Discussion on dating the transition of smectite to illite in Palaeozoic mudrocks using the Rb-Sr whole-rock technique

Abstract

D. M. D. James writes: It is exciting and important that the variable time intervals between burial, the smectite-illite transition in mudrocks and cleavage formation/uplift which might be expected in the tectonically active Welsh Basin can now be quantified. This will allow estimation of syn-depositional tectonic tilts between different elements of the basin, subject to tilt corrections for the dip of the seabed and for any non-vertical heatflow, i.e. factors jointly determining the dip and depth of the 90°C isotherm. Such corrections are likely to be extremely small in the samples studied, particularly for the 'basin-floor' samples 1 and 2 of the paper which probably had low depositional dip and lie at adequate elevation above the relief of top crystalline basement to suggest that lateral heat-flow complications are minor. The very sparse and, as yet, statistically inconclusive tilt geometries suggested by the smectite-illite dates can, however, be readily confirmed by the abundance of statistically conclusive data available for the degree of illite diagenesis/metamorphism; i.e. illite crystallinity. Not only is the smectite-illite transition pre-cleavage/uplift (and by implication pre-folding, assuming this to be syn-cleavage) but so also are the higher metamorphic grades, to low epizone. Across central Wales, where no anomalously high-strain zones are reported and where equal values of illite crystallinity (isocrysts) may thus reasonably relate directly to isotherms, the distribution of isocryst values displayed on BGS sheets 178 and 179 (1994 (1993) bears no obvious relation to folding. The map pattern is thus not the result of folding of isotherms