Organic geochemistry of the Lower Toarcian Posidonia Shale in NW Europe

Abstract

A suite of core and outcrop samples of the Lower Toarcian Posidonia Shale and time-equivalent deposits was collected from multiple locations, including southern Germany (Swabian Alb and Franconian Alb), northern Germany, the east coast of Great Britain, the Netherlands, and Luxembourg, in order to study major differences and similarities with respect to depositional environment and redox potential based on molecular geochemical data. All samples are immature or in the early oil window with the highest thermal maturity occurring in the profile from Great Britain (about 0.7% vitrinite reflectance). Molecular geochemical maturity parameters were used to quantify maturation in detail.During shale deposition in the West Cleveland Basin, Great Britain, H2S was immediately consumed in the water column by terrigenous input, supplying abundant reactive iron to form pyrite. These samples are characterized by low dibenzothiophene/phenanthrene ratios (<0.2) and gammacerane indices (<0.1) and are accompanied by high sulfur contents and abundant tiny pyrites (>95% of<7μm in diameter), generated within the water column due to the immediate reaction of Fe2+ and H2S. In contrast, sulfidic (euxinic) bottom waters, possibly with enhanced salinity and stratified water column, prevailed due to a more restricted iron supply in Luxembourg, northern Germany and the Franconian Alb of southern Germany. A more oxygenated (dysoxic) bottom water column associated with anoxic sediments, i.e., anoxic/dysoxic boundary close to the sediment/water interface, prevailed in the Swabian Alb of southern Germany. Despite observed changes within each vertical profile at each site, large scale regional trends in depositional conditions can be clearly identified for the Early Toarcian.