Clay mineralogy of Gulf of Papua Shelf and Pandora Trough deposits constrains sediment routing during the last sea‐level cycle

Abstract

AbstractThe Gulf of Papua inner mid‐shelf clinothem and lowstand deposits in Pandora Trough record sediment source and routing through the last sea‐level cycle on 20 kyr cycles. Clay mineralogy tracked dispersal of sediment from the two types of rivers (wide versus narrow floodplains) to constrain the contributions of river systems to the Gulf of Papua clinothem and Pandora Trough deposits. Fly River sediment has higher illite:smectite than clays from the small mountainous rivers (Bamu, Turama, Kikori and Purari rivers) that drain regions with more limestones. X‐ray diffraction shows high illite:smectite proximal to the Fly River delta that decrease towards the north‐east. Downcore mineralogy of inner mid‐shelf cores reveals that the largest shifts in illite:smectite correspond to changes in sediment units. The relict clinothem emplaced on the Gulf of Papua shelf during Marine Isotope Stage 3 has lower illite:smectite than the Holocene clinothem that has been building since 2 ka and the Marine Isotope Stage 5a relict clinothem. In the inner mid‐shelf, downcore decreases in illite:smectite during Marine Isotope Stage 3 suggest that this region received less clay from the Fly River and more contributions from small mountainous rivers. During Marine Isotope Stage 3, the exposed physiography and narrower shelf in this region may have deflected Fly River sediment more south‐eastward, where it bypassed the inner shelf via the Kiwai, Purutu and Umuda valleys and was deposited in the Pandora Trough. The Fly River may have been more susceptible to valley incision because of its limited shelf accommodation and higher ratio of water to sediment discharge. Such bypass of the inner mid‐shelf by Fly River sediment during the Marine Isotope Stage 2 sea‐level lowstand is recorded in Pandora Trough deposits with high illite:smectite ratios. Inner mid‐shelf clinothems with compositional shifts on the order of 20 kyr may be influenced by shelf physiography, accommodation and the variable incision by small and large rivers.