Shallow-marine impact origin of the Wetumpka structure (Alabama, USA)

Abstract

The Wetumpka structure, an arcuate, 7.6 km diameter, rimmed feature of the inner Coastal Plain, Alabama, is a Late Cretaceous shallow-marine impact crater. In this paper, we show unequivocal evidence of Wetumpka’s impact origin. Within and about this structure, pre-existing Upper Cretaceous stratigraphy was resedimented and(or) deformed, thus creating distinctive intra-structure and extra-structure terrains. These terrains are located, respectively, within Wetumpka’s crystalline-rim terrain and adjacent to the structure on the southern side. Core drilling near the structure’s geographic center revealed that Wetumpka’s basin-filling sequence has two distinctive units, suggestive of a two-stage filling process consisting of (1) fall-back plus resurge followed by (2) a later secondary seawater resurge event. Wetumpka’s lower subsurface unit includes polymict impact breccias, which contain quartz grains displaying shock-characteristic multiple sets of planar deformation features. Selected subsurface samples of this breccia also contain elevated Ir, Co, Ni and Cr concentrations indicative of a minor extraterrestrial component.