The Influence of Carbonate Platforms on the Geomorphological Development of a Mixed Carbonate‐Siliciclastic Margin (Great Barrier Reef, Australia)

Abstract

Sedimentation regimes on the Great Barrier Reef margin often do not conform to more conventional sequence stratigraphic models, presenting difficulties when attempting to identify key processes that control the margin's geomorphological evolution. By obstructing and modifying down-shelf and down-slope flows, carbonate platforms are thought to play a central role in altering the distribution and morphological presentation of common margin features. Using numerical simulations, we test the role of the carbonate platforms in reproducing several features (i.e., paleochannels, shelf-confined fluvial sediment mounds, shelf-edge deltas, canyons, and surface gravity flows) that have been described from observational data (seismic sections, multibeam bathymetry, sediment cores, and backscatter imagery). When carbonate platforms are present in model simulations, several notable geomorphological features appear, especially during lowstand. Upon exposure of the shelf, platforms reduce stream power, promoting mounding of fluvial sediments around platforms. On the outer shelf, rivers and streams are re-routed and coalesce between platforms, depositing shelf-edge deltas and incising paleochannels through knickpoint retreat. Additionally, steep platform topography triggers incision of slope canyons through turbidity currents, and platforms act as conduits for the localized delivery of land and shelf-derived sediments to the continental slope and basin. When platforms are absent from the topographic surface, the model is unable to reproduce many of these features. Instead, a more typical "reciprocal-type" sedimentation regime arises. Our results demonstrate the essential role of carbonate platform topography in modulating key bedload processes. Therefore, they exert direct control on the development of various geomorphological features within the shelf, slope, and basin environments.Plain Language Summary The modern Great Barrier Reef sits atop the skeletal remains of its ancestors. These remains form large (50-200 km 2 ) columns of chalk (or carbonate platforms) that rest on the northeast Australian continental shelf. By comparing observational data with computer simulations, we find that these platforms majorly disrupt and modify the flow of rivers and deep-sea density currents during periods of lower sea level. When platforms are exposed, they become hills, forming steep topographic high points that are large enough to re-route rivers and promote incision on the continental slope. On the modern seafloor, evidence of this activity is preserved in the form of ancient deltas, paleochannels, submarine canyons, and sediment flows that stretch across the abyssal plain. The morphology and distribution of these seafloor features are more robustly accounted for when carbonate platforms are present, and many of them do not appear in computer simulations where carbonate platforms are absent. Our work shows that carbonate platforms can alter seascapes in ways that are traditionally less understood.

Controls on submarine canyon connection to the shoreline: a numerical modelling approach

“…Submarine erosion corresponds to a hyperpycnal submarine extension of the subaerial fluvial network, with A restricted to the subaerial drainage extent, and S modified to Seff (Petit et al, 2015;Thran et al, 2020):…”

“…Submarine erosion and entrainment by turbidity currents is initiated when the sediment-water mixture within the hyperpycnal flow network reaches a pre-defined critical density (1000.04 kg/m 3 ). The critical density is required to be low as the model is unable to episodically trigger turbidity currents, therefore erosion by turbidity currents is modeled over longer timescales (Thran et al, 2020). Deposition (dprop) from turbidity currents occurs as a function of the local slope and a dimensionless scaling parameter (a) (Lowe, 1976):…”

“…This numerical description of turbidity current erosion and deposition has been used successfully to recreate the geomorphic evolution of canyons on the Great Barrier Reef slope, Australia (Thran et al 2020). Creep and slope failure are accounted for in the model by a linear diffusion law (e.g.…”

“…Once the critical shear stress drops below the threshold, or the seabed depth drops below the maximum wave base (set at 20 m), deposition occurs (Thran et al 2020).…”

“…Modifications of the stream-power law for the purpose of modelling submarine canyon erosion has been applied previously by Petit et al (2015), who reconstructed canyon evolution in the NW Mediterranean, and by Thran et al (2020), who showed how carbonate mounds control submarine erosion and canyonisation in the Great Barrier Reef.…”