Seismic stratigraphy and development of the shelf-edge reefs of the Great Barrier Reef, Australia

• Seismic interpretation of the shelf-edge reefs of the GBR reveals major reflectors.
• Two distinct seismic units of regressive and transgressive nature were identified.
• Strong morphologic control by past sea level and antecedent topography was inferred.
• 3D reconstruction of past reef morphologies in the depth domain.
• LGM and postglacial depositional history of the shelf-edge of the GBR.

Extensive shelf-edge reefs (SERs) and terraces are common along the seaward margin of the Great Barrier Reef (GBR). We aim to better understand the architecture of the SERs, deposited during a poorly constrained period: the Last Glacial Maximum (LGM) and early postglacial. A dense array of Topas seismic lines was interpreted at Hydrographers Passage in the southern central GBR following a seismic stratigraphic approach ground-truthed against Integrated Ocean Drilling Program Expedition 325 (IODP Exp. 325) sediment core data. We interpreted two prominent sub-bottom reflectors, an upper R1 and a lower R2, that separate three seismic units. According to our borehole-to-seismic correlation in a representative location, R1 corresponds to a downward increase of impedance which is also reflected in lithological and diagenetical changes. R2 is poorly constrained, but likely also represents a lithological contrast. Seismic velocities were also estimated in the same borehole location for inter-reef deposits and reef framework. Together with the geomorphic interpretation, we reconstructed velocity maps for the study area, leading to the generation of a 3D geomorphic model of the antecedent topography or substrate, and volumetric calculations. Geomorphic and thickness trends suggest a strong relationship between the antecedent topography and the morphology of the SERs. The distribution of seismic facies identified in the dataset, coupled to geomorphic trends and published sea level curves, allowed us to reconstruct the LGM to postglacial depositional history of these deposits. Our interpretation suggests significant regressive reefal accumulations formed on top of a marine flooding surface represented by reflector R2 during the sea level fall to the LGM. R1 is the distal equivalent of an unconformity previously identified in the GBR inner- and mid-shelf, widely affected by subaerial exposure, on top of which a postglacial, transgressive, mainly reefal Unit 1 was deposited. Two zones are distinguished: (1) a distal zone (below 80 mbsl) with thick deposits displaying fringing-reef morphologies, influenced by the late-LGM fall and subsequent early-postglacial rise in sea level and by the updip availability of substrate; and (2) a proximal zone (above 80 mbsl) of enhanced vertical growth with unfilled lagoons, resulting from the lack of lateral substrate and the rapid sea level rise. Our seismic stratigraphic model of the development of the SERs, sediment volumetrics, and 3D reconstruction of the antecedent topography provides a foundation for future forward modeling stratigraphic studies.

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