Numerical and experimental analysis of the Wetumpka (Alabama USA) impact crater

Introduction: The Wetumpka impact structure is a Late Cretaceous marine-target crater located in central Alabama, USA [1][2]. The target region was comprised of weathered crystalline rock of the Piedmont metamorphic terrane, which was overlain by poorly consolidated sediments from the Upper Cretaceous Tuscaloosa Group and Eutaw Formation. The water depth is interpreted to have been approximately 35-100 m [1][3]. The current crater is heavily eroded and exhibits asymmetric rims, due to collapse of the southwest section, reaching a maximum NE-SW diameter of 7.6 km [1][3]. Wetumpka’s surficial geology consists of a deformed, semi-circular, crystalline-rim, and a relatively lower relief area, composed by deformed sediments and mega-blocks from sedimentary and crystalline target rocks, as well as resurge chalk deposits [4].

Not unlike other marine-target craters, the layer of sea water may have played an important role on Wetumpka’s crater unique features, such as the collapsed rim and the distinctive moat-filling sequence. The water may influence since early stages of crater formation, as transient crater depth and diameter, until late formation stages, as tsunami-influenced sediment transport and an aqueous-dominated, moat-filling sequence. This study aims to understand the effect of water depth, tsunami formation, and mechanical parameters of target materials, on crater development and final morphology.

Methodology: The formation of Wetumpka was simulated using iSALE-2D, an extension of the SALE hydrocode developed to model impact crater formation [5][6][7][8]. The current study focuses on an axisymmetric approximation of the original impact problem and a resolution of 32 CPPR (cells per projectile radius). A lower resolution simulation (9 CPPR) was also used to obtain a rough but faster estimate of the visible processes during the crater evolution. The main questions to be explored in this study are the influences of impact velocity, water layer, and target properties on crater formation.

The target consisted of three layers: (1) metamorphic bedrock as granite; (2) a layer of sediment as either wet tuff or quartzite; and (3) the topmost sea water layer. A spherical impactor of 400m diameter traveling at 12 and 20km/sec was considered. Simulations were achieved using different water depths (60m and 125m), different sediment material (quartzite and wet tuff) and 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 better estimate of material parameters.

Results and Discussion: Three main processes were identified in 9 CPPR simulations: (1) crystalline rim collapse, (2) sedimentary rim collapse, and (3) tsunami resurge. For 32 CPPR simulations, a total of 24 distinct simulations were performed with different combinations of initial water depth scenarios (60m and 125m), impact speed (12km/sec and 20km/sec), sediment layer thickness (100, 200, and 300m), and EoS used for sediment (wet tuff and quartzite). Tensile and compressive strength values, obtained by split-Brazilian and compressive tests, were used to estimate cohesion (17.12 MPa) and friction angle (0.373) of metamorphic bedrock. These values were then used to set more realist input parameters for the 32 CPPR simulations.

While the simulations are ongoing, results are presented for approximately 70 seconds after the impact. The main differences observed between the simulations are connected to the impact velocity and sediment layer thickness. The higher velocity impact shows a maximum diameter of the transient crater developed by about 13 seconds, with an approximate 6 km diameter and 1.8 km depth. For the lower velocity impact, the maximum transient crater size was attained at about 10 seconds, with an approximate 5 km diameter and 1.5 km depth. In both cases, the rim started to collapse at about 25 seconds, and the ejecta curtain started to fall on top of the water layer, creating tsunami waves that move outwards. This turbulent flow carried sea floor sediments and blocks of the impacted target rocks, which are more abundant in the proximities of the crater. 

Simulations with different sediment layer thickness show differences in the composition of the crater rim and upper crater wall. According to our simulations, the crater rim was composed mostly by sedimentary material that can extend to the upper crater walls. Thicker initial sediment layer result in greater areas of crater wall composed of sedimentary material.

Higher velocity simulations also show higher peak pressure and temperature, reaching about 47GPa and 7700K at 0.1 seconds, whereas lower velocity impacts produce peaks of about 42 GPa and 4500K. Temperature peaks act on a greatly small volume of rock on lower speed simulations. This could lead us to relate the lack of melt fragments in the crater area with the lower velocity impact.

References: [1] King D.T. Jr. et al. (2002) EPSL 202, 541-549. [2] Wartho J.-A. et al. (2012) MAPS 47, 1243-1255. [3] King and Ormö (2011) GSA SP 483, 287-300. [4] King D. T. Jr. et al. (2006) MAPS 41, 1625–1631. [5] Melosh H.J. et al. (1992) JGR 97, no. E9, 14735-14759. [6] Ivanov B.A. et al. (1997) Int. J. Impact Eng. 20, 411-430. [7] Collins G. et al. (2004) MAPS 39, 217-231. [8] Wunnemann K. et al. (2006) Icarus 180, 514-527.

Acknowledgements: The authors are grateful to the CSIC financial support for international cooperation: I-LINK project LINKA20203 “Development of a combined capacity of numerical and experimental simulation of cosmic impacts with special focus on effects of marine targets”.