The distribution of megablocks in the Ries crater, Germany: Remote sensing, field investigation, and statistical analyses

Abstract

The Ries crater is a well-preserved, complex impact crater that has been extensively used in the study of impact crater formation processes across the solar system. However, its geologic structure, especially the megablock zone, still poses questions regarding crater formation mechanics. The megablock zone, located between the inner crystalline ring and outer, morphologic crater rim, consists of allochthonous crystalline and sedimentary blocks, Bunte Breccia deposits, patches of suevite, and parautochthonous sedimentary blocks that slumped into the crater during crater modification. Our remote sensing detection method in combination with a shallow drilling campaign and geoelectric measurements at two selected megablocks proved successful in finding new megablock structures (>25 m mean diameter) within the upper approximately 1.5 m of the subsurface in the megablock zone. We analyzed 1777 megablocks of the megablock zone, 81 of which are new discoveries. In our statistical analysis, we also included 2318 ejecta blocks >25 m beyond the crater rim. Parautochthonous megablocks show an increase in total area and size toward the final crater rim. The sizes of allochthonous megablocks generally decrease with increasing radial range, but inside the megablock zone, the coverage with postimpact sediments obscures this trend. The size-frequency distribution of all megablocks obeys a power-law distribution with an exponent between approximately -1.7 and -2.3. We estimated a total volume of 95 km3 of Bunte Breccia and 47 km3 of megablocks. Ejecta volume calculations and a palinspastic restoration of the extension within the megablock zone indicate that the transient cavity diameter was probably 14-15 km.