ABSTRACT

The refined biostratigraphic framework that has been developed for Jurassic sediments of northwest Europe enables precise environmental distinctions to be made on a time-equivalent basis. Using such control, it can be demonstrated that shelf sedimentation in the Lower Jurassic (Toarcian) of southern England was dominated by the gradual migration of a large sand bar. Despite the general homogeneity of its detrital sandstones and siltstones, this bar may be subdivided into four principal environments of deposition which include 1) bar (beach and upper shoreface), 2) fore-bar (middle shoreface), 3) back bar, and 4) tidal channel. These environments are distinguished on the basis of qualitative and quantitative textural and compositional criteria: succession of sedimentary structures, bio urbation, grain size, sorting, skewness, and petrography.

The bar complex is 150 ft thick, and is transitional downward and southward into condensed limestones and argillaceous siltstones (40 ft average thickness). The limestones are principally intramicrites, the intraclasts being of the same composition as the host sediment (biomicrite), and often display oxidation rims. Such deposits are considered to be of shallow, marine origin. On the other hand, the argillaceous siltstones, which may contain up to 20 percent micrite, represent deeper marine conditions of deposition. The bar complex is succeeded upward and to the north by condensed limestones of variable composition, principally oomicrites, pisomicrites, and biomicrites. These limestones (12 ft average thickness) represent a platform interior sand blanket which was developed under shal ow marine conditions in the lee of the large bar complex. Because 1) new sediment was constantly added to the southward side of the bar, and 2) subsidence did not quite keep pace with sediment supply, the bar was forced to accrete continually in a southward direction, at a rate of 1 mile per 80,000 years. The gradual southward migration of the bar resulted in preservation of vertical sequences which, when completely developed, comprise a basal argillaceous and calcareous facies (the Upper Lias Clay Formation), succeeded by an arenaceous facies (the Upper Lias Sand Formation), and capped by a calcareous facies (the Cephalopod Bed Formation).

Sediments comprising these three facies were deposited in two distinct basins of deposition, separated by an east-west trending structural high (the Mendip Axis). Both basins subsided to receive a maximum thickness of some 340 ft of marine sediments. Because of the southward migration of the bar, the southern basin received its major sediment input only after the northern basin had already been filled.

The nature and rate of sediment supply remained constant throughout the 6 million years duration of the Toarcian, the principal source being a meta-igneous complex located in the present western approaches to the English Channel.