THE HORSESHOE-SHAPED RIM OF WETUMPKA MARINE IMPACT CRATER -- FROM OBLIQUE IMPACT OR HETEROGENEITY IN THE RIM?

Wetumpka impact structure is a Late Cretaceous impact feature of the inner Gulf coastal plain of Alabama. The shape of Wetumpka's rim is similar to a horseshoe (encompassing ~ 270 degrees of arc). The southwestern quadrant is topographically open (i.e., the land is rather flat) and free of crystalline rock exposures that characterize the balance of the rim. In view of experimental results with oblique impactors, Wetumpka would appear to be a clear-cut example of an low-angle impact where the trajectory was southwest (causing rim blow-out on the southwest). However, the underlying geology of the area and the marine-impact origin of this structure point to a different scenario for development of the horseshoe shape. The target was composed of a water layer, underlain by semi-consolidated Upper Cretaceous sedimentary units (~ 100 m thick) lying on hard crystalline rock. Wetumpka impact occurred ~ 25 km offshore in water ~ 30 to 100 m deep. Final rim height exceeded water depth, and a continuous (360 degree) rim likely persisted for some time after impact enabling a short-lived terrestrial ecosystem to form within. Evidence for this history was acquired during core drilling at crater center, which revealed a paleosol layer at ~ 67 m depth. This paleosol separates > 100 m of lower fall-back and wash-back breccias from overlying displaced sedimentary materials. Based on their structures and textures, these upper displaced materials appear to be the deposits of a late-stage, catastrophic rim collapse that washed sediment-water slurries and entrained sedimentary megablocks onto the crater floor. Local bedrock dip (~ 15m/km) to the southwest explains target sedimentary cover that was thicker on the southwestern quadrant of the impact structure than elsewhere. This effect likely resulted in the southwestern rim being composed mainly of Upper Cretaceous sedimentary target, whereas the balance of the rim was formed mainly of hard, crystalline target (Piedmont bedrock). This heterogeneity of rim material seems more likely to account for the rim's subsequent failure and resulting catastrophic flood emplacement of the crater-floor sedimentary cover that obliquity of the impactor. This instance shows how difficult it is to assess obliquity of impact based solely upon surface morphology of an impact structure.