Expression of the Toarcian Oceanic Anoxic Event: New insights from a Swiss transect

A sedimentological, biostratigraphical and geochemical (stable isotopes and Rock-Eval parameters) analysis was performed on four Swiss successions, in order to examine the expression of the Toarcian Oceanic Anoxic Event along a north-south transect, from the Jura through the Alpine Tethys (Sub-Brianconnais and Lombardian basins). The locations were selected to represent a range of palaeoceanographic positions from an epicontinental sea to a more open marine setting. The Toarcian Oceanic Anoxic Event was recognized by the presence of the characteristic negative carbon-isotope excursion in carbonate (ca 2 to 4 parts per thousand) and organic matter (ca 4 to 5 parts per thousand) at the base of the falciferum ammonite Zone (NJT6 nannofossil Zone). The sedimentary expression of the Toarcian Oceanic Anoxic Event varies along the transect from laminated mudstone rich in total organic carbon (11wt.%) in the Jura, to thin-bedded marl (5wt.% total organic carbon) in the Sub-Brianconnais Basin and to hemipelagic reddish marly limestone (total organic carbon <005wt.%) in equivalent levels from the Lombardian Basin. The carbon-isotope excursion is thus independent of facies and palaeoceanographic position. The low nannofossil abundance and the peak in Calyculaceae in the Jura and the Sub-Brianconnais Basin indicate low salinity surface waters and stratified water masses in general. Sedimentological observations (for example, obliquely-bedded laminae and homogeneous mud layers containing rip-up clasts) indicate the presence of dynamic conditions, suggesting that water mass stratification was episodically disrupted during the Toarcian Oceanic Anoxic Event. The proposed correlation highlights a stratigraphic gap and/or condensed interval between the Pliensbachian-Toarcian boundary and the Toarcian Oceanic Anoxic Event interval (most of the tenuicostatum ammonite Zone is missing), which is also observed in coeval European sections and points to the influence of sea-level change and current dynamics. This transect shows that the sedimentary expression of the Toarcian Oceanic Anoxic Event is not uniform across the Alpine Tethys, supporting the importance of local conditions in determining how this event is recorded across different palaeoceanographic settings.