LATE QUATERNARY DELTAIC AND CARBONATE SEDIMENTATION IN THE GULF-OF-PAPUA FORELAND BASIN - RESPONSE TO SEA-LEVEL CHANGE

The rivers that drain the wet, mountainous island of New Guinea discha rge about 1.5 billion tonnes/yr of sediments into the adjacent seas, i ncluding the foreland basin between New Guinea and Australia, Despite this huge sediment input, there appears to have been only limited depo sition in the Gulf of Papua during the (Holocene) postglacial rise in sea level, Seismic and core data indicate that the transgressive syste ms tract in the Gulf of Papua is thin and patchy, It is confined to re gions within and north of an incised, east west-trending shelf-valley system, Of the possible explanations for the absence of a significant transgressive systems tract, inland storage and along- and off shelf t ransport of the sediment are of greatest significance. Reef growth up to the latitude of the east west-trending incised-valley system in the southern Gulf of Papua is considered to have been facilitated by a no rthward flowing coastal boundary current, the Coral Sea Coastal Curren t. This current now sweeps turbid, brackish waters and terrigenous sed iments discharged by the rivers northwards away from the reefs. An obs erved northward offset in transgressive sediments in relation to the a xis of the shelf valleys suggests that such a northward-flowing shelf current operated during the late Pleistocene and early Holocene, The n orthern limit of the Great Barrier Reef could thus be controlled by th e balance between fluvial sediment supply and northward advection of s uspended sediment by the Coral Sea Coastal Current, This current may a lso be important in maintaining a supply of clear water to the eastern Gulf of Papua, thus enabling photosynthesis and the flourishing of ca lcareous algae (Halimeda) bioherms or biostromes at depths of up to 10 0 m over much of the middle and outer shelf, directly offshore of the modern Fly Delta, These carbonate sediments represent the exposed maxi mum flooding surface and condensed section. Modern highstand delta dep osits have begun to prograde over this layer on the inner shelf.