HR: 08:00h
AN: OS11A-01    [PDF]
TI: Holocene Evolution of the Middle Fly River, Papua New Guinea
AU: * Chappell, J
EM: john.chappell@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AB: The Fly River headwaters descend steeply from high tropical mountains in Papua New Guinea (PNG) and enters a lowland platform of very low relief: 800 km from the sea, the floodplain is only ~18 m above sea level. From this point, river sediment is muddy sand. At Everill Junction, 400 km downstream, the Fly is joined by the Strickland River, which has a larger discharge (~3100 vs 2250 m3s-1) and larger sediment load (~6x107 vs 8x106 t yr-1). The river continues across the lowlands for a further 300 km and enters the Gulf of Papua through a large tide-dominated delta. The Fly floodplain comprises a tract of highstand sediments (HST) deposited during the last 7000 years of relatively stationary sea level, which overlies a transgressive tract (TST) deposited during the Post-glacial sea level rise (PSR). Radiocarbon-dated boreholes show TST sedimentation rates were 5-15 times greater than HST rates, reflecting a change in the fluvial system from trapping to bypassing, driven by changing accommodation and sea level (a change of sediment input is unlikely: despite lower Pleistocene treelines, Pleistocene yields from PNG highland catchments apparently remained fairly constant). The delta apex arguably migrated up the paleo-valley early in TST deposition; towards the PSR culmination about 7000 yrs BP it may have been only 60 km below Everill Junction. Downstream of the junction, Strickland River sediment drove fluvio-deltaic development during the TST and HST, but for a distance of about 80 km above the junction, TST sedimentation only just kept pace with rising base level, and shallow off-river water bodies probably were more extensive than today. However, the system remained fluvially continuous, and the landward limit of sea-level influence at the PSR culmination lay close to the upstream limit of today's floodplain. The skeleton of today's Fly was established at the PSR culmination, including meander-dominated reaches in the middle and upper floodplain. Three factors interacted in development of the HST: progradation of the delta, ponding in the Fly above Everill Junction induced by Strickland-driven sedimentation, and isostatic warping driven by post-glacial rising seas. The first two factors induce HST aggradation whereas isostatic warping gently steepens the basement gradient, relative to a 7000 BP horizontal surface, arguably inducing minor incision. Although there is no HST incision, radiocarbon dating shows that net HST aggradation in the middle Fly is relatively slight, especially in floodplain lagoon and oxbows. The HST river reworks itself through meandering, and much of the catchment-derived sediment apparently passes through, or did so before establishment of Ok Tedi mine increased sediment input several-fold. Comparison with Holocene evolution of the Sepik - a river of similar size and sediment input in northern PNG - suggests that rivers of this type more efficiently transmit their load seawards, once coastal progradation effectively ceases.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting