WETUMPKA IMPACT STRUCTURE (ALABAMA) AND ITS OUTWARDS TSUNAMI GENERATION AND SUBSEQUENT RESURGE

Wetumpka impact structure, a 5-km Late Cretaceous marine-target crater, is situated at the boundary between the Appalachian piedmont and the coastal plain. At the time of impact, 84.4 +/- 1.4 m.y. ago, the target was a shallow continental shelf with 100 m of mainly unconsolidated coarse clastic sediments covering a basement schist-gneiss. Marine water, a few 10s of meters in depth, was swept away from the impact site and the near surroundings. Seawater eventually returned as a debris flow-like resurge. The main aqueous deposit from this resurge is a chalky deposit that is akin to the Mooreville Chalk of the adjacent coastal plain, but contains polymict ejecta and marine components otherwise not encountered in this location relative to the paleoshore. These discontinuous chalky deposits vary widely in thickness from several 10s of meters to only a few centimeters. Here, we areanalyzing the mechanisms behind the formation of these deposits by means of sedimentological studies and simple 2D numerical simulation of the impact event. The simulation shows that the ejection of the water layer follows the seafloor crater ejecta curtain (i.e., no "outer crater" forming in the water layer as at Lockne). However, it can be seen how an outwards tsunami is generated by the emplacement of the ejecta layer. The ejecta pushes the water away from an area well beyond that to be covered by the continuous ejecta layer (i.e., one crater radius beyond the rim). It takes several minutes before water returns to the crater site. The rim prevents a forceful entry of the resurge. However, geological evidence tell that the southern sector of the rim was unstable, possibly due to high content of unconsolidated target sediments, and collapsed to allow the debris/laden resurge flow. The composition of the resurge deposits indicates that some of the material has its origin far offshore from the impact site. The location of the deposits in low-lying parts of a hummocky crater interior shows that the resurge occurred well after the slumping of the unconsolidated target sediments and followed depressions in the terrain.