Deformation of dry and wet sandstone targets during hypervelocity impact experiments, as revealed from the MEMIN Program

Abstract

Abstract-Hypervelocity impact experiments on dry and water-saturated targets of fine-grained quartz sandstone, performed within the MEMIN project, have been investigated to determine the effects of porosity and pore space saturation on deformation mechanisms in the crater's subsurface. A dry sandstone cube and a 90% water-saturated sandstone cube (Seeberger Sandstein, 20 cm side length, about 23% porosity) were impacted at the Fraunhofer EMI acceleration facilities by 2.5 mm diameter steel spheres at 4.8 and 5.3 km s )1 , respectively. Microstructural postimpact analyses of the bisected craters revealed differences in the subsurface deformation for the dry and the wet target experiments. Enhanced grain comminution and compaction in the dry experiment and a wider extent of localized deformation in the saturated experiment suggest a direct influence of pore water on deformation mechanisms. We suggest that the pore water reduces the shock impedance mismatch between grains and pore space, and thus reduces the peak stresses at grain-grain contacts. This effect inhibits profound grain comminution and effective compaction, but allows for reduced shock wave attenuation and a more effective transport of energy into the target. The reduced shock wave attenuation is supposed to be responsible for the enhanced crater growth and the development of ''near surface'' fractures in the wet target.

Ries Bunte Breccia revisited: Indications for the presence of water in Itzing and Otting drill cores and implications for the emplacement process

“…Experimental cratering studies in Seeberger sandstone targets (~23% porosity, ~89% quartz, ~10% phyllosilicates), conducted on dry and wet samples, revealed a shift to finer grain sizes in the ejecta for wet samples (Buhl et al. ; Sommer et al. ).…”

Formation of shatter cones in MEMIN impact experiments

“…The quartz grains are subrounded and cemented by quartz, phyllosilicates, and accessory phases (Buhl et al. , ). Uniaxial compression tests showed strength values of ~70 MPa for dry samples and ~57 MPa for wet samples (Poelchau et al.…”

“…; Buhl et al. , ), and also to search for shatter cones and to constrain the formation conditions (Shoemaker et al. ; Moore et al.…”

The Steinheim Basin impact crater (SW-Germany) – Where are the ejecta?

“…This could have been responsible for constant water saturation of the Ries area but temporary water table drawdown in the Steinheim area. A significant influence of water saturation or the temporary lack of groundwater in the target rocks on the rate of compaction of porous target rocks and suppression of impact ejecta can be assumed (e.g., Buhl et al, 2013) but we are not aware of any detailed crater-scale investigations on this effect. The degree of water-saturation in the target rocks of the Steinheim Basin and the influence of this parameter on the suppression of impact ejecta in our investigation unfortunately remains unclear.…”