New stratigraphical constraints on the Durness Group of NW Scotland

Abstract

Synopsis New field observations are combined with strontium isotope and trace element analyses in a stratigraphical investigation of the Durness Group at Balnakeil Bay, NW Scotland. The karst fissures and breccias used by Palmer et al. (1980) as evidence of a major unconformity between Lower Cambrian (Sailmhor (III) Formation) and overlying Ordovician strata (Sangomore (IV) Formation), are here interpreted as being of Tertiary or Recent age. Similar features are visible at outcrops along the entire coastline in association with modern cemented beach-sand. For the first time in the Palaeozoic, strontium isotope stratigraphy is used to date rocks whose age has remained unresolved by conventional bio- and litho-stratigraphy. All formations at Balnakeil Bay display consistent Lower Ordovician, Middle-Upper Cambrian 87Sr/86Sr ratios when compared with established data for this time interval (Donnelly et al. 1990; Keto and Jacobsen 1987). The 87Sr/86Sr ratios of Lower Toyonian archaeocyaths from Labrador (supported by currently unpublished data for the Tommotian and Atdabanian of Siberia) suggest that Lower Cambrian 87Sr/86Sr seawater ratios were significantly lower than those in the late Middle and Upper Cambrian. The increase in 87Sr/86Sr ratios may correlate with the Toyonian archaeocyath extinctions and mark the beginning of a global regressional event (which reached its maximum during the Late Cambrian). As there is no such excursion present within the Sr isotope curve for Balnakeil Bay, this implies that the upper Eilean Dubh (II) Formation is of Middle Cambrian age or possibly younger.