Kord Ernstson 1, Michael R. Rampino 2, and Michael Hiltl, 3

1 Fakultät für Geowissenschaften, Universität Würzburg, kord@ernstson.de; 2 New York University & NASA Goddard Institute for Space Studies, mrr1@is3.nyu.edu; 3 MHiltl@gmx.de

In fracture mechanics, spallation is a well-known process typically combining compressive and tensile stress. Spallation occurs when a dynamic compressive pulse impinges on a free surface, where it is reflected as a rarefaction pulse. The associated tensile stress may lead to internal spallation fractures and to complete spalls detaching from the free surface.

In impact cratering, spallation is well known to occur in the near-surface zone of the target [1], has been described for lunar micrometeorite craters [2], and has been studied in the laboratory by experimental impacts in, e.g., gabbro targets [3, 4].

Here, we report on the natural occurrence of spallation in Triassic Buntsandstein conglomerates induced by shock waves from nearby large impacts. The conglomerates are exposed in a large area (several 1000 km²) and surround the Azuara impact structure and especially its newly established [5, 6] Rubielos de la Cérida companion impact structure in NE Spain.They are in general composed of quartzite cobbles in a sandy matrix and are well known among geologists because of the distinctly pock-marked and cratered cobble surfaces. Commonly, these deformations are considered to have resulted from pressure dissolution by overburden or/and tectonic compression. Recent studies [7] show that there is no dissolution evidence and that the peculiar features and a distinct macroscopic and microscopic sub-parallel fracturing are better explained by dynamic deformation in the Azuara/Rubielos de la Cérida impact event. Among the different aspects of these deformations [7], we consider spallation a very effective and macroscopically significant process.

Fig.1 and 2 show typical macroscopic spallation features in cobbles sampled from the Buntsandstein exposures. The samples have sets of distinctly open sub-parallel fractures with a very small spacing. There is no shearing, and the materials are not broken to pieces. An origin from quasi-static tectonic deformation is hardly to understand physically. We specially point out the spall fractures (arrows) mirroring the surface of the cobble. For geometrical reasons, this is expected to occur as a result of the reflection of a dynamic pulse at the free surface. In the case of the Buntsandstein cobbles, there was not a free surface in the strict sense but a distinct contrast of shock-wave impedance between the very dense quartzite cobbles and the sandy, porous matrix. Perfect opposite spall craters are shown in Fig.3. We point to the sharp contours of the craters as a result of brittle fractures and exclude any dissolution processes. Spallation on a smaller scale is shown in Fig.4 (thin section) and frequently occurs also on a microscopic scale [7].

To investigate the conditions of spall-fracture formation in cobbles, we performed shock experiments on artificial conglomerates at the Fraunhofer Institute for High-Speed Dynamics (Ernst-Mach-Institut) in Freiburg/Germany using a single-stage light-gas gun. We impacted quartz spheres (rock crystal, diameter 14 mm) embedded in an epoxy matrix and confined in a steel container with steel projectiles of truncated-cone shape. The impact velocities ranged between 25 and 115 m/s corresponding to impact initial pressures between about 0.55 and 2.5 GPa. Even at the lowest impact velocity (25 m/s) we observe clear spall fracturing in the sphere only indirectly impacted and otherwise macroscopically untouched. At higher impact velocities, up to five spalls were observed to occur in one sphere together with strong deformations described in more detail in [7]

Conclusions. - The Triassic Buntsandstein conglomerates in northeastern Spain display clear and frequent spallation effects resulting from dynamic deformations well known in fracture mechanics. Because of the vicinity of the exposed conglomerates to the Azuara impact structure and the newly suggested Rubielos de la Cérida companion crater, the spallation is ascribed to shock waves from these impact structures having led to internal accelerations within the conglomerates, multiple collisions of the components, strong reverberations, and, by focussing effects in the spherical bodies, to high compressive and tensile stresses even at very low impact velocities. From impact cratering considerations [1], low impact velocities, like those produced in our experiments, may occur at distances several tens of kilometers away from a very large impact. Because of reverberations and focussing effects in spherical components, we conclude that strong spallation may occur in conglomerates even at distinctly lower velocities (< 10 m/s ?) and at even larger distances.

Because of this, spallation features in conglomerates are suggested as a strong macroscopic diagnostic tool to identify nearby impact sites where other signature is weak or absent. Since the spallation may be overprinted and partly or totally removed by later true pressure dissolution or tectonic processes, thin-section inspection of the interior of cobbles [7] could helpfully identify microscopic spallation.

References. - [1] Melosh, H.J., 1989, Impact cratering. A geologic process,Oxford University Press. [2] Hörz, F. et al.,1971, J. Geophys. Res., v. 76, p. 5770-5798. [3] Lange, M.A. et al., 1984, Icarus, v. 58, p. 383-395, [4] Polanskey, C.A., and Ahrens, T.J., 1990, Icarus, v. 87, p. 140-155. [5] Ernstson, K. et al., this volume; [6] Hradil, K. et al., this volume. [7] Ernstson, K. et al., Geology, v. 29, No. 1, pp. 11–14.