Discussion on new seismic evidence for a major geololgical boundary at shallow depth, N Devan

Abstract

M. G. Bassett & J. C. W. Cope write: Pretannia and the shallow basement below north Devon: The recent paper by Brooks, Hillier & Miliorizos (1993) adds valuable seismic evidence for the interpretation of shallow crustal structure below the southern Bristol Channel and north Devon. Of particular interest is the identification of a major, southerly dipping lithostratigraphical unit below the Devonian outcrop and in apparent stratigraphical (as opposed to tectonic) contact with it. The authors conclude that the nature and age of this unit remain uncertain, but we believe that much of this uncertainty can be resolved by analysing wider regional facies patterns and stratigraphy, together with evolving palaeogeographies. Most of the conclusions resulting from such an analysis have been published previously (Cope & Bassett 1987; Cope 1987 ) and are readily reconciled with the Brooks et al. data. The original presence of a metamorphic Precambrian basement below southwest England now appears to be unequivocal from the combined geophysical and sedimentological evidence. This is the positive sediment source that we christened Pretannia (Cope & Bassett 1987, p.326), and whose presence is confirmed amply by the high mica content in persistent southerly-derived late Precambrian through Silurian sequences across southern Wales. Lithologically, Pretannia clearly comprised extensive areas of gneisses and/or quartz mica schist. Brooks et al. (p. 134) point to the evidence for such lithologies in south Devon, to which we would add the significant presence of metamorphic xenoliths with a pre-Variscan fabric 'emplaced within Devonian sediments by basic magma' in west Cornwall