The petrography of the Lower Purbeck Limestones of Dorset

Abstract

The petrography of the limestone types found in the Lower Purbeck of Dorset is described. Four main types are distinguished namely algal limestones, pellet-limestones, shell-limestones; and replacement-limestones produced by the replacement of gypsum and anhydrite by calcite. The distribution of these limestones especially as revealed by coastal sections, is tabulated. From these studies it is deducted that the principal environment of deposition was a lagoon, the seaward margin of which was restricted by an algal barrier. On the landward side of this barrier evaporites, pellet-limestones and shell limestones were deposited in a lateral sequence. There is evidence to suggest the occurrence of lower salinities to the west than to the east. This lateral change in salinity is probably due to dilution by river water in the west, especially during Hard Cap times. The origin of two post depositional breccias is also discussed.