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 Oligocene strata of the region north of the Aral Sea and northern Usturt Plateau (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 northeastern part of the Paratethys sea. Clastic and carbonate deposition occurred in a range of environments from fluvial and lacustrine in coastal plains (northern Priaral) to relatively deep marine in the north Usturt trough. Stratal geometries and sedimentation rates indicate that the deposition of the northern Priaral sections occurred under tectonically quiet conditions, whereas 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 subsidence and sediment supply rates. We therefore conclude that eustasy played a major role in controlling sedimentation in the region of study, especially in the northern Priaral. Through sequence stratigraphic analysis, we identified four sequence boundaries resulting from regional sea-level falls. Two major sea-level falls are identified in the earliest Oligocene and at the early–late Oligocene boundary, 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 δ 18 O record. In addition, paleoclimatic data from the region indicate that regional periods of cooling are coincident with the intervals of inferred sea-level falls. We therefore conclude that global glacioeustasy represents the main controlling force for the sea-level record we observe in the northern Priaral and north Usturt trough. This makes the region of study very useful for reconstruction of eustatic history for late Eocene–Oligocene time.