Uniquely extensive soft-sediment deformation in the Rhaetian of the UK: Evidence for earthquake or impact?

摘要: The lower part of the Cotham Member in Penarth Group (latest Triassic, Rhaetian) UK incorporates a uniquely extensive metre-scale horizon soft-sediment deformation. Interpreted as seismite, it shows evidence for only single seismic event even at its thickest development. It is recorded from more than forty sites across least eight discrete sedimentary basins covering N250,000 km 2 , and originally must have covered still larger area. Such widespread deformation, unique Phanerozoic implying exceptional magnitude, difficult to account by conventional terrestrial mechanisms. Contemporaneous volcanism Central Atlantic Magmatic Province (CAMP) was too far distant cause tectonic setting region not conducive earthquakes on this scale. Slump fold long axes suggest an epicentre broadly southern Irish Sea or St. George's Channel. Impact km-scale asteroid here potentially could produce observed sedimentological effects UK, but any late Triassic impact structure would now be concealed younger strata. At development, Northern Ireland, seismite succeeded rip-up breccia hummocky- wave-rippled cross stratification. These facies, their position immediately above are consistent with tsunami arising directly event. Tentative tsunamite age has also been reported France. putative Ireland desiccation-cracked hiatus which may correlate similar truncating England. England correlates closely δ 13 C isotope excursion that traced eastern Europe western North America associated significant biotic changes. ultimate remains unclear. No crater appropriate location currently known other time best equivocal. considered caused Lilstock Formation area, necessarily contributory factor generation either changes through Triassic–Jurassic boundary interval.