Microfacies Analysis of Upper Eocene Shallow-water Carbonates from the Rodnei Mountains (N Romania)

Abstract

A Priabonian carbonate ramp developed in the NW Rodnei Mountains as a part of a Paleogene sedimentary cover that overlies the metamorphic basement of the Eastern Carpathians (N Romania). Two sections, namely Iza Valley and Btrâna Spring, were sampled at different locations along the carbonate ramp. The deposits accumulated in a shallow environment, in which relative sea level change is seen as the main controlling factor of facies distribution. A shore facemiddle rampinner ramp transition was recorded in both sections. Associations of large benthic foraminifers and high siliciclastic input are the dominant features in the northern part of the ramp, while in the south corals and coralline algae thrived in an environment characterized by low hydrodynamic energy and clastic input. A 4-5 m thick intercalation identified in the middle ramp deposits of both studied sections shows a temporary change in fossil assemblages, with foraminifers almost completely replaced by red algae and rhodoliths, and extensive bioturbation caused by ostracods and gastropods. Towards the top of the carbonate deposits, large bivalve colonies and an increase in clastic input indicate a relative sea-level fall, which ultimately led to the cessation of carbonate sedimentation and the deposition of black shales on a basin-wide scale during the Lower Oligocene.