Sedimentology and Petrography of Sajau Coal Formation in Berau Basin, East Kalimantan, Indonesia

The coal maceral identification have been undertaken to determine the sedimentology and petrography characteristics of the Sajau Formation coal in the Berau Basin, East Kalimantan and their depositional environment. The coal bearing Sajau Formation are represented by clays, carbonaceous clay, sandstones, sandy and lime clays and up to 13 coal seam (A ? M coal seam). The Sajau coal belong to the lithotype category of matrix soft brown coals, immature stage and at low phase of coalification represents a transition from peat to lignite. The microscopic investigations revealed the coal is characterized by great maceral diversity, which dominated by huminite group, which constitute between 72.08?95.36 vol. %vol of the coal; following by liptinite macerals with range of 5.24 to 15.24 vol. %, and inertinite approximately 2.68% - 8.60 vol.%. The Sajau coal can be separated into three microfacies based on maceral assemblages: (1) high humocolinite coal represents the in the upper section. This facies was derived from herbaceous plant in lower delta plain, (2) moderately humocolinite coal together with high humotelinite which are developed in the upper delta plain in the wet to dry and (3) low humocolinite coal in the lower section of the seams was origin in the fluviatile dominated influence environment (overbank mire). The coal petrological properties, as well as the indices of the coal facies shown low TPI, GI, VI and moderately GI, reveal that the coal was deposited under a rheotrophy - mesotrophy to an ombrogenous hydrology regime in continuously wet limnic to limno-telmatic conditions with occasionally dry season. The coals are a result of deposition of either from the dominantly herbaceous vegetation, mixed with woody trees, forming in three depositional environments from fluviatile dominated environment (overbank mire), upper delta plain and lower delta plain. The varied pollen assemblages which correlate with the palynofloras of bog-forest, fresh water and mangrove affinity suggested that the Sajau coal deposited within a number of mire environments, from fresh water, wet swamp to bog forest as same conclusion with the maceral analysis.