SHALLOW STRATIGRAPHIC CROSS-SECTION OF WETUMPKA IMPACT STRUCTURE, ALABAMA

Wetumpka impact structure (central Alabama) is a marine-target, Late Cretaceous ((U-Th)/He age = 84.4 ± 1.4 Ma) impact feature, which consists of three surficial terrains: (a) crystalline rim (coarse megablocks of schist-gneiss and coarse schist-gneiss breccias); (b) interior (intracrater sediments and broken formations); and (c) exterior (structurally disturbed target formations). The crystalline rim has a diameter of ~ 5 km and the lobate exterior terrain extends ~ 3 km beyond the southern rim. Wetumpka’s Upper Cretaceous target formations include (in reverse age order): lower beds of Mooreville Chalk, the clastic paralic Eutaw Formation, and the clastic fluvial Tuscaloosa Formation, all of which rested upon weathered crystalline Piedmont metamorphics.

A total of 8 shallow holes have been drilled within the Wetumpka impact structure for scientific purposes. Five of the holes are less than 100 m; 3 are 200 m or more. Holes drilled so far penetrate the upper 15 to 20 percent of the whole of the crater-filling sequence. Six of the holes were cored; 2 provided cuttings. These holes (with dates) are as follows: Schroeder (1998); Reeves (1998); Eason (cuttings/2002); Inscoe (cuttings/2006); Gardner (2009); Buck Ridge Road (2009); Bailiff (2009); and Wadsworth (2009). Also, we have measured several key stratigraphic sections, e.g., Breccia Hill.

Shallow drilling and measured sections have revealed distinctive interior lithic types, including crystalline blocks in sand matrix, crystalline megablocks, glauconitic chalk (resurge), impactite sands with sedimentary megablocks, polymict impact breccia, and slumped sedimentary target formation megablocks (Eutaw and Tuscaloosa). Of these rock types, impactite sands, polymict impact breccia, and glauconitic chalk contain shocked grains. A shallow stratigraphic transect based on cores, cuttings, and outcrops shows a central crater-filling vertical sequence of crystalline megablocks, slumped sedimentary target megablocks (Eutaw and Tuscaloosa), and polymict impact breccia (at top). Between the central crater area and the crystalline rim, the sequence is impactite sands, polymict impact breccia, slumped sedimentary target megablocks, and glauconitic chalk deposits (at top). Supported in part by NASA grant NNX09AD90G awarded to King and Ormo.