SHALLOW WATER, TRANSGRESSIVE BLACK SHALES

The paleobathymetry of transgressive black shale deposition has long been a source of contention. This partly arises from the different connotations applied to the term "shallow water deposition". This can either mean absolutley shallow (i.e. deposition in less than 20 m water depth for example) or relatively shallow in the sense that deeper water facies occur down-dip. Sequence stratigraphic analysis of transgressive black shales has indicated that the majority occur in association with maximum flooding surfaces and are thus not "shallow water" in any sense of the term. However, some black shales also occur at the base of transgressive systems tracts (TSTs) when water depths were probably absolutely shallow. In addition, new examples of relatively (and absolutely) shallow water black shales from the Late Jurassic of northern France will be presented. These occur in the basal portion of parasequences developed within TSTs and can be demonstrated to pass down-dip into better oxygenated, bioturbated mudstones. This new black shale variant, termed a transgressive, nearshore variety, is characterised by highly dynamic, benthic redox fluctuations, probably recording deposition from beneath a small volume of sub-pycnocline bottom water subject to frequent changes of oxygen levels. Why such conditions only pertain during rising stages of relative sea-level change remains enigmatic.