Early Jurassic Marine Basin (Anabar-Lena Sea) in the Northeast Siberia: Palaeogeographic, Microbiotic, Sedimentological and Geochemical Events

Boris Nikitenko

The Lower Jurassic deposits in the west of northeastern Siberia are represented by marine and shallow-water clays, silts and sandstones, sometimes with interlayers of continental and sub-continental sandstones at the base, and characterized by rare ammonites and rich and diverse microfossils. In the east of the region, the coeval deposits mainly consist of clays and silts. A large marine transgression took place in the beginning of the Hettangian, but stable marine regime only occurred in the eastern part of Anabar-Lena Sea. The environmental conditions were unfavorable for microbenthos development in the western part of this basin during the Hettangian and Early Pliensbachian. A marine transgression and climatic warming occurred at the beginning of the Late Pliensbachian and caused the invasion of a number of thermophilic migrant taxa in the microbenthic communities. The regressive stage of the palaeobasin began at the end of the Late Pliensbachian. The presence of glendonites suggests low bottom water temperatures in the latest Pliensbachian, while the wide distribution of thermophilic taxa in the microfauna communities and low-latitude vegetation in the earliest Toarcian indicate a considerable climate warming throughout Arctic regions. In the earliest Toarcian, the Anabar-Lena Sea was characterized by the leveling of the environments, and black clays were formed everywhere, corresponding to the well-documented Toarcian Oceanic Anoxic event (T-OAE). Sedimentological and micropalaeontological data indicate that the T-OAE in the area coincided with a major eustatic sea-level rise, significant temperature rise that resulted in a strong and rapid reduction of the taxonomical diversity of microbiota (mass-extinction). The T-OAE is associated with a marked 6‰ negative carbon isotope excursion, with the lowest values (–32‰) recorded in the lowermost part of the falciferum Zone. This sharp decrease of δ13CTOC value is accompanied by a marked rise of TOC contents and coincides with the appearance of bituminous shales in the eastern areas of the Anabar-Lena Sea. The fossiliferous bituminous shale interval indicates reducing conditions, high nutrient input under low sedimentation rates and periodic (seasonal-like) changes in depositional conditions. The organic matter (OM) was mostly composed of colloalginite; geochemical parameters confirm that OM is mainly composed of marine components with a minor terrigeneous contribution. The high-resolution biostratigraphic control of geochemical data obtained from Siberian sections allows direct correlations with other T-OAE sites from Western Europe and definition of the global and regional aspects of this event. In the second half of the Toarcian, the differentiation of depositional environments between western and eastern parts of the Anabar-Lena Basin resumed, with sandy and silty deposits dominating in western areas and clayey and siltly deposits dominating in eastern areas.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013