Shock-induced phenomena in limestones in the quarry near Ronheim, the Ries Crater, Germany


Authors: Skála R

Published in: Bulletin of Geosciences, volume 77, issue 4; pages: 313 - 320; Received 14 May 2002; Accepted in revised form 6 August 2002;

Keywords: impact structures, shock metamorphism, limestone, calcite, cell dimensions, Ries Crater, X-ray diffraction, infra-red spectroscopy,

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Shock-induced effects in Malmian limestone sequences sampled in the quarry near Ronheim in the southeastern rim of the Ries impact structure in Germany were studied using electron microscopy with local chemical microanalysis, X-ray powder diffraction, IR spectroscopy, stable isotope study and MAS NMR spectroscopy. The most pronounced deformation observed is a pervasive brecciation resulting in the formation of parautochthonous to completely allochthonous limestone megablocks (so-called 'Schollen') on a megascopic scale. On a macroscopic scale, this process gives rise to monomict breccias (so-called 'Gries'). The applied analytical methods also revealed the damage of the limestones on submicroscopic and atomic scales. The peaks in X-ray diffraction patterns are broadened due to increased domain size and microstrain. Bands in the MAS NMR spectra show also broadening - most probably due to the random shifts of CO32- ions in the calcite crystal structure. The overall degree of shock-induced solid-state deformation is, however, low. No decomposition, stable isotope fractionation or melting occurring at high degrees of shock metamorphism were found. This allows to speculate on fast attenuation of peak shock pressure towards the crater rim in the most superficial regions of the target. Also the role of brittle behaviour of carbonates occurring in a relatively thin layer deposited over more competent crystalline basement revealing considerably higher degrees of shock metamorphism in large volumes must be taken into account in interpreting the low degree of shock metamorphism in the studied limestones.