Middle–Late Eocene Dinocysts from the Aktumsuk Section (Ustyurt Plateau, Uzbekistan): Biostratigraphy and Paleoenvironments

Resumo

The results of palynological study of the mid-upper Eocene sediments of the Aktumsuk key-section (Ustyurt Plateau, Uzbekistan), calibrated with nannoplankton data, are presented. Dinocyst stratigraphical distribution permitted to recognize five Dinoflagellate Zones from the Peri-Tethys zonation: Costacysta bucina, Enneadocysta pectiniformis, Rhombodinium draco, Rhombodinium perforatum, and Thalassiphora reticulata. First-order calibration with nannoplankton zones permitted to precise the lowermost occurrences of the key dinocyst species. Based on the dinocyst and nannoplankton data, two important stratigraphic hiatuses, corresponding to the upper Ypresian and upper Bartonian-lower Priabonian, were recognized. Analysis of quantitative fluctuations of different groups of palynomorphs through the section indicates paleoenvironmental changes in this part of the Peri-Tethys during the Lutetian-Priabonian: the beginning of significant transgressive phase in the early Lutetian, relative deepening in the mid Lutetian; after the break in sedimentation at the end of Bartianian-beginning of Priabonian the sedimentation continues again in the open-marine (outer neritic) environement.