Plankton biostratigraphy and magnetostratigraphy of the Santonian–Campanian boundary interval in the Mudurnu–Göynük Basin, northwestern Turkey

The Santonian–Campanian boundary interval close to the village of Göynük in northwestern Turkey (Bolu province) was recorded and examined with respect to nannofossil and foraminiferal biostratigraphy, magnetic polarity and magnetic susceptibility. During the Late Cretaceous, the Mudurnu–Göynük Basin was located on the Sakarya continent situated in the northwestern Tethyan Realm, north of the Neo-Tethys. The sections assessed for the present study comprise hemipelagic to pelagic deposits. Five localities were examined and a composite record spanning the Santonian–Campanian boundary was established. The stratigraphically older parts are characterised by uniform reddish limestone, while we frequently recorded shaly marls and marly limestones with recurrent tuff intercalations in the younger subsections. A biostratigraphic investigation of planktonic foraminifera and calcareous nannoplankton assemblages, together with magnetostratigraphy, provides a stratigraphic framework that allows to home in on the Santonian–Campanian boundary. Thus, biostratigraphic data suggest an age that ranges from the late Santonian Dicarinella asymetrica to the early Campanian Globotruncanita elevata planktonic foraminifera biozones, and calcareous nannofossil zones UC12–UC14 and CC16–CC18. The magnetostratigraphic assessment finds a palaeomagnetic reversal that can be recognised as the basal Campanian reversal C33r above polarity chron 34n, the Long Cretaceous Normal. Field magnetic susceptibility (MS) data show a distinct cyclic pattern in the lower Campanian. Sinusoidal patterns in the MS signature curve can presumably be interpreted as the 400 kyr orbital eccentricity cycle, and subsequent smaller-scale cycles of obliquity and precession can be identified even though volcanic activity impacted sedimentation. A sediment accumulation rate of 12 mm/kyr can be inferred for one of the sections.