DALRYMPLE, Robert W., Department of Geological Sciences, Queen's University, Kingston, ON, K7L 3N6 Canada,dalrymple@geol.queensu.ca; BAKER, Elaine K. and HUGHES, Michael, Ocean Sciences Institute, University of Sydney, Sydney, NSW 2006, Australia, ebaker@extro.ucc.su.oz.au and hughes@beryl.es.su.oz.au; HARRIS, Peter T., Antarctic CRC, University of Tasmania, Hobart, TAS 7001, Australia, p.harris@antarc.utas.edu.au

The Fly River delta (tidal range 3.6-5 m; significant wave height < 2 m) displays a funnel-shaped set of distributaries which extend more than 110 km seaward from the point of bifurcation. At their seaward end, they occupy 90 km of the Gulf of Papua coast. They are 2-10 km wide and have high-tide depths of 5-15 m. Elongate tidal bars subdivide these channels into mutually evasive, ebb-dominant (continuous) and flood- dominant (discontinuous) portions. Because of preferential deposition of coarser sand further inland, most of the sediment in the delta is fine to very fine sand, silt and clay. Suspended-sediment concentrations are very high and fluid muds occur widely. Thus, channel floors are underlain mainly by rhythmically interlaminated sand and mud, despite near-bed, spring-tide current speeds > 1 m/s. Single-tide mud layers may exceed 1 cm in thickness. Dunes do occur, but are rare. Tidal bars and point bars typically coarsen upward to about the mid-channel depth; above this they fine upward. As a result, the identification of channel facies may be difficult. Channel-floor erosion in inactive distributaries creates thin lags of sand and mud pebbles, but erosion in active channels places muddy rhythmites on older muddy rhythmites. The distributary mouth bar (DMB) consists of shore-normal tidal bars and is shallower on average than the distributaries. The sediments consist of well sorted sand due to resuspension of mud by waves. Thus, the DMB deposits are coarser-grained than those of the channels which feed it. The tidal bars of the DMB are sharply based, presumably due to lateral (along-coast) migration.