Abstract

S. Carroll writes: Recent detailed mapping of the Lower Devonian succession in the northeastern Midland Valley by the British Geological Survey partially confirms the conclusions of Marshall et al. 1994 that the Siluro-Devonian succession in Kincardineshire is thinner than previously suspected and that tectonism strongly influenced sedimentation. However the interpretation of Marshall et al. is open to question in some respects.

Marshall et al. assume an average geothermal gradient of 30°Ckm-1 which they consider to be 'conservative in view of the ... contemporaneous volcanism'. However they consider their calculated gradient of 23°Ckm-' for the Lower Carboniferous of East Fife, a succession which also contains evidence of significant contemporaneous volcanism (Forsyth & Chisholm 1977), to be 'about crustal average'. Allen & Allen (1990) indicate the geothermal gradient of normal sedimentary basins to range from 25-30 °Ckm-1, while Mann er al. 1983 and Royden (1985) suggest that in some circumstances, pull-apart basins may have lower than average geothermal gradients. The value for the lower end of the range of 'normal' geothermal gradients would allow the calculated total burial depth for the Stonehaven Group to be about 7 km, rather than the 5.4 km suggested by Marshall et al. 1994.

They note the lack of systematic variation in reflectivity values in a stratigraphically well-constrained section of the Dundee Formation and add that caution should be exercised in interpreting such data. They suggest that this situation probably reflects 'as much local variation in igneous heating as minor differences in burial'. This caution is reasonable

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