Sand ridges on the Celtic Sea shelf edge

Celtic Sea linear tidal sand ridges, the Irish Sea Ice Stream and the Fleuve Manche: Palaeotidal modelling of a transitional passive margin depositional system
by James Scourse, Katsuto Uehara, and Adam Wainwright
Marine Geology 259 (2009) 102–111

https://pdfs.semanticscholar.org/3d50/6a07a449df3641bab203ce0d8df2d5d26e6d.pdf

Abstract

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.


That is not the easiest Abstract to understand (nor is the paper), but the gist is that these big ridges in the Celtic Sea, out towards the shelf edge and in water depths of between 130m and 200m, are probably not glacially streamlined features or De Geer moraines, but the products of tidal “pumping” in areas with huge sediment supplies. The sediments shifted about might well have had glacial origins, since the Devensian ice certainly did reach the area to the west of the Isles of Scilly (and according to the latest paper by Praeg et al it probably reached the shelf edge. The idea is that the sediments were pumpted over the shelf edge and into a set of distributory channels which carried them down to the Celtic Deep Sea Fan and the Armorican Deep Sea Fan in deeper water. But I am still mystified as to why there are ridge forms — as the authors explain, some of the ridges are "as much as 200 km in length, 55 m high and 15 km wide, with a ridge-to-ridge wavelength of some 20 km”. I am not sure that the artice does this, but maybe somebody will explain to me why these features have elongated ridge forms and why we do not simpley see an unduating thick spread of sea floor sediments……….

Anyway, a very useful paper, with some interesting maps added. Figure 3 is fascinating, illustrating the peak bed stress vectors for the Celtic Sea in “timesteps” since the peak of the Devensian Glaciation. The evolving shoreline is shown; so we see that 21,000 years ago the Celtic Sea arena , Bristol Channel and St George’s Channel were dry land. That means that any glacier ice still present would have been grounded. By 19,000 BP some lakes were forming to the west of embrokeshire, and these get bigger and bigger until around 15,000 BP. Then the rising sea breaks through, and the centre of St George’s Channel has sea water flowing through it around 14,000 years ago. The shoreline creeps closer and closer to the westernmost cliffs of Pembrokeshire and by about 10,000 BP it is not far from the Pembrokeshire islands of Ramsey and Skomer. By 8,000 BP the water is flooding into St Bride’s Bay and Carmarthen Bay.

The other interesting map is Figure 2, showing not only the famous elongated ridges but also a reconstruction of the Devensian British and Irish Ice Sheet (or Celtic Ice Sheet, as it is now being called). I know this is a bugbear of mine, but I am intriged by the reconstructed flowlines. All of them look perfectly sensible, perpendicular to proposed ice edges — except for the Celtic Sea lobe, where everything looks cockeyed. The eastern edge of the lobe should be much further east — but we’ll forgive that, since this paper is almost 10 years old. But the flowlines should be fanning out, not converging, and ice should be flowing towards the Scilly Islands and the tip of Cornwall from the NW. Much as I admire geomorphologists, there seems to be a corporate mental blockage on this issue.

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