Biostratigraphy and paleoenvironments of the Oligocene succession (İhsaniye Formation) at Karaburun (NW Turkey)

Abstract

The biostratigraphy and paleoenvironments of the İhsaniye Formation exposed at Karaburun in northwest Turkey is describedbased upon the study of abundant and well-preserved foraminifera, calcareous nannofossils and palynomorphs. The studied succession isEarly Oligocene in age, with calcareous nannofossil zones upper NP21 (Subzone CNO1) to lower NP23 (Subzone CNO3) and planktonicforaminifera zones O1 (~P18) and O2 (~P19) represented, and palynological assemblages suggestive of zones D13 to D14a. Based on thesenew data, a revised interpretation of the stratigraphic succession is presented. Deposition was controlled by a now inverted normal fault,with deposition of older stratigraphy (upper NP21 to NP22) restricted to the original hanging wall. During NP23, deposition commencedon the footwall, resulting in progressive onlap of an exposed Eocene reefal limestone (Soğucak Formation). Three primary sedimentaryfacies are present: marls with thin calcareous siltstones, marls with synsedimentary slumps and debris flows, and coarse pebbly sandstones.The coarse pebbly sandstones were deposited in a fan-delta/shoreface paleoenvironment and represent the initial phase of onlap duringbiozone NP23 onto a rocky shoreline on the footwall side of the fault. The marl-dominated facies represent deposition in outer shelf–upperbathyal environments. The succession demonstrates evidence for a near-end Eocene relative sea-level fall. Changes in the abundance ofplanktonic foraminifera and the onlap onto the footwall demonstrate maximum subsequent transgression within NP23. This reflects eustasyrather than Paratethyan relative sea-level. No interpretation of sea-water salinity reduction can be made for the sediments deposited duringbiozone NP23 in the studied sections, although this is noted in coeval sediments in parts of Paratethys (the “Solenovian Event”). Togetherwith the open marine nature of the diverse and abundant fossil assemblages, it is suggested that deposition of the Karaburun section wasstrongly influenced by a connection to the global ocean, via the Çatalca Gap, as suggested in a recent study.