Constraining upland erodibility and marine deposition: source-to-sink sediment transfer in the Gulf of Papua

The Papua New Guinean (PNG) landscape is young and rapidly evolving, with high uplift rates and precipitation rates that exceed 10 m/yr producing high erosion rates. Consequently, the Gulf of Papua, an active foreland basin in southern PNG, receives a large terrigenous sediment flux of ~384 Mt/yr. The fresh and tectonically active Papuan landscape makes the Gulf an ideal location for studying tectonic and climatic impacts on source-to-sink surface processes and basin stratigraphy. This thesis aims to (1) constrain a range of parameters, including tectonic uplift rates, upland erodibility and marine sediment distribution, in order to calibrate a numerical model simulating the Late Quaternary landscape evolution of the Gulf of Papua foreland basin. In addition, this study aims to (2) use the calibrated model to identify temporal and spatial patterns in erosion, fluvial sediment transportation and sediment distribution and composition in the basin. This thesis uses the open-source landscape evolution code Badlands to simulate surface processes in PNG. However, to simulate source-to-sink sediment transfer in the Gulf of Papua it was first necessary to constrain a range of parameters with tectonic and geological constraints specific to the basin. The magnitude, timing and spatial variations in uplift across the Papuan Peninsula were constrained using the present-day elevation of remnants of late Miocene volcanics that were deposited near sea level during the Miocene and have subsequently been uplifted. To constrain upland erodibilities, a series of simulations of the landscape evolution of the Gulf of Papua were run in Badlands, testing the impact of a range of erodibility coefficients on the sediment flux into the Gulf. The sediment loads of simulated rivers were compared to the present-day natural sediment loads of the Fly, Kikori and Purari Rivers to find both an averaged erodibility coefficient for the Gulf of Papua catchment and lithology-specific coefficients. Comparing simulation results to observed sediment accumulation rates in the deep-sea Gulf assisted in constraining the distribution of sediment in the marine environment in Badlands. The calibrated model was then used to simulate the sediment routing system in the Gulf of Papua catchment. Results show an increase in the Plio-Quaternary uplift rate from negligible at sea level along the south-western coast of the Papuan Peninsula to > 600 m/Myr in the peaks of the Owen Stanley Range. Comparisons with present-day fluvial sediment loads constrain the averaged erodibility coefficient in Badlands for the Gulf of Papua to within the range 5.53 – 7.41 x 10−6 yr−1, with an estimated averaged erodibility coefficient of ~7 x 10−6 yr−1 for a resolution of 500 m. Erodibility coefficients for specific lithologies range from ~6.5 x 10−6 yr−1 for metamorphics and granites to ~13.0 x 10−6 yr−1 for soft sediments. Subsequently, the marine accommodation variable was constrained to within the range 0.005 − 0.009, so that the largest depocentres in the Gulf are on the continental slope. A sensitivity analysis reveals that the simulated sediment flux into the Gulf of Papua is more sensitive to changes in precipitation rate than to changes in the uplift rate. These findings permit the simulation of sediment transfer processes in the Gulf of Papua during the Late Quaternary, informing our understanding of the catchment’s surface evolution and the stratigraphy of the deep sea basin. Simulation results suggest that sediment delivered to the eastern Gulf is largely composed of metamorphics and volcanics sourced from the Owen Stanley Range. Changes in sea level have a limited impact on sediment deposition in the east as the narrow, steep continental shelf allows rivers to deliver sediment directly into the deep-sea depocentres, which are compositionally distinct from one another. The mature rivers in the west of the catchment transfer sediment derived from soft sediment, mafics, metamorphics and granites in the Papuan Orogen to the western Gulf of Papua. Marine transgression and highstand in the last 14 Kyr have submerged the wide continental shelf in the western Gulf and prevented rivers from delivering sediment directly into the deep sea. Thus, there has been increased sediment accumulation on the submerged shelf. These results highlight basin morphology, the surface distribution of lithologies and the glacio-eustatic cycle as major controls on the sediment distribution and composition in the marine environment