Transgressions and Regressions in the Dinantian: A New Synthesis of British Dinantian Stratigraphy

Summary In the area between the Southern Uplands of Scotland and the Mendip Hills the Dinantian succession below the D 1 Zone is shown to comprise four major cycles each characterized by transgressive and regressive phases recognized by changes in lithology and fauna. The transgressive phases on the shelf areas are represented by bioclastic limestones, and the regressive phases by oolitic limestones and calcite mudstones, the latter often dolomitized and containing stromatolites and interpreted as the early stages of evaporite deposition. Regressions are indicated also by the frequent presence of pebble-beds, breccias and non-sequences in the marginal areas of deposition, indicating that there was an actual lowering of sea level rather than a mere filling-up of the areas of deposition. Dolomitization of limestones underlying the regressive phases is a common feature, and it is suggested that this is a consequence of the evaporation induced by regressions and that it offers an explanation of the distribution of many of the dolomitic limestones of the Carboniferous Limestone. The major cycles are traced and correlated throughout the area. Each major cycle can be shown to include a number of minor transgressions and regressions when it is traced into the Northumberland Trough where minor cyclicity occurs throughout. Above the base of the D 1 Zone this minor cyclicity becomes evident over the whole area. It is concluded that the major influence on Dinantian sedimentation (apart from any due to local tectonic effects) was repeated eustatic change in sea level, and in support it is shown that similar changes in sea level have taken place in extra-British areas. It is suggested that various stratigraphical breaks and unconformities, hitherto regarded as of tectonic origin, can be more easily explained by regressions. The recognition of these cycles provides an improved means of chronostratigraphical division of the Dinantian. Each major transgression brings with it distinctive faunas and a new scheme for the division of the Dinantian is proposed, which necessitates at least partial abandonment of the Vaughanian Zones, and their replacement by divisions based on the major cycles.