Tidal bedding and tidal cyclicities within the intertidal sediments of a microtidal estuary, Dyfi River Estuary, west Wales, U.K.

Abstract

In the microtidal Dyfi River Estuary, west Wales, the characteristic sedimentary structure within the intertidal sediments is tidal bedding. This consists of sand-mud couplets. By means of thickness measurements and detailed observations, the following conclusions can be drawn. (1) The thicknesses of, and structures in, both mud layer and sand layer of each couplet vary systematically downward in the studied cores and trenching profiles. Variations are mainly due to differences in the current speed and tidal range between tides, to difference of individual neap-spring tides, and to the possible effect of ‘filtering’ of currents due to being in the intertidal zone. (2) The patterns of sand-mud couplet thickness variations reflect two main types of tidal cyclicities: diurnal inequality, and the neap-spring-neap tidal cycle, which is in turn related to the lunar cycle. (3) The mud layer of each couplet represents deposition from suspension at times of tidal slack (no ebb slack water period during low tide in the intertidal zone, thus, no ‘ebb’ mud deposition during low water), whilst the sand layer is deposited by asymmetrical bidirectional tidal currents in either the ebb or flood half of a tidal cycle. (4) The tidal regime derived from these variation patterns is characterized by a mixed, predominantly semi-diurnal form, which is comparable with the present tidal regime. (5) Finally, it can be concluded that tidal bedding patterns are the structural expressions of tidal cyclicity.