Sequence stratigraphy, tectonics and depositional history in eastern Thrace Basin, NW Turkey

Abstract

The eastern Thrace Basin forms the eastern portion of the Turkish Thrace Basin in NW Turkey. The pre-Tertiary crystalline basement shows platform and deep basin depositional morphology which results in facies pattern and thickness distribution localization during the early stages of basin development in Middle to Late Eocene time. Normal fault controlled early and lateral⧸reverse fault controlled later stage tectonic events characterize basin geometry and the present topography, respectively. The early stage normal faulting caused subsidence and sediment accumulation in the Late Eocene and Early Oligocene, while later stage tectonic events caused intense deformation and structuration in the Early to Middle Miocene. As a result, southeastern and northern regions were folded or uplifted and deeply eroded. Outcrop, well log, core and reflection seismic studies reveal that the sedimentary succession can be subdivided into five characteristic depositional sequences bounded at their base by unconformities. These unconformities were dated using paleontological data. The sequence boundaries, underlying consecutively Sequences-1, 2, 3, and 5 were dated at 39.5, 36, 30, and 25.5 Ma from older to young in order of succession, corresponding to the major erosional breaks between the Middle and Upper Eocene, the Upper Eocene and the Lower Oligocene, the Lower and Upper Oligocene, and the Upper Oligocene and the Lower Miocene. The sequence boundary underlying Sequence-4 was determined to lie within the Late Oligocene succession and, as an age designation could not be assigned, it was termed intra sequence boundary (ISB). The first two sequences correspond to relative sea level rises and a normal fault controlled tectonism coincides with them. Reefal carbonates, coarse grained clastics and marine shales, with abundant volcanic flows or layers represent the sedimentary facies of these two early sequences. The third and fourth sequences correspond to either a decreased relative rise of sea level or a relative still stand of sea level and a sediment-induced subsidence related to thermal relaxation provides accommodation. Prograding delta sediments of varying extent constitute the sedimentary packages of these two sequences. The Depositional Sequence-5, which makes up the uppermost part of the sedimentary succession in the basin, corresponds to a relative sea level fall and a tectonic style shaped by lateral and inversion faulting characterizes this period. Nonmarine alluvial fan, fluvial channel and various lagoonal deposits constitute the sedimentary facies of this last sequence. Porous reefal limestones and shallow marine beach sandstones of Sequence-1 and deltaic sandstones of Sequences-3 and 4 provide good reservoir rocks for the hydrocarbons produced in the study area. The marine shales and marls of the second and the third sequences provide good source rocks as they are rich in disseminated organic matter content. Although the non-marine alluvial and fluvial channel deposits of Sequence-5 are good reservoirs, they lack effective seals. The time interval in which Sequence-5 was deposited represents a period of major structural and trap development in the basin.