Evidence of orbital forcing in lake level fluctuations in the Middle Eocene oilshale bearing lacustrine successions in the Mudurnu Göynük Basin NW Anatolia Turkey
Mudurnu-Göynük basin of the Sakarya Zone in NW Anatolia comprises ca. 1500 m thick Paleocene–Eocene terrestrial to shallow marine succession overlying the Late Cretaceous deeper marine progradational fore-arc sediments. Formed in a foreland setting in relation to southerly situated İzmir–Ankara suture zone, this terrestrial succession (regionally known as Kızılçay group) comprises a thin (<200 m) oil shale-bearing lacustrine section with very good cyclic patterns that potentially serves the quantification of stratigraphy and enlightening the origin of cyclicities of various hierarchy. Our detailed facies analysis on three correlative measured sections showed that mudstone, oil shale and thinner limestone alternations characterize the relatively deeper part of the Eocene lake with probable marine intervention, while thicker limestone, coal, marl and occasional oil shale alternations typify the southern relatively freshwater shoal areas. These facies are frequently organized as meter-scale symmetric to asymmetric transgressive–regressive cycles. Spectral analysis of the mudstone beds and the cycles within the lacustrine succession strongly indicates the occurrence of full bands of Milankovitch with the shortest precession cycle (19 ka) at ca. 2.30 m. Our observations further revealed quite rhythmic thin couplets with estimated durations of 365–730 yr that might represent abrupt climatic changes during deposition. On the other hand, longer duration (ca. 1 Ma) of shoaling and deepening trends in the studied sections were attributed basically to varying subsidence due to tectonic loading in the southerly suture zone. Lastly, regarding the distribution of depositional environments we propose that the oil shale exploration activities should be carried out within a 20 km wide E–W running belt while the southern limits of this belt is more prolific for coal resources
