Anaerobic degradation of organic matter controlled by productivity variation in a restricted Late Triassic basin

The organic-rich late Triassic Kössen Formation (Hungary) was deposited in a shallow basin, situated between a carbonate platform and the continent. A large number (107) of samples of a 240 m thick, steeply dipping core section of the Kössen Fm., built up of mostly non-bioturbated sediments have been analysed by Rock-Eval pyrolysis. Organic petrographic studies, sulphur and iron speciation (total sulphur, HCl-soluble sulphate sulphur, pyrite iron and HCl-soluble iron) and CO2 measurements have been carried out on smaller sample sets. According to Rock-Eval pyrolysis and organic petrography the immature organic matter (OM) is of dominantly marine origin throughout the section. Unicellular planktonic algae are remains of the most important organic constituents but microbial-algal mat material is also of importance. The carbonate and TOC contents and HI values display much scatter and upward decreasing trends. Type II-S OM is common in the lower third of the section and it occurs in its middle interval, too. The marked downward decrease of Tmax values is explained by the downward increase of the OM sulphur content. The original organic carbon content (TOCor) was calculated. The degree of OM preservation, expressed in term of TOC/TOCor, varies between 0.36 to 0.89 and shows an upward decreasing trend, interrupted by high values around the middle of the section. This strengthening of the degradation during the black shale event was the result of the parallel decreasing rate of sedimentation and OM deposition. The high TOC/TOCor values found around the middle of the section are mostly the result of a temporary acceleration of OM deposition. Sulphur richness of the OM shows no positive correlation with the degree of organic preservation. Sulphur partitioning between pyrite and OM was controlled by the ratio of reactive iron to OM. Average rates of planktonic productivity (Cprod) vary between 6–9 and 60–88 t Corg/m2/Ma. The rough covariance of the degree of preservation and Cprod suggests that variations in planktonic productivity exerted a strong control on preservation in the suboxic, low to medium productivity Kössen Formation.