Doctor of Philosophy
Planetary Science and Exploration
Osinski, Gordon R.
Longstaffe, Fred J.
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.
Summary for Lay Audience
Meteorite impact events are well known for their destructive force, imparting massive amounts of energy into the surface of rocky bodies throughout the solar system. If an impact event is big enough, the energy from the impact can potentially create pools of molten rock, which can retain their heat for thousands of years as they gradually cool. On Earth, and potentially on ancient Mars, craters made by meteorite impacts can also infill with lake systems. In cases where water begins to fill an impact crater that still has hot masses of rock the heat causes that water to circulate through the cracks and fractures in the impact structure, potentially creating ideal conditions for early forms of life. The interaction between an impact crater lake and hot rocks created by the impact are potentially recorded in the rock record as the lake deposits accumulate over time. This thesis investigates the interface between rocks made by impact crater lakes and the previously molten rocks created by the impact event itself to better understand the environmental conditions in the early crater lake environment. The Ries impact structure in southern Germany, being one of the best preserved and well-studied impact structures on Earth, makes an excellent candidate for this case study. The rocks representing the earliest parts of the Ries crater lake were characterized, and their sedimentary structures and mineralogy indicated that they likely represented debris flows and the formation of alluvial fans. Additionally, the minerals in the fractures and voids of these rocks indicated that slightly alkaline, high temperature 100–130 ˚C fluids were present, which cooled to ~20 ˚C. High temperature minerals were more abundant in central regions of the crater lake environment than they were towards the edge of the lake system. Not only is this crater lake environment spatially diverse, but the processes that formed it were also shown to complicate the preservation of plumes of dust launched into the air by the impact event, making them less useful as markers for timing than previously thought.
Svensson, Matthew J. O., "The Origin and Evolution of Impact Crater Lakes: A Case Study of the Ries Impact Structure, Germany" (2022). Electronic Thesis and Dissertation Repository. 9079.