Massive siliciclastic discharge to slopes of the Great Barrier Reef Platform during sea-level transgression: constraints from sediment cores between 15°S and 16°S latitude and possible explanations

Abstract

The northeast Australian continental margin, including the Great Barrier Reef (GBR) Platform, provides an outstanding modern analogue for understanding sedimentation in tropical mixed siliciclastic/carbonate depositional systems. Previous studies of cores from this margin have suggested that, in contrast to widely accepted models, maximum siliciclastic fluxes to slopes over the last 30 ky occurred when sea level transgressed the shelf ca. 11–7 ka rather than when sea level was at a lowstand before 18 ka. However, climate and bathymetry can affect regional sediment deposition significantly, and the cores examined to date have come from the southern Queensland Trough where the margin has a wet tropical climate and a broad shelf (>50 km) with gradual slopes. In this study, we examine the abundance and accumulation of siliciclastic material in four cores from lower slopes of the northern Queensland Trough where rivers drain a drier region and debouche onto a narrow shelf with steep, incised slopes. Similar to previously examined cores, all cores contain a dark greyish brown horizon within the upper few meters. This horizon represents a low in carbonate content and corresponding high in siliciclastic material, primarily clay, and oxygen isotope stratigraphy shows that it was deposited during late transgression, nominally between 11 and 7 ka. Moreover, mass accumulation rate calculations demonstrate that this horizon reflects a significant rise in siliciclastic fluxes. When combined with the previous work to the south, it appears that slopes all along a large portion of the northeast Australian continental margin received greatly enhanced inputs of siliciclastic material during late transgression, independent of proximal climate and bathymetry. Two general explanations are offered for this observation: (1) climate change induced highly variable riverine discharge over the past 30 ky with a prominent maximum coincident with late transgression, or (2) the shelf stores large quantities of siliciclastic sediment during lowstand and releases this material to the slope during late transgression. In either case, siliciclastic accumulation on the northeast Australia continental margin behaves completely opposite to predictions from conventional models.