NEW STRATIGRAPHICAL CONSTRAINTS ON THE DURNESS GROUP OF NW SCOTLAND

New field observations are combined with strontium isotope and trace e lement analyses in a stratigraphical investigation of the Durness Grou p at Balnakeil Bay, NW Scotland. The karst fissures and breccias used by Palmer et al. (1980) as evidence of a major unconformity between Lo wer Cambrian (Sailmhor (III) Formation) and overlying Ordovician strat a (Sangomore (IV) Formation), are here interpreted as being of Tertiar y or Recent age. Similar features are visible at outcrops along the en tire 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- a nd litho- stratigraphy. All formations at Balnakeil Bay display consis tent Lower Ordovician, Middle-Upper Cambrian Sr-87/Sr-86 ratios when c ompared with established data for this time interval (Donnelly et al. 1990; Keto and Jacobsen 1987). The Sr-87/Sr-86 ratios of Lower Toyonia n archaeocyaths from Labrador (supported by currently unpublished data for the Tommotian and Atdabanian of Siberia) suggest that Lower Cambr ian Sr-87/Sr-86 seawater ratios were significantly lower than those in the late Middle and Upper Cambrian. The increase in Sr-87/86 Sr ratio s may correlate with the Toyonian archaeocyath extinctions and mark th e beginning of a global regressional event (which reached its maximum during the Late Cambrian). As there is no such excursion present withi n the Sr isotope curve for Balnakeil Bay, this implies that the upper Eilean Dubh (II) Formation is of Middle Cambrian age or possibly young er.