Changes in the bivalve assemblage of an early Jurassic mudstone sequence (the Fjerritslev Formation in the Gassum 1 well, Denmark)

In the Gassum 1 well cores were cut at regular intervals through the Lower Jurassic Fjerritslev Formation in which the dominant lithologies are dark grey mudstone and silt-streaked mudstone. Bivalves are found in most samples though generally in small numbers and form the basis of a palaeoecological study. The proportions of epifaunal suspension feeders, infaunal suspension feeders and infaunal deposit feeders changes through the sequence independent of the lithology and a development from a normal shale facies fauna to a restricted shale facies fauna is seen. Gassum 1 is located centrally in the Danish Basin and a restricted shale facies fauna is encountered here in the Lower Pliensbachian while contemporaneous, comparable lithologies from more marginal localities have normal shale facies faunas. The recorded change in the fauna composition in Gassum 1 is therefore related to an increase in water depth. The diversity of three benthic groups, bivalves, ostracods and foraminifera, changes simultaneously through the sequence. These biological changes are not reflected in the lithology, which indicate some other source for the environmental change, probably variations in the content of dissolved oxygen in the bottom water related to phases of increase in water depth. The latter can be shown to correlate with some of the phases of eustatic rise of sea level recognized by Hallam.