Sequence Stratigraphy and Growth of Shelfal Carbonates in a Deltaic Province, Kutai Basin, Offshore East Kalimantan, Indonesia
Arthur Saller, Sena W. Reksalegora, Philip Bassant, 2010. "Sequence Stratigraphy and Growth of Shelfal Carbonates in a Deltaic Province, Kutai Basin, Offshore East Kalimantan, Indonesia", Cenozoic Carbonate Systems of Australasia, William A. Morgan, Annette D. George, Paul M. (Mitch) Harris, Julie A. Kupecz, J. F. (Rick) Sarg
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The Kutai Basin of East Kalimantan, Indonesia, has been dominated by deltaic deposition during the Neogene, but carbonate buildups are also common. Carbonate—siliciclastic interactions were studied in 3D seismic and well data in the upper Pleistocene and uppermost Miocene to Pliocene, offshore north of the Mahakam delta. Upper Pleistocene carbonates accumulated over the siliciclastic shelf margins during ∼ 110 kyr eustatic cycles. U-Th dating supports deposition of ∼ 77–100 m of carbonate on the shelf margin during each of the last two eustatic cycles (18–130 ka; 130–240 ka). This sedimentation rate of 70–90 m/100 kyr is higher than other documented carbonate systems for that length of time. Pleistocene shelf-margin carbonates contain a lower transgressive ridge (TST) overlain by landward-dipping clinoforms interpreted as the highstand systems tract (HST) and capped by subaerial exposure surfaces formed during falls of eustatic sea level. Coral-rich packstone and grainstone dominate the upper Pleistocene shelf-margin buildups, though partially dolomitized wackestones and siliciclastic mudstones are associated with maximum flooding surfaces. The uppermost Pleistocene shelf-margin buildups are ∼ 0.5 km wide and occur along a 50 km trend. Siliciclastic strata prograded across the inner and middle-shelf during high and falling sea level. Uppermost Pleistocene buildups grew on siliciclastic strata on the middle shelf during the last eustatic sea-level rise (after 18 ka). The middle-shelf buildups are roughly circular to elongate in map view, vary from 5 km to less than 100 m across, are commonly 30–50 m thick, and preferentially grew on the upthrown side of faults, though they are observed in many other places.
Carbonates were deposited within two sequences in the uppermost Miocene and lower Pliocene. Shelf-margin carbonate buildups grew above siliciclastic shelf margins during transgressions, and then those buildups were buried by basinward-prograding siliciclastics during the following highstand. Mio-Pliocene carbonate buildups on shelf margin are commonly 255 m thick, 5 km long, 1 km wide, and composed largely of bioclastic packstone and grainstone. The carbonate buildups are thicker with more relief and steeper margins to the north, away from the main influx of siliciclastics. High-relief buildups to the north contain multiple stacked carbonate sequences that were not covered at earlier stages by siliciclastic mud, in contrast to thinner buildups in the south. Although porous, most of the Mio-Pliocene shelf-margin buildups are filled with water, probably because overlying downlapping siliciclastics do not provide a robust seal. Middle-shelf buildups, equivalent in age to Mio-Pliocene shelf-margin carbonates, also grew during transgressions, but the middle-shelf buildups are thinner (15 m) because they had less accommodation space, and because the middle-shelf spent more time with siliciclastics deposition. Biostratigraphic dating suggests that one of these Mio-Pliocene carbonate—siliciclastic sequences is ∼ 0.6 Myr in duration, resulting in a carbonate accumulation rate of ∼ 425 m/Myr.
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The Cenozoic carbonate systems of Australasia are the product of a diverse assortment of depositional and post-depositional processes, reflecting the interplay of eustasy, tectonics (both plate and local scale), climate, and evolutionary trends that influenced their initiation and development. These systems, which comprise both land-attached and isolated platforms, were initiated in a wide variety of tectonic settings (including rift, pas-sive margin, and arc-related) and under warm and cool-water conditions where, locally, siliciclastic input af-fected their development. The lithofacies, biofacies, growth morphology, diagenesis, and hydrocarbon reser-voir potential of these systems are products of these varying influences. The studies reported in this volume range from syntheses of tectonic and depositional factors influencing carbonate deposition and controls on reservoir formation and petroleum system development, to local studies from the South China Sea, Indonesia, Kalimantan, Malaysia, the Marion Plateau, the Philippines, Western Australia, and New Caledonia that incor-porate outcrop and subsurface data, including 3-D seismic imaging of carbonate platforms and facies, to un-derstand the interplay of factors affecting the development of these systems under widely differing circum-stances. This volume will be of importance to geoscientists interested in the variability of Cenozoic carbonate systems and the factors that controlled their formation, and to those wanting to understand the range of po-tential hydrocarbon reservoirs discovered in these carbonates and the events that led to favorable reservoir and trap development.