Stratigraphy and faunas of the Durness Group (Cambrian–Middle Ordovician) of Northwest Scotland: constraints on tectonic models and the development of the Great American Carbonate Bank

Abstract

The Durness Group of NW Scotland records deposition on the Laurentian margin from the basal Miaolingian (Cambrian, 509 Ma) to the Dapingian–Darriwilian boundary interval (Middle Ordovician, 470.3–468.9 Ma). The 930 m thick succession of peritidal and subtidal carbonates was deposited on the Scottish promontory, a nearly 120° deflection in the Palaeozoic continental margin between the Appalachian and Greenland sectors. These sediments were deposited as part of the Great American Carbonate Bank, a non-uniformitarian, continent-scale carbonate platform developed on the peneplaned craton. Measurement and description of a bed-by-bed composite section through the Durness Group provide a high-resolution reference framework that integrates conodont biostratigraphy, chemostratigraphy and sequence stratigraphy, including correlation with the Sauk megasequence and its subdivisions. The Sauk II–Sauk III sequence boundary marks the base of the group. The top of the group is faulted against rocks of the Moine thrust zone, generated by the Scandian orogeny, but sedimentation was probably terminated by the earlier Grampian arc–continent collision at 470–469 Ma. The highly mature quartz arenites of the underlying Ardvreck Group (Cambrian Series 2) indicate that there was no source-to-sink depositional continuity from the Hebridean foreland to the Dalradian Supergroup, which has coeval clastic sedimentary rocks of contrasting composition.