Stratigraphy and Larger Foraminifera of the Middle Eocene to Lower Oligocene Shallow-Marine Units in the Northern and Eastern Parts of the Thrace Basin, NW Turkey

Authors: GYÖRGY LESS, ERCAN ÖZCAN, ARAL OKAY

Abstract: The shallow-marine Eocene Soğucak Limestone and Oligocene Ceylan Formation were studied in the northern and eastern parts of the Thrace Basin with detailed biometric analysis of the full spectrum of larger benthic foraminifera (mainly nummulitids and orthophragmines). This allows us to establish a high-resolution biostratigraphy in the context of the shallow benthic zonation (with SBZ zones) of the Tethyan Palaeogene since larger foraminiferal assemblages show a very strong Western Tethyan affinity. Only two species (Heterostegina armenica and Orbitoclypeus haynesi) are unknown so far to the west of the Thrace Basin. The age of particular larger foraminiferal sites is determined based on (i) the occurrence and developmental stage of different species of Heterostegina (H. armenica hacimasliensis n. ssp. is introduced here), (ii) the presence/absence of giant Nummulites, (iii) the presence/absence of Spiroclypeus, (iv) the developmental stage of reticulate Nummulites, (v) the occurrence and developmental stage of orthophragmines, (vi) the occurrence of particular Operculina and radiate Nummulites. Six larger foraminiferal horizons could be established. They correspond to (i) the vicinity of the early/late Bartonian boundary (SBZ 17/18), (ii) the middle late Bartonian (SBZ 18B), (iii) the latest Bartonian (SBZ 18C), (iv) the early Priabonian (SBZ 19), (v) the late Priabonian (SBZ 20) and (vi) the early Rupelian (SBZ 21). Three main shallow-water depositional environments could be recognized in both the late Bartonian and Priabonian: two of them took place in the middle shelf; one with low and another with high water-energy (back-bank and Nummulites-bank facies) whereas the third one refers to the outer shelf (fore-bank facies). Biostratigraphical and palaeoenvironmental observations allow us to reconstruct three subregions in the northern and eastern parts of the Thrace Basin with different depositional histories: (i) The eastern part of the territory, with an İstanbul Zone basement was flooded at the beginning of the middle late Bartonian (SBZ 18B), but the carbonate platform was drowned in the latest Bartonian (SBZ 18C). (ii) The Çatalca block, lying on the Istranca Massif, formed a palaeohigh in whose peripheries a similar depositional history to for the former sub-region can be reconstructed, although the central part was transgressed only in the late Priabonian and was not drowned at all. (iii) The northern margin of the recent Thrace Basin (also lying on the Istranca Massif) was flooded only in the latest Bartonian (SBZ 18C) or in the early Priabonian (SBZ 19) and the Priabonian carbonate platform had only partly and shallowly been drowned. This subregion very probably formed the real northern margin of the whole Thrace Basin in the Palaeogene.

Keywords: Northern and Eastern Thrace, larger benthic foraminifera, biometry, taxonomy, biostratigraphy, Palaeogene, depositional history

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