Upper Cretaceous volcaniclastic complexes and calcareous plankton biostratigraphy in the Western Pontides, NW Turkey

Abstract

This study describes and interprets Upper Cretaceous volcaniclastics interbedded with hemipelagic to pelagic limestones, marls,and turbidites from the Western Pontides, northwestern Turkey. The Dereköy Formation, the Unaz Formation (red pelagic limestoneunit), and the Cambu Formation can be distinguished, overlain by the turbiditic Akveren Formation. Biostratigraphic ages from thepredominantly volcaniclastic Dereköy Formation indicate Turonian (Dicarinella concavata planktonic foraminifera zone, CC13/UC8-9nannofossil zones) to middle/late Santonian ages up to CC17/UC13. The Unaz Formation, deposited during the late Santonian (UC13,Dicarinella asymetrica Zone), is overlain by the volcaniclastic Cambu Formation of latest Santonian (CC17b/UC13) to early/middleCampanian (CC20/UC15a) age. However, turbidite intercalations are already present diachronously from CC19/UC14d onwards, andthe turbiditic Akveren Formation ranges up into the late Maastrichtian (Abathomphalus mayaroensis Zone). Geochemistry of thevolcaniclastic units reveals information about the volcanic series and tectonic setting, although element mobility has to be considered.An overall trend of calc-alkaline to high-K calc-alkaline series and basaltic to basalt-andesitic rock types was identified. A negativeNb anomaly with respect to Th and Ce in all samples confirms the existence of a volcanic arc, also indicated by a negative Ti anomaly.Biostratigraphic age constraints give evidence for volcanic arc activity mainly between Turonian and early Campanian, ca. 91 - 79 Ma.Various geochemical data, especially element variations normalized to N-MORB, show two types of volcanic arc basalts (VAB): (1)Calc-alkaline to high-K calc-alkaline VAB have increased Nb and Zr values, (2) more tholeiitic VABs are depleted in Nb, Zr, Ti, V. Thevolcaniclastics successions were deposited in an arc setting triggered by different stages of subduction of the Neotethys Ocean to thesouth and contemporaneous spreading in the Western Black Sea Basin to the north. An extensional regime caused formation of smallbut deeper-water sedimentary basins along the southern shore of the Black Sea where volcaniclastic complexes interfingered withinshort lateral distances with pelagic and mass-flow deposits.