WETUMPKA IMPACT STRUCTURE, ALABAMA: RESULTS FROM NEW CORE-HOLE DRILLING DURING 2009

The Late Cretaceous Wetumpka marine-target impact structure (located near the town Wetumpka, in the inner Coastal Plain of Alabama) has a wide, semi-circular, horseshoe-shaped crystalline rim, which is approximately 5 km in diameter. The whole of the impact structure includes an interior structure-filling unit and an exterior disturbed terrain (located south and southwest, respectively, of the crystalline rim). The crater rim spans 270 degrees of arc and is open to the southwest, the same side as the structurally disturbed terrain. The crystalline rim, central polymict breccia, interior marine chalk deposits and reworked glauconitic sands, and southern rim are the specific sites drilled during 2009 and reported on here. Each of the four drill sites displays rock types and physical relationships that reveal information about the probable physical processes during excavation and modification of Wetumpka impact structure, including their relative timing. In the crystalline rim (particularly in well #09-01 in the northwestern quadrant), there is good evidence of excavation-phase overturning of a crystalline flap. During early modification, sedimentary mega-blocks were emplaced by gravity sliding into the transient crater bowl (e.g., interior mega-blocks in the lower parts of wells #09-03 and #09-04). This mega-block movement was attended by slumping of less stable rim materials, i.e., the collapse of sedimentary units comprising the southern rim seen in the upper part of well #09-04 and the crystalline block-bearing polymict breccia in well #09-02, located in the central breccia area. We have interpreted these early modification-related events as being coeval with the development of the exterior disturbed terrain. The foregoing events were followed by modification-stage aqueous resurge, which emplaced units like the green-grey chalk and calcareous shale drilled at site #09-03. Supported by NASA grant NNX09AD90G awarded to King and Ormö.