Paleobiologic Effects of the Late Cretaceous Wetumpka Marine Impact, a 7.6-km-Diameter Impact Structure, Gulf Coastal Plain, USA

Abstract

The 7.6-km-diameter Wetumpka impact structure in Alabama, USA, formed by a bolide impact within the shallow epicontinental Gulf of Mexico, during the Late Cretaceous (late Santonian to early Campanian). Water depths for the epicenter of this event are estimated to have been 30 to 100 m, and this feature probably formed within ∼ 25 km of the local barrier-island shoreline. All indications are that this impact would have been a locally devastating event. For example, the infra-red flash-burn radius extended to the local shore area and substantial seismic waves followed by a strong atmospheric blast wave would have reached the shoreline tropical forest very shortly after impact. Further, the radius of discontinuous ejecta would have extended beyond the local shoreline and other effects such as local tsunami run-up extended into the local shore-area tropical forest as well. The lower crater-filling unit of Wetumpka contains an impactentombed fossil record: there is considerably more lignite (fragments and finely divided material) in some of the impact breccias and sands than existed in any of the target strata. This implies that some comminuted wood from coeval, shore-area tropical forests was swept up within the returning marine-water washback-surgeback event -- or was air-borne in returning winds -- and thus became incorporated into the deep parts of the crater fill. Wetumpka’s crater fill also includes mixed trace and body fossils from slumped target materials and blocks displaying relic sedimentary facies of target units. Since impact, Wetumpka’s rim structure, mainly composed of crystalline basement rocks, appears to have been an enduring subaerial feature. Rim height estimates indicate that this crystalline feature (comprised today of a remnant ∼ 270° arc of elevated schists and gneisses) was subaerially exposed after impact. Deep tropical soil development (saprolitization) characterizes most of the higher elevations of the rim, probably attesting to long-term rim exposure since Late Cretaceous. We theorize that a limited terrestrial ecosystem could have existed at higher elevations upon the crater floor area as well during the interval between end of fall-back sedimentation and subsequent late-stage rim collapse; however only an intra-crater paleosol unit atop the fall-back crater fill remains as evidence of this possible period of stasis. Late-stage rim collapse resulted in catastrophic sedimentation across the crater floor area.