Britain’s own impact

While evidence has been accumulating for the influence of asteroid and comet strikes elsewhere, the British geological community has had a disproportionate share of sceptics; those who thought it was all a matter of “whizz-bang” science.  It is welcome news that we now have our own “piece of the action”, for geoscientists from Aberdeen University and the Open University have a discovered a well-preserved impact horizon in Late Triassic terrestrial sediments that contain both devitrified glass spherules and shocked quartz grains (Walkden, G. et al. 2002.  A Late Triassic Impact Ejecta Layer in Southwestern Britain.  Science Express –, 15 November 2002).  It is not associated with the Triassic-Jurassic boundary, which witnessed on of the “Big Five” mass extinctions, but is dated at 214±2.5 Ma, within error of the major impact at Manicougan (~100 km diameter; Quebec; 214±1 Ma) the lesser Rochechouart structure (~25 km diameter; France; 214±8 Ma).  The Ar-Ar dating did not use spherule glass, but authigenic potassium feldspar that postdates the spherules, but may have formed from potassium released when they became hydrated.  Given its size and position relative to Britain on a Triassic plate reconstruction, Manicougan is a likely culprit.  However, despite its considerable size, there are no signs of significant faunal changes at the time of the Manicougan impact.  The host Triassic rocks in Somerset rest directly on Carboniferous limestones, and primitive mammal remains are known from infillings of a palaeokarst surface in the Mendip Hills.  Now the deposit has come to light, the search is on for similar materials in Late Triassic marine sediments.

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