--> Cyclic Deposits and Hummocky Cross-Stratification of Probable Storm Origin in Upper Paleogen Rocks of the Ujoh Bilang Formation, Upper Kutai Basin, Borneo, Indonesia

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Cyclic Deposits and Hummocky Cross-Stratification of Probable Storm Origin in Upper Paleogen Rocks of the Ujoh Bilang Formation, Upper Kutai Basin, Borneo, Indonesia

Abstract

The Oligocene Ujoh Bilang Formation, exposed in the Belayan River of Tabang Area, Upper Kutai Basin, Borneo, is a typical a storm wave deposits based on sedimentology characters and fossil analysis. Hummocky cross-stratification is an important structure formed on the shoreface and shelf by waves. It characterizes a wave-dominated facies. Many of the important features of storm deposits are displayed in the Oligocene Ujoh Bilang Formation that contains numerous storm deposits can be cyclic of storm condition. Storm waves operated contemporaneously and probably contributed toward sediment entrainment, as evidence by hummocky cross-stratification. Hummocky Cross-Stratification as an Indicator of Storm Dominated Shallow-Marine Environments. Each sand unit has a interbeded with thin calcareous shale with scoured base and comprises to parallel lamination that grades upward into hummocky cross-stratification on top of sand body. These features in turn are overlain by horizontal lamination, with an uppermost interval consisting of small-scale wave ripples containing abundant horizontal burrows (thallasinoides) and feeding trails. HCS comprises 20–80% of the primary sedimentary structures in these beds. The cycles are interpreted as shallowing-upward prograditional sequence. However, the result analysis of fossils suggests that facies of Oligocene Ujoh Bilang sequence is shoreface facies with cyclic of storm condition.