Sedimentologie des roches detritiques de la Formation de Keşan (Paleogene): Un facies a turbidites au Sud-Ouest de la Thrace turque

Abstract

Two thousand meters of Paleogene clastic sediments (mainly epiclastics) exposed in the Keşan area, southwestern Turkish Thrace (southwestern Alpine Ergene basin) have been studied and the stratigraphic sequence divided into two major lithostratigraphic units: (1) the Keşan Formation (sandy); (2) the Yenimuhacir Formation (muddy). The Keşan Formation consists of non-cyclic Çinarlidere (Upper Eocene) and cyclic Şaplidere (Lower Oligocene) members. Volcanic rocks occur at three different localities of the Çinarlidere member and are interbedded (contemporaneously) with clastics. Clastics beds are generally gently tilted (at 15°–25° towards the north) and present a long outcrop along the strike. Two key horizons were followed along the strike approximately 10 km from west to east. On the basis of certain sedimentary structures, the Keşan Formation has been defined as a “sandy turbidite sequence” and statistical treatment of paleocurrent directions shows that the mean current direction of two members of this formation are in opposite directions. The quantitative analysis of structural and textural features makes it possible to distinguish two groups of turbidites: “proximal” and “distal”. These two groups of sandstones are designated as “sedimentary facies” and their occurrence is explained on the basis of the flow regime of the currents. The compositional analysis of light and heavy fractions of both member sandstones of the Keşan Formation showed marked petrological variations. The existence of two distinct “compositional facies” was established by computational statistics. Analysis of composition and current-direction data suggests that two main sources have provided materials for the formation of the Keşan sandstones. These are: (1) the Biga massif to the south-southeast, for the Çinarlidere sediments; and (2) the Rodop massif to the west-northwest, for the Şaplidere sediments. Finally, the sedimentation history of the Paleogene Keşan Beds in relation to volcanic activity and the facies development in the Keşan basin have been examined and the origin of the upper member turbidite cyclicity has been discussed. Conclusions are as follows: 1. (1) Although the direction of the axis of the depositional basin, (the Keşan as well as the Ergene basin) remained constant during the Paleogene, its axial plunge has changed its direction twice; once west-northwest (in Çinarlidere) and once south-southeast (in Şaplidere). 2. (2) The epiclastic and volcanic sedimentary (“pyroclastics”) deposits of the Keşan Formation have not been produced by the contemporaneous volcanic activity in the area: their source appears to be outside the basin of deposition. 3. (3) The tectonic structures of the Keşan area show that north—south oriented major stresses have been dominant and here tectonic evolution was younger than deposition. 4. (4) It can be postulated that the Ergene basin started to become a “stratigraphic basin” either at the beginning of or possibly earlier than the Paleogene.