Wetumpka impact structure (Alabama), a Late Cretaceous marine-target crater

King David T. J.; Neathery Thornton L.; Petruny Lucille W.

Wetumpka impact structure (N32 degrees 31.5'XW86 degrees 10.5') is a deeply eroded 6-km diameter feature characterized by a substantial impact-deformed crystalline rim and a crater-filling melange unit. Wetumpka's age (Campanian?) is constrained by youngest target fragments within its resurge deposits, whereas its position within shallow (100 m?), epicontinental Gulf waters is determined from well-documented paleogeographic reconstructions of the target area. Borehole drilling shows that Wetumpka's melange has two main parts: [1] a surficial resurge unit (approximately 50 m thick) and [2] an underlying impact-breccia unit (over 140 m thick). Wetumpka's resurge deposits are conglomeratic clayey sands and sandy clays, which display graded and contorted bedding and incorporate local, megablocks of target strata. Wetumpka's impact breccia contains intercalated monomict and polymict (shocked-quartz bearing) breccias and sizeable (up to 10 m) blocks of target strata and crystalline basement. Wetumpka's inferred cross-section is more like a partially filled, simple bowl-shaped crater than a typical land-target, complex (or "terraced") crater of comparable size. Wetumpka's bowl could be the deep, central part of the characteristic 'inverted sombrero' form, which is typical of impact excavation in marine-target situations involving relatively shallow marine waters. Unlike some marine-impact craters, Wetumpka may not have preserved resurge channels, and its potentially broad, shallow outer rim is not evident based upon our studies thus far. However, this situation may be attributable entirely to substantial post-impact erosion.