Log-sequence analysis and environmental aspects of the Lower Jurassic Fjerritslev Formation in the Danish Subbasin

Abstract

The lithostratigraphic, biostratigraphic, and chronostratigraphic subdivisions of the Lower Jurassic Fjerritslev Formation in the Danish Subbasin are discussed in relation to a log-sequence analysis. The formation is subdivided into three depositional sequences. The upper and lower sequences comprise 15 minor log-sequences, which can be correlated between most well sections. These minor log-sequences are separated by strata boundaries, which by means of biostratigraphic data are shown to be synchronous. The depositional environment is discussed on the basis of previously published data and new organic geochemical data, and in relation to the published sealevel curve. Five major events of eustatic sea-level rises seem to have influenced the benthonic faunas; faunal diversity and density decreased during the sea-level rises, and subsequently increased during the higher sea-level stillstands. The level of oxygen in the bottom waters decreased during the Early Jurassic transgression.