NUMERICAL ANALYSIS OF THE WETUMPKA IMPACT CRATER WITH FOCUS ON THE SOUTHERN COLLAPSED RIM

The Wetumpka impact structure, located in central Alabama, is a 7.6-km simple crater formed during the Late Cretaceous in a near-shore marine environment with approximately 35-100m of depth. The target region was comprised, from bottom to top, of weathered crystalline rock from the Piedmont metamorphic terrain, overlain by poorly consolidated sediments from the Upper Cretaceous Tuscaloosa Group and Eutaw Formation. The current crater exhibits asymmetric rims due to collapse of the southwest, ocean-facing, section. The interior area has a lower relief and is composed by deformed sediments from target rocks, as well as resurge chalk deposits. This study aims to understand, from a numerical perspective, the formation of the Wetumpka crater and the effect of different input parameters, such as velocity, water depth, and sediment thickness, on the development and final morphology of the structure.

Methodology: The formation of Wetumpka was simulated using iSALE-2D, focusing on an axisymmetric approximation of the original impact problem and a resolution of 32 CPPR (cells per projectile radius. The target model consisted of three layers: a) crystalline basement represented by granite; b) the sediment layer, which is represented by the wet tuff, and c) the uppermost sea water layer. A spherical impactor of 400m diameter traveling at 12 and 20km/sec was considered. Simulations were achieved using different water depths (62.5m and 125m) and different sediment thicknesses (100, 200, or 300m), while maintaining the impactor and target properties. Samples were collected from the crystalline rim for split-Brazilian and compressive tests, as per ASTM standards, to obtain more realistic values of cohesion and friction angle for target material input parameters.

Results and discussion: The model that better approximates the field and drill core observations with respect to the southern rim section, show a simple crater formed by a 400m asteroid, impacting at 12km/s on a 62.5-meter water depth sea, with 200 meters of sediment layer overlaying the bedrock. The crater filling sequence in the model is also consistent with the gravity analysis predicting higher density material underlying the lower density sedimentary fill. Peaks of pressure and temperature in the model are consistent with shock petrological studies.