Ries impact crater sedimentary conglomerates: Sedimentary particle 'impact pre-processing', transport distances and provenance, and implications for Gale crater conglomerates, Mars

Abstract

Terrestrial impact cratering events produce anomalous rock fragmentation and alteration, and form sedimentary basins that are out of equilibrium with the local geomorphological environment. Assessement of their initial deposits, crater erosion and infilling, lacustrine environments and biology can provide important insights into similar features on Mars. Here we report on fluviodeltaic conglomerates associated with the 26km diameter Miocene Ries impact structure (target: sediments overlying crystalline basement) with respect to shock features, grain size distribution, roundness, and potential parent rocks (provenance). The poorly sorted conglomerates form coarsening-upwards cycles associated with a delta-mouth bar system, with subangular to subrounded pebbles derived from crystalline rock impact breccia deposits. Pebbles comprise a wide range of lithologies and states ranging from fresh and solid to intensively impact-fractured, friable crystalline rocks, including shatter cones. Shocked quartz grains, however, are restricted to the sand fraction. The latter observation points to suevite as an additional, but subordinate source of the conglomerates. Granulometric analysis indicates that, in contrast to being derived from weathering of bedrock, impact brecciation of the parent crystalline rocks generated the initial grain size distribution of the conglomerates (i.e., preprocessing). This concept of “impact preprocessing” helped to defne the provenance, the post-impact sedimentologic history and the sediment transport distances of the conglomerates. Post-impact fluvial transport leads to the outwash of fine fractions into the prodelta subenvironment, and an increase in roundness and mean diameter in the residual conglomerates (i.e. hydraulic sorting). On the basis of the characteristics of the ejecta deposits and the present-day locations and occurrences of crystalline rock impact breccias and blocks in the northwestern Ries crater segment analyzed, a transport distance of less than 3.5km is derived for these impact-preprocessed conglomerates. Reconstruction of the initial distribution and composition of the primary ejecta layer indicates that the conglomerates composed solely of crystalline rock had a maximum transport distance less than 6.5km, i.e., within the closed basin of the Ries impact structure. In contrast to traditional interpretations of conglomerates, requiring long transport distances from bedrock source regions to deposition to ensure sufficient rounding and sorting, the rounding and sorting associated with these short transport distances are explained by preprocessing of parent rocks by impact brecciation and enhanced susceptibility to abrasion. Consequently, transport distances of conglomeratic fragments associated with impact craters, such as Gale crater on Mars, are likely to have been only a few kilometers, and derived from nearby ejecta, most likely from the inner crater walls. This Ries crater-based preprocessing and transport model implies that the unusual Gale crater conglomeratic fragments are likely to represent the immediate target material excavated by the Gale cratering event, rather than fragments transported laterally from greater distances outside the crater.