SHOCK METAMORPHIC CHARACTERISTICS AND AGE OF THE WETUMPKA IMPACT STRUCTURE, ALABAMA

The Wetumpka structure (7.6 km diameter; 32°31.2'N, 86°10.4W) is a shallow-marine impact structure that formed in water about 35-100 m deep. Wetumpka has a crystalline rim and a sedimentary floor. Recent core drilling revealed a basin-filling stratigraphy consisting of an upper layer of highly deformed and broken target strata and a lower layer of clast- and matrix-supported polymict breccias. These lower breccias are mainly coarse, poorly stratified polymineralic, crystalline clast-bearing sands. The lower breccias are interpreted to represent fall-back ejecta mixed with resurge-deposited material. Quartz grains within the clastic matrix of lower breccias display shock-characteristic multiple sets of planar deformation features, and deformation-induced twinning and brittle fractures. Shock features are also conspicuous within grains of muscovite, garnet, feldspar, and kyanite (derived from both the target Appalachian basement and overlying immature Lower Cretaceous sediments). Illite aggregates (in forms that mimic larger micas) occur sparingly in the lower breccia matrix and are inferred to be pseudomorphs formed by thermal and mechanical decomposition of muscovite. The stratigraphic age of the Wetumpka impact structure is Late Cretaceous (probably Campanian) based on the age of the youngest deformed target strata (i.e., the Mooreville Chalk). The complete stratigraphic age bracket for Wetumpka is Campanian to late Pliocene. The younger age is from flat-lying gravel units on tops of hills in and around the structure. We are undertaking a program of dating shock-metamorphosed K-bearing minerals from the lower sands in ANIMAL (the Auburn Noble Isotope Mass Analysis Laboratory). Our initial laser fusion analyses of individual, shock deformed muscovite crystals (exhibiting polysynthetic, mechanical twins) yielded ages approximately coeval with regional Appalachian basement deformation (c. 300 Ma) suggesting little loss of accumulated radiogenic 40Ar during shock deformation. Our subsequent analytical approaches will shift to incremental heating analysis of single crystals, and the illite pseudomorphs, to better evaluate the extent of argon loss and timing of argon retention in these crystals.