Discussion of Longmyndian dates

Abstract

Dr P. Toghill writes: In his most interesting paper Dr Bath concludes that the Eastern Longmyndian cannot be older than 600 m.y. Thus if we accept the base of the Cambrian at 570 m.y., this leaves 30 m.y. in which to deposit the Western Longmyndian, and to fold up the Uriconian and Longmyndian into its accepted syncline, before the start of the Cambrian. This at first sight appears plausible. However, if we take Dr Bath's dates for the Eastern Longmyndian before allowing for dewatering, i.e. 529 ± 23 m.y. for the Lightspout Group, this date could represent a metamorphic episode in the late Cambrian, which affected the Longmyndian W. of the Church Stretton Fault (CSF) and E. of the Pontesford–Linley fault line, but which is later than the main folding of the Longmyndian. It is well-known that the Cambrian sequence E. of the CSF has a number of non-sequences and unconformities, which presumably reflect tectonic disturbance elsewhere. One of these breaks is at the base of the ‘upper’ Cambrian at about 515 m.y. and the other at the base of the Tremadoc Shineton Shales at about 500 m.y. (dates from Cowie et al. 1972 , p. 12). These could be the effect on the shelf area of a metamorphic episode which Dr Bath has dated in the Longmyndian at 529 ± 23 m.y.W. of the CSF. It must be realized that the Longmyndian strata on the W. of the CSF, now very close to the Cambrian strata E. of