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Sequence Stratigraphy of an Oligocene Carbonate Shelf, Central Kalimantan, Indonesia

SALLER, ARTHUR, and RICHARD ARMIN, Unocal S & T, Brea, CA, LA ODE ICHRAM, Unocal Indonesia, Jakarta, Indonesia, and CHARLOTTE GLENN-SULLIVAN, University of Houston, Houston, TX

Interpretations of Oligocene shelfal limestones from Central Kalimantan, Indonesia, suggest caution in predicting sea-level lowstands from seismic reflector patterns or published sea-level curves. Three major depositional sequences, each 200-400 m thick, were delineated in outcrops and seismic lines: late Eocene to early Oligocene (34-38 Ma), middle Oligocene (29.7-32 Ma), and early late Oligocene (28-29.7 Ma). The lowest sequence is mainly shale with thin sandstones and limestones (large-foram wackestone). The middle and upper sequences are carbonate with transgressive systems tracts (TSTs) overlain by highstand systems tracts (HSTs). TSTs contain large-foram wackestone-packstones and coral wackestone-packstones. HSTs are characterized by (1) shale and carbonate debris flows deposite on the lower slope, (2) argillaceous large-foram wackestones on the upper slope, (3) discontinuous coral wackestones and boundstones on the shelf margin, (4) bioclastic packstones and grainstones on backreef flats and shelf-margin shoals, and (5) branching-coral and foraminiferal wackestones in the lagoon. Bases of sequences are characterized by transgression and onlap. Deepening and/or drowning of the carbonate shelf occurred at the top of the middle and upper sequences. Internally, the two carbonate sequences have vertically building shelf margins with landward-dipping, shingled clinoforms in the lagoon indicating landward progradation of shallow carbonate environments from the shelf margin. Basinal strata that apparently onlap the middle and upper carbonate shelf margins might be mis nterpreted as lowstand deposits, although regional studies indicate they are prodelta sediments baselapping against the shelf. Shallowing and subaerial exposure of the carbonates might be expected during the large mid-Oligocene (29.5-30 Ma) sea-level drop of Haq et al. (1987), instead of the observed deepening and local drowning.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)