Pleistocene Shelf-to-Basin Depositional Systems, Offshore East Kalimantan, Indonesia: Insights into Deep-Water Slope Channels and Fans
Stratigrapher and Exploration Geologist, Cobalt International Energy, Houston, Texas
3D seismic data show the depositional history of shallow Pleistocene shelf margin, slope and basinal strata in offshore East Kalimantan, Indonesia. Siliciclastic sequences on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. During the last two lowstands of sea level (˜18 and ˜130 ka), coarse siliciclastics were generally not deposited in deep-water environments because lowstand deltas did not prograde over the underlying shelf margin. During the lowstand of sea level that ended at ˜240 ka, deltas prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.
During the late Pleistocene, siliciclastic sediment supply determined the depositional characteristics of the slope. Channel-levee complexes developed on the slope where deltaic sediment supply was large; in contrast, incised valleys/canyons formed on the slope where siliciclastic input was limited. Pleistocene channel-levee complexes can be traced upslope to lowstand deltas associated with the paleo-Mahakam River. In areas with limited sediment supply, rivers and deltas were generally not present on the outer shelf, including areas upslope from incised slope valleys and canyons. Strata on the basin floor downslope of the slope valleys and canyons are dominated by mass-transport complexes, suggesting that slope valleys and canyons formed by mass failures of the slope, not by erosion associated with turbidite sands derived from rivers or deltas.
In the area with limited sediment supply, one small river was present on the shelf margin during the upper Pleistocene, and sediments originating from its lowstand delta filled a pre-existing slope valley/canyon and formed a basin-floor fan. That slope valley/canyon has a lower fill that consists of amalgamated, sinuous channel deposits and an upper fill consisting of a shale-rich, channel-levee complex. The basin-floor fan also has two parts: a lower fan containing broad lobes with relatively continuous reflectors and an upper fan with a shale-rich, sinuous channel-levee complex that prograded over the lower fan and fed sheet-like lobes on the upper, outer fan. These shallow Pleistocene systems serve as analogs for deeper, more poorly imaged reservoir systems.
AAPG Search and Discovery Article #90159©2012 AAPG Foundation Distinguished Lecturer Series 2012-2013.