The sediment on the floor of the southern Irish Sea

Abstract The southern Irish Sea has been surveyed using Asdic, continuous reflexion profiling, echo sounding and bottom samples and the results presented in the form of a sediment facies chart. In a section on the bathymetry some of the major sea-bed features have been interpreted using seismic data. Bed forms, principally sand waves, sand ribbons and sand streams, have been identified and their hierarchical associations established; these have provided insight into the patterns of sediment circulation. The form of these patterns is directly in the control of the tidal regime, which, in the southern Irish Sea, is the reverse of that normally found around British coasts. Along the Irish coast the tidal form consists of high velocities associated with negligible range: on the Welsh side low velocities and a pronounced range are the usual association. The effects of these tidal peculiarities on the sediment have been to create a bedload separation on the Irish shelf, and a coalescence on the Welsh side. Sediment studies indicate a wide compositional range for the potentially mobile material across the area. The sub-littoral sands are essentially quartzose with a strong rock debris component, carbonate contents being generally low (5%). Offshore and in mid-channel, there is a variable but significant and frequently dominant carbonate component which is bimodal as a result of whole shell imposition. Macrofauna communities, broadly defined in association with bottom characteristics, illustrate relationships between source material and detrital organogenic sediment.