Complex Impact Cratering in a Soft Target: Evidence from Ground Penetrating Radar (GPR) for Three Structures in the Chiemgau Meteorite Impact Strewn Field, SE Germany (1.3 km-Diameter Eglsee, 250 m-Diameter Riederting, 60 m-Diameter Aiching)

Abstract:

Impact cratering generally distinguishes between simple, bowl-shaped small craters and larger complex structures with a central peak and/or inner rings. In the modification stage of the latter, the transient crater is largely re-filled by centripetal movements particularly due to gravitational collapse of the crater rim. The transition from simple to complex craters is generally assumed to occur at about 1.5 - 4 km diameter of the final crater. Here we report on small craters, where in a modification stage the primary bowl has changed into a complex crater with internal peak or ring morphology seen on GPR profiles through the crater center. The Eglsee crater, which has a comparable size as the famous Barringer (Meteor) crater, belongs to the Chiemgau impact event (900-600 B.C.) and has a depth-to-diameter ration of roughly 1:70 comparable to much larger complex impact structures (like e.g. the Ries crater). The Eglsee crater complex nature of formation is underlined by GPR that reveals a buried ring-like mound below the otherwise flat crater interior. The nearby Riederting crater has a diameter of 250 m and, with a maximum depth of 5 m, is comparably shallow as the Eglsee crater and other complex craters. GPR through the flat basin shows a complexly sculptured wavy layering with a central depression several meters deep, which is surrounded by a steplike inner rim wall of roughly 60 m diameter. With a diameter of 60 m and a depth of about 10 m, the Aiching semi crater has indeed the typical bowl shape of a simple meteorite crater. The GPR on the flat crater floor however shows a very complex, wave-like layer structure and a central mound with a diameter of about 30 m, which reaches up to about 2 m below the crater floor in the center. We conclude from the GPR that in a soft target such as the unconsolidated, water-saturated Quaternary material in the strewn field of the Chiemgau impact, small craters may well have quite flat complex morphologies with central peak or ring-like mound in the subsurface. Similar to large complex craters, the partial collapse of the previously formed ring wall may have been effective in a modification stage.