Ries structure, southern Germany, A review

Abstract

In southern Germany a prominent north-northwest-facing scarp is interrupted by a roughly circular depression known as the Ries basin. Surface topography and geophysical investigations reveal a complicated crater geometry consisting of an inner zone roughly 8 km in diameter with only modest surface expression, followed outward by a concentric zone of flat to hummocky relief, which itself is bordered by the above-mentioned scarp to the south, and by a less prominent rim to the north. The diameter of the entire depression is 22–24 km. The present crater floor is a flat plain as a result of crater filling by fallback breccias as well as fresh-water lake and alluvial deposits. The distribution, structure, texture, composition, and mechanism of deposition of the various ejecta are important clues for reconstructing the cratering mechanism. The undisturbed crystalline basement of the Ries originally was overlain by a sequence of Mesozoic sedimentary rocks, roughly 600 meters thick. The ejecta consist of a variety of sedimentary breccias (Bunte Trummermassen) and crystalline breccias (monomict breccias, polymict breccias, suevite) derived from these preexisting rocks. Volumetrically the sedimentary breccias dominate, forming a discontinuous ejecta blanket especially in the south. Most crystalline breccias are confined to the crater itself or to the proximity of the rim, though sporadic patches occur as far away as one crater radius. Ejecta block size decreases with increasing distance from the crater. In general, where discernible the pre-Ries stratigraphic sequence tends to be reversed within the ejecta blanket. Suevite is the single most important breccia derived from basement rocks. It contains a variety of crystalline rock types in all stages of shock metamorphism. Its most characteristic components are aero dynamically shaped glass bombs. Detailed petrographic investigations indicate peak shock pressures in excess of 600 kb and 2000°C. Suevite is responsible for strong local negative magnetic anomalies. Oriented samples show reversed remanent magnetism. Gravity measurements have revealed a strong negative anomaly corresponding to about 7×106 g mass deficiency. The Ries event has been dated by K-Ar and fission track techniques at 14.8±0.7 m.y. and 14.0±0.6 m.y., respectively, and these dates are compatible with stratigraphic evidence (Upper Tortonian).