Stratigraphy of the Upper Cretaceous and Paleogene in Yığılca-Bolu (NW Turkey)

Abstract

The post-Triassic rocks of the Yığılca area extend in age from late Santonian/early Campanian to Ypresian or probably Lutetian, and constitute a marine sequence intervened by submarine and subaerial unconformities. The sequence is represented primarily by volcaniclastic sandstone and conglomerate, derived from a provenance of mafic volcanic rocks, and subordinately by epiclastic sandstone. The Cretaceous and Paleogene rocks of the Yığılca area have a marked lithic and sequential similarity to those in the northerly-lying Ereğli area. The Cretaceous is suggested to have been deposited by the progressive onlap of an apparently southward transgressing sea. The coeval but differing rocks of the uppermost Cretaceous and Paleogene in the Yığılca, and southerly-lying Bolu and Mengen areas, suggests a structural divide to the south of the Yığılca area, formed by the latest Cretaceous. The totally 200 m thick Cretaceous olistostromal interval of the Yığılca sequence, which has a 4 m thick correlative in the Ereğli sequence, has apparently a significant bearing on the stratigraphy of the so-called «Ankara melange». The arcal structure is characterized by a southward recumbent syncline whose northern limb is thrusted on the southern. High-angle faults are suggested to be originally shear fractures related to a nearly northeast-trending horizontal acute bisector, and later to have acted as extensional. High-angle faults postdate the thrust, and both are probably post-Lutetian in age.