Sequence stratigraphy and sea-level history of Oligocene strata of the northern Aral Sea region (Kazakhstan): Implications for glacioeustatic reconstructions

We have applied sequence stratigraphic techniques to the upper Eocene to Ol igocene strata of the region north of the Aral Sea and northern Usturt Plat eau (,western Kazakhstan) and compared the inferred sea-level history with other records derived from different parts of the world. During late Eocene to early Miocene time, the entire region of study was within the northeast ern part of the Paratethys sea. Clastic and carbonate deposition occurred i n a range of environments from fluvial and lacustrine in coastal plains (no rthern Priaral) to relatively deep marine in the north Usturt trough. Strat al geometries and sedimentation rates indicate that the deposition of the n orthern Priaral sections occurred under tectonically quiet conditions, wher eas noticeable syndepositional subsidence took place in the central part of the north Usturt trough. The same Oligocene sequences have been identified in these areas despite significant spatial variations in tectonic subsiden ce and sediment supply rates. We therefore conclude that eustasy played a m ajor role in controlling sedimentation in the region of study, especially i n the northern Priaral. Through sequence stratigraphic analysis, we identified four sequence bounda ries resulting from regional sea-level falls, Two major sealevel falls are identified in the earliest Oligocene and at the early-late Oligocene bounda ry, and two smaller sea-level falls took place in late early Oligocene and latest Oligocene time. Each of these events can be correlated to peaks in the global delta(18)O re cord. In addition, paleoclimatic data from the region indicate that regiona l periods of cooling are coincident with the intervals of inferred sea-leve l falls. We; therefore conclude that global glacioeustasy represents the ma in controlling force for the sea-level record we observe in the northern Pr iaral and north Usturt trough. This makes the region of study very useful f or reconstruction of eustatic history for late Eocene-Oligocene time.