Akademik Veri Yönetim Sistemi
GEOMORPHOLOGY, cilt.165, ss.7-24, 2012 (SCI-Expanded)
We present the results of detailed field investigations of the fluvial succession exposed along the Euphrates valley adjoining the Ataturk Dam in the northern part of the Arabian Platform within SE Turkey. This work, which has used Differential GPS surveying to obtain accurate heights of deposits and Shuttle Radar Topographic Mission imagery for location purposes, has included documentation of many fresh sections exposed by quarrying. The work has been supplemented by unspiked K-Ar dating of late Middle Miocene to Late Miocene basalt flows, which are widespread in the region, providing a chronology for the early stages of development of this river system following regional emergence above sea-level in the early Middle Miocene. For example, beside the Ataturk Dam Lake at Siverek Iskelesi, basalt dated to 10.24 +/- 0.22 Ma (+/- 2 sigma) caps a polymict Euphrates gravel some 80 m above the modern river; this is the oldest Euphrates terrace currently recognised. However, amounts and rates of fluvial incision are shown to vary across the northern Arabian Platform in a complex manner, due to the interaction between regional uplift and localised vertical crustal motions caused by slip on active reverse faults beneath anticlines. The study reach downstream of the Ataturk Dam includes the footwall of one such fault, beneath the Bozova Anticline; we estimate that the resulting rate of localised subsidence, superimposed onto the regional uplift that has also been occurring, has been similar to 0.01 mm a(-1) during the present phase of crustal deformation, which began at similar to 3.7-3.6 Ma, but was higher, maybe similar to 0.03 mm a(-1), during the previous phase, which began at similar to 6 Ma, when the pattern of plate motions in the surrounding region was different. A large palaeo-lake centred north of the present study region around the city of Adiyaman is inferred to have existed during this similar to 6 Ma to similar to 3.7-3.6 Ma phase of plate motion, apparently because the relatively rapid localised hanging-wall uplift on the northern flank of the Bozova Anticline 'dammed' the Euphrates valley near the site of the modern Ataturk Dam. The uppermost part of a thick fluvial aggradation west of the Euphrates near Karababa Bridge, inferred to date from around the Mid Pliocene, reaches similar to 100 m above the modern river and, after correction for an estimated similar to 30 m of localised subsidence and by similar to 50 m for subsequent downstream lengthening of the Euphrates channel, indicates similar to 180 m of subsequent regional uplift. The Euphrates then incised to within similar to 20 m of its present level by the mid Early Pleistocene, then a switch to regional subsidence accompanied the deposition of a second thick fluvial aggradation, the top of which is similar to 70 m above the modern river. Subsequent renewed regional uplift, following the Mid-Pleistocene Revolution, has resulted in the development of a succession of fluvial terraces, typically inset into these older aggradations, which are correlated with the cold stages of Milankovitch climate cycles. The early part of the Euphrates chronology, constrained mainly by the disposition of basalt flows, indicates similar to 800 m of regional uplift since the early Middle Miocene, of which an estimated similar to 330 m is masked by the subsequent downstream lengthening of the river.