Tuesday, January 23, 2007
Impact-related soft-sediment deformation in the UK Triassic?
In the latest issue of Palaeogeography, Palaeoclimatology, Palaeoecology a new explanation is offered for an extraordinarily widespread horizon of soft-sediment deformation in the lower part of the Cotham Member of the Penarth Group (latest Triassic, Rhaetian). The most parsimonious interpretation is for a major seismic event triggering in situ foundering of poorly consolidated sediments, but its exact cause remains enigmatic. The consistent position of the ‘seismite’ horizon in the lower part of the Cotham Member, the lack of any evidence for more than one deformational event, and the absence of similarly widespread phenomena in contiguous parts of the geological column, favour a single-event scenario. The authors argue that volcanism is unlikely to have been responsible and that the impact of a 2-3 km diameter asteroid offers a more attractive explanation for such a powerful seismic shock. However, no impact crater of the right age has yet been located so the bolide impact interpretation remains speculative.
Simms, M. J., ‘Uniquely extensive soft-sediment deformation in the Rhaetian of the UK: evidence for earthquake or impact?’, Palaeogeography, Palaeoclimatology, Palaeoecology 2007;244(1-4):407-423.
Abstract. The lower part of the Cotham Member in the Penarth Group (latest Triassic, Rhaetian) of the UK incorporates a uniquely extensive metre-scale horizon of soft-sediment deformation. Interpreted as a seismite, it shows evidence for only a single seismic event even at its thickest development. It is recorded from more than forty sites across at least eight discrete sedimentary basins covering > 250,000 km2, and originally must have covered a still larger area. Such a widespread horizon of soft-sediment deformation, unique for the UK Phanerozoic and implying a seismic event of exceptional magnitude, is difficult to account for by conventional terrestrial mechanisms. Contemporaneous volcanism in the Central Atlantic Magmatic Province (CAMP) was too far distant to cause the deformation, and the tectonic setting of the region was not conducive to earthquakes on this scale. Slump fold long axes suggest an epicentre broadly in the southern Irish Sea or St. George's Channel. Impact of a km-scale asteroid here potentially could produce the observed sedimentological effects across the UK, but any late Triassic impact structure would now be concealed by a km or more of younger strata. At its thickest development, in Northern Ireland, the seismite is succeeded by a rip-up breccia and hummocky- and wave-rippled cross stratification. These facies, and their position immediately above the seismite, are consistent with the effects of a tsunami arising directly from the seismic event. Tentative evidence for a tsunamite of this age has also been reported from southern France. The putative tsunamite in Northern Ireland is succeeded by a desiccation-cracked hiatus which may correlate with a similar hiatus truncating the seismite at sites in southern England. The hiatus in southern England correlates closely with a δ13C isotope excursion that has been traced from eastern Europe across to western North America and is associated with significant biotic changes. The ultimate cause of the seismite and associated tsunamite remains unclear. No impact crater of appropriate age or location is currently known and other evidence for impact at this time is at best equivocal. It is considered here that impact of a km-scale asteroid may have caused the observed sedimentological effects in the Lilstock Formation across the UK area, but was not necessarily a significant contributory factor in the generation of either the isotope excursion or of the biotic changes through the Triassic-Jurassic boundary interval.
Simms, M. J., ‘Uniquely extensive soft-sediment deformation in the Rhaetian of the UK: evidence for earthquake or impact?’, Palaeogeography, Palaeoclimatology, Palaeoecology 2007;244(1-4):407-423.
Abstract. The lower part of the Cotham Member in the Penarth Group (latest Triassic, Rhaetian) of the UK incorporates a uniquely extensive metre-scale horizon of soft-sediment deformation. Interpreted as a seismite, it shows evidence for only a single seismic event even at its thickest development. It is recorded from more than forty sites across at least eight discrete sedimentary basins covering > 250,000 km2, and originally must have covered a still larger area. Such a widespread horizon of soft-sediment deformation, unique for the UK Phanerozoic and implying a seismic event of exceptional magnitude, is difficult to account for by conventional terrestrial mechanisms. Contemporaneous volcanism in the Central Atlantic Magmatic Province (CAMP) was too far distant to cause the deformation, and the tectonic setting of the region was not conducive to earthquakes on this scale. Slump fold long axes suggest an epicentre broadly in the southern Irish Sea or St. George's Channel. Impact of a km-scale asteroid here potentially could produce the observed sedimentological effects across the UK, but any late Triassic impact structure would now be concealed by a km or more of younger strata. At its thickest development, in Northern Ireland, the seismite is succeeded by a rip-up breccia and hummocky- and wave-rippled cross stratification. These facies, and their position immediately above the seismite, are consistent with the effects of a tsunami arising directly from the seismic event. Tentative evidence for a tsunamite of this age has also been reported from southern France. The putative tsunamite in Northern Ireland is succeeded by a desiccation-cracked hiatus which may correlate with a similar hiatus truncating the seismite at sites in southern England. The hiatus in southern England correlates closely with a δ13C isotope excursion that has been traced from eastern Europe across to western North America and is associated with significant biotic changes. The ultimate cause of the seismite and associated tsunamite remains unclear. No impact crater of appropriate age or location is currently known and other evidence for impact at this time is at best equivocal. It is considered here that impact of a km-scale asteroid may have caused the observed sedimentological effects in the Lilstock Formation across the UK area, but was not necessarily a significant contributory factor in the generation of either the isotope excursion or of the biotic changes through the Triassic-Jurassic boundary interval.
# posted by Nemagraptus @ 8:29 AM 
