Paleogene biostratigraphy of the North Caspian region based on dinocysts and nannofossils from the Novouzensk borehole. Article 2: Biotic events and paleoecological settings

Abstract

The relatively complete late Danian-initial Lutetian succession of biotic events, which are suitable to serve as biostratigraphic reference levels for interregional correlations are defined in the section of the Novouzensk reference borehole based on dinocyst and nannoplankton assemblages. The analysis of palynological and nannofossil assemblages allows dynamics of basin development and sea-level fluctuations to be traced. Three major stages reflecting different sedimentation regimes are established in the borehole section: (1) Danian with terrigenous-carbonate sedimentation regime corresponding to the Algai and Tsyganovka formations; (2) Selandian-Thanetian (initial Eocene included) characterized largely by siliceous-terrigenous sediments of the Syzran, Novouzensk, and, partly, Bostandyk formations; (3) Ypresian-Lutetian with the carbonate-terrigenous sedimentation regime corresponding to the upper part of the Ypresian Bostandyk and Lutetian Kopterek formations. The composition of dinoflagellate and nannoplankton assemblages characterizes the North Caspian basin as an internal largely shallow sea with a changeable temperature regime. The paleoecological analysis of phytocoenoses made it possible to define basin fluctuations comparable with third-order rhythms. Four Danian rhythms definable in the Algai and Tsyganovka formations indicate conditions of an open warm-water basin. Dynamics of the step-wise shoaling of the basin was characteristic of the Selandian-early Thanetian ages with subsequent restoration of the marine regime in the late Thanetian. The Syzran Formation corresponds to two Selandian rhythms, which were formed in shallower settings of the restricted sea basin. Four Thanetian rhythms are established in the Novouzensk Formation. The early Thanetian basin was characterized by the shallowest disturbed marine conditions, which were followed by the restored normal marine regime in the late Thanetian. Six Ypresian rhythms correspond to the Bostandyk Formation (Yp-0, Yp-1, Yp-5-Yp-8) reflecting transition from initial Ypresian coastal (Yp-0, Yp-1) to normal marine conditions during the Yp-5 rhythm. The early (Yp-2-Yp-4) and late (Yp-9-Yp-10) Ypresian corresponded to the sedimentation break. Most abundant and diverse microbiotic assemblages characteristic of the Yp-7 rhythm are correlative with the EECO event. The marly Kopterek Formation corresponding to the early Lutetian rhythm (Lu-1) is characterized by diverse thermophilic biota.