catastrophic post-impact rim collapse, which allowed sea water to rush into the crater (thus depositing a thick layer of sediment containing mega-blocks from the rim). [Vulcan Materials Company of Birmingham, Alabama provided all resources for Wetumpka drilling.]