A PRELIMINARY-STUDY OF SEDIMENTATION IN THE TIDALLY DOMINATED FLY RIVER DELTA, GULF OF PAPUA

High resolution seismic profiles, surface sediment samples and cores document sedimentary facies of the tidally dominated Fly River Delta. Tidal currents dominate in the transport of sandy sediments throughout the Fly Estuary. On the delta front (5-17 m water depth) surface waves generated by southeasterly trade winds rework the muds and sands to winnow out the fine grained sediments, from March to November. During the northwest monsoon (December-March), minimal surface wave activity results in the deposition of a mud drape, resulting in seasonal sand-mud interbeds (varves). Prodelta deposits extend below this depth from 17 to 45 m and contain massively bedded muds accreting at a mean rate 4 cm year-1. A facies model for the deltaic sequence is presented and employed to derive a preliminary sediment budget for the delta. The budget demonstrates that of the 85 million tonnes year-1 of sediment discharged by the Fly, about 47 million tonnes year-1 is deposited in the delta area adjacent to the months of the Fly River and less than 2% is deposited in Torres Strait to the south. Suspension load transport removes sediments northeastwards into the Gulf of Papua and westwards along the coast, whereas fluid muds may remove sediments eastwards onto the shelf and, possibly, to the adjacent Coral Sea Basin. The budget also demonstrates that a approximately 38 m vertical section, encompassing distal delta, prodelta and delta front facies. is deposited in about 1 100 to 1600 years. The application of the facies model in interpreting other tidally dominated deltaic deposits is discussed.