^1,2Elmar Buchner, ³Martin Schmieder
¹HNU – Neu-Ulm University, Wileystraße 1, D-89231 Neu-Ulm, Germany
²Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstraße 18, 70174 Stuttgart, Germany
³Philamlife Village, Pueblo de Oro, Upper Carmen, Cagayan de Oro, 9000 Philippines

The ∼24 km Nördlinger Ries and the ∼3.8 km Steinheim Basin in southern Germany are thought to represent a ∼14.8 Ma old impact crater doublet. The complex craters of the Steinheim Basin with its crater fill deposits and the Nördlinger Ries and its voluminous impact ejecta blanket are still widely preserved. Although located in an environmental setting that presumably underwent the same erosional history as the Ries crater, field geologic studies suggest that no proximal or distal ejecta of the Steinheim impact event are presently preserved. Generally, the lack of the ejecta blanket around the crater could be explained either by intense erosion, the scarcity of outcrops, or it never formed. In contrast to the lack of ejecta, fluvial and lacustrine Middle Miocene sediments deposited prior to, synchronous with, and shortly after the impact are preserved in many places in the surroundings of to the Steinheim Basin.

On low-density asteroids or planets with highly porous target rocks (⩾ 30-40% effective porosity), impact structures can form without significant ejecta outside the craters due to the compaction of porosity and a concordant drastic reduction of the ejecta velocity. In the Steinheim area, the target rocks comprised loose, porous Miocene sands, Upper Jurassic limestones and Middle Jurassic porous sand- and claystones. The average porosity of the entire sedimentary target suite may have reached 20-30% or even higher values assuming the existence of open karst cavities in the Upper Jurassic carbonates. Compaction of the porous target rocks, resulting in the reduction of ejected material, in combination with erosion could explain the apparent lack of impact ejecta in the wider periphery of the Steinheim impact structure.

Reference
Buchner E, Schmieder M (2015) The Steinheim Basin impact crater (SW-Germany) – where are the ejecta? Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2014.12.026]

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