Thermal and Structural History of Impact Ejecta Deposits, Ries Impact Structure, Germany

Abstract

AbstractThe Ries impact structure (Germany) contains well‐preserved ejecta deposits consisting of melt‐free lithic breccia (Bunte Breccia) overlain by suevite. To test their emplacement conditions, we investigated the magnetic properties and microstructures of 26 polymict breccia clasts and a stratigraphic profile from the clasts into the suevite at the Aumühle quarry. Remanent magnetization directions of the Bunte Breccia clasts fall into two groups: those whose directions mostly lie parallel to the reversed field during impact carried mostly by magnetite, and those whose directions vary widely among each clast carried by titanohematite. Basement clasts containing titanohematite acquired a chemical remanent magnetization (CRM) during the ejection process and then rotated during turbulent deposition. Clasts of sedimentary rocks grew magnetite after turbulent deposition, with CRM directions lying parallel to the paleofield. Suevite holds a thermal remanent magnetization carried by magnetite, except for ∼12 cm from the contact with the Bunte Breccia, where hematite concentrations increase due to hydrothermal alteration. These observations lead us to propose a three‐stage model of (a) turbulent deposition of the melt‐free breccia with clast rotation <580°C, (b) deposition of the overlying suevite, which acted as a semi‐permeable barrier that confined hot (<300°C) oxidizing fluids to the permeable breccia zone, and (c) prolonged hydrothermal activity producing further alteration which ended before the next geomagnetic reversal. Basement outcrops have significantly different magnetic properties than the Bunte Breccia basement clasts with similar lithology. Two basement blocks situated near the inner ring may have been thermally overprinted up to 550°C.