Abstract 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.

ABSTRACT In the Kutai Basin, regional compression reactivated basement extensional faults, inverting the Paleogene depocenters as anticlines that are often flanked on one side by basement thrusts. Over most of the basin, the overlying Neogene section is detached near the top of an overpressured zone and deformed as a thin-skinned fold-thrust belt. Fieldwork and hydrocarbon explorationonthe northern marginof the Kutai Basin, where the sedimentary section is relatively thin compared with the basin center, has provided data in an imaging window where both structural deformation styles can be observed. We contend that even in the deeper parts of the basin center, basement-involved inversion beneath the overpressure zone has influenced the shallower structures that are present in the Mahakam Delta depocenter. In the Mahakam Delta depo-center, the subsequent response to inversion of the Paleogene rift section was controlled in part by heterogeneity in the shallow section, including structures formed through syndepositional loading, delta progradation, normal faults, and marked facies changes. It is proposed that the structural model derived from study of the northern area is applicable to all of the Kutai Basin, including the basin depocenter, an area where only the thin-skinned deformation can be imaged because of the thickness of the Neogene section.