REVIEWED STRATIGRAPHY OF UPPER CRETACEOUS - EOCENE DEPOSITS FROM THE NORTHERN EDGE OF ISTRIA BASIN, WESTERN BLACK SEA.

The Istria Basin, part of Western Black Sea (WBS) represents a major tectonic unit of the Romanian Black Sea shelf, with prolific hydrocarbon bearing structures. A significant amount of exploration and development wells were drilled on this basin’s northern edge. Core and cuttings data from 12 boreholes provide important details on this basin and the WBS stratigraphy, especially for the two of the main hydrocarbons bearing deposits Upper Cretaceous and, respectively, Eocene, on which we have achieved an age interpretation, using calcareous nannofossil and foraminiferal assemblages. In this paper, we focus on the Upper Cretaceous comprised of Cenomanian, Turonian, Coniacian-Santonian, Campanian and Maastrichtian stages and on the Eocene ones, containing i.e., Ypresian, Lutetian and Bartonian stages. Our interpretation confirms the presence of three major unconformities. The oldest one is placed at the end of the Cretaceous, with a time gap of almost 10 My, which is estimated based on the ranges of the overlying Eocene planktonic foraminifera and calcareous nannofossils assemblages. The Paleocene series has not been not encountered in the studied wells. The second unconformity is intra-Eocene in age, referring to the upper part of the Lower Eocene (the upper part of Ypresian) is absent in several wells, especially in the southern part of our study area. The newest and most important unconformity marks a regional event at the beginning of Oligocene, well known in the literature as “Base Oligocene”, a regional marker for the WBS. In the Upper Cretaceous deposits, the presence and high diversity of planktonic foraminifera and calcareous nannofossils indicates that the water column was not deeper than ~200m and implicitly, it was an open marine environment. The Eocene microfossil assemblages also indicate a shallow marine environment, which may offer an indication of occasional subaerial exposure of the shelf area; this setting was the main factor in determining the occurrence of the Upper Cretaceous and Eocene first two unconformities. The origin of the third unconformity is miscellaneous, as Oligocene deposits are deep-water type. [ABSTRACT FROM AUTHOR]