Evidence of Barremian/Albian Cretaceous Anoxic Oceans Interrupted by Turbulent Mixing

Are Early Cretaceous environmental changes recorded in deposits of the Western part of the Silesian Nappe? A geochemical approach

Authors:

Wójcik-Tabo et al

Abstract:

The organic matter (OM)-rich, relatively monotonous sequence of upper Barremian–Albian deposits in the western part of the Silesian Nappe in the Polish Outer Carpathians, was investigated, using geochemical proxies to show environmental changes. The amounts of organic carbon (0.4–4.32 wt.% of TOC) co-vary with the concentration of S and/or redox sensitive trace elements (RSTE: Mo, U, Co, V, As). Kerogen is present as Type III and mixed Type II/III. Maceral assemblages consist of vitrinite, inertinite and liptinite. The carbon isotopic composition of the organic matter (δ13Corg) varies between − 25.38 and − 21.31‰ with an outlier at − 15.14‰. During the Early Cretaceous the Proto-Silesian Basin was the site of OM accumulation in a detrital oceanic anoxic event as a result of increased sedimentation rates of terrestrial organic matter. Runoff of terrestrial organic matter was accompanied by the production of plankton on an occasional basis. The marine matter contribution is inferred from occurrence of siliceous microfauna, the domination of short-chain n-alkanes, the distribution of sterane and prevalence of cholestane, and enrichment in nutrient elements (P2O5, Ba, Ag, Cd). The bottom waters could have been dysoxic to anoxic, with a mid-water oxygen minimum zone (OMZ) developed owing to the oxidation of organic matter. Anoxia is inferred from the lamination, pyritization patterns, RSTE concentration and low Pr/Ph and C23t/C30 H ratios. Intervals enriched in TOC and RSTE in the Veřovice Formation record pulses of marine productivity, preceded by volcanic (teshenite) and hydrothermal activity. They are referable to the following episodes: the Taxy (Barremian–Aptian), early Aptian Selli, Fallot and Paquier during the late Aptian and at the Aptian–Albian boundary. The initiation of turbidite deposition during the Albian led to bottom ventilation. The Lhoty Formation records the late Albian Toolebuc Episode, marked by a TOC peak, which resulted from the detrital flux. Acidic solution was responsible for reducing conditions in the sediment.