--> ABSTRACT: Late Quaternary Stratigraphy and Sedimentology of the Offshore Mahakam Delta, East Kalimantan (Indonesia), by Harry H. Roberts and Johan Sydow; #90906(2001)

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Harry H. Roberts1, Johan Sydow2

(1) Louisiana State University, Baton Rouge, LA
(2) BP Amoco, Houston, TX

ABSTRACT: Late Quaternary Stratigraphy and Sedimentology of the Offshore Mahakam Delta, East Kalimantan (Indonesia)

Late Quaternary shelf stratigraphy, over which the modern Mahakam River delta is prograding, indicates alternating dominance of shelf carbonate sediments and siliciclastic deltaic deposits in response to eustatic sealevel changes. Shelf deposits have been influenced by strong north-to-south flowing currents from Makassar Strait. Holocene prodelta sediments have been confined to the inner shelf in the northern and central sectors of the delta front, but are skewed to the south creating a broad facies tract. The middle-outer shelf is dominated by mounded topography, individual and aggregate bioherms. Mound relief, accentuated by intervening erosional lows, varies from a few to over 30 m (avg. 20 m). The mounds are constructed Halimeda in foraminifera-rich terrigenous mud, established on a transgressive ravinement surface. Inner shelf bioherms are being slowly buried by the Holocene advance of the Mahakam delta. Below the Holocene-Pleistocene transgressive surface, deposits defining a falling-stage systems tract and associated lowstand systems tract consist of entrenched fluvial networks, incised alluvial valley fill, aggraded delta plain and platform deposits, and prograded delta lobes. Over 50% of the stratigraphy in the lowstand systems tract is related to aggradation during the lowstand turnaround. Deltaic deposits bury massive carbonate bioherms resting on a prominent ravinement. A maximum flooding surface runs through the bioherms. Deltas have prograded to the shelf edge in all sectors. Steep shelf edge slopes from aggredation of carbonates, coupled with active regional tectonics, promotes faulting and mass wasting on many scales. Numerous canyons occupy the present slope, suggesting sediment by-passing to deepwater depositional sites.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado