ALBERT

Description: The Plio-Pleistocene Whalers Bluff Formation of the offshore Otway Basin is composed of mixed siliciclastic-carbonate sediments. In seismic cross sections, this formation includes an interval that consists of higher amplitude seismic reflections that display alternating depressional ponds and raised ridges. This interval is shallowly buried and lies between 40-150 ms two-way travel time below the present-day seafloor. In this study, we used 2-D and 3-D seismic datasets in combination with the available shallow subsurface well logs, to characterize the geomorphology and investigate the origin of these enigmatic features. The ponds are expressed as densely packed, circular to polygonal, and in some cases, hexagonal-shaped features in time slice maps, and closely resemble previously documented honeycomb structures. In our study area, the honeycomb-like structures (HS) are comprised of large (200 to 800 m diameter range) depressed ponds that are separated by narrow (∼20 m at the top) reticulate ridges. In total, these HS cover an area of 760 km2. GIS analysis shows that the ponds of HS, especially those in the NE of the study area, are aligned along the NW-SE trend-lines. There are several possible origins for the HS. The most probable mechanism is that the HS are resulted from the bulk contraction of soft sediment, associated with shallow burial diagenesis processes such as subaqueous dewatering of the fine-grained successions within the Whalers Bluff Formation. Interestingly, irregular furrows of various lengths on the seafloor correspond to the ridges of the HS, and we hypothesize that these furrows may have formed due to differential compaction of the underlying alternating ponds and ridges. Our results demonstrate the benefits of using seismic reflection datasets in combination with geospatial analysis to investigate the buried paleo-geomorphologic features and their impact on the present-day seafloor physiography.

Description: 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.