Scaling craters in carbonates: Electron paramagnetic resonance analysis of shock damage

Abstract

Carbonate samples from the 8.9-Mt nuclear (near-surface explosion) crater, OAK, and a terrestrial impact crater, Meteor Crater, were analyzed for shock damage using electron paramagnetic resonance (EPR). Samples from below the OAK apparent crater floor were obtained from six boreholes, as well as ejecta recovered from the crater floor. The degree of shock damage in the carbonate material was assessed by comparing the sample spectra to spectra of Solenhofen and Kaibab limestone, which had been shocked to known pressures. Analysis of the OAK Crater borehole samples has identified a thin zone of allocthonous highly shocked (10–13 GPa) carbonate material underneath the apparent crater floor. This ∼5- to 15-m-thick zone occurs at a maximum depth of ∼125 m below current seafloor at the borehole, sited at the initial position of the OAK explosive, and decreases in depth towards the apparent crater edge. Because this zone of allocthonous shocked rock delineates deformed rock below, and a breccia of mobilized sand and collapse debris above, it appears to outline the transient crater. The transient crater volume inferred in this way is found to be 3.2±0.2×10^6 m^3, which is in good agreement with a volume of 5.3×10^6 m^3 inferred from gravity scaling of laboratory experiments [Schmidt et al., 1986]. A layer of highly shocked material is also found near the surface outside the crater. The latter material could represent a fallout ejecta layer. The ejecta boulders recovered from the present crater floor experienced a range of shock pressures from ∼0 to 15 GPa with the more heavily shocked samples all occurring between radii of 360 and ∼600 m. Moreover, the fossil content, lithology, and Sr isotopic composition all demonstrate that the initial position of the bulk of the heavily shocked rock ejecta sampled was originally near surface rock at initial depths in the 32 to 45-m depth (below sea level) range. The EPR technique is also sensitive to prehistoric shock damage. This is demonstrated by our study of shocked Kaibab limestone from the 49,000-year-old Meteor (Barringer) Crater Arizona. We found shock damage present in the β member of the Kaibab Formation exposed in the crater walls corresponding to peak shock stress in the 0.3- to 0.6 GPa range. Carbonate ejecta recovered from within the crater experienced shock pressures of up to 0.6 GPa. Assuming shock damage levels of 0.3 to 0.6 GPa for the lightly shocked carbonate on the walls of the Meteor crater, combined with the shock pressure versus distance model of Moss [1988] and Lamb et al. [1991], Meteor Crater impact energies of 2.4 to 8.9 Mt are obtained. This approximately agrees with energies of 3.3 to 7.1 Mt calculated from the crater scaling of Schmidt and Housen [1987].

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