Biostratigraphic data from the Çetmi Melange, northwest Turkey: Palaeogeographic and tectonic implications

The Çetmi accretionary melange is cropping out in the Biga Peninsula of northwest Turkey. It is characterised by an isolated position, relatively far from the accretion complexes of the nearest suture zones, which raises the question of its lateral correlations. A detailed biostratigraphic investigation of the limestone and radiolarite blocks and the matrix of the Çetmi melange allowed to propose a solution for this palaeogeographic problem. Scarce red nodular limestones in the Han Bulog facies represent the oldest lithology in the melange. Their Late Scythian–Ladinian age is based on Chiosella gondolleloides, the co-occurrence of Gladigondolella sp. and Nicoraella cf. kockeli, and Paragondolella fuelopi. Light grey limestone blocks are a characteristic feature of the Çetmi melange. They occur in two distinct facies. Facies A consists of packstone to grainstone, and is characterised by unsorted and poorly washed pelbiosparites. Facies B consists of wackestone to packstone, and is characterised by poorly washed biopelmicrites to biopelsparites. The foraminiferal assemblage of Facies A, containing Triasina hantkeni, is of Late Norian to Rhaetian age. The foraminiferal assemblage of Facies B never contains T. hantkeni, and is characteristic of a Late Triassic (Carnian? to Norian–Rhaetian) age. Radiolarian cherts are widely distributed in the Çetmi melange. They record fully pelagic sedimentation from the Upper Bajocian to the Aptian. The matrix of the Çetmi melange consists of brown to black shales, sometimes silty or siliceous, intercalated with dark grey greywackes. Palynomorphs of one sample of brownish silty shale yielded an Early to Middle Albian age, based on the co-occurrence of several dinoflagellate cysts. The age of the matrix, representing the youngest lithology within the melange, and of the unconformable overlaying section (latest Albian–Cenomanian) indicate that the melange-forming process stopped between the Early Albian and the latest Albian–Cenomanian. At a regional scale, the Çetmi melange has little in common with the melanges from the İzmir–Ankara and Intra–Pontide sutures of northwestern Turkey precluding a direct correlation. On the other hand, the Çetmi melange shares several characteristics with the melange-like units of the eastern Rhodope Zone (Bulgaria and Greece), like a major Cenomanian transgression, the reworking of Triassic limestones and Middle Jurassic–Lower Cretaceous radiolarians, and the absence of Jurassic–Cretaceous pass