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Lower Cretaceous Turbidites of the Pontides and the Opening of the Black Sea

Abstract

A huge Lower Cretaceous submarine turbidite fan, measuring 300 km by 60 km crops out in the Central Pontides in northern Turkey and extends north to the Black Sea. The turbidite fan is known as the Çağlayan Formation in the east and the Ulus Formation in the west. Both of these formations have source and reservoir characteristics and have been drilled onshore for hydrocarbons. Geological field studies and clastic zircon U-Pb dating in the sandstones have shown that the Çağlayan and Ulus formations are deposits of the same basin, which to a large extend was sourced from the Ukranian shield north of the Black Sea. This implies that during the Early Cretaceous (Barremian-Aptian) there was no “Black Sea” between the Pontides and the East European Platform. A second implication is with regard to the reservoir characteristics of the sandstones in the Çağlayan (Ulus) Formation. Previously, the source for the Lower Cretaceous turbidites were considered to lie in the south, in the large metamorphic area south of the Central Pontides, which is made up predominantly of phyllite and metabasite. New Ar-Ar age data have shown that this metamorphic area is largely of Early Cretaceous age, and therefore cannot be a source for the Çağlayan (Ulus) Formation. Hence, the sandstones in the Çağlayan (Ulus) Formation were largely sourced from the granites and gneisses of the Ukranian shield and may have suitable reservoir characteristics, a major problem in the Black Sea petroleum exploration. We envisage a large river system draining the Ukranian shield south to the Tethys ocean via the Central Pontides. A major phase of deformation, metamorphism and uplift occurred during the Albian in the Central Pontides. The distal parts of the Barremian-Aptian turbidite fan, which extended south to the Tethyan ocean, was entrapped in the subduction zone and was metamorphosed. Oceanic crustal rocks metamorphosed in the eclogite and blueschist facies during the Albian were accreted to the southern margin of Laurasia. Turonian to Coniacian-Santonian pelagic limestones lie unconformably over the deformed Lower Cretaceous turbidites and mark the opening of the Black Sea basin.