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Analysis of Depositional Environment and Sedimentation Dynamics of Upper Ngrayong Formation Based on Surface Stratigraphic Observation in Polaman, Jiken, and Sedan Area, North East Java Basin.

Proceedings Title : Proc. Indon. Petrol. Assoc., 41st Ann. Conv., 2017

As one of productive reservoirs in North East Java Basin, Ngrayong Sandstone has significant meaning in stratigraphic study. A lot of stratigraphic study has been conducted both of surface and subsurface in this area with excellent stratigraphic outcrops found along Rembang Anticlinorium. This research was conducted on several outcrops in sandstone quarries in Polaman, Sedan, and Jiken Areas. By comparing stratigraphic outcrops in these areas, we can observe the characteristic of upper part of Ngrayong formation that represents last stage deposition of this formation. Several lithofacies found are shale, quartz sandstone, carbonaceous sandy shale, lignite coal, shaly sandstone, calcareous shaly sandstone, and foraminiferal limestone. Sedimentary structures found in the sandstone body are cross-bedding, ripple marks, cross-lamination, and vertical burrows that indicate high energy environment in open marine conditions. Several lignite coal and carbonaceous shales found in several sections indicate transitional environment. From the data, we find that these facies were deposited in terrestrial (marsh) to open marine (sand flat/shore face) condition as part of a deltaic succession. Vertically upward, facies changed from deltaic product represented by shale, calcareous muddy sandstone, and quartz sandstone into deeper marine that is represented by foraminiferal limestone. Foraminiferal limestone dominated by larger foraminifera such as Cycloclypeus sp. (Cyclocypeus indopacificus, Katacycloclypeus annulatus, and Katacycloclypeus martini) and Lepidocyclina sp. (L. oneataensis, L. decilata, etc). These fossils indicate deeper marine condition at middle Miocene. From these conditions, we conclude that Ngrayong formation was deposited in lagoon and deltaic until open marine environment, and ended by transgression process at middle Miocene. Keywords: depositional environment, Ngrayong formation, lithofacies, transgression.

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