SEDIMENTARY AND TECTONIC EVOLUTION OF THE CENOZOIC ILI BASIN (NORTHERN TIEN SHAN, KAZAKHSTAN)

The Ili basin is a triangle-shaped Cenozoic broken foreland basin located between the Northern Tien Shan and Dzungarian Alatau thrust fronts in Kazakhstan. It is disrupted by several thrust-related basement uplifts.

The most representative sections occur in the piedmont of the Katutau basement range. Distal deposits emerge in the Aktau anticline while proximal strata overlie the Paleozoic basement in the north and west. The Aktau succession whose base is not exposed starts in the middle Oligocene (mammal fossils; Indricotherium horizon) with fluvial deposits of a large river system and varying flood-plain deposits exhibiting intense soil formation (calcretes and gypsisols). Transport directions and quartz content of these sediments suggest they were not sourced from the nearby mountain ranges present today. They are followed by late Oligocene to Miocene strata reflecting the transition from an evaporitic lake/playa system to freshwater lacustrine conditions, eventually grading into a large sandy river system.

In the proximal settings, alluvial and fluvial deposits rest on deeply weathered paleosurfaces. Transport is mainly to the south. Alluvial fans dominated by tabular conglomerate sheets are sharply overlain by fine-grained sandy mudstones with frequent calcareous paleosols. These represent terminal alluvial fan and playa deposits interfingering with freshwater lake deposits, finally also giving way to sandy fluvial strata reminiscent of the modern Ili river. The basal fluvial deposits can be confidently correlated from the Aktau to the proximal successions on the basis of facies and clast spectra. Up section the correlation is not as clear, although the main lacustrine episodes and the transition to the sandy fluvial system may coincide.

The rapid tapering from the Aktau section into much thinner, more coarse-grained and more proximal successions may in part reflect restricted sediment transport in a very arid environment. In addition, it is apparently mediated by NNW-trending normal faults displacing the Cenozoic strata at least up to the Upper Miocene. Some of these normal faults were later reactivated as dextral strike-slip faults. The Ili Basin apparently originated as a mildly extensional basin in Late Paleogene time and evolved into a foreland basin possibly as late as Pliocene time.