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 Derekoy Formation, the Unaz Formation (red pelagic limestone unit), and the Cambu Formation can be distinguished, overlain by the turbiditic Akveren Formation. Biostratigraphic ages from the predominantly volcaniclastic Derekoy Formation indicate Turonian (Dicarinella concavata planktonic foraminifera zone, CC13/UC8-9 nannofossil 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/middle Campanian (CC20/UC15a) age. However, turbidite intercalations are already present diachronously from CC19/UC14d onwards, and the turbiditic Akveren Formation ranges up into the late Maastrichtian (Abathomphalus mayaroensis Zone). Geochemistry of the volcaniclastic 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 negative Nb 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. The volcaniclastics successions were deposited in an arc setting triggered by different stages of subduction of the Neotethys Ocean to the south and contemporaneous spreading in the Western Black Sea Basin to the north. An extensional regime caused formation of small but deeper-water sedimentary basins along the southern shore of the Black Sea where volcaniclastic complexes interfingered within short lateral distances with pelagic and mass-flow deposits.
Highlights
During the Mesozoic to Paleogene, the western Tethys consisted of a complex array of opening and closing oceanic basins (e.g., Stampfli and Borel, 2002; Schettino and Turco, 2011)
Fifty-five samples were evaluated for calcareous nannofossils, 44 of them provided a biostratigraphically significant nannofossil assemblage, 64 samples were processed for planktonic foraminiferal stratigraphy, the washing residues of 24 samples yielded biostratigraphically relevant data, and 16 samples of volcaniclastics were analyzed in detail for geochemistry, mineralogy and petrology
At sections Kuşçu, Çayaltı, and Köseli, a small time gap and slight unconformity may be present at the base of the Unaz Formation, but no significant time interval is missing, as sediments below (Dereköy Formation) and above (Unaz Formation) the unconformity can be dated into the same nannofossil zones (CC17 and UC12-13) of the middle to late Santonian
Summary
During the Mesozoic to Paleogene, the western Tethys consisted of a complex array of opening and closing oceanic basins (e.g., Stampfli and Borel, 2002; Schettino and Turco, 2011). BOEHM et al / Turkish J Earth Sci volcaniclastic complexes alongside the southern Black Sea coast in northern Turkey give biostratigraphic age constraints on volcanism, allow the reconstruction of paleogeography, and provide information on the type of magmatism, sources and the magmatic evolution in time Both Upper Cretaceous volcanic successions in the Eastern Pontides (Eyüboğlu, 2010; Eyüboğlu et al, 2011; Sipahi et al, 2014) and volcaniclastic occurrences in the Western Pontides are of increasing interest and thoroughly studied (Keskin et al, 2003; Çinku et al, 2009; Şahin et al, 2012; Tüysüz et al, 2012; Keskin and Tüysüz, 2017). The area studied in this work is situated in the northwestern Turkey, in the Western Black Sea Section, in Bartın, Kastamonu, and Sinop Provinces (Boehm, 2015)
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