Numerical simulation of the shallow marine target Wetumpka impact crater (Alabama, USA)

Abstract The Wetumpka impact structure, which has a NW-SE diameter of ~5 km and a NE-SW diameter of ~7.6 km, is located in central Alabama (USA), where it was formed in a shallow, nearshore marine environment during the Late Cretaceous. This impact structure has been studied previously through field investigations, shallow core drilling, and gravity modeling. In this paper, we build upon those studies by performing hydrocode modeling using iSALE-2D to investigate the transient crater evolution and the crater-filling sequence. The present study helps explain the unusual collapsed, southwestern, seaward-facing, quadrant of the rim, which is thought to have contributed substantially to the upper part of the crater-filling sequence. We chose a three-layer model to approximate the initial target layer morphology: water on top; sediments in the middle; and granite at the bottom. We performed compression and tensile strength tests on intact micaceous schist (approximated as granite in the model) collected from the crater rim, which were used to obtain values for cohesion in the iSALE-2D damage model. We performed simulations with different combinations of water depths (62.5–125 m), impact velocities (12–18 km/s), and sediment thicknesses (100–300 m) to assess five different impact scenarios. We observed that simulations involving a 400-m-diameter impactor, impacting at 12 km/s (vertical component) on a target with 62.5 m water depth and a 300-m-target sediment layer, resulted in a final crater model that best approximates what has been observed in the field as well as by drilling and gravity modeling. Finally, we compared the pressures predicted by iSALE-2D with previous studies of Wetumpka's shock petrography. Our numerical results show a relatively close correlation between both the geological relationships and shock levels observed within and among the crater-filling units. This study not only enhances our understanding of the Wetumpka impact structure but also demonstrates the potential of numerical modeling in reconstructing impact crater evolution, offering a foundation for future research in both terrestrial and planetary impact cratering.