3. Geology and Sedimentary History of Modern Estuaries

Modern estuaries are part of a continuum of coastal depositional environments within which the variation in geomorphology is closely related to the dominant one of three main processes affecting sedimentation, viz waves, tides or rivers. The present location of the coast is controlled by sea-level rise brought about by the release of water from continental ice sheets following the glacial maximum around 20,000 years ago. The current form of the coast is partly inherited from the shape of the precedent land surface flooded by the rising sea, which is then modified by a combination of ongoing local erosion and/or deposition of sediment transported by rivers from the adjacent land mass or submarine erosion, and then redistributed by the locally dominant marine processes. Once eustatic sea level stabilised around 6–7000 years ago, sediment was able to progressively infill the topographically lower areas, except in areas where glacial rebound is ongoing. In some cases, where the rate of sedimentation is relatively high, infill of coastal indentations may have been completed, and the coast is now prograding seaward. Elsewhere, where sedimentation rates are lower, or waves and tides are able to effectively move sediment away from the point of river entry, infill may have only partially proceeded, and the coast has been modified into characteristic forms. Where waves dominate over tides, features made from coarse-grained sediments such as barriers, beaches and bars, form parallel to the general trend of the coast. These establish less-energetic environments isolated from the full force of the ocean, where fine-grained sediments can accumulate. Where tidal forces are relatively dominant, the coarser-grained bars tend to orient at right angles to the coast, and fine-grained sediments accumulate in the intertidal areas as mud flats, and marshes.