Abstract— Impact ejecta eroded and transported by gravity flows, tsunamis, or glaciers have been reported from a number of impact structures on Earth. Impact ejecta reworked by fluvial processes, however, are sparsely mentioned in the literature. This suggests that shocked mineral grains and impact glasses are unstable when eroded and transported in a fluvial system. As a case study, we here present a report of impact ejecta affected by multiple fluvial reworking including rounded quartz grains with planar deformation features and diaplectic quartz and feldspar glass in pebbles of fluvial sandstones from the “Monheimer Höhensande” ?10 km east of the Ries crater in southern Germany.
Abstract Isotopic and stratigraphic ages of the ~ 80 km diameter Puchezh-Katunki (Russia; 220 ± 10 to 167 ± 3 Ma) and the ~ 20 km diameter Obolon (Ukraine; 215 ± 25 to 169 ± 7 Ma) impact structures are associated with significant age uncertainties. As a case study, reconstructions of the palaeogeography at the time of crater formation (Late Triassic to Middle Jurassic) based on recent palaeogeographic maps help further to constrain impact ages. Palaeogeographic studies suggest that Puchezh-Katunki is older than 170 Ma and that Obolon is younger than 185 Ma. This also rules out that Obolon formed during a ~ 214 Ma Late Triassic multiple impact event as recently discussed.
Abstract A recrystallized band of pale feldspathic impact melt in a gneissic impact breccia from the approximately 10 km Paasselkä impact structure in southeast Finland was dated via 40 Ar/ 39 Ar step‐heating. The newly obtained plateau age of 228.7 ± 1.8 (2.2) Ma (2σ) ( MSWD = 0.32; p = 0.93) is equal to the previously published pseudoplateau age of 228.7 ± 3.0 (3.4) (2σ) for the impact event. According to the current international chronostratigraphic chart and using the most recent published suggestions for the K decay constants, a Carnian (Late Triassic) age for the Paasselkä impact structure of 231.0 ± 1.8 (2.2) Ma (2σ) is calculated and considered the most precise and accurate age for this impact structure. The new plateau age for Paasselkä confirms the previous dating result but is, based on its internal statistics, much more compelling.
Introduction: Construed as the result of a collapsing ejecta plume since 1977 [1], the formation and emplacement of the Ries suevite was recently reinterpreted as a result of (a) ground-hugging impact melt flows [2], or (b) ‘phreatomagmatic eruptions‘ that were caused by the interaction of surficial water with an impact melt pool [3,4]. Furthermore, [5] pointed out a striking similarity between structural features in suvites and ignimbrites (compare Figs. 1 and 2). Ignimbrites are deposits of pyroclastic density currents (pyroclastic flows), a hot suspension of particles and gas driven by the collaps of an eruptive column. The deposits are composed of a poorly-sorted mixture of volcanic ash and pumice, commonly with scattered lithic fragments; various stages of welding and reomorphic flow structures can be observed [6]. They usually exhibit a fine-grained, often nonerosive, basis (surge), followed by ash layers that contain inversely graded rock fragments. Bottom-up, ignimbrites are dominated by pumice-rich ash layers, overlain by very fine-grained fall-back ashes [7]; elutriation (degassing) pipes are frequently developed at the top.
Abstract The ~3.8 km Steinheim Basin in SW Germany is a well-preserved complex impact structure characterized by a prominent central uplift and well-developed shatter cones that occur in different shocked target lithologies. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and electron probe microanalysis have revealed, for the first time, the occurrence of rare metals on the Steinheim shatter cone surfaces. Shatter cones produced from the Middle Jurassic (Aalenian) Opalinus Claystone (‘Opalinuston’), temporarily exposed in the central uplift in spring 2010, and shatter cones in Upper Jurassic (Oxfordian) limestones from the southeastern crater rim domain are commonly covered by faint coatings. The Opalinus Claystone shatter cone surfaces carry coatings dominated by Fe, Ca, P, S and Al, and are covered by abundant small, finely dispersed microparticles and aggregates of native gold, as well as locally elevated concentrations of Pt. On several surfaces of the claystone shatter cones, additional Fe, Ni and Co was detected. The Ca–Mn-rich coatings on the limestone shatter cone surfaces locally include patches of Fe, Ni, Co, Cu and Au in variable amounts and proportions. The intriguing coatings on the Steinheim shatter cones could either stem from the impacted Lower Jurassic to Palaeogene sedimentary target rocks; from the crystalline-metamorphic Variscan crater basement; or, alternatively, these coatings might represent altered meteoritic matter from the Steinheim impactor, possibly an iron meteorite, which may have been remobilized during post-impact hydrothermal activity. We here discuss the most plausible source for the rare metals found adherent to the shatter cone surfaces.
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