Bunte Breccia of the Ries: Continuous deposits of large impact craters

Abstract

The Ries Crater, an impact structure of 26 km diameter in south Germany, is the largest terrestrial crater where substantial amounts of ejecta are preserved, on occasion >100 m deep. Further, the target stratigraphy is well known, and it is possible to relate specific clasts and breccia lithologies to initial target depth. As a consequence the continuous deposits of the Ries, also known as Bunte Breccia, may be studied with exceptional field control. We report field observations and laboratory analyses obtained from 560‐m core materials, taken at nine different locations that range from 16 to 37 km in radial distance from the impact center. The objective is to relate the Ries observations to ejection, and to emplacement processes of large‐scale, planetary crater deposits. The observations regarding the modal‐stratigraphic characteristics of the Bunte Breccia may be summarized as follows: only <0.15% (weight) of the total deposit consists of crystalline clasts larger than 1 cm that are derived from depths of >600 m; some 0.7% is composed of Triassic clasts, originating from 300 to 600‐m depths; Lower and Middle Jurassic horizons (approximately 300–150 m) constitute some 2.3%, and Upper Jurassic (0–150 m) makes up some 31.5%. In addition, the Bunte Breccia contains Tertiary materials in the form of >1‐cm clasts (29.1%) and as highly comminuted, fine‐grained “matrix” (<1 cm) accounting for the remaining 36.3%; these Tertiary materials constituted the immediate crater environment, i.e., a substrate, onto which the Ries ejecta were deposited. These substrate materials were thoroughly mixed into the continuous deposits. The ratio of “primary crater ejecta” to local substrate components decreases with increasing radial range. There is, however, no vertical stratification with regard to modal‐stratigraphic composition at any specific location; modal‐stratigraphic composition is highly variable on meter scales; Bunte Breccia appears to be a chaotic mixture resulting from a highly turbulent depositional environment. Also, the orientation of clasts larger than 1 cm is random. Detailed grain size data reveal progressively decreasing grain sizes with increasing radial range of both primary crater ejecta and local substrate materials. In addition, progressive comminution of primary ejecta related to increasing target depth is observed. Components shocked to >10 GPa constitute <0.1% (weight) of the entire deposit, which indicates that Bunte Breccia was emplaced at essentially ambient temperatures. When possible, the above observations are quantified via linear regressions throughout the text. All of these observations are consistent with, if not predicted by, a ballistic emplacement scenario as postulated by Oberbeck and co‐workers: primary crater ejecta are expelled ballistically and will form secondary craters in the local substrate; a mixture of primary and secondary ejecta results and combines into a highly turbulent, ground‐hugging debris surge as the final phase of ejecta emplacement. Total emplacement time for the Bunte Breccia (⪖200 km³) is estimated to be of the order of 5 min only. These findings are compared with cratering theory relating to a number of ejecta thickness decay models and with the so‐called Z model, addressing material flow during various stages of crater formation. Qualitative to fair agreement of observations and predictions results. An initial crater radius of 6.5 km, an excavation depth of 1650 m, an excavation volume of 136 km³, and an associated transient cavity volume of aproximately 230 km³ appear to be reasonable estimates. Approximately 170 km³ of material was involved in slumping and restoration of the transient cavity for the above radius and Z=2.7. The modal composition of Ries ejecta with regard to preimpact target stratigraphy indicates that materials contained in the continuous deposits of large, complex planetary craters are predominantly derived from depths as small as one‐hundredth the apparent crater diameter. A number of implications are addressed regarding remote sensing of planetary surfaces and investigation of lunar and meteoritic impact breccias.