Stratigraphy and Larger Foraminifera of the Eocene Shallow-marine and Olistostromal Units of the Southern Part of the Thrace Basin, NW Turkey

The Eocene marine sequence in the southern part of the Thrace Basin (NW Turkey) involves a variety of platform and deep-marine olistostromal units, the stratigraphy of which have been vigorously debated in the past. A detailed analysis of larger foraminifera in these either foraminifera or foraminifera-coral-coralline algae-dominated platform and associated comparatively deeper-marine units permits us to establish a high-resolution biostratigraphy in the context of shallow benthic zonation (with SBZ zones) of Tethyan Paleogene. The oldest zone (SBZ 5, corresponding to the basal Ypresian) was observed only in olistoliths. An old erosional remnant of a transgressive shallow-marine to basinal sequence (Disbudak series; late Ypresian-? middle Eocene) was recognized below the regionally most widespread carbonate platform unit, the Sogucak Formation. The Disbudak sequence, previously considered to belong to the Sogucak Formation and formally introduced recently, contains larger foraminifera, such as orthophragmines, nummulitids and alveolinids in its shallow-marine package referred to SBZ 10 (late Ypresian). The Sogucak Formation, which often exhibits patch reef developments, contains a rich and diverse assemblage of orthophragmines (Discocyclina, Orbitoclypeus and Asterocyclina), nummulitids (reticulate and other Nummulites, Assilina, Operculina, Heterostegina and Spiroclypeus), and other benthic taxa (Silvestriella, Pellatispira, Chapmanina, Orbitolina, Linderina, Gyroidinella, Fabiania, Halkyardia, Eoannularia, Sphaerogypsina, Asterigerina, Planorbulina and Peneroplis). Their assemblages, referred to SBZ 15/16, 17, 18, 19 and 20 Zones, provide a precise tool for recording the history of marine events which resulted in the deposition of the Sogucak Formation during four main periods. Their spatial distribution, recorded as late Lutetian, early Bartonian, late Bartonian and Priabonian, suggests a marine transgression from WSW to ENE. The Cengelli flysch sequence overlying the Sogucak Formation in a limited area to the east of the Gelibolu Peninsula, contains benthic foraminifera, mainly from limestone olistoliths mostly derived from the Sogucak Formation, and also in the turbiditic strata. The assemblages in the olistoliths reveal the existence of various shallow marine limestone sequences ranging in age from late Bartonian to early Priabonian.