Fluvial and oceanographic controls on clinoform architecture in the Gulf of Papua

Abstract:

Receiving sediment input from the large-floodplain Fly River and small mountainous rivers, the Gulf of Papua (GOP) is an ideal environment to study how clinoforms record the response of dispersal systems to sea level rise. Contributions from the Fly River and small mountainous rivers to the clinoform have varied due to differing responses to rising sea levels since the last glacial maximum. Near-bed currents that advect sediment to the northeast further complicate this signal. Pairing geophysical and geochemical data from the 2004 NSF MARGINS Source-to-Sink program, we imaged clinoform architecture with CHIRP seismic profiles, identified clinoform sediment provenance, and constrained depositional timing with radiocarbon dates. Sediment provenance can be identified from higher illite:smectite and quartz:feldspar ratios in Fly River sediment than that from small mountainous rivers draining volcanoclastics. Increasing illite:smectite ratios in surficial sediment imply that the Fly River is presently building out clinoforms; however, for most of the late Holocene, the northeastern rivers contributed the bulk of the sediment. The time-lag for Fly River sediment flux to the clinoform suggests sediment storage within its wide floodplain during the rapid transgressions, compared to shorter time-lags in the small mountainous rivers during transgressions. CHIRP profiles reveal consistent patterns of oblique stacking on top of preexisting topography, creating topographic highs and intervening lows. Aggradation on topographic highs occurs during the lower-energy Monsoon season; however, this sediment is winnowed away by energetic currents during the Trade Wind season and advected to the northeast. Northeast progradation is supported by greater sediment thicknesses and accumulation rates along the northeast sides of GOP promontories. Teasing out influences of oceanographic currents can illuminate contributions of large and small-floodplain rivers in the same basin.