LOWER JURASSIC EPICONTINENTAL CARBONATES AND MUDSTONES FROM ENGLAND AND WALES - CHEMOSTRATIGRAPHIC SIGNALS AND THE EARLY TOARCIAN ANOXIC EVENT

Sections through Lower Jurassic epicontinental carbonates from Souther n Britain (Junction Bed and equivalent) show a positive carbon-isotope excursion (delta(13)C(carbonate)), detectable in bulk rock, in the fa lciferum Zone of the lower Toarcian. Isotopic data from organic matter in more clay-rich sections from Wales and north-east England, togethe r with determinations on belemnite calcite, indicate that highest delt a(13)C values are localized in the upper exaratum Subzone of the falci ferum Zone. Levels of particular enrichment in organic carbon were dev eloped in the early to mid-exaratum Subzone and hence pre-date this de lta(13)C maximum. These phenomena reflect the impact of the early Toar cian oceanic anoxic event in the British area. Similar isotopic trends have been recorded in other Toarcian sections from Tethyan Europe and are interpreted as reflecting the chemistry of sea water. On the assu mption of isotopic correlation between the English and Tethyan section s, the delta(13)C maximum would be everywhere dated as latest exaratum Subzone in terms of the north European ammonite scheme. Absolute oxyg en-isotope values in carbonates probably reflect both early diagenetic cementation and later temperature-related burial diagenesis, although a palaeotemperature maximum is tentatively identified as characterizi ng the early falciferum Zone. Subsequent climatic deterioration may ha ve been triggered by drawdown of CO2, related to regional excess carbo n burial during the oceanic anoxic event. Using the positive delta(13) C excursion as a correlative level in sections from different faunal p rovinces (Britain, Italy and Spain) implies that lower Toarcian zonal stratigraphy is diachronous between northern and southern Europe. Ther e is evidence for partitioning of water masses between the north Europ ean shelf and the Tethyan continental margin during the Early Jurassic .