Biofacies, stratigraphic distribution and depositional models of British onshore Jurassic black shales

The majority of British Jurassic dysaerobic and anaerobic facies (black shales) accumulated in rapidly subsiding basinal areas. Exceptionally, during the earliest stages of transgression, organic-rich facies expanded their distribution into more marginal areas. The biofacies of the black shales display a range of species richness values which, when ranked, record an oxygen gradient that is also reflected by gradients in geochemical and sedimentological data. Truly anaerobic facies (no benthic taxa present) are very rare in the British Jurassic. More common are fissile organic-rich shales with discrete, low-diversity horizons of benthos here defined as lower dysaerobic biofacies. These are interpreted to have formed in a generally anoxic environment punctuated by benthic colonization events in response to ephemeral improvements of oxygen levels. Upper dysaerobic biofacies are characterized by slightly higher diversity faunas scattered throughout poorly fissile strata. The favoured depositional model for the British Jurassic black shales attributes the high preservational values of organic carbon to accumulation in oxygen-deficient waters trapped beneath a stratified water column in the deepest, basinal areas. We propose an ‘expanding puddle’ model for transgressive black shales. Deep water conditions are envisaged to become proportionally more extensive during early transgression due to the combined effects of subsidence, a rapid rate of sea-level rise and, unique to this portion of the sea-level curve, sediment starvation.