Facies analysis of the Portland Beds

The Portland Group, previously known as the Portland Beds, is the highest division of the regressive marine Jurassic succession in England. The strata are well exposed on the coast of Dorset and the Bas Boulonnais of North-east France and poorer exposures occur in Wiltshire, Oxfordshire and Buckinghamshire. The rocks are also known from boreholes in Hampshire, Surrey, Sussex and Kent. This thesis comprises a facies analysis of the Portland Group using stratigraphical, sedimentological and palaeo- ecological data. In spite of being familiar to most geologists, if only in name, these rocks are surprisingly poorly documented and there was ample scope for a detailed study of the sediments. Because refined correlation by fossils has not been established a regional approach is used and the successions in each area are described and interpreted in turn. Correlations are suggested between the widely separated areas.using phases of regression and transgression, supplemented by ammonite evidence as much as possible. The thickest sections exposed are in Dorset where the sequence reaches nearly 80m. This area provides the basis for description and interpretation of the areas where the successions are thinner, reaching a minimum of 6m in the far north-eastern outcrop in Buckinghamshire. The Portland Group in Dorset is divided into the Portland Sand Formation overlain by the Portland Limestone Formation, described in separate chapters. The Portland Sand Formation is an upward continuation of the sandy and silty development in the Upper Kimmeridge Clay. The sediments are dolomitic and the highest beds are virtually pure fine grained dolomite which is interpreted as having formed in relatively deep water. An environmental model for the deposition of the Portland Sand Formation is discussed ... [See pdf file for full abstract.]