ENVIRONMENT OF THE JET ROCK, A CLASSIC BLACK SHALE IN THE LOWER TOARCIAN OF YORKSHIRE, ENGLAND

The Jet Rock, a laminated black shale named after the lignified driftwood it contains, marks one of the most striking episodes of sea-level rise in the Jurassic, and an important mass extinction event involving benthos and nektobenthos. Two significant carbon isotope excursions have been recognised. The younger, positive one has been generally accepted as marking an oceanic anoxic event, while the older, pronounced negative excursion has a disputed origin, one group of workers favouring the release of isotopically light CO₂ that had built up below a relatively deep pycrnocline, the other group favouring an episode of methane hydrate release. Strontium isotope analysis of belemnites supports an earlier study of compound calcite concretions in indicating a high level of stratigraphic condensation during Jet Rock deposition. If the striking kerogen-clay laminae, about 40 microns thick, are assumed to be annual varves, their counting gives a result indicating only a fraction of the time implied by radiometric dating. Therefore either the laminae do not represent varves or there are stratigraphic gaps in the succession. The pyrite framboid population in typical of euxincity, with ORB 3-4 in the classification of Wignall and Hallam. There is limited evidence from barite concentrations of increased surface water productivity copared with older and younger strata. An apparent coincidence has been noted between the anoxic event and contemporary extinction with the peak of Karroo volcanicity, and some preliminary stomatal index studies of contemporary plants tentatively suggest increased atmospheric CO₂, which should have led to higher temperatures. Limited oxygen isotope data are supportive, but the results could have been distorted by diagenesis.