Detailed sedimantological study of oligocene aged deltaic sediments in the Thrace basin by using subsurface data

This study has been performed as a Ph.D. dissertation to be submitted to the Department of Geological Engineering, Institute of Science at the University of Çukurova. Study has been continued from 1995 to 1997. Main objective of this dissertation is a detailed sedimentological study by using subsurface^ data of the upper sequence of sediments belonging to the Yenimuhacir group deposited in various subenvironments of a deltaic complex throughout the Oligocene in the northwest of the Thrace Basin. The field study needed to achieve the main objective was conducted during the summer months of 1995. The Thrace Basin forms one of the largest Tertiary sedimentery basins in Turkey. Paleontological and sedimentological evidences suggest that sedimentation and basin formation commenced by a major transgression from the soutwest in the Middle to Late Middle Eocene. Sedimentological studies indicate that two major depositional cycles prevailed during the formation of the Basin. The lower sequence of sediments (first cycle) belonging to the Keşan group were deposited in deep marine environments by turbidity currents as submarine fans (App.3500 meters in thickness) At this time, the rate of subsidence was higher than the rate of sedimentation. The upper sequence of sediments (second cycle) belonging to the Yenimuhacir group deposited in various subenvironments of which deltaic depositional conditions were the most prominent and 2600 m. thick sediments were laid down. Deltaic deposition started in the beginning of Oligocene when the rate of subsidence slowed down and the basin was relatively filled to its maximum capacity and shallow marine conditions became prevalent. The total sedimentary thickness (6100 m.) deposited until the end of Oligocene indicate that rates of subsidance and deposition were quite high. Sedimentation in the Basin continued from the beginning of Middle Eocene to the end of Oligocene. There was no break in sedimentation. Depositional environment changed from deep marine fan deposits to deltas.Deltaic sediments, which stratigraphically deposited in between the Keşan group (Middle to late Eocene) and the Ergene group (Plio-Quaternary in age) consist of the Mezardere, Osmancık, and Danişmen formations each representing one of the three subenvironments of the delta. Deltaic sequence has been divided into 13 coarsening upward depositional cycles by using GR-SONIC logs of 37 wells located in the study area. These cycles have been correlated in 7 stratigraphic sections flattened at a tuff level observed in GR-SONIC logs at the upper most part of the Osmancık formation as marker datum. Thickness of the cycles range from 21 m to 523 m. These correlations show that progradation of the delta is from north, northwest and west to south, southeast and east. Using GR logs, lithological separations and sand ratios of each cycle have been determined. Based on these determinations and thickness distributions of each cycle in the deltaic sequence, several exploration areas are delineated in the study area in which stratigraphic traps may have the most potential. Key Words : North of Thrace Basin, Oligocene Delta, From West to East Progradation, Log, Lithological Seperation, Sand and Thickness Maps, Sedimantology.