MICROFOSSIL (FORAMINIFERA AND OSTRACODA) ASSEMBLAGES, UPPER PLIENSBACHIAN TO LOWER TOARCIAN STRATIGRAPHY, PALAEOBIOGEOGRAPHY AND BIOFACIES OF THE ARCTIC BASIN

The microbiotic event at the Pliensbachian-Toarcian boundary is the most drastic that occurred during the evolution of Jurassic microbenthos. The investigation of biotic and abiotic events occurring in the Arctic Basin and northwestern regions of Western Europe during the Pliensbachian - Toarcian indicates that this event was almost simultaneous in these two regions. The Early Toarcian of these regions is marked by the development of organic rich mudstone, often bituminous. At the beginning of the Toarcian, a microbiota crisis took place that is recorded in numerous sections. The beginning of the Late Pliensbachian is characterized by a transgression. Taxonomic diversity of microbenthos in the Arctic Basin and the seas of Europe increased, reaching its maximum. The transgression, combined with the warmer climate, caused the invasion of a number of thermophilic taxa in the microbenthos communities of the Arctic paleoseas. The regressive stage of the Arctic paleobasin began at the end of the margaritatus phase. Together with a eustatic sea-level fall, a rather sharp cooling has been observed. The climate gradually changed to arid. The regression apparently caused changes in the current system. Simultaneous eustatic sea-level fall and climate cooling caused a decrease of microbenthos diversity at the species and generic levels. In the beginning of the Early Toarcian, a climatic warming and major eustatic sea-level rise took place. The analysis of microbenthos distribution at the Pliensbachian - Toarcian boundary suggests the absence of continuity between Pliensbachian and Toarcian microbenthos communities. In the base of the Toarcian (tenuicostatum zone), almost 100% of the Arctic foraminiferal species have been replaced. Generic composition of ostracoda were completely changed. At the falcifer phase, a new stage of development of microbenthos communities began. At that stage, there were reliable links between the microbenthos associations of the Arctic and European seas caused by transgression and climatic warming. The Early Toarcian microbiotic crisis was much sharper in the Arctic than in the European basins. The trigger of this biotic mass extinction event may have been large-scale eruptions in the Karoo-Ferrar volcanic province at the end of the Pliensbachian and/or beginning of the Toarcian which would have caused catastrophic climatic changes.