Tectonostratigraphic observations in the western Thrace Basin in Greece and correlations with the eastern part in Turkey

New observations on the Tertiary tectonostratigraphy of the western part of the Thrace Basin in Greece enabled the distinction of several tectonostratigraphic formations ranging between Middle Eocene and Late Oligocene. The first major conclusion was that two NE-SW trending dextral strike-slip fault zones – the Soufli FZ in the south and the Ardas FZ in the north-dissect the western part of Thrace Basin into three sub-basins: the Alexandroupolis SB in the south, the Orestias SB in the middle and the Petrota SB in the north.
   The Alexandroupolis SB consists of two stratigraphic sequences separated by an angular unconformity. The lower sequence comprises the Kirki Formation, made of sandstones, shales and conglomerates of reddish colour, overlain by a 30 m thick sandstone member and by the Chorafaki Formation made of alternations of sandstones and pelites. The age has been determined as Middle Eocene (nannofossil biozone NP17, 39.8-36.8 Ma). The upper sequence comprises the Avas Formation, made of neritic limestones followed by the Pylaea Formation, made of marls, sandstones and some limestone interbeds. The age has been determined as Late Eocene-Early Oligocene (NP19/20-NP23, 36.2-30.0 Ma). At the area around Feres the Pylaea Formation contains thick volcanic rocks and pyroclastics.
   The Orestias SB is featured only by the upper sequence, comprising the Metaxades Formation which is equivalent to the Avas Formation of Alexandroupolis SB and the Pythion Formation, which is equivalent to the Pylaea Formation of Alexandroupolis SB. A characteristic stratigraphic member is the Congeria-bearing limestone of Early Oligocene age. Volcanic rocks are practically absent from Orestias SB.
   The Petrota SB has a basal clastic formation of sandstones and conglomerates of Late Eocene age, overlain by marls of Oligocene age.
   The pre-Tertiary basement is different in the three sub-basins of western Thrace. The low-grade metamorphic Makri unit (part of the Circum Rhodope Unit) is observed below the western margin of Alexandroupolis SB, whereas the Melia not metamorphosed diabases and flysch are observed below the central part of the sub-basin. On the contrary, medium-high grade metamorphic rocks are observed below the southern margin of Orestias SB and also below the western margin of Petrota SB.
   The above tectonostratigraphy can be correlated to that of the southern part of Eastern Thrace in Turkey around Tekirdag- Keşan - Kallipolis. Thus, Kirki Fm is equivalent to Fiçitepe Formation, Chorafaki Formation is equivalent to Keşan Formation, Avas Formation is equivalent to Soğucak Fm, Pylaea Formation is equivalent to Ceylan and Mezardere Formations. The Congeria-bearing sediments are indicating the northern margin of the Thrace basin both in the western part (e.g. Didimoticho, Pythion Fm) and the eastern part (e.g. Pinarhisar).