Devonian precessional and other signatures for establishing a Givetian timescale

Abstract

Abstract Palaeozoic radiometric dates are so isolated and crude as to offer little numerical precision in stratigraphy. In a Givetian sequence of pelagic micrites in the Montagne Noire of southern France there is a succession of small-scale microrhythms which appear to represent precessional signatures. Correlations can be made with other sections in the Tafilalt of southern Morocco and, for the late Givetian, in the Marble Cliff Beds of north Cornwall. Now that the IUGS has approved boundaries defining the upper and lower limits of the Givetian it is possible to estimate the number of putative precessional signatures for the whole Givetian and, using the calculation of periods for these in the Devonian, to estimate the duration of the Givetian and its conodont zones independantly from radiometric dates. Graphical correlation techniques are used to establish a time framework for the Givetian and this is used to estimate the duration of zones in relation to the preliminary precessional scale, and hence to estimate their duration in real time. Biological changes and sedimentary events can similarly be constrained against a scale for the Givetian. It is suggested that such techniques, using a stage for a base, is likely to be useful long before continuous orbital forcing timescales (OFT) can be established. It would be a start in eliminating present crude estimates of the length of stages based on the false presumption that zones within them are of equal duration. Given boundaries, defined internationally by IUGS at Global Stratotype Section and Points, such OFT scales would enable events and zonal durations to be given in relation to OFT graphic composite scales and on the basis of estimated precessional signatures.