Integrated biostratigraphic, palaeoenvironmental and wireline log characterisation of upper Miocene to lower Pliocene successions of the eastern Dacian Basin (SE Romania)

Abstract

This paper presents a biostratigraphic study of upper Miocene to lower Pliocene sedimentary successions from SE Romania based on palynological and micropalaeontological analyses integrated with geophysical wireline log profiles. The data derive from five wells drilled in an area of the Dacian Basin where no previous biostratigraphic data are available. A detailed correlative framework of palynological datums for the studied interval is proposed, which complements and supplements the ostracod and mollusc-based biostratigraphy traditionally used for age determination in the Neogene of the Dacian Basin. Interpretation of the microfossil assemblages in terms of depositional environments enables correlation of the recorded bioevents with regional palaeoenvironmental changes recognised in more marginal areas of the Dacian Basin. Two major marine incursions into the basin are represented by influxes of the benthonic foraminifer Ammonia beccarii and used to define the Khersonian – Meotian and the Meotian – Pontian boundaries. Between these two transgressions, the Meotian is interpreted as a period with at least a certain degree of marine influence. The occurrence of dinocyst taxa usually found in Mediterranean and Atlantic records suggests a connection between Paratethys and the Mediterranean, which probably occurred via the Black Sea. After the Meotian-Pontian marine flooding, a diverse endemic microflora derived from the Pannonian Basin is first recorded in the lower Pontian succession. The signature of a reduction in water level during the middle Pontian within the study area is interpreted to be represented in the successions by an interval dominated by distinctive sand units. The endemic Pannonian dinocysts experienced a significant increase in abundance in the late Pontian, indicating the development of brackish conditions and a water level rise during this period. Due to the absence of foraminifera, or any other indicator of marine conditions, it is probable that this water level rise was related to a positive hydrological balance, as suggested in the Focşani region. The Pontian-Dacian boundary is determined by a transition from brackish to fresh depositional environments, which is associated with a change in the microfloras and the first occurrence of coal layers.