Barrier island to tidal shelf transition in the early Cambrian Eriboll Sandstone

Abstract

Synopsis The Cambrian Eriboll Sandstone, 150–225 m of quartz arenites deposited on the western margin of the Iapetus Ocean, consists of a Lower Member (75–125 m thick) overlain by the Pipe Rock Member (75–100 m thick). The basal 10 m consists of tabular cross-bedded sands deposited within channels of a mesotidal barrier island system. These sands are truncated by an extensive ravinement surface overlain by thin cross-bedded and parallel-bedded shoreface sands with bimodal palaeocurrents. The ravinement surface reflects erosional retreat of the barrier foreshore and upper shoreface during transgression. Lower shoreface deposits were dominated by storm and tidal processes. Palaeocurrents in these were enhanced by superposition of northwards flowing geostrophic storm currents. Shoreface sands pass upwards into tidal shelf sands deposited as dunes and sandwaves. Sandwave cosets thicken upwards in the Lower Member indicating increased tidal current velocities. This may reflect resonant amplification of tides as the shelf became wider during sea level rise. As the transgression proceeded the bioturbated Pipe Rock was deposited. Proximal deposition was by large compound sandwaves, similar to those below, and grading offshore into small, simple dunes and sandwaves. Distal fades were storm dominated. Both were populated by suspension feeding animals which produced abundant Skolithos burrows.