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Mixed Carbonate–Siliciclastic Sedimentation Along the Great Barrier Reef Upper Slope: A Challenge To the Reciprocal Sedimentation Model
Results of studies involving numerous cores and ODP holes along the Great Barrier Reef (GBR) margin and adjacent Queensland Trough and Queensland Plateau have challenged the use of a reciprocal sedimentation model to describe the sedimentary response of slope and basin settings to glacioeustatic sea-level fluctuations. Upper-slope sedimentation results from the relationships between sea-level fluctuations, antecedent topography, and regional climate that play an important role in the type and amount of sediment deposited on the upper slope during glacial, deglacial, and interglacial times. During the Last Glacial Maximum (LGM, > 20 ka ago) upper-slope sediments generally lacked siliciclastic material and are characterized by very low accumulation rates, whereas early deglacial-time (Termination I, TI) deposits are dominated by a siliciclastic and neritic carbonate pulse. Siliciclastic sedimentation was significantly reduced in the Holocene, while carbonate sedimentation remains elevated. A new borehole, IODP Expedition 325 Hole M0058A (Hole 58A), recovered 82% of a 40.4 m hole on the upper slope east of Noggin Passage on the central GBR margin near Cairns, Australia. Hole 58A provides a detailed sedimentary record during Termination II (TII), Marine Isotope Stage 6/5e (MIS-6/5e), deglacial transition, and through most of interglacial MIS-5. This hole, along with two others (ODP Leg 133 Holes 820A and 819A from the upper slope east of Grafton Passage), show carbonate–siliciclastic cyclicity as the result of glacioeustatic change with the GBR shelf. Sedimentation at Hole 58A is consistent with that of previous studies along the GBR margin (focusing on the LGM to present), and extends the upper-slope sedimentary record back to TII and interglacial MIS-5. A siliciclastic pulse similar to the one during TI occurred during the penultimate deglaciation, TII; however, the maximum neritic aragonite export to the upper slope occurred not during peak MIS-5e highstand when sea level was a few meters above modern position, but subsequently during a time (MIS-5d to 5a) when lowered sea level fluctuated between 30 and 50 m below present sea level. Siliciclastic sediments were reworked and exported to the upper slope when the lowstand fluvial plain was re-flooded, whereas neritic carbonate export to the slope reached a maximum when sea level fell and much of the mid to outer shelf re-entered the photic zone, subsequent to a drowning interval. Thus, this analysis refines the mixed-sedimentation models of upper-slope sedimentation along the central GBR margin during the penultimate deglacial transgression and subsequent interglacial early and late highstand. This study provides further evidence that mixed carbonate–siliciclastic margins do not always behave in a predictable manner and that mixed margins both modern and ancient would benefit from detailed study of sediment transport in the context of sea-level rise and fall.
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