Shock-induced effects in calcite from Cactus Crater

Shock metamorphism of calcite from coralline limestone samples retrieved from a borehole drilled into the rocks beneath Cactus Crater, a nuclear explosion crater at Eniwetok Atoll, has been detected and quantified using electron spin resonance (ESR). ESR spectra of Mn^(2+), present as a trace constituent in the coral samples, show a consistent decrease in hyperfine peak splitting with decreasing depth of sample. A similar variation was observed in coral samples experimentally shocked to progressively higher pressures. It is speculated that the decrease in hyperfine peak splitting reflects a decrease in crystal field splitting and hence, small (< 0.01 Å) increases in cation-anion distances produced by mechanical energy input during the shock process. Two alternative crater models are suggested by the ESR results. One depicts a steady decay of the shock wave, from a maximum stress level of 4.5 GPa, at a rate, calculated in terms of post-flow co-ordinates, of d^(−5.7); this high attenuation rate may be due to the rocks underlying Cactus Crater having been displaced downward 5–6 m. The second delineates a breccia lens, possibly stratified, with a breccia-bedrock interface at 20 ± 5 m