A new impact structure in NE Spain? - Evidence from striated rock surfaces

osachs [ at ] awi-bremerhaven.de


New occurrences of striated pebbles have been recently discovered in NE Spain (SACHS & PFLUG 2002; SACHS 2002 in prep.). Usually these pebbles have been considered to be a typical feature of ejecta from impact craters (CHAO 1977; POPE et al. 1999). To decide whether these scratch marks are produced during an impact event, striated rock surfaces of similar rocks from the well documented Nördlinger Ries impact crater (CHAO 1976) have been compared with the pebbles from Spain. The investigated pebbles from the Nördlinger Ries impact structure and the pebbles from the salt diapir near Peñacerrada in NE Spain are Jurassic and Cretaceous limestones, respectively. The striae producing minerals are in both cases mainly quartz fragments as shown by semiquantitative EDX analyses. The surfaces of the pebbles were studied under the scanning electron microscope. The striae of both pebble suites show many similarities, but also some significant differences. On the sample surfaces from the Nördlinger Ries, for example, occur strongly curved striae whereas pressure solution phenomena at the contact scratching grain/pebble are restricted to the samples from Spain. These findings suggest that it is possible to distinguish striated pebbles formed by different processes. The striated pebbles of the salt diapir in Spain can easily be formed by plastic deformation within a clay-mineral matrix under relatively high confining pressure in the centre of an overthrusted anticline. Therefore striated pebbles do not necessarily indicate impact structures.Acknowledgement: Many thanks to Prof. R. Pflug for his support during fieldwork in Spain.References:CHAO, E.C.T. (1976): Mineral-produced high pressure striae and clay-polish: Key evidence for nonballistic transport of ejecta from Ries Crater.- Science, 194: 615-618.CHAO, E.C.T. (1977): The Ries Crater of Southern Germany a Model for Large Basins on Planetary Surfaces.- Geol. Jb., A 43, Hannover.POPE, K. O., OCAMPO, A. C., FISCHER, A. G., ALVAREZ, W., FOUKE, B. W., WEBSTER, C. L., VEGA, F. J., SMIT, J., FRITSCHE, A. E. & CLAEYS, P. (1999): Chicxulub impact ejecta from Albion Island, Belize.- Earth Planet. Sci. Lett., 170: 351-364.SACHS, O. (2002): Unpublished Diploma thesis University of Freiburg, Germany (in prep.)SACHS, O. & PFLUG, R. (2002): Erzeugung ähnlicher Oberflächenstrukturen auf Gesteins-fragmenten mittels zweier verschiedener geologischer Prozesse: plastische Deformation durch Faltung und durch ein Impaktereignis (Abstr.).- Erl. Geol. Abh., Sonderb. 3: 83-84, Erlangen.

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PanGeo Austria 1, 28-30 Juni 2002, Salzburg, Österreich..
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Sachs, O. (2002): A new impact structure in NE Spain? - Evidence from striated rock surfaces , PanGeo Austria 1, 28-30 Juni 2002, Salzburg, Österreich. .

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