WETUMPKA IMPACT STRUCTURE AND THE RETURN OF SEA-WATER

Wetumpka is a Late Cretaceous marine-target impact structure in the inner Coastal Plain of Alabama. The structure is characterized by a wide, horseshoe-shaped crystalline rim, an interior region of broken and disturbed sedimentary formations, and an extra-structure terrain (on the south-west quadrant) composed of structurally disturbed target formations. The extant crystalline rim spans 270 degrees of arc and is open on the southwest, the same side as the structurally disturbed terrain just noted. The northwest-southeast diameter of the crystalline rim is approximately 5 km. We have performed core drillings at two locations on the crystalline rim. The Eason well on the western rim penetrated 67 m of schist, whereas the Inscoe well on the southeastern rim penetrated 30 m of schistose ejecta resting upon a few meters of schist, possibly uplifted basement. In 1998, drilling to 200 m depth at the structure's center revealed 100 m of sedimentary mega-breccia overlying polymict impact breccia mixed with target rock blocks. In addition, we have augmented drill-core and field-based information with numerical simulations of the cratering event. The focus with these simulations has been the formation of the rim and its interaction with the resurging seawater. We used iSALE, a multi-material, multi-rheology extension to the SALE (Simplified Arbitrary Lagrangian Eulerian) hydrocode, with several improvements. Numerical simulation shows that the rim would have included only a small percent of the relatively strong basement rocks, and thus was not very stable. At the modeled water depth of 72 m, which is in the deeper part of the possible depth range suggested for the target water sea (30-100 m), collapse of the rim is to be expected. Collapse would likely have opened up a part of the rim for extensive slumping of extra-crater disturbed sediments as well as resurge of surrounding seawater although resurge sediments have yet to be confirmed within the crater. The simulation indicated instability of the rim as noted in the south-western quadrant of the structure where there is no rim today – only an extensive area of slumped sediments. Thus we think the modeling supports our ground-based interpretations about the return of sea water.