Ostracods as palaeoenvironmental indicators in the Lower Carboniferous Yoredale Series of northern England

Abstract. The ostracod fauna and vertical changes in its composition were examined in a shale sequence of the 5-Yard Limestone cyclothem (Yoredale Series) at a locality in Bishopdale, N. Yorkshire. The ostracod tax a are mainly confined to three superfamilies: Kirkbyacea, Healdiacea and Bairdiacea. The co-occurrence of several relatively short-ranging species such as Kirkbya quadrata, Cribroconcha insculpta and Bairdiolites elevatus suggests a Late Brigantian age. Four new species are described: Cornigella posteroextensa, ? Eriella minima, Rectobairdia bavarica and Roundyella binoda.

Fluctuations in influx of terrigenous mud and water turbulence, related to delta growth, appear to have been the main environmental parameters that controlled the ostracod distribution and abundance along a nearshore-offshore gradient. Three different ostracod assemblages are recognized. A Roundyella-Cribroconcha assemblage, dominated by kirkbyacean and healdiacean ostracods, represents a quiet, nearshore environment with a fairly high depositional rate of terrigenous mud. A Bairdia assemblage, higher in the section, is dominated by bairdiacean ostracods that lived in a more turbulent, relatively offshore environment with less input of terrigenous sediment. A third, intermediate assemblage consists of almost equal amounts of bairdiacean and kirkbyacean ostracods, and reflects a transition from the Bairdia to the Roundyella-Cribroconcha assemblage.