Published in Geology, vol. 31 (June 2003), pp.557-560

Abstract: A 2-4 m thick seismite, in places overlain by a previously unreported tsunamite, can be traced across >250,000 km2 of the UK outcrop/subcrop of the latest Triassic (Rhaetian) Cotham Member of the Penarth Group, an extent unique for the British Phanerozoic. Its consistent thickness, intensity of deformation, and preferred orientations of slump-fold axes, indicate a seismic event of M>10 with an epicenter >600 km W or NW of central Britain. The magnitude of the event is incompatible with known terrestrial mechanisms (fault, volcano) but is consistent with a major bolide impact. An interval of short, but unknown, duration separates the top of the Cotham Member seismite from major geochemical and biotic perturbations associated with the end-Triassic extinction, although a direct link between the seismite and these other events remains equivocal. The exceptional extent of ‘mega-seismites’ such as this may prove a useful indicator of previously undocumented bolide impacts.

The map shows the locations around Britain and Northern Ireland where the unique seismite has been found. Red dots are where it has been seen at the surface, in cliffs or quarries. Blue dots are where it lies beneath the surface and has been found in samples recovered from boreholes.

Three localities were examined in detail and the 3 columns show sketches of the rocks in which the seismite is found. At Larne the seismite is nearly 4 m thick and overlain by more than a metre of sediments deposited by the waning effects of the tsunami which followed the earthquake. At the two other sites, Lavernock and Manor Farm, only the lower part of the seismite is present; the tsunamite and upper part of the seismite appear to have been eroded away a short while after they were deposited.

Key: dark grey=mudstone; pale grey=siltstone and fine sandstone; yellow=limestone; orange=sandstone