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

The organic-rich late Triassic Kossen 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 secti on of the Kossen Fm., built up of mostly non-bioturbated sediments have bee n analysed by Rock-Eval pyrolysis, Organic petrographic studies, sulphur an d iron speciation (total sulphur, HCl-soluble sulphate sulphur, pyrite iron and HCl-soluble iron) and CO2 measurements have been carried out on smalle r 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 or ganic constituents but microbial-algal mat material is also of importance. The carbonate and TOC contents and HI values display much scatter and upwar d decreasing trends. Type II-S OM is common in the lower third of the secti on and it occurs in its middle interval, too. The marked downward decrease of T-max values is explained by the downward increase of the OM sulphur con tent. The original organic carbon content (TOCor,) was calculated. The degr ee 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 ar ound the middle of the section. This strengthening of the degradation durin g the black shale event was the result of the parallel decreasing rate of s edimentation 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 wit h the degree of organic preservation. Sulphur partitioning between pyrite a nd OM was controlled by the ratio of reactive iron to OM. Average rates of planktonic productivity (C-prod) vary between 6-9 and 60-88 t C-org/m(2)/Ma . The rough covariance of the degree of preservation and Cprod suggests tha t variations in planktonic productivity exerted a strong control on preserv ation in the Suboxic, low to medium productivity Kossen Formation. (C) 2000 Elsevier Science Ltd. All rights reserved.