Sabkha Facies and the Preservation of A Falling-Stage Systems Tract At the Sauk II–III Supersequence Boundary In the Late Cambrian Eilean Dubh Formation, NW Scotland

Abstract

The Durness Group of northwest Scotland represents a nearly one-kilometer-thick succession of mid-Cambrian to lower Middle Ordovician carbonates. These rocks crop out in a narrow belt along the foreland of the Caledonian orogen and in the Moine Thrust zone, stretching some 170 km from Loch Eriboll southwestwards to the Isle of Skye. The Cambrian parts of the succession represent deposition on a paleo-southeast-facing, low-latitude, passively subsiding continental margin, situated at around 20° S. Carbonate deposition in Scotland formed part of the extensive Great American Carbonate Bank. The Sauk II–III supersequence boundary occurs above an interval of thin, meter-scale shallowing-upward peritidal cycles containing intraclast dolorudstones, stromatolites, tepees, siliciclastic sand, and surface karst. Evidence of former evaporites in the succession comprises pseudomorphs of nodular and bedded anhydrite, crystal-shaped calcite-filled vugs representing growth of sulfate, dissolution breccias, and local halite casts. These features represent sabkha facies arranged into cycles that constitute a preserved falling-stage systems tract (FSST). The top of the FSST is marked by a pronounced karst surface of sandstone-filled fissures. The section preserves a rare snapshot of a seldom-preserved environment on the Cambrian Laurentian margin. Its preservation during a time of falling relative sea level under slow tectonic subsidence is discussed.