Depositional sequences and palaeoceanographic change in the Ordovician of the Siberian craton

Nine major depositional sequences separated by regional unconformities are recognized in the Ordovician of the Siberian Craton. These sequences are typically characterized by an absence of lowstand systems tracts, relatively thin transgressive systems tracts and somewhat thicker shallowing upward highstand systems tracts. The most prominent unconformities, marking forced regressions are associated with the base of the Baykit depositional sequence (Early Darriwilian) and the top of the Ordovician. Across the former, the tropical-type carbonate factory was terminated and siliciclastic sediments (Baykit Sandstone) were deposited. These changes in lithology inferably correspond to tectonic events along the western margin of the Siberian continent. With major flooding events during the Volgino, Kirensk-Kudrino and Mangazeya depositional sequences phosphatization became much more widespread and carbonate rock compositions shift from tropical-type to temperate-type. This lithological shift is due to a combination of upwelling of cool water, increased nutrient influx and increased water turbidity. Major transgressions marked by widespread black shales and/or deep-water marine red bed facies occurred during the Volgino, Kirensk-Kudrino (Late Darriwilian), Mangazeya (Early Sandbian) and Kety (Middle Katian) depositional sequences. The Ordovician depositional sequences of the Siberian Craton can be tentatively correlated with those developed in the Baltica palaeocontinent, which suggests that most of them were controlled mainly by eustasy. The pattern of long-term lithological changes in the Ordovician of the Siberian Craton and our established sea-level curve demonstrate more similarities with North America than with Baltica.