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Mixed Carbonates and Siliciclastics North of the Mahakam Delta, Offshore East Kalimantan, Indonesia

Abstract

For the last 7 million years, carbonates have been mixed with siliciclastics north of the Mahakam delta, offshore East Kalimantan, Indonesia. The distribution of carbonates is controlled by the location of deltas, currents transporting siliciclastics, rates of sea level rise, and depositional bathymetry. At the present time, modern carbonates are deposited locally north of the delta while large amounts of sand and shale are coming out of the delta. In the late Pleistocene, carbonate mounds and shelf margin carbonates grew during transgressions when siliciclastic shorelines moved landward. Mounded carbonates preferentially occur on the upthrown side of faults, while shelf margin carbonates grew on underling siliciclastic or carbonate shelf margins. Thin shales accumulated in lows between carbonate mounds. During highstands of sea level, siliciclastics prograded basinward across the shelf. Many carbonate mounds drowned and were covered with siliciclastics. At the shelf margin, carbonates prograded landward during highstands of sea-level. During the last 7 m.y., shelf margins have been generally backstepping landward north of the Mahakam delta because of rapid subsidence and currents transporting most deltaic clays to the south. Carbonates repeatedly grew during transgressions. During the latest Miocene and Pliocene, thick very elongate carbonates grew on the underlying siliciclastic shelf margins while thinner roughly circular mounds grew on depositional and structural highs of the shelf interior. Most of the carbonates are covered by highstand siliciclastics. The shelf margin and mounded carbonates are mainly lime grainstone and boundstones with good porosity. Unfortunately, shales covering carbonates are downlapping packages that were generally not effective seals.