Steinheim suevite-A first report of melt-bearing impactites from the Steinheim Basin (SW Germany)

Abstract

The 3.8-km Steinheim Basin in SW Germany is a complex impact crater with central uplift hosted by a sequence of Triassic to Jurassic sedimentary rocks. It exhibits a well-preserved crater morphology, intensely brecciated limestone blocks that form the crater rim, as well as distinct shatter cones in limestones. In addition, an impact breccia mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones is known from drilling into the impact crater. No impact melt lithologies, however, have so far been reported from the Steinheim Basin. In samples of the breccia that were taken from the B-26 drill core, we discovered small particles (up to millimeters in size) that are rich in SiO2 (-50-wt%) and Al2O3 (-28-wt%), and contain particles of Fe-Ni-Co sulfides, as well as target rock clasts (shocked and unshocked quartz, feldspar, limestone) and droplet-shaped particles of calcite. The particles exhibit distinct flow structures and relicts of schlieren and vesicles. From the geochemical composition and the textural properties, we interpret these particles as mixed silicate melt fragments widely recrystallized, altered, and/or transformed into hydrous phyllosilicates. Furthermore, we detected schlieren of lechatelierite and recrystallized carbonate melt. On the basis of impactite nomenclature, the melt-bearing impact breccia in the Steinheim Basin can be denominated as Steinheim suevite. The geochemical character of the mixed melt particles points to Middle Jurassic sandstones ("Eisensandstein" Formation) that crop out at the center of the central uplift as the source for the melt fragments. © The Meteoritical Society, 2010.