Holocene evolution of the River Bann estuary and adjacent coast, Northern Ireland

Abstract

From a variety of published and unpublished geomorphological and stratigraphical information, supported by 14C dates, an attempt is made to reconstruct the Holocene evolution of the estuary of the River Bann and the adjacent stretch of coastline, Northern Ireland. Submerged and buried peat beds have yielded early Holocene pollen assemblages and 14C dates that demonstrate a low stand of relative sea-level (RSL) at this time. Peat formation was terminated by the deposition of sands and diatomaceous muds, reflecting RSL rise as a result of both eustatic controls and a declining level of isostatic recovery. Holocene maximum RSL occurred at c. 6000 years bp and reached +2 to +3 m OD (Belfast). A gravel barrier developed at the estuary mouth and provided the anchor point for later aeolian dune development. Along the adjacent coast, basalt shore platforms were either formed or modified, having been within the range of effective wave erosion since the early Holocene. Relative sea-level then fell rapidly. By c. 5500 years bp, aeolian dunes had formed below OD at one site within the estuary. By c. 4000 years bp, the primary elements of the geomorphology of the Bann estuary were in place. Since that time, several phases of aeolian dune and sand sheet development are apparent from the occurrence of buried soils and archaeological materials, but the primary mechanisms controlling dune stability/instability phases are not known with certainty. Periods of temporary supra-elevated water levels, probably resulting from storm conditions, have also been recognized in the coastal sedimentary record; interbedded peat and silty sand with rounded clasts forms part of a terrace sequence at +6 to +8 m OD. Radiocarbon dates suggest that several such events occurred between c. 2300 and 1700 years bp, as peat was accumulating. Recent changes include the construction of estuary mouth training walls and river dredging. Resulting from these activities, aeolian dunes have extended their area, and increased wave erosion within the estuary has been recorded. Human activities and any future RSL rise, consequent upon global warming, seem likely to lead to continuing instability.