The “suevite” conundrum, Part 2: Re-examining the type locality at the Ries impact structure, Germany

Abstract

One of the most common types of allochthonous impactite produced in hypervelocity impact events is impact breccia that contains melt particles. In numerous terrestrial hypervelocity impact structures such melt-bearing breccias have been termed “suevite,” after the type locality at the Ries impact structure, Germany. Despite its widespread occurrence, the origin, emplacement, and classification of suevite remains debated. In this contribution, we re-examine the nature and origin of suevite at the Ries impact structure. The results of new field and laboratory investigations, when combined and synthesized with results from previous studies, lead to a multi-stage model for the origin and emplacement of allochthonous impactites during the Ries impact event. Following the creation of a transient cavity the so-called Bunte Breccia and “megablocks” were emplaced via ballistic sedimentation and subsequent radial flow during the excavation stage to form a continuous ejecta blanket. At the end of the excavation stage, a mixture of melt and lithic fragments formed a lining to the transient cavity and it is this material that later became the crater, dike, and outer suevite (OS) units. The crater suevite represents the material from the displaced zone of the transient cavity that was transported and mixed but never left the cavity. The emplacement of dike suevite occurred during the modification stage as the crater suevite was intruded into fractures in the underlying crater floor. The OS and rare impact melt rocks overlying the ballistic (Bunte Breccia) ejecta deposits were emplaced as outwards-directed ground-hugging flows largely during the modification stage of crater formation. The OS flows varied both spatially and temporally in terms of the flow characteristics, from being dominated by solid particles and gas (cf. pyroclastic density currents) to a mixture of solid particles, liquid (impact melt), and minor gases (i.e., particulate impact melt-rich flows). These particulate impact melt-rich flows dominated by far. Minor “fallback” of material from an ejecta plume is evidenced by accretionary lapilli in the Nördlingen 1973 core. In summary, allochthonous impactites at the Ries impact structure are not unusual but are consistent with observations from other terrestrial and planetary craters, where melt-rich impactites overly ballistic ejecta deposits both outside and inside crater rims and where melt-rich impactites occur in crater interiors.