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

The Wetumpka impact structure (~5km NW-SE and ~ 7.6km NE-SW diameter), which is located in central Alabama (USA), was formed in a shallow near-shore marine environment during Late Cretaceous. This impact structure has been studied previously through field investigations, shallow core drilling, and gravity modeling. In this paper, we build upon these 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 (62.5-125m), impact velocity (12-18km/s), and sediment thickness (100-300m) to assess 5 different impact scenarios. We observed that simulations involving a 400m diameter impactor, impacting at 12km/s (vertical component) on a target with 62.5m water depth and a 300m 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.

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