Authors: Hinestrosa, G., Webster, J.M., Beaman, R.J., Anderson, L.M.
Publication: 30th IAS Meeting of Sedimentology, 2-5 Sep 2013. International Association of Sedimentologists, Manchester, U.K.
During the Last Glacial Maximum (LGM) and early postglacial (~19 to ~10 ky BP) extensive coral reefs developed along the shelf-edge of north-eastern Australia (Great Barrier Reef), as evidenced by geophysical surveys and recent drilling in four sites by the Integrated Ocean Drilling Program (IODP), Expedition 325. At two of the drilling sites, Hydrographers and Noggin passages in the central Great Barrier Reef, we used Topas sub-bottom seismic profiles and constraints provided by IODP Exp. 325 drill holes to investigate the seismic stratigraphy of the shelf-edge reefs. Distinct seismic reflectors were identified across the surveys and seismic facies were interpreted in both sites, for reef and inter-reef locations.
At the southern site, Hydrographers Passage, two sub-bottom reflectors were identified: R1, corresponding to the glacial-postglacial unconformity; and R2, corresponding to a precedent marine-flooding surface. The integration of our interpretation with drilling cores and published literature reveals a complex depositional history influenced by the rising sea level and varying hydrodynamic conditions as the shelf was exposed and reflooded during the glacial-postglacial cycle. Our interpretation suggests a substantial influence of the antecedent substrate topography and of inter-reef channel transport (fluvial/tidal) in the deposition of the shelf-edge reefs.
Evidence suggests that the configuration of the antecedent substrate contributed directly to the persistence through time of reef structures at similar geomorphic locations, despite the extreme environmental variations occurring since the onset of the LGM. A strong morphologic differentiation is also evident, responding partly to lateral and vertical substrate availability. LGM and postglacial fringing-reefs occur in the distal, deeper areas characterized by a gently sloping, terraced substrate; and barrier-reefs with extensive, thin lagoonal deposits occur landward, matching ridges and platform flats recognized in reflector R1.
The northern site, Noggin Passage, does not have well developed terraces, and consists mainly of a single barrier-reef formation with smaller terraces, and an extensive backreef lagoon. The slope seaward of the ~90 m contour is also steeper than at the south. As in the southern site, broad inter-reef channels interrupt the reef ridges, connecting the lagoon to the Coral Sea. Seismically, the scattering of the signal by reefs did not allow a straight forward identification of reflectors and seismic facies. However, reflector R1 (possibly corresponding to the postglacial unconformity surface) was identified in key locations, and seismic facies were interpreted, particularly in the backreef, where abundant patch reefs are observed on top of R1.
In this study we compared both areas to establish depositional models that are consistent with the singularities of each location. Our results are a step forward toward the full understanding of these poorly known structures, which constitute a window to the LGM and early postglacial depositional history. Moreover, our 2D and 3D seismic reconstructions constitute an ideal input for numerical models of reef accretion. Together this will allow us to directly test what factors (e.g. shelf width and substrate slope, sea level change and reef growth rates) have most influenced the development of the shelf-edge reef system that are potentially applicable to other geological times and locations. Another important result of our study is the volumes of carbonate accretion, to be integrated into studies on the carbon cycle.