BRITTLE DAMAGE IN SANDSTONE FORMED DURING HYPERVELOCITY IMPACTS: FIELD AND EXPERIMENTAL OBSERVATIONS FROM THE SERPENT MOUND IMPACT STRUCTURE, SOUTH-CENTRAL OHIO

Deformation bands are common strain localization features in sandstone, and form at a wide range of loading conditions during impact cratering. We analyze the formation of deformation bands from the Serpent Mound Impact Structure (SMIS) in south-central Ohio using field and experimental techniques. A Split-Hopkinson Pressure Bar (SHPB) and triaxial apparatus are used to study deformation in sandstone at a range of strain-rates (10^-2-10⁴ s^-1) and short duration times (10^-4-10⁵ s). Experiments within this strain-rate spectrum enhance our understanding of processes that occur during each phase of impact cratering, including shock compaction (10²-10⁶ s^-1), excavation (10⁰-10⁵ s^-1), and modification (10^-2-10⁴ s^-1). Deformation bands in core from the transition zone and central uplift, extracted from depths of up to 600 m, were analyzed using computed tomography (CT) scanning. We compare these bands to compressive shear and dilation bands formed during our SHPB experiments. Macroscopic deformation bands in core from the SMIS are predominantly shear and compaction bands and dilation bands have not been observed. Dynamic uniaxial compression tests produced cataclastic shear bands similar to those found in core from the SMIS. In dynamic tension experiments strain is accommodated by the formation of dilation bands and pore space expansion that can result in localized increases in porosity of up to 10%. Whereas compaction and shear bands produced under uniaxial compression are dominated by intense cataclasis that reduces porosity, dilation bands form by grain boundary failure with minimal intragranular fracturing. We analyze the nucleation of deformation bands in sandstone units at the SMIS in comparison to our experiments using micromechanical models modified for dynamic loading and P-Q (mean stress-differential stress) space analysis. Our experimental results suggest that the absence of dilation bands may result from the transient nature of tensile damage in porous media formed during impact cratering. Fluid overpressure and post emplacement mineral growth in dilation bands is required in order to preserve a record of tensile loading. These observations have important implications for mapping the full extent of damage and estimating the energy budget of hypervelocity impacts in the geologic record.