Inner-Shelf Sedimentation in the Gulf of Papua, New Guinea: A Mud-Rich Shallow Shelf Setting

Abstract

Each year, approximately 4 × 108 tons of sediment from multiple river sources is discharged to the Gulf of Papua. This large supply enters a system where physical energetics vary seasonally and accumulation of mud and sand occurs on the inner shelf. Sediment supplied by the largest of these fluvial sources, the Fly River, is advected northeastward by the prevailing currents, where it joins with sediment supplied from other rivers. Three sedimentary deposits are identified along the 20-meter isobath: (1) slowly accumulating (∼0.6 cm y−1) massive sandy mud in the west; (2) interbedded mud and sand with higher accumulation rates (1.9–2.7 cm y−1) in the central region; and (3) slowly accumulating (∼0.7 cm y−1) bioturbated muds in the east. Trends in the preserved strata reflect the interaction of fluvial, tidal, and wave processes. In the tidally energetic western region, massive sandy units dominate, whereas interbeds of mud and sand are observed in the central region. The interbeds (centimeters thick) are hypothesized to be related to seasonal changes in the hydrodynamics of the Gulf: deposition and erosion of sediment associated with changes between monsoon and trade-wind conditions. On finer scales (millimeters), the sedimentary structures observed in the Gulf are typical of those found in tide-dominated deltaic settings. Bioturbated sedimentary structures characterize the eastern Gulf, as a result of slow accumulation by fine material and less energetic physical processes. Several important relationships are recognized for the Gulf of Papua: (1) the large supply of fine-grained sediment allows it to accumulate at significant rates on the inner shelf; (2) quiescent monsoon conditions probably aid sediment retention on the inner shelf; and (3) coalescence of supply from multiple river sources causes maximum accumulation rates to occur in the central Gulf of Papua.