Nannofacies analysis as a tool to reconstruct paleoenvironmental changes during the Early Toarcian anoxic event
Résumé
The black shales of lower Toarcian (Schistes Carton in France, Jet Rock in England, Posidonia Shale in Germany) are sediments formed by fine alternations of millimetric to sub/millimetric clear and dark laminae. A study of polished core surfaces (Dotternhausen core, SW Germany) was carried out in SEM in order to characterize the micro‐fabrics of different laminae and their content in nannofossils. Two main intervals were recognized where fabrics and nannofossil content are significantly different. In the clay‐rich interval, characterized by high wt.%TOC and a large negative carbon isotope excursion (both in carbonates and organic matter) the dominant micro‐fabrics are well foliated or lumpy, giving evidence of the absence or reduction of benthic life. Nannofossils are scarce and only concentrated in discrete laminae separated by large, abiotic intervals. Assemblages are indicative of a deep nutricline, but sporadic nutrient inputs occurred, as demonstrated by fluctuation in abundance of specimens of the Biscutaceae. Nutrients were probably delivered to the German basin via riverine influx. Different sized framboids of pyrite were recognized in this interval, the smallest ones (..̃2 μm) being probably precipitated in an anoxic and sulphidic water column. The clay‐rich interval was likely deposited during low sea level in a restricted basin. These conditions were followed by a sea‐level rise and better water‐mass circulation within the German basin. Sedimentation records an increase in carbonate deposition, organic matter content decreased, and carbon isotopes came back to pre‐excursion values. Micro‐fabrics became less markedly laminated, pyrite framboids nearly disappeared, and rich and diversified nannofossil assemblages are recorded. Sea‐water conditions were probably highly fluctuating, as indicated by alternation of intervals dominated in turn by Schizosphaerella spp., flourishing under generally oligotrophic conditions, where nutrients were temporarily available in surface waters due to storms, and by Crepidolithus spp., a deep‐dweller. The results of this work fit well with a silled‐basin model.