Evolution and morphology of rafted blocks in an ancient deepwater mass-transport complex (Exmouth Plateau, offshore North West Australia)

Abstract

Submarine mass wasting plays a fundamental role in transporting substantial volumes of sediments basinward including gigantic slide blocks. However, the understanding of processes involved in block generation and their associated deformation until flow arrest remains limited, especially in data-starved deepwater settings. Here, 2D and 3D seismic reflection data from the Exmouth Plateau, offshore North West Australia are used to investigate the architecture of large blocks preserved within an ancient mass-transport complex (MTC) and their interaction with the basal shear surface (BSS). The evolution of the investigated MTC (MTC-BDF) is related to instability along the slope and flanks of an underlying bifurcative Miocene canyon. The MTC-BDF spans approximately 75 × 35 km containing at least 32 well-imaged blocks (within the 3D seismic coverage) encapsulated in a well-deformed debrite background. These carbonate blocks interpreted as rafted blocks have lengths ranging from 0.48 to 3.40 km with thicknesses reaching up to 165 m. Interestingly, the blocks are more abundant in a region characterized by moderate- to high-amplitude debrites. Erosional morphologies encompassing a unique groove and other circular- to irregularly shaped depressions mapped along the BSS provide evidence for the erosive nature of the flow. The origin of the groove is related to transported blocks gouging the BSS. Importantly, intrablock deformations are recorded within these blocks as fault and fold systems. This suggests a complex flow regime within MTC-BDF, with the deformations arising either during block transportation or possibly upon arrest of the failed mass while interacting with bathymetric elements. Our findings suggest that inherent deformations within these blocks may serve as high-permeability conduits with implications for deepwater drilling operations within this segment of the Exmouth Plateau and elsewhere in other hydrocarbon-rich deepwater settings.