ORGANIC RICH MUDSTONES BEFORE AND AFTER THE END-TRIASSIC EVENT IN THE DANISH BASIN

The end-Triassic biotic crisis is generally explained by massive input of CO₂ and/or methane to the atmosphere during the formation of the Central Atlantic Magmatic Province (CAMP), with subsequent ocean acidification and intense global warming affecting the biota across the Triassic-Jurassic boundary (TJB). Recently biomarker analysis, supported by palynological data showing restructuring of organic phytoplankton communities; from pre-crisis dinoflagellate dominated associations to post-crisis acritarch and prasinophyte dominated ones, across the extinction event have suggested that widespread marine photic zone euxinia in both the epicontinental sea of the Tethys and in the Panthalassic Ocean played a major part in the extinction scenario.

The stratigraphically well constrained marine TJB succession at Stenlille in the Danish Basin was situated on the northeastern shelf of the epicontinental sea c. 100 km or more southwest of the coast of the Baltic Shield. Organic rich mudstones were deposited in connection to flooding surfaces, both prior to and after the end-Triassic crisis, and appear similar with regards to e.g. sedimentary characteristics, general lack of bioturbation and depositional conditions. Rock-Eval data show a dominance of immature type-III kerogens, with minor type-II kerogens, suggesting that the organic matter was primarily derived from subaerially oxidized terrestrial tissues and marine organic matter. The geochemistry of the organic rich mudstones differs before and after the extinction, suggesting changing environmental conditions. High sulphur content, elevated levels of molybdenum and lack of bioturbation may indicate anoxic or dysoxic conditions during the transgression (MFS7), but similar conditions are not as apparent in the post-crisis mudstones. A substantial loss of dinoflagellate taxa is registered during the extinction interval. Nevertheless, the dominating marine phytoplankton associations present in the pre-crisis mudstones returned after the extinction interval without major restructuring of the community. Here we present and discuss palynological, geochemical and sedimentological characteristics of the organic rich mudstones of the Stenlille succession with the aim to assess the changes in environmental conditions under which they formed.