The Ordovician–Silurian boundary at Keisley, northern England

Abstract

AbstractThe relationships of the Keisley Limestone, an isolated carbonate mudmound in the Cross Fell Inlier of northern England, have long been problematical with respect both to the other upper Ordovician as well as the Silurian strata of the inlier. Bedded limestones, separated by gaps from the Keisley mudmound below and from atavus Biozone graptolite shales above, yielded a prolific shelly fauna to Temple in 1968 which he regarded as being of lower Llandovery age. This age is controversial. To try to establish the precise ages of the rock units, the position of the Ordovician–Silurian boundary, and the detailed nature of the changes which took place at this boundary, a critical section was excavated. Bed by bed lithological and faunal collections reveal from beneath the known atavus outcrop (a) five bentonite bands, unexpected in the context of northern England, (b) graptolites of both acuminatus and persculptus Biozones above the bedded limestones and (c) a similar but distinct assemblage from siltstones associated with these limestones which contains Hirnantia elements. From the palaeontological evidence there is little doubt that the topmost part of the carbonate sequence is of Hirnantian age.