Sedimentological, geophysical and oceanographic studies of postglacial and contemporary sedimentary processes of the NE Menai Strait and Conwy Bay (Wales, U.K.)
Various sedimentological, geophysical and oceanographic techniques have been used to study postglacial and contemporary sedimentary processes in the NE Menai Strait and Conwy Bay. Seismic records show three main reflectors, which delineate three depositional sequences (glacial, post-glacial and modern). The maximum recorded depth of the Pre Quaternary bedrock is 99m O. D. Newlyn, northeast of Puffin Island in a channel-filled structure which is probably an extension of the Dinorwic Fault. Variations in thickness of modern sediment deposits indicate the dynamic nature of sedimentation in the area. Tidal currents have been measured over spring/neap cycles at several stations. It is shown that residual currents in the NE Menai Strait are to the southwest though near Great Ormes Head (in the northern part of the study area) they are to the northeast. It is shown that sedimentation in this macrotidal coastal area is controlled primarily by tidal currents. Most of the seabed is covered by sands with local occurrences of gravelly sands, muddy sands, and sandy muds. The area can be subdivided into 5 subareas on the basis of sediment type, statistical grain size parameters, configuration of grain size distribution curves, and percentages of organic matter and carbonate. In Area A (near Bangor Pier) the sediments are very fine sands to medium silts, poorly and very poorly sorted, very positive skewed. The sediments are dominated by intermittent suspension and suspension populations. The sediments of Area B are: unimodal, fine and very fine sands, very well sorted, positively skewed, leptokurtic, and the dominant mode of transport is intermittent suspension. In the main channel area of the Menai Strait (Area C), mean grain size ranges from gravel to fine sands, sorting is spatially very variable, skewness is very negative, and sediment transport modes are surface creep and intermittent suspension. Northeast of Puffin Island (Area D) sediments are: poorly to very poorly sorted, negative and very negative skewed, and sediment transport modes are surface creep, intermittent suspension and suspension. Near Great Ormes Head (Area E), sediments are poorly to very poorly sorted, very positively skewed and the dominant transport mode is suspension. Small-scale ripples are the most abundant surficial bedforms in the intertidal areas where the sediments are fine grained and current velocities are relatively low. More than 80% of the observed megaripples are found in those areas where the water depth is between 9m and 11m and mean grain size >3.04. Sandwaves predominate in those areas with water depth >11m, where the grain size is <2.750. Sandwaves also occur in intertidal areas where water depths are 4-6m at high water and sediment size is 2.0-2.50. Calculated sediment transports at 12 hydrographic stations (subtidal area) vary from 1 to 136 kg cm-1 per tide depending mainly on sediment grain size and current velocity. In intertidal areas, the rate of flood dominant sediment transport increases away from the Arfon coastline towards the main channel in the Menai Strait and increases to the southwest as the Menai Strait narrows. Residual sediment transport is generally in the flood direction. It is shown that the net sediment transport in the area is to the southwest since: (1) bedform asymmetries are in the flood direction; (2) trends of grain size fining, regionally improved sorting, and a change from negative to positive skewness are to the southwest; (3) calculated residual transport vectors are also to the southwest.