Abstract
Impact ejecta formation and emplacement is of great importance when it comes to understanding the process of impact cratering and consequences of impact events in general. Here we present a multidisciplinary investigation of a distal impact ejecta layer, the Blockhorizont, that occurs near Bernhardzell in eastern Switzerland. We provide unambiguous evidence that this layer is impact-related by confirming the presence of shocked quartz grains exhibiting multiple sets of planar deformation features. Average shock pressures recorded by the quartz grains are ~ 19 GPa for the investigated sample. U–Pb dating of zircon grains from bentonites in close stratigraphic context allows us to constrain the depositional age of the Blockhorizont to ~ 14.8 Ma. This age, in combination with geochemical and paleontological analysis of ejecta particles, is consistent with deposition of this material as distal impact ejecta from the Ries impact structure, located ~ 180 km away, in Germany. Our observations are important for constraining models of impact ejecta emplacement as ballistically and non-ballistically transported fragments, derived from vastly different depths in the pre-impact target, occur together within the ejecta layer. These observations make the Ries ejecta one of the most completely preserved ejecta deposit on Earth for an impact structure of that size.
Highlights
Impact cratering is a ubiquitous process throughout the solar system, its effects are obscured and/ or removed on Earth due to its active geological nature (e.g., Ref.[1])
The occurrence of shocked quartz in Cretaceous–Paleogene (K–Pg) boundary layers around the world was key evidence for understanding that ejected material from the Chicxulub impact had spread across E arth[9]
Shocked quartz is characterized by the presence of planar deformation features (PDFs)[1]
Summary
Impact cratering is a ubiquitous process throughout the solar system, its effects are obscured and/ or removed on Earth due to its active geological nature (e.g., Ref.[1]). Characterized distal ejecta layers can give important clues on impact events for which the impact crater is not preserved (or unknown), as for the Australasian tektite strewn field, e.g., Refs.[2,7,8], or act as the only source of information on the Earth’s early bombardment history[2]. We apply a multi-disciplinary approach that involves detailed petrographic analyzes, Universal stage (U-stage) characterization of shocked quartz grains (for confirmation of the mode of formation of the planar microstructures and for the estimation of the experienced shock pressures), U–Pb dating of bentonite horizons adjacent to the Blockhorizont to determine the age of deposition, as well as whole-rock element analysis, C- and O-isotopes, and paleontology to constrain the provenance of the ejecta components
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