Abstract

In offshore East Kalimantan, Indonesia, three-dimensional seismic reflectors can be traced downslope from a lowstand delta to a basin-floor fan, giving insight into depositional processes controlling the distribution of sands that serve as hydrocarbon reservoirs in many ancient deep-water settings. The studied interval includes the last three Pleistocene cycles (10–330 ka; each ∼110 k.y. in duration). Cycles on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. Progradational packages are separated by parallel reflectors and carbonate buildups of the transgressive systems tracts. During the last two lowstands of sea level (∼18 and ∼130 ka), coarse clastics were not deposited in deep-water environments because growth faults and regional subsidence prevented lowstand deltas from reaching the slope. During the lowstand of sea level that ended at about 240 ka, a delta prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.

Strata on the slope and basin floor show how a deep-water depositional system evolved during a single cycle of eustatic sea level. A canyon on the slope connects the 240-ka lowstand delta to a coeval basin-floor fan. The canyon has a sinuous, bipartite fill that consists of a lower, amalgamated channel complex and an upper channel-levee complex. The basin-floor fan formed at the toe of the slope also has two parts. The stratigraphically lower part of the basin-floor fan has broad lobes with relatively continuous reflectors. The stratigraphically higher part has a sinuous channel-levee complex that prograded over the lower fan and fed sheetlike lobes on the outermost fan. The amalgamated channel fills on the slope and sheetlike lobes on the basin-floor fan have moderate- to high-amplitude reflectors and are inferred to represent sand-rich, early lowstand deposits. The channel-levee complexes on the slope and basin floor are dominated by low-amplitude reflectors and are inferred to be mud-rich strata deposited during the late lowstand. Unlike classic sequence-stratigraphic models, these lowstand strata do not onlap the slope; instead, deep-water clastics extend from the last clinoforms of lowstand deltas. In this system, lowstand deltas determined when and where sand-rich sediments entered preexisting canyons on the slope to feed basin-floor fans.

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