Distribution, petrography and shock metamorphism of the ejecta of the Ries crater in Germany—a review

Abstract

The lithologies and distribution of the various ejecta formations from the Ries crater, Germany, are described. They may be subdivided into: (1) low shock level (< 10 GPa), represented by the Bunte Breccia (sediments from the upper 600 m of the target area), megablocks of sedimentary rocks, and megablocks and monomict breccias derived from the crystalline basement, and (2) high shock level (> 10 GPa) represented by polymict crystalline breccias, fall-out suevites, crater suevite, and tektites (moldavites). The ejecta from the Mesozoic sedimentary column do not show effects of shock pressures exceeding 10 GPa, and only basement gneisses were shock melted to yield suevite glasses. It is therefore concluded that the maximum impact energy was transferred from the impactor to the target rocks at a depth of at least 1000 m below ground level, where a complex of gneisses underlies shallower granitic intrusions. The estimated depth of the transient crater was about 2700 m and its diameter was 12–13 km. Bunte Breccia, megablocks of sedimentary rocks, and crystalline breccias and megablocks were ejected in ballistic trajectories and form the earliest ejecta deposits, extending up to a radius of 22 km. Fall-out suevites are understood to have been lofted from the transient crater by an expanding plume of vaporized rocks and deposited outside the crater as fluidized turbulent masses on top of the earlier ejecta. In contrast, the crater suevite never left the crater cavity; it represents the main mass of suevite. Moldavites are understood to have been formed during the first encounter of the impactor with the ground by condensation from vaporized and ionizised Tertiary sands, covering the surface of the target area.