STRUCTURE-FILLING STRATIGRAPHY OF THE LATE CRETACEOUS WETUMPKA MARINE-TARGET IMPACT STRUCTURE, ALABAMA

Wetumpka impact structure is the only known impact feature in the inner coastal plain of the southeastern U.S. Wetumkpa is a deeply eroded, arcuate, 7.6-km diameter Late Cretaceous feature located near the fall line in Elmore County, Alabama. This distinctive structure was produced by a cosmic impact into shallow marine water, probably less than 30 m deep. Wetumpka has three distinctive surficial geologic terrains produced by impact-related processes. These exposed terrains include Wetumpka's crystalline rim and two sedimentary terrains: (a) an interior unit comprised of impactite sands and broken sedimentary target formations (also known as the resurge unit, or the Wetumpka Melange), and (b) an adjacent extra-structure unit (or the structurally deformed unit). This deformed unit is located only outside the rim on the structure's southern and southwestern sides. Core drilling near the structure's geographic center revealed that Wetumpka's structure fill has a distinctive (unnamed) subsurface unit as well. Drilling initially penetrated the upper, resurge-deposited unit (~ 60 m thick; same as the “interior unit” above), but at depth encountered an underlying lower, structure-filling breccia unit comprised of fall-back ejecta layers, slumped target-rock blocks, and wash-back sandy breccias and sands. This unique subsurface unit, ~ 130 m thick, contains shocked minerals within the fine clastic matrix. Drilling did not penetrate the full thickness of the unnamed breccia but the breccia likely grades into an autochthonous bedrock breccia, which characterizes the deepest reaches of most complex terrestrial impact structures.