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ABSTRACT: Detailed Sedimentological Study in Deltaic Upper Sequence of the Thrace Basin, Northwest Turkey

M. Zihni Aksoy

The Thrace basin forms one of the largest Tertiary sedimentary basins in Turkey. Paleontological and sedimentological evidence suggests that sedimentation and basin formation commenced by a major transgression from the southwest in the middle to late middle Eocene.

Sedimentological studies indicate that two major depositional cycles prevailed during the formation of the Thrace basin. The lower sequence of sediments (first cycle) was deposited in deep marine environments by turbidity currents as submarine fans (approximately 3500 m thick). The upper sequence of sediments (second cycle) was deposited in various subenvironments, of which deltaic depositional conditions were the most prominent and 2600 m thick sediments were deposited. The total sedimentary thickness (6100 m), deposited until the end of the Oligocene, indicates that rates of subsidence and deposition were quite high. Deltaic deposition started in the beginning of the Oligocene when the rate of subsidence slowed. At this time, the Thrace basin was relatively filled to its maximum cap city and shallow-marine conditions became prevalent. It is seen that initially the rate of sedimentation was higher than the rate of subsidence. Sedimentation in the basin continued from the beginning of the middle Eocene to the end of the Oligocene, with no break in sedimentation. The depositional environment changed from deep marine fan deposits to deltas. Geometry and sand-body thickness along with the sand/shale ratio throughout the delta sequence have been determined using data obtained from 29 wells. Based on this study, deltaic sand bodies in the Thrace basin are now among the major hydrocarbon exploration targets.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990