National Symbol of Papua New Guinea

Preface

The island of New Guinea and its surrounding areas present a natural laboratory for the study of a wide range of environmental conditions affecting margin development. Examples of some of these environments are presented herein, illustrating the diversity of opportunities presented by the New Guinea candidate site for MARGINS.

By virtue of its geographic location and present rates of uplift and denudation, the island of New Guinea is a major source of sediment to the oceans. In general, rivers draining wet-tropical basins are important gateways for terrestrial water, solutes and particles, supplying more than 50% of the world's budgets for these components (Meybeck, 1988; Milliman and Syvitski, 1992). A disproportionate amount of this supply is provided by the islands of the Indo-Pacific archipelago, the six high-standing East Indies islands alone supplying about 25 percent of the fluvial sediment discharged to the global ocean (Milliman et al., 1999). The combined sediment discharge from rivers draining New Guinea alone is estimated to be about 1.5 times that of the Amazon River, although the island has only one-eighth the drainage area. The geologic setting and oceanic processes of New Guinea that control the fate of its discharge represent important contrasts to those processes operating near the large point sources characterizing other regions of fluvial discharge.

Although high siliciclastic fluxes characterize New Guinea, sedimentary deposits rich in calcium carbonate surround the island. These carbonate environments occur ubiquitously beyond fluvial influence, where the fluxes are small. As an example, coral reefs, which are unable to survive in turbid conditions, characterize much of the New Guinea shelf where terrigenous influx is small, resulting in mixed siliciclastic-carbonate depositional environments (e.g., Harris et al., 1993) which are well represented in rocks formed in ancient tropical environments. Changes in the processes that supply sediment from land or distribute it in the ocean can affect the balance between muddy seabeds and reefs, thus altering the spatial distribution of both substrates.

Some of the highlights of New Guinea as a MARGINS candidate site include:

• Massive sediment inputs resulting in large, measurable signals, and representing a sedimentologically important physiographic region of Earth - wet tropical environments.
• Presently active, and hence measurable, sedimentary processes and transfers from hillslopes to channels, from channels to floodplains, and from river to coastal, shelf, slope and basin environments (i.e., source to sink).
• Reasonably sized and constrained dispersal systems allowing quantification of sediment budgets, and providing the opportunity to contrast point-source and line-source systems.
• High-Resolution sedimentary record resulting from high inputs and, in some areas, tectonically produced accommodation.
• Close interplay between carbonate and siliciclastic environments.
• Narrow, active margins on the north side of the island and broad passive margins to the south, offering both high- and low-stand analogs.
• Ongoing Australian (e.g., TROPICS), Indonesian, and PNG studies presenting opportunities for collaboration, and assistance with logistics.

As described in example sections, recent studies of New Guinea and surrounding areas provide adequate background information to propose focused studies addressing key issues for MARGINS.
 
 

Geological Conditions for New Guinea Margin Sedimentation

The rocks and topography of the Central Range of Papua New Guinea are the product of ~25 million years of collision between the northern margin of the Australian continental plate (including New Guinea) and the Pacific Plate (Pigram and Davies, 1987). Convergence rates from GPS measurements across the easternmost margin near the Huon Peninsula vary from 20 to 50 mm/year (Tregoning et al., 1998), resulting in crustal thickening and mountain building. Integrated over geologic time, such convergence has produced a stack of folds and thrusts (Abers and McCaffery, 1988), which are thought to be propagating southward (Pigram et al., 1989), consistent with accepted models for fold- and thrust-belt evolution (Boyer and Elliot, 1982; Davis et al., 1983). Deformed oceanic and continental rocks form the Mobile belt, the older central part of the New Guinea orogen whose initial uplift is recorded by late Oligocene sediments in a foredeep (Pigram et al., 1989). Fission-track data indicate that this central portion is eroding at long-term rates of ~0.2 to 2 mm/year (Crowhurst et al., 1996), and seismic data indicate minimum left-lateral slip rates of 5 to 25 mm/year along large strike-slip faults (Abers and McCaffrey, 1988) that have largely dismembered pre-existing structures. Propagation of deformation further southward along with infilling of the foredeep is recorded by the first appearance of siliciclastic sediments in the Coral Sea during the Mid-Miocene (Andrews et al., 1975). Sediment cores from the Gulf of Papua indicate an acceleration of subsidence from ~10-50 meters per million years to 65-330 meters per million years entering the Pliocene, accompanied by the predominance of siliciclastic sediment over carbonates (Wang and Stein, 1992). The source for these sediments is the Papuan fold and thrust belt, a stack of folds and thrust sheets of Mesozoic and Cenozoic limestone and siliciclastic rocks initiating between 5 and 6 million years ago as recorded by stratigraphy (Haig and Medd, 1996) and apatite fission-track data (Hill and Gleadow, 1989). These structures are the result of ~100 km of crustal shortening (Hobson, 1986) and their local geometry determines regions of high rock uplift rates whose steep topographies are primary sources for sediment. In the cores of large anticlines from this belt, long-term erosion rates are estimated to be 0.7 to 1 mm/year using apatite fission-track data from Hill and Gleadow (1989).

The uplift of New Guinea has created elevations on the island that locally exceed 4000 m, and combined with the wet-tropical setting result in annual sediment yields greater than 1000 t/km² (Chappell, 1993) and hence massive sediment discharge. A number of canyons and the major Pandora Trough descend from the shelf to the Coral Sea floor. The northeast coast of New Guinea is part of the South Bismarck plate, which is undergoing arc-continent collision as the Australian plate moves northward (Honza, et al., 1987). The island is undergoing rapid uplift with areas in the east (especially the Huon peninsula) rising at rates of 3-10 mm/y (Chappell, 1974; Crook, 1989); thus, the northeast shelf is narrow and the margin is steep—dropping to over 1000 m depth within 20 km or less of the shoreline.

The major rivers (Fly, Sepik and Mamberamo) all have significant lowland floodplain environment where sediment transfer processes occur and trap much of the coarse sediment.

One apparently important sink is the mangrove forests that rim the New Guinea coast, especially in the Gulf of Papua (Robertson et al., 1991). Sedimentary deposits in the mangrove shorelines of the Gulf have a major effect on the particulate terrestrial carbon budget (Robertson and Alongi, 1995), and probably are a significant sink for lithogenic sediment as well. The microtidal conditions on the northeast coast restrict development of mangrove shorelines, limiting its importance in trapping sediment.

The broad southern continental shelf is another potential sink for New Guinea sediment. Approximately 40% of the modern Fly sediment discharge, for example, accumulates on the shelf as a subaqueous delta (Harris et al., 1993). Most of the remainder is transported eastward forming a mid-shelf deposit of muddy sediment. The Coral Sea Coastal Current sweeps the Gulf of Papua in a clockwise fashion, and, although details of circulation associated with river effluent are complex (Wolanski et al., 1994), sediment transport apparently also moves in a general clockwise motion (Harris et al., 1993; Alongi and Robertson, 1995). This explains the presence of relict carbonate substrates in southern and western regions and the suggested off-shelf escape of sediment down the Pandora Trough (Brunskill et al., 1995).

Sediment accumulation on the northeast shelf of New Guinea (Sepik region and Huon Peninsula), is probably less important, because it is very narrow. Previous studies (Chappell, 1993) have suggested that the valley of the Sepik River was a brackish coastal embayment trapping sediment until it was filled (and uplifted) several thousand years ago. Since then, terrigenous sediment has escaped onto and bypassed the shelf, burying outer shelf-upper slope coral reefs in the process. Westward equatorial currents that sweep the north side of New Guinea disperse fluvial sediment somewhat westward of river mouths and beyond the shelf break (Fine et al., 1994).

Another sink for New Guinea sediment is the continental slope and beyond. On the northeast coast and in the Gulf of Papua, submarine trenches and canyons/troughs appear filled with sediment (Hamilton, 1979; Winterer, 1970). In some cases rivers discharge directly into submarine canyons, and observations of divergent sediment plumes extending onto the slope have been reported (Kineke et al., in press). Deltaic deposits have been observed by side-scan sonar surveys to depths > 1400 m on the south side of the Huon peninsula (Liu et al., 1995), and similar deposits are probable on the north side. These observations strongly imply that terrestrial sediment is escaping beyond the shelf break. However, they give little information regarding processes of deposition, age of the deposits, rates of accumulation or volumes of sediment.