Celtic Sea linear tidal sand ridges, the Irish Sea Ice Stream and the Fleuve Manche: Palaeotidal modelling of a transitional passive margin depositional system

The linear tidal sand ridges (LTSR) of the Celtic Sea constitute the largest examples of their bedform type on Earth. Previous sedimentological and seismic stratigraphic interpretation suggests that the LTSR are moribund tidally remobilised sediments representing the transgressive systems tract. This interpretation is supported by two-dimensional finite-difference model reconstructions of the M₂ tide, forced using the output from a glacial isostatic adjustment model to derive palaeotopography, and a global ocean model to derive the tides on the ocean boundary, used here to reconstruct peak bed stress vectors for the Celtic Sea for timesteps covering the transgression since the Last Glacial Maximum (last 21 ka). These data are coupled with interpretation of recent published observations on the outer shelf depocentres of the Fleuve Manche and the Irish Sea Ice Stream (ISIS) to confirm that the LTSR distribution is consistent with modelled sand transport paths during transgression. The main phase of LTSR growth was between 20 cal ka and 12 cal ka. Ridge axis orientations reflect the final phase of LTSR construction around 12 cal ka, with some later growth of the most southerly LTSR as late as 10 cal ka. LTSR growth was from the SW across the shelf towards the NE. Strong tidal pumping of sediments into slope canyon heads on the outer shelf occurred between 20 cal ka and 12 cal ka, contributing to turbidite activity and the growth of the Celtic and Armorican deep sea fans. It is proposed that 1. the Fleuve Manche shelfal estuary-delta, and the Irish Sea Ice Stream (ISIS) shelf fan, were the main sediment depocentres supplying the growing LTSR, 2. the lack of sediments available in the western Channel limited LTSR growth in this area, and that the easterly termination of the LTSR in this sector results from sediment starvation, and 3. the northeasterly termination in the Celtic Sea sector is a function of declining peak bed stresses rather than sediment starvation. The Celtic Sea margin represents a passive margin depositional system transitional between true glacial ice stream-trough mouth fan systems (to the north) and the fluvial canyon systems characteristic of the margin to the south. This interpretation therefore complements studies of macroscale sedimentation linked to glaciation on the continental margins of the North Atlantic.