Late Cretaceous to Early Eocene sedimentation in the Sinop-Boyabat Basin, north-central Turkey : facies analysis of turbiditic to shallow-marine deposits

The Sinop-Boyabat Basin is a SE-trending, elongate basin in the Sakarya Zone of Central Pontides, northern Turkey, filled with a succession of Early Cretaceous to Middle Eocene deposits, nearly 7 km thick. The basin evolved from a forearc graben controlled by the process of Western Black Sea rifting into a foreland trough increasingly influenced and eventually inverted by compressional deformation associated with the Pontide/Tauride orogeny. This thesis summarized the results of a detailed sedimentological study of the Late Cretaceous to Early Eocene part of the basin-fill succession, ca. 2 km thick, which comprises three formations (Gürsökü Fm., Akveren Fm. and Atbaşı Fm.). The aim of the study was to reconstruct the basin’s palaeogeographical and tectonic history on the basis of sedimentary facies analysis, supplemented with petrographical, micropalaeontological and ichnological data. Special emphasis was on the depositional processes involved and on the basin’s responses to active tectonics and relative sea-level changes. The basin-fill succession studied has been described and its sedimentation processes interpreted in terms of eight component sedimentary facies, ranging from hemipelagites and turbidites to tempestites and shallow-marine calcarenites/limestones. These facies constitute four facies associations, stacked conformably upon one another with gradational boundaries: predominantly siliciclastic turbidites (FA 1), calcareous turbidites (FA 2), turbiditic to tempestitic carbonate-ramp deposits (FA 3) and ramp-drowning, predominantly hemipelagic deposits (FA 4). The Campanian-Maastrichtian Gürsökü Formation (up to 1200 m thick) consists of sheet-like turbidites interbedded with calcareous mudstones and marlstones (FA 1). These deposits represent the medial to distal reaches of a deep-marine turbiditic system that was supplied with siliciclastic and increasingly abundant bioclastic sediment from the basin’s uplifted southwestern margin and was formed by unconfined, lowto high-density turbidity currents flowing eastwards along the basin axis. A sinuous turbiditic palaeochannel occurs in the lowermost part of the formation, but the system was subject to gradual headward backstepping and it is thus unknown if its proximal part involved many more channels. The Campanian uplift correlates with the accretion of Kirşehir microcontinent to the Sakarya margin of Eurasia, and this first pulse of compression, superimposed on the rifting-driven subsidence, marked the onset of a foreland regime in the basin. The overlying Maastrichtian-Palaeocene Akveren Formation (up to 600 m thick) consists of sheet-like calcarenitic turbidites interbedded with marlstones and calcareous mudstones (FA 2), passing upwards into a tempestite-dominated succession overlain by shoreface calcarenites and shallow-marine reefal limestones (FA 3). These deposits represent a turbiditic carbonate slope apron evolving into a carbonate ramp increasingly influenced by storms and covered by a basinward-expanding reefal platform. The rapid upward shallowing, from bathyal to littoral conditions, indicates a decline in subsidence, probably accompanied by mild compressional uplift of the basin floor. This pulse of compressional deformation correlates with the trench closure and the onset of continental collision in the adjacent Tauride-Anatolide Zone to the south. The Late Palaeocene-Early Eocene Atbaşı Formation (up to 200 m thick) consists of variegated calcareous mudstones interspersed with thin calcarenitic tempestites and turbidites (FA 4). These deposits represent a rapid rise in relative sea level that caused shoreline backstepping, cessation of sand supply, reduced sedimentation rate and extensive seafloor oxidation in the sediment-starved basin. This change indicates a strong pulse of rifting, with a rapid wholesale sinking of the basin and its footwall margin. The overlying Early-Middle Eocene Kusuri Formation (up to 1400 m thick), not covered in detail by the present study, is a siliciclastic turbiditic succession recording the last stages of the basin development, until its closure by tectonic inversion that correlates with the culmination of the Tauride-Anatolide orogeny.