EUSTASY AND TECTONICS: SEDIMENTARY RHYTHMS AND STRATIGRAPHICAL UNITS IN BRITISH DINANTIAN CORRELATION

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Abstract
SUMMARY: The six ‘Major Cycles’ of sedimentation formally recognised by Ramsbottom in the British Dinantian succession are based on alternations of rock types and facies, ascribed to ‘regression’ and ‘transgression’ of the sea. Conceptually interpreted on a hypothesis of eustasy, they are different in kind from the six regional stages of the recent Special Report of the Geological Society, which were empirically and arbitrarily founded on the basis of a biostratigraphical exercise and in another sense reflect contemporary episodes of earth-movement; and, in practice as in principle, cycle and stage diverge in their application to sediments of the several depositional provinces. Where there are radical facies changes, the ‘Major Cycles’ may be uncertainly identified or completely misidentified, being without definitive internal criteria, and they have no place in most of the Scottish deposits or in the Culm. The six regional stages also have their shortcomings, of boundary stratotypes and of criteria in extended correlation. A hypothesis of eustasy as a major control on sedimentation diminishes the place of contemporary tectonism in the palaeogeographical profile, whereas pulsed diastrophism during British Dinantian times was of a magnitude to subsume and obliterate most or all signs of oceanic changes in sea level and engendered stratigraphical variation and nonsequence in ways independent of a ‘universal’ or ‘world-wide’ eustasy. In the result, the sedimentary rhythms everywhere to be seen, including the ‘Major Cycles’, lack an intrinsic ‘universal’ parallelism between one tectonic province and another; and the systematic use of eustatic criteria in correlation, even if they could be locally recognised, is impracticable in such a structurally complex area as Britain. Still less are such criteria available for more extended application in terrain beyond Britain.

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