Geophysical/sedimentological studies of a Quaternary tidal delta system
An attempt has been made to define the Quaternary development of a tidal delta system and the surrounding area of the Menai Strait and Caernarfon Bay. The morphologies and processes of a tidal inlet and its associated flood and ebb-tidal deltas are described and a model of sediment transport pathways prevailing in the study area has been proposed. Seismic stratigraphic evidence suggests that the Quaternary deposits in Caernarfon Bay are primarily composed of three depositional sequences i. e. glacial, postglacial, and recent. The oldest sequence i. e. glacial sequence, lies unconformably on bedrock. The bedrock topography was successfully defined using a sparker seismic system and the results suggest that the bedrock surface is very uneven in nature and that there are two NE-SW trending valley features which can be linked to the Berw and Dinorwic faults. However, their typical U-shaped form may be attributed to erosion during the late Devensian ice advance. Various types of progradational reflection patterns within the postglacial sequence suggest a rapid rise in sea level after c. 9000 years B. P. It appears that the completion of the present morpho-hydrodynamic system most probably took place c. 5000 years B. P. The newly evolved morpholgy and flood and ebb tidal currents resulted in the accumulation of thick deposits of recent sediments in and outside the inlet i. e. flood and ebb-tidal deltas. A comparison of these tidal deltas with standard models suggest that whilst the flood-tidal delta shows hardly any resemblance to the standard model, the ebb-tidal delta is strikingly consistent in morphology and processes with the standard model. A model of the sediment transport pathways, proposed on the basis of comprehensive studies of bedform characteristics, grain size distribution trends, and to some extent on the basis of sediment transport calculations, suggests that there is a net sediment transport towards the southwest through the Menai Strait. The study of temporal variations in the characteristics of bedforms within the Menai Strait during a neap/spring tidal cycle suggest that the bedforms are ebb-oriented during most of the tidal states. A partial flood orientation occurs only during high spring tide. The extensive occurrence of sand ribbons with superimposed small scale megaripples (1-2m in wavelength) in the deeper parts of the Caernarfon Bay suggest a relatively lesser net sediment transport northeastwards which turns towards the north as it approaches the ebb-tidal delta body.