Redox conditions, productivity, and volcanic input during deposition of uppermost Jurassic and Lower Cretaceous organic-rich siltstones in Spitsbergen, Norway

Abstract

In this paper, uppermost Jurassic and Lower Cretaceous black organic-rich siltstones (Rurikfjellet Formation) from Carolinedalen (Isfjorden, Spitsbergen) are studied using integrated palynological, petrographic, and geochemical methods (organic and inorganic geochemistry as well as Rock-Eval pyrolysis) in order to decipher the depositional conditions prevailing during their sedimentation, the type of organic matter from which they were formed, and their potential for generating hydrocarbons. The age of these investigated sediments encompasses an interval from the upper Tithonian-Berriasian to the Hauterivian, dated on the basis of dinocyst biostratigraphy. The ammonoids found in loose form at the base of the investigated section, such as Laugeites groenlandicus, are indicative of the upper part of the middle Tithonian. Based on palynology, biomarkers, and Rock-Eval data, the kerogen in the investigated siltstones from Isfjorden is of the mixed II/III type; the organic matter is mixed marine and terrestrial in origin, and represents the early to peak ‘oil window’ maturation stage (Tmax around 440 °C).These siltstones were deposited under a high-productivity regime with oxic/dysoxic bottom water conditions, as evidenced from the size distribution of pyrite framboids, in which pyrites with diameters >5 μm predominate. Values of Th/U and Corg/P ratios generally above 3 and 30, respectively, along with pristane/phytane ratios >2 and sterane/hopane ratios <0.3, also indicate oxic to suboxic sedimentary conditions in the water column. Anoxia, if present, must have been short-lasting and formed oxygen minimum zone in the water column. Elevated productivity in the photic zone may have been generated by an increase in volcanic activity, as confirmed by higher Hg contents and/or terrigenous nutrient supply.