Shelf-edge reefs of the Great Barrier Reef: A time-capsule from the last glaciation

Abstract

A detailed investigation of the internal and external architecture of the shelf-edge reefs (SERs) of the Great Barrier Reef (GBR), Australia is presented here, constituting the most comprehensive seismic stratigraphy study of these drowned reefs. In two sites of the central GBR, seismic reflectors and facies were identified, ground-truthed against core and downhole data from the Integrated Ocean Drilling Program, Expedition 325. Marked depositional differences between the two sites were found and linked to local and regional physiographic and environmental contrasts. A sequential stratigraphy framework was established for these sites, which exhibit a complete depositional sequence dominated by transgressive reefs, bounded by two flooding surfaces. The postglacial flooding of the GBR shelf was also simulated. The measured parameters suggest a strong influence of the local antecedent substrate and of the interplay of regional physiographic variations and sea level change in the development of the SERs. Supported in these new interpretations, local and regional SERs CaCO3 accumulation were estimated. It was found that the Pleistocene SERs of the GBR are equivalent to ca. 20 % of the GBR's Holocene reef mass. Both the magnitude and the timing of the shelf-edge reef accumulation suggest that the drowned reefs in the GBR (and globally) had the potential to influence postglacial climate change. In addition, forward stratigraphic simulations were run on models based on this dataset, which suggest that the ensemble of conditions for reef growth deteriorated as the transgression advanced, resulting in shelf-edge reef demise. The role of the basement substrate was significant, but limited. All together, the multidisciplinary reconstructions in this study represent a useful framework to constrain the development of these under-studied formations, which according to the findings had a significant role in shaping the Quaternary GBR and, possibly, in postglacial climate change.