THE MORPHOLOGY OF SUBMARINE CHANNELS ON THE FLY RIVER CLINOFORM AND THEIR ROLE IN SEDIMENTATION

Bathymetric surveys over the delta front of the Fly River, Papua New Guinea have revealed broad channels cutting across the topset and foreset of the Fly River clinoform. Their locations suggest that they are actively participating in sediment transfer from the estuarine environment of the Fly River delta to the zone of sediment accumulation on the foreset of the clinoform. These features were investigated by a coordinated study that included detailed high-resolution (300 kHz) multibeam mapping, seabed coring, and boundary-layer instrument deployment. Cores were x-rayed to reveal sedimentary structure and additional laboratory analysis included measurement of grain size and ²¹⁰Pb activity.

All data indicate that the channels compose a dynamic sedimentary environment. X-radiography shows that cores are dominated by physical stratification. The absence of biological mixing indicates that the seabed is too unstable to allow colonization by benthos. Below ~150 cm in the cores, ²¹⁰Pb activity corresponds to background values for the Gulf of Papua. Above ~150 cm, activities abruptly increase to a relatively uniform level of excess activity that extends to the surface. This type of profile indicates a period of non-deposition or erosion followed by relatively rapid sedimentation of the upper 150 cm.

Sediments within 30 cm of the seabed surface exhibit variable activities due to reworking by currents. Within the channel, currents are dominantly tidal during the quiescent monsoon season, and are focused along channel. Currents act to transport sediment down-channel during spring tides. Multibeam mapping of the delta front reveals along-shelf heterogeneity for the Fly River delta. The area directly seaward of the river mouth has a higher concentration of channels than the area to the northeast of the river mouth, which has a smoother morphology. In the channelized region, areas between channels are also smooth, and have a different pattern of accumulation than the channels themselves.

The origin of the channels is unclear, but potentially could provide insight to erosional processes (e.g., gravity flows) occurring on an otherwise accreting clinoform. Similarly, the locations of the channels are interesting and might be related to distributary channels on the Fly delta. Continuing studies will help address these issues.