Late Quaternary Stratigraphy and Sedimentology of the Offshore Mahakam Delta, East Kalimantan (Indonesia)
Harry H. Roberts, Johan Sydow, 2003. "Late Quaternary Stratigraphy and Sedimentology of the Offshore Mahakam Delta, East Kalimantan (Indonesia)", Tropical Deltas of Southeast Asia—Sedimentology, Stratigraphy, and Petroleum Geology, F. Hasan Sidi, Dag Nummedal, Patrice Imbert, Herman Darman, Henry W. Posamentier
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Late Quaternary stratigraphy of the shelf over which the modern tropical Mahakam River delta is prograding indicates an alternating dominance of shelf carbonate sediments and siliciclastic deltaic deposits. Control of these major facies shifts is in response to high-frequency cyclic relative sea-level changes. Seismic facies and stratigraphy of the two latest Pleistocene depositional episodes reflect interplay of deltaic and shelf processes as well as accommodation during the last two glacio-eustatic cycles. Over 3000 line km of high-resolution seismic and side-scan sonar data calibrated to 380 bottom samples and 97 vibracores amd piston cores help characterize seismic facies of the Holocene and latest Pleistocene shelf deposits, which have been influenced by a strong north-to-south flowing current from the Makassar Strait. Holocene prodelta sediments are confined to the inner shelf of the northern and central sectors of the delta front, but they are skewed to the south, creating a broad facies tract in the southern sector. The middle and outer parts of the shelf are dominated by mounded topography characterized by both individual and aggregate bioherms. Mound relief varies from a few meters to over 30 m, but the average is about 20 m. This topography is locally accentuated by intervening erosional lows. Cores through the upper 6 m of these features indicate that the mounds are constructed of flakes of the calcareous green alga Halimeda in a matrix of foraminifera-rich terrigenous mud. The Halimeda bioherms are established on a ravinement surface formed during the Pleistocene to Holocene transgression. Inner-shelf bioherms are now being slowly buried by the Holocene advance of the Mahakam delta.
Below the Pleistocene to Holocene ravinement surface are the deposits associated with the preceding relative sea-level fall. Deposits defining this 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. Pleistocene deltaic deposits have prograded to the shelf edge along the entire shelf–slope break. Over 50% of the stratigraphy in the lowstand systems tract is related to aggradation during the lowstand turnaround. The Pleistocene deltaic deposits bury massive carbonate bioherms and aggregates of bioherms that mimic their Holocene counterparts in morphology and scale. A prominent ravinement surface underlies the bioherm facies, and a maximum flooding surface runs through these features.
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It is the objective of this volume to bring to the fore a category of deltas with which many sedimentologists and stratigraphers are, at best, vaguely familiar. It is expected that this volume also will stimulate new research on tropical deltas by highlighting how their facies and stratigraphic architectures differ from mid- and high-latitude ones, by emphasizing their significance to the global sediment budget, and by stressing their uniqueness within a petroleum systems framework. This special publication emphasizes the need for models intrinsic to tropical deltas of Southeast Asia to supplement the more conventional general models currently in vogue, based on past studies of large and small mid-latitude deltas. The papers in this book explore how the combination of these complex factors has shaped deltas in this region. Sedimentological surprises such as distributary channels floored by thick accumulations of fluid mud lend a bit of “mystery” to tropical deltas. We hope that, rather than being merely a summary of tropical deltas, this book may open the door to a new and active phase of sedimentological and stratigraphic research in tropical environments across the globe.