Construction of an evolutionary deglaciation model for the Irish midlands based on the integration of morphostratigraphic and geophysical data analyses

摘要

Alternative, established models for the deglaciation of the midlands of Ireland are tested against an interpretation of a suite of deglacial sediments covering an area of 600 km~2 in the east central midland area. Interpretation of the sediments is based on geomorphological mapping, lithostratigraphic characterization of exposures and geotechnical data supported by electrical resistivity tomography (ERT) and ground penetrating radar (GPR). GPR depicted small-scale sedimentological and deformational structures within low-conductivity soft sediments, such as cross-bedding, planar bedding, channel-like features and faulting planes, and revealed the internal architecture of eskers, glaciodeltas, subaqueous fans and raised bogs. ERT data permitted the detection of depth to bedrock and the lithological characterization of unconsolidated sediments. The ten sites presented were surveyed by traditional mapping methods and/or geophysical techniques. This allowed the construction of a local model of the deglaciation of the area which recognized five main stages. An ice sheet covering most of Ireland withdrew as a single body as far as the midlands. At this stage, two main directions of ice retreat are identified from the spatial distribution of meltwater/overflow channels, esker and morainic ridges, and ice-marginal glaciolacustrine deposits. A pattern of deglacial sedimentation into an expanding ice-marginal glacial lake is depicted. The glacial lake was dammed to the west by two ice dome fronts, one decaying to the north-west and another to the south-west, and by the Shannon Basin watershed to the east. Glacial lake outlets identified along the watershed and the altitude of the topset/foreset interface zone depicted in glaciodeltaic deposits allowed the identification of three lake water levels. The highest level is at 87-89m Ordnance Datum (OD), the second lake level at 84m OD and the third at 78m OD.