Sedimentology and Stratigraphy of a Tide-Dominated, Foreland-Basin Delta (Fly River, Papua New Guinea)
Robert W. Dalrymple, Elaine K. Baker, Peter T. Harris, Michael G. Hughes, 2003. "Sedimentology and Stratigraphy of a Tide-Dominated, Foreland-Basin Delta (Fly River, Papua New Guinea)", 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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The delta of the Fly River is tide dominated, both morphologically and sedimentologically: the spring tidal range reaches 5 m, with near-bed tidal currents that commonly exceed 1 m/s, whereas wave influence is minimal, except during the monsoon period, when winds blow onshore. The delta plain displays a classic funnel-shaped geometry, with three main distributaries that flare seaward from a common bifurcation point 110 km inland. Of these, the southernmost is the main route for sediment export; the northernmost channel is effectively abandoned at present and is experiencing widespread erosion by tidal currents. Despite the net export of mud and sand, mutually evasive tidal-current patterns have created a series of elongate tidal bars within both active and abandoned distributary channels and in the distributary-mouth-bar area. The later is a zone of bedload convergence, with net seaward transport on its landward side and net landward transport on its seaward flank. This limits the offshore movement of sand.
The deposits of the Fly delta are dominated by mud, because the river flows along the low-gradient axis of the foreland basin before reaching the sea, causing deposition of most sediment coarser than fine to very fine sand farther inland. Distributary-channel bases are floored by a thin unit of cross-bedded and rippled sand and mud-pebble conglomerate. These are typically overlain abruptly by mud deposits formed by dense fluid-mud bodies that form in the channel bottoms during spring tides. The mud layers in this facies are anomalously thick (commonly > 1 cm), and channel-floor deposits are characterized by interbedding of the coarsest and finest sediments. Above these muds, the sediments are pervasively heterolithic and show a net upward coarsening to about the mid-depth level on the tidal bars because of the thinning of the mud layers. These bars may contain 50% (or more) mud and display lateral-accretion bedding. Bioturbation is scarce to absent. The sediments then fine upward into the intertidal zone. Clear indications of a tidal origin for the heterolithic stratification are relatively uncommon, although tidal rhythmites are present locally, including within active channels. The mouth-bar deposits are predominantly sand and also contain lateral-accretion bedding. The delta-front facies are heterolithic, with both millimeter- and decimeter-thick sand/mud alternations, all with a limited degree of bioturbation. The prodelta consists of biologically homogenized mud. The stratigraphic organization of the deposits reflects the fact that the delta plain aggraded vertically during the last part of the postglacial sea-level rise, producing a complex stack of channel deposits, followed by progradation of the mouth bars by as much as 40–50 km.
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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.