--> Abstract: Short Phase of Bioclastic Development in Siliciclastic Epicontinental Upper Jurassic Deposits, West Siberia, by Konstantin Zverev, Elena R. Chuhlanceva, and Vladimir V. Fedorcov; #90082 (2008)

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Short Phase of Bioclastic Development in Siliciclastic Epicontinental Upper Jurassic Deposits, West Siberia

Konstantin Zverev, Elena R. Chuhlanceva, and Vladimir V. Fedorcov
Tyumen Oil Co, TNK-BP Russia, Moscow, Russian Federation

Cores from exploratory wells drilled in Vassyugan suite (Upper Jurassic) in the south-eastern part of Uvat, West Siberia, cover transition zone between bioclastic carbonate and siliciclastic deposits formed in a wave- and current-dominated siliciclastic epicontinental basin. The antecedent topography and sea-level change are the main factors controlling stratigraphic architecture and sediment type. In West Siberia the Late Kimmerigian period tectonically was a relatively quiet period when the complex relict basin was flooded by a marine transgression. During the transgression a siliciclastic depocentre migrated landward and created an ecological window that allowed bioclastic carbonates to develop around the islands emerged in place of basement highs. A bioclastic carbonate sequence (10 m thick) is identified in siliciclastic deposits (the upper part of Vassyugan suite). The sequence was accumulated during a relatively short-time phase of active shallow-marine organic development as the result of sea-level stabilization or regression. The bioclastic carbonate sequence has a basal boundary that is hardground covering barrier system siliciclastic deposits formed during the first phase of transgression. The carbonate sequence passes upward from argillaceous nodular wackstone to thicker-bedded pack-and grainstone. They are overlain by skeletal grainstone with interbeds of conglomerate and glauconitic fine-grained sandstones which are interpreted as very shallow shoalwater deposits formed due to storm-wave and current sweeping in the nearshore zone. The carbonate sequence was capped by sandy skeletal-rich phosphatic rock indicating distinctive hardground formed with the beginning of the new flooding phase. The sequence is overlain by deep water bituminous black shales of Bazhenov suite representing the period of maximum transgression.

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