The Çetmi Group in the Biga Peninsula (NW Anatolia-Türkiye): Reassessment of Its Nature and Tectono-stratigraphic Significance

The pre-Liassic basement rocks of the Biga Peninsula, located in the westernmost part of the Sakarya Zone, consist of metamorphic units thought to represent different continental basements to the north and south, with a suture (ophiolitic/mélange) zone in between. The Çetmi “mélange”, considered to be located between the Sakarya Zone in the south and the Ezine Zone in the north, is interpreted as a subduction accretionary mélange belonging to the Intra-Pontid Ocean. Consequently, it has been assumed that a suture passes through this part of the peninsula. The units to the east and west of the Kazdağ Massif and those west of Karabiga differ from each other in terms of Mesozoic cover units. While the metamorphic basement rocks in the southeastern part of the Kazdağ high are covered by a platform carbonate sequence, a clastic, high-energy environment represented by the Çetmi Group developed in the west-northwest of the Kazdağ high during the same period. The Çetmi Group is the first and only unit in the region that has not undergone metamorphism and is directly related to both different basement rocks and the overlying sedimentary sequences. The Çetmi Group overlies the high-grade metamorphic rocks of the Kazdağ Massif and the low-grade metamorphic rocks of the Örenli metamorphics tectonically in the west of the Kazdağ Massif, while it unconformably overlies low-grade metamorphic rocks of Ediacaran-Late Paleozoic age in the west of Karabiga. Accordingly, the Çetmi Group consists of a sedimentary-volcanic-volcanoclastic matrix and blocks within this matrix, displaying an olistostromal character with exotic blocks of varied lithologies. Field evidence from the Karabiga Massif reveals an unconformable stratigraphic contact between the Çetmi Group and the underlying low-grade metamorphic basement, further supporting its interpretation as a post-exhumational sedimentary succession. This unit shows similarities with Mesozoic units distinguished further east in the Western Pontides. In particular, both areas record a similar transgressive–regressive sedimentary cycle and the development of olistostromal levels within a Cretaceous clastic–volcanic succession, suggesting a comparable paleogeographic and tectonic setting along the western Pontide margin.