Submarine Rivers of Mud and Sand: Channels Dispersing Sediment Across the Fly River Clinoform

The subaqueous delta of the Fly River has a clinoform structure prograding onto transgressive shelf deposits. The clinoform contains numerous channels, which appear to have ancestral origins. Perhaps they were subaerial stream valleys during lower stands of sea level, but they presently can be buried partially or completely by modern and late Holocene sediment deposits - especially toward shore (i.e., beneath topset deposits). Some of the larger channels extend from the topset across the foreset region, and seem to be linked to distributary channels of the Fly delta. Some extend onto the bottomset and outer shelf. Others can be recognized only in subbottom profiles. The channels can be dramatic features with steep walls rising 50 m, and can contain 20 m or more of infilled sediment. Most extant channels (i.e., still morphological depressions) appear to be active conduits for rapid transport seaward of sediment and carbon. The boundary-layer regions of the water column within channels contain high concentrations of sediment (>1 g/l) and the seabed is relatively unconsolidated and accumulating rapidly. Sedimentary structures found in the thalwegs of channels indicate graded deposits resulting from density flows, which may be shelf turbidity currents. The channelized morphology and resulting mechanisms for sediment dispersal differ from other portions of the clinoform structure in the Gulf of Papua. They also differ from modern shelf clinoforms elsewhere in the world, although they probably have ancient analogs (Cretaceous Seaway of North America).