Numerical and experimental analysis of Wetumpka impact crater, Alabama.

Abstract

Impacts are the most crucial process in the formation of the solar system, since planets and other bodies are considered to have formed by accretion of small objects through collisional processes. By studying craters on Earth, we can have a better understanding on how this process works on other planetary surfaces and thus improve our knowledge on the formation of our solar system. The process of impact cratering in marine target settings, which is the focus of this project, has been a topic of significant interest because of its connections to ancient oceans on other planetary bodies such as Mars. The Wetumpka impact crater, located in central Alabama, is a good candidate for this study, since it was formed in a shallow sea environment, approximately 85 million years ago, during the late Cretaceous. The impact structure has been studied previously through field studies and core drilling. This study is built on the field and drill-core studies and performed hydrocode modeling using iSALE-2D to analyze the transient crater evolution and crater filling sequence with emphasis on the collapsed, southern, seaward section of the rim because that part of the rim is considered to have had the greatest influence on the crater fill sequence. With the intention to acquire experimentally based values for material input parameters, especially in the crystalline rim terrain, we collected, prepared, and submitted samples from that terrain for tensile and compressive tests, which used to estimate values of cohesion and friction angle. These values were included in the iSALE-2D damage model for simulations with different water depth, impact velocity and sediment thickness scenarios. The best fit model considers the impactor as a granitic sphere of 400m in diameter traveling at 12km/sec, striking a three-layered target: a) crystalline basement; b) 200 meters of sediments, and c) the uppermost sea water layer with 62.5 m of depth. Modeling results confirm field observations in relation to crater dimensions. The crater filling sequence predicted by iSALE-2D matches the drill-core observations. Finally, the pressures predicted by iSALE-2D are consistent with shock petrography observations.