EARLY PALEOGENE TRANSGRESSIONS: STRATIGRAPHICAL AND SEDIMENTOLOGICAL EVIDENCE FROM THE NORTHERN PERI-TETHYS

Abstract

Our study in lithologically diverse lower Paleogene deposits of the former southern USSR and West Siberia resolves three sedimentary provinces: (i) Southern (Crimea-Caucasus and Central Asia), dominated by biogenic calcareous sediments of the deepest basinal portions; (ii) Transitional (southern Russian craton and Turan plate), displaying calcareous and siliceous clayey deposits; and (iii) Northern (central Russian craton and West Siberian plate), showing biogenic siliceous and terrigenoussiliceous sediments of basinal margin. We used standard and regional scales based on seven microfossil groups (planktonic and benthic foraminifers, nannoplankton, radiolaria, diatoms, silicoflagellates, and dinocysts) to correlate various sediment facies among ∼80 reference sections across the study area. Correlation was performed for the Transitional province, where the presence of both calcareous and siliceous facies affords the use of calcareous and siliceous planktonic and dinocyst scales with varying resolutions. Besides, in the Transitional province, we established regional sedimentary cycles corresponding to the late Thanetian, Ypresian, and late Lutetian–Bartonian. These cycles are traceable into the Northern province, enabling us to determine or refine ages of the sequences, and into the Southern province. Each sedimentary supercycle falls into three units, often with hiatus at the base, correlatable to 3rd order eustatic cycles and featured by distinctive facies. The facies succession appears to reiterate through the upper Thanetian, Ypresian, and upper Lutetian–Bartonian cycles. The lower units of each supercycle are composed of calcareous sediments rich in nannofossils and foraminifers and are traceable from the Southern province into the Transitional province, the late Lutetian unit extending as far as the Northern province. The middle unit is high in organic matter in the Southern province and passes into terrigenous-siliceous sediments rich in radiolaria, diatoms, and dinocysts in the Transitional and Northern provinces. The upper unit is terrigenous-carbonate and, in places, terrigenous-siliceous in the Southern province, becoming terrigenous-siliceous and biogenic siliceous (spongolites, diatomites) in the Transitional and Northern provinces. Calcareous plankton (lower unit) giving way to siliceous (upper unit) might record changes in basinal circulation due to Peri-Tethys communicating with the North Atlantic and/or Arctic. The sedimentary provinces shifted southward as the Peri-Tethys shrank and climate changed between the Thanetian and Bartonian.