The Origin and Evolution of Impact Crater Lakes: A Case Study of the Ries Impact Structure, Germany

Matthew J. O. Svensson, The University of Western Ontario

Abstract

Impact events are known to generate hydrothermal systems, which can subsequently vent into an overlying crater lake and potentially create ideal conditions for some microbial life-forms. Thus, early post-impact sedimentary deposits would be excellent targets for Mars sample return, and as such, the robust characterization of such deposits on Earth is critical. In this thesis, we establish an improved understanding of how the Ries crater lake formed, and how an active impact-generated hydrothermal system influenced its early evolution. The ~14.8 Ma Ries impact structure hosts the majority of its paleolake deposits within the structure's central basin with some deposits situated at higher stratigraphic positions beyond the central basin's edge. This research highlights and reconciles differences between the basal sedimentary deposits within the central basin sampled by the Nördlingen drill core, and those beyond the edge of the central basin sampled by the Wörnitzostheim drill core. We suggest that the Wörnitzostheim sedimentary deposits likely represent the transition from a back stepping alluvial fan to a playa lake system. The basal conglomerates representing the Wörnitzostheim alluvial fan host 100–130 °C mineral deposits localized to void and fracture spaces. Illite, kaolinite and particularly smectitic clay minerals were major constituents throughout the alluvial fan to playa lake transition, and their δ¹⁸O and δ²H indicate formation from weakly alkaline, local meteoric water at ~20 ˚C. The basal sedimentary deposits of the Nördlingen drill core likely represent a water-laden debris flow, as opposed to previous interpretations suggesting subaerial deposition from an ejecta plume. These deposits lack accretionary lapilli and were affected by pervasive alteration; they host 100–200 °C, void-filling mineralization in their basal conglomerate and gravelstone sections. This re-interpretation implies that ejecta plume fallback deposits are not always well-preserved and that they may not always be ideal marker beds, and points to concomitant fluvio-lacustrine deposition and hydrothermal activity. Overall, the results of this thesis have shown evidence of spatially diverse lacustrine processes during early sedimentary deposition and contributes the first mineralogical evidence of hydrothermal alteration at temperatures of ~100 ˚C or greater in early post-impact sedimentary deposits in an impact structure on Earth.