A.E. Dodonov

Loess stratigraphy of Central Asia: Palaeoclimatic and palaeoenvironmental aspects

Abstract The loess-palaeosol succession of Central Asia is comparable with that of the Loess Plateau in North China. Palaeomagnetic investigations and palaeontological studies in South Tadjikistan suggest a date of 2–2.5 Ma for the oldest loess, indicating that dust storms became frequent in the region since 2–2.5 Ma BP. Under the prevalently arid and semi-arid environments of Central Asia, aeolian processes had a gradually increasing influence on subaerial sedimentation during the Pleistocene. However, atmospheric aeolian processes in Central Asia have mainly developed in accordance with Pleistocene climatic cyclicity, without extremely high intensification during the late Pleistocene as implied by the previous TL dating results. The identification of loess and palaeosol horizons with respective glaciations and interglacials is sometimes disputable. Nevertheless the available data, in spite of limited evidence, characterise well-developed palaeosols of the middle and late Pleistocene, as warm and wet intervals associated with interglacials. Palynological studies of loess sections show that loess and palaeosol horizons have palynological spectra that are sometimes difficult to interpret. Interregional loess-palaeosol correlation with glacial-interglacial events as well as with oxygen isotope records should be supplied by multidisciplinary investigations.

Thermoluminescence dating of the orkutsay loess section in Tashkent region, Uzbekistan, Central Asia

Abstract The chronology of loess in Central Asia was investigated by means of thermoluminescence dating (TL) in the 1970s, when age estimates ranging from 20 ka to over 800 ka were taken as providing an absolute chronology. This has caused major misinterpretation of Central Asian loess stratigraphy and palaeoclimatic reconstruction. We present a new TL study of a loess section at Orkutsay in the Tashkent Region, Uzbekistan. Nine samples spreading over a large stratigraphic interval yield apparent TL ages ranging from 20 ka to 134 ka. We interpret our TL results with reference to recent TL dating studies of loess from other regions. We consider most of our TL age estimates obtained by the regeneration method to be underestimated. A single TL date of 92.8 ka obtained by the total bleach method for loess just below the first pedocomplex (PC 1) of the last interglacial is also significantly younger than expected. We attribute the underestimation to sensitivity change induced by the laboratory optical bleach and instability of the TL signal. We conclude that the current TL techniques as employed in this study cannot provide a reliable absolute chronology for the sequence below PC 1 and cannot provide even a relative chronology for the loess below PC2. For the Orkutsay section the dating limit is around 130 ka. The results of this study suggest that the previous Central Asian loess chronology based on early TL studies is not valid.

Cenozoic tectonics and evolution of the Euphrates valley in Syria

Abstract Late Cenozoic tectonics affected the evolution of the Euphrates river valley in northern Syria. Data on the height and composition of terraces and new K–Ar dating of overlying basalts are presented for the area between the Assad Reservoir and the town of Abou Kamal. The presence of the Late Cenozoic Euphrates Fault, longitudinal with respect to the valley, is established by the lower height of the terraces on the NE side of the valley compared with the same terraces on the SW side. Geophysical profiling (dipole axial sounding; correlation refraction method and georadar) across the southern side of the valley (opposite the town of Ar Raqqa) confirms the offset on the fault as >25 m. Movements along the transverse Rasafeh–El Faid fault zone and the Halabiyeh–Zalabiyeh deformation zone have resulted in local uplift and the splitting of river terraces. During the Pliocene–Early Pleistocene, uplift and strong incision of the Euphrates valley propagated from near the Syrian–Turkish border to near the Iraq–Syrian border. The Euphrates began to deposit alluvium onto the pre-existing low-lying Mesopotamian Foredeep at c. 3.5 Ma. Intense incision began by late Late-Pliocene time to form terrace IV. Comparable incision further downstream began during the Early Pleistocene to form terrace III.