Sedimentological, structural and geochemical investigations of the Suevite of the impact crater Nördlinger Ries, Germany

Le résumé fourni par la source

The processes of formation and transport of particles in suevite during impact crater formation are investigated at the 15 Ma old, 25 km wide Ries crater in southern Germany. The suevite of the Ries crater occurs in three different geological settings: 1) crater suevite in the central crater cavity, 2) outer suevite on top of the continuous ejecta blanket, and 3) dikes in the crater basement and in displaced megablocks. For suevite genesis, the following processes have been discussed to-date in literature: 1) Fall-back of material into the crater and its periphery upon collapse of an ejecta plume, and 2) Horizontal transport of ejected material, akin to a) an impact melt flow, b) a pyroclastic flow, or c) initiated by phreatomagmatic explosion. In order to differentiate between the two emplacement modes of fall-back and horizontal transport, the shape fabrics of suevite components from two localities outside the Ries crater were analyzed by fitting shape fabric ellipsoids and by applying high-resolution X-ray computed tomography to analyze the three- dimensional shape and orientation of the suevite particles. It could be shown that the preferred orientation of particles is aligned either radially or concentrically with respect to the crater center. These observations indicate that suevite material was not only derived from an ejecta plume but was transported by lateral flow under viscous conditions. For the further differentiation of this emplacement mode the grain shape, grain size distribution, and content of particles in several drill core sections inside and outside the crater were investigated. These stereometric results imply a secondary comminution process after the impact cratering, where the clasts will be comminuted and sorted as a function of their size, density, and distance to the crater center, and where particle-particle interactions could occur. A secondary milling and sorting process in a gas dominated suspension seems to be reasonable. Only the upper most portion of the inner crater suevite seems to be have fallen out of the ejecta plume. Today, the hydrothermal formation of the suevite matrix is widely accepted. However, which primary material was transformed into the clayey matrix is still a matter of discussion. This work compares the chemical composition of the melt particles of suevites at the four different suevite drill cores with their corresponding matrix to identify the original composition of the suevite matrix. Finally, the primary melt content of the Ries suevite is estimated. The results show that the suevite can be distinguished in the following subunits: 1) Strongly altered suevite, 2) Suevite with differentiation in K-poor and K-rich melt with either silicate matrix similar in chemistry to the melt or non existing silicate matrix, 3) Suevite with similar melt and matrix composition, which can be considered as original impact melt, and 4) Suevite with different melt and matrix composition. The volume of the impact melt of the Ries crater is estimated to be 4 km, which is still less compared to impact craters similar in size to the Ries crater. The following five stages can be distinguished for the formation and deposition of the Ries suevite: 1) An early ejecta plume deposited as a 2 m thick, 2) After a hiatus, a phreatomagmatic explosion, 3) A basal pyroclastic surge, 4) A pyroclastic flow, 5) A second ejecta plume, where accretionary lapilli are formed.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.