Stratigraphy, source rock and paleoclimatic characteristics oftertiary deposition in the Keşan region

The Thrace Basin, which has been studied by many researchers in terms of structural geology, stratigraphy and volcanism due to its rich oil and natural gas resources, and the Keşan Formation, one of the most important formations in this basin, were examined in terms of source rock (provenance) and paleoclimate in this study. Geological studies in the Thrace basin have been continuing since the 19th century (Ternek, 1949; Boer, 1954; Kopp, 1955-1965; Andrews, 1960; Schindler, 1958 and 1959 etc.; L. Gökçen, 1967; Ünal, 1967; Şenol, 1980; Kasar et al.,1983; Türkecan and Yurtsever, 2002; Siyako, 2005 and 2006b; Perinçek, 1987 and 1991; Perinçek, 2010a, 2010b and 2010c; Perinçek, et al., 2015; Şengüler, 2022; Perinçek, 2024). The Thrace Basin consists of Tertiary and Quaternary sedimentary and magmatic volcanic rocks. The oldest unit in the study area is the Eocene Keşan Formation. The Keşan Formation consists of conglomerate, sandstone, siltstone and volcanic units. The Keşan Formation is overlain by the Eocene Yenimuhacir Formation consisting of mudstone and sandstones, the Oligocene claystone, shale and lignite levels, the Hisarlıdağ volcanics consisting of andesitic, basaltic volcanics and ignimbrites, and the Çanakkale Formation consisting of Miocene conglomerate, sandstone and claystones, and the Quaternary Alluvium at the top. The Istranca massif, which consists of Precambrian, Paleozoic and Mesozoic magmatic and granitic rock assemblies that have undergone different degrees of metamorphism and are unconformable to each other in the north of the Thrace basin, has been exposed to tectonic deformations belonging to the Cadomian, Variscan and Alpine tectonic periods, and the cover on it has been eroded before the Tertiary basin deposition, and the Eocene Koyunbaba Formation (Figure 4-4), which is formed by the terrestrial clastics of the deposition, has taken material from the basic units and overlain it with an angular unconformity. The stack, especially in the north, is overlain by the Ceylan formation clastic and carbonate units consisting of shale, marl, sandstone, tuffite and clayey limestones, which present sedimentation up to the shelf, with rapid deepening, especially due to the effect of the faults opening the basin. The Keşan region constitutes the part of the basin where it has partially deepened, and no reefal sedimentation is observed. In the south of this part of the basin, metamorphic units related to the Ezine Zone and Kazdağ Massif uplift crop out. The basement Uplift source, which occurs in the tectonic classification of the turbiditic sandstones of the Keşan formation, is a provenance observed especially in areas where the basement uplift is widespread with metamorphic and magmatic feeding, and represents areas where deep crystalline rocks with structural uplift in the continental areas are surfaced and corresponds to a deposit compatible with the Istranca uplift. Due to the decomposition of the rocks belonging to the Keşan formation, which is located in the Thrace basin and contains different types of rocks, under mild and humid climate conditions, detrital and euthygenic clay minerals have developed. It is seen that the mineralogical composition of the source rock, climatic factors (humid/semi-humid, arid), paleotography, depositional environment and dianetic factors affect the type of clay that can develop. The product of the submarine fan deposits, which constitute a large part of the study area, generally has rhythmic sedimentary features, and distinct layers with trace fossils, and widespread measurable outcrops have been observed on the ridges extending along the southeast of Keşan and along the ridge extending in the Yenimuhacir direction to the south of the E-85 highway. The layers and strata belonging to the Eocene-Oligocene deposits commonly exhibit WNW oriented, low angle, northeastward inclinations. As can be seen in the prepared contour and rose diagrams, it has been observed that the layer planes were repositioned under the influence of the ENE-oriented compressions reflecting the effects of the east-northeast oriented Alpine Orogeny. In the region where rocks with different geological conditions and lithological characters are located from Eocene to present, it has been observed that fold mechanisms have developed in rocks of different ages and under various stress conditions under the influence of compression and shear tectonics in the Eocene and later periods. Tectonic and sedimentary folds have been identified, and in the stratigraphic upper levels of the Keşan formation, there is the development of folds in the form of slump structures that are coeval with sedimentation. These density structures and ruptures, resulting from differences in deformation behavior, are observed in the levels where clayey, silty-sandy units alternate, reflecting the flysch character in the region and where rhythmic sedimentation is observed. In the middle sections of the Keşan formation, in the levels where successive volcanics are found, in addition to the clastic and carbonate levels, fracture cleavages reflect a special deformation style as a fairly regular crack system, especially in the clayey zones. Except for these levels where rod-like fractures are remarkable, the dominant fracture-crack system of the region consists of shear cracks with distinct diamond-shaped appearances. Contour diagrams were prepared with direction and slope measurements collected from these cracks, which are generally unfilled and have millimeter-scale openings, and tensor analyses were performed to reveal the deformation structure of the region. The faults detected during the mapping studies carried out in the Keşan region are generally thought to have formed with a high angle, normal component dominant and approximately north-south tension tectonic regime. It can be said that these faults correspond to the north-south tension that opened the basin in the Eocene period. These faults with syn-sedimentary formation are evident with very distinct offsets, drag folds, shear fibers, breccia structures and morphologies at the base levels of the Keşan formation, and the kinematic features and principal stress axes of the faults detected in the field can be seen in poly stress analyses. In addition to the tectonic period that tightened the basin in the post-Eocene period, the fault traces reflected in the stress analyses of the tectonic conditions that formed the shear regime and conjugate fractures frequently observed in the region were not observed in the field. It has been observed that small-scale folds have developed tectonically. In the conglomeratic levels of the Ergene formation, which constitutes the youngest sediment in the region and presents widespread outcrops in the south, changes in layer orientations in the form of quite flat undulations are observed as folded structures in the sections.