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

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 deep-water settings. Here a 2D and 3D seismic reflection data from the Exmouth Plateau, offshore NW Australia is 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 flanks of an underlying bifurcative Miocene canyon. MTC-BDF spans ∼75 km by ∼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 km to 3.40 km with thicknesses reaching up to 165 m. Interestingly, the blocks are more abundant in a region characterized by moderate-high amplitude debrites. Erosional morphologies encompassing a unique groove and other circular to irregular-shaped depressions mapped along the BSS provide evidence for the erosive nature of the flow. The origin of the groove is related transported blocks gouging the BSS. Importantly, intra block deformations recorded within these blocks as fault and fold systems suggest a complex flow regime within MTC-BDF, with the deformations arising either during block translation or also possibly upon the arrest of the failed mass in interaction with bathymetric elements. Our findings suggest inherent deformations within these blocks may serve as high-permeability conduits with implications for deep-water drilling operations within this segment of the Exmouth Plateau and elsewhere in other hydrocarbon-rich deep-water settings.