Middle Jurassic ostracoda from southern England and northern France
A taxonomic and biostratigraphic study of Middle Jurassic (mainly Bathonian) ostracods from southern England and northern France is here presented. A fauna of 111 species of 55 genera, belonging to the Order Podocopida, is described. Thirty four species, 3 genera, Strictocythere, Angliaecytheridea and Konarocythereis and 1 subgenus, Blanoacanthocythere, are new; a further 11 species are left under open nomenclature owing to lack of material. The biostratigraphic application of the ostracods is a continuation of the work of Bate (1978) in constructing a zonation scheme based on ostracods for the correlation of the English Bathonian. Such a zonation is necessary because of the rarity, and often absence, of ammonites within the Stage and because of the highly complex interfingering of facies belts, making correlation on lithologies very tenuous. The Zones are named, fully defined and applied to a much wider geographical area than before. These are the rimosa, confossa, polonica, blakeana, and falcata Zones, the lowermost rimosa Zone (commencing in the topmost Bajocian) being sub-divided into the rimosa, batei, and postangusta Subzones. Considered as time Zones they are shown to be highly reliable in dating sediments of varying facies and to permit a direct correlation of the beds in southern England with those of northern France. On the basis of the zonation two lithostratigraphic units in the French succession, the Calcaire de Reviers and the Caillasse de Fontaine-Henry are no longer considered valid as discrete units and are regarded as being equivalent to the Calcaire de Blainville and the Caillasse de Blainville respectively. Certain discrepancies in the ostracod faunas are seen between the areas sampled and these are used to construct a slightly modified palaeogeographic interpretation to that currently accepted.