THE LATE QUATERNARY TECTONO-STRATIGRAPHIC EVOLUTION OF THE LAKE VAN, TURKEY

Abstract

Many of the terraces around the Lake Van record a relatively short period of the much lon- ger geological history of the Lake Van Basin. Their deposition took place during the last ca. 125 ka BP. They accumulated in a large array of shallow lake and lake margin environments, such as alluvial fan/braided river, beach, Gilbert-type delta, nearshore lake and offshore la- ke. Variability of their lithofacies provides evidence for climatic and tectonic controls of the- ir depositional conditions. During their deposition high relief areas in the watershed delive- red abundant detritus to the coastal areas of the lake. The sedimentation was therefore domi- nated by terrigenous clastic deposits. The highest concentrations of the coarse clastic sedi- ments were at the mouths of the major streams where they formed Gilbert-type deltas. The- se river-dominated lacustrine deltas formed during rising lake levels and are relatively more abundant and thicker in the eastern margin of the lake, indicating that this margin mostly had a low-energy coast sheltered from the prevailing westerly winds. However, some areas of the same margin adjacent to deltas were also supplied with sediments by waves and storm-indu- ced longshore currents to form beaches. Storms and storm-generated traction currents were perhaps active agents along the shores of the Lake Paleo-Van as suggested by the presence of the coarse-grained material in its nearshore facies. Somehow, during times of lake high- stands, turbidity currents seem not to have played an important role in the sediment transpor- tation along the lake margin, because the nearshore sediments hardly show any evidence of turbidite depositon, such as graded bedding and sole-marks. The offshore lake was relatively quite standing water, depositing laterally persistent, thinly-bedded to varved and fine-grained sandstones and mudstones in part with hydroplastic disruptions, such as slumps and convo- lute beddings. Because of the lack of sufficient datings of the terrace sequences around La- ke Van, we cannot correlate them unequivocally. However, the absence of large-scale cycli- city within a given terrace sequence in each locality suggests that deposition of each terrace occurred during a separate lake level fluctuation each reaching to higher level than the mo- dern lake level followed by a regression. The available age data suggest that high lake levels, reaching up to 1760 m asl, occurred during the last interglacial (MIS 5; 123-71 ka BP), 26- 24 ka BP, 22-21 ka BP and 10-6 ka BP. The younging of the terrace deposits along with the decrease in elevation suggests either a gradual decline of lake level with time, or the effect of the cumulative uplift with time or both. The fluctuating lake level was probably due to a combination of climatic, volcanic and tectonic processes. Considering the hydrologically clo- sed nature of the lake, climate probably played more important role than the others. Since the formation of the youngest terrace sediments of 6 ka BP, perhaps the climate in the basin has been mostly relatively more arid and evaporative. During the entire history of the lake (last 600 ka), geology of the area has been characterized by the neotectonic régime of Turkey with active dip- and strike-slip faults, resulting in the offshore lake the characteristic slump struc- tures and convolute beddings, and eruptions of mainly the Nemrut Volcano.