Mass transport complexes (MTCs) are one of the most sedimentologically and seismically distinctive depositional elements in deep-water depositional systems. Seismic reflection data provide spectacular images of their structure, size, and distribution, although a lack of borehole data means there is limited direct calibration between MTC lithology and petrophysical expression, or knowledge of how they may act as hydrocarbon reservoir seals. In this study, we evaluate the lithological and petrophysical properties, and seismic characteristics of three deeply-buried (>2300 m/7546 ft below the seabed), Pleistocene MTCs in the northern Gulf of Mexico. We show that: (i) MTC lithology is highly variable, comprising a mudstone-rich debrite matrix containing large (4.5 km3/1.08 mi3), deformed, sandstone-rich blocks; (ii) MTCs are generally acoustically faster and are more resistive than lithologically similar (i.e. mudstone-dominated) slope deposits occurring at a similar burial depth; (iii) MTC velocity and resistivity increase with depth, likely reflecting an overall downward increase in the degree of compaction; and (iv) the lowermost 15-30 m (49-98 ft) of the MTCs, which represent the basal shear zones, are characterised by relatively high P-wave velocity and resistivity values, likely due to shear-induced over-compaction. We conclude that detailed analysis of petrophysical data, in particular velocity and resistivity logs, may allow recognition of MTCs in the absence of high-quality seismic reflection data, including explicit identification of the basal shear zone. Furthermore, the relatively thick basal shear zone, rather than the overlying and substantially thicker MTC itself, may form the primary permeability barrier and thus seal for underlying hydrocarbon accumulations.

Mass-transport complexes (MTCs) are deposits of subaqueous mass flows, and comprise slides, slumps and debrisflows (Dott, 1963; Nardin, Hein, Gorsline, & Edwards, 1979; Posamentier & Kolla, 2003). MTCs are found along all continental margins, and can play a major role in sediment transfer from the continents to the deep ocean (e.g. Hjelstuen,

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Lateral spread (or ‘spreading’) and submarine creep are processes that occur near the headwalls of both terrestrial landslides and submarine mass-transport complexes (MTCs). Both submarine creep and spread deposits may contain giant (km-scale) coherent blocks, but their transport processes remain poorly constrained. Here we use 2D and 3D seismic reflection data to determine the geometry, scale, and origin of an ancient (Late Miocene) mass-transport complex (MTC) located in the Kangaroo Syncline, offshore NW Australia. We show that this large remobilised mass of carbonate ooze is c. 170-300 m thick and covers an area of at least c. 1050 km2. The deposit is defined internally by two distinct seismic facies: (i) large, upward-tapering blocks (up to 210-300 m thick, 170-210 m wide, and 800-1200 m long) with negligible internal deformation, which decrease in height and spacing along the transport direction (identical, but in situ, seismic facies forms undeformed slope material immediately updip of the deposit headwall); and (ii) troughs (160-260 m thick, 190-230 m wide and 800-1200 m long) comprising moderately deformed strata, which contain ‘v’-shaped, pipe-like structures that extend upwards from the inferred basal shear surface to the top surface of the MTC. The lack of deformation within the blocks, and their correlation to adjacent in-situ deposits, suggests they underwent very limited transport (c. 50 m-70 m). The relatively high degree of deformation within the intervening troughs is attributed to the vertical expulsion of fluids and sediment during hydraulic failure of the sediment mass. We present a hydraulic failure model that accounts for the styles and patterns of intra-MTC deformation process. This model invokes evacuation of the lower slope by a pre-cursor MTC that formed the space to trigger the lateral spread event. Our study provides new insights into the genesis and rheology of subaqueous lateral spreads, enabling improved assessments of the threats posed to critical seafloor infrastructure. The genetic links identified between mass wasting and spatially-focused fluid flow indicate that, as well as disturbing the deep seafloor, submarine landslides may also create important deep-sea biodiversity hotspots.

The offshore area of the Otway Basin (south‐eastern Australia) is dominated by multibranched canyons where mass‐transport complexes are widely distributed. This study integrates high‐resolution multibeam and seismic data to investigate the importance of mass‐transport complexes in dictating the evolution of canyons. The study interprets three regionally distributed mass‐transport complexes that fail retrogressively and affect almost 70% of the study area. Within the mass‐transport complexes, seven canyons that initiated from the continental shelf edge and extended to the lower slope are observed. Although the canyons share common regional tectonics and oceanography, the scales, morphology and distribution are distinctly different. This is linked to the presence of failure‐related scarps that control the initiation and formation of the canyons. The retrogressive failure mechanisms of mass‐transport complexes have created a series of scarps on the continental shelf and slope. In the continental shelf, where terrestrial input is absent, the origin of the canyons is related to local failures and contour current activities, occurring near the pre‐existing larger headwall scarps (ca 120 m high, 3 km long). The occurrence of these local failures has provided the necessary sediment input for subsequent gravity‐driven, downslope sediment flows. In the continental slope, the widespread scarps can capture gravity flows initiated from the continental shelf, developing an area of flow convergence, which greatly widens and deepens the canyons. The gradual diversion and convergence through mass‐transport complex related scarps have facilitated the canyon confluence process, which has fundamentally changed the canyoning process. Thus, this study concludes that the retrogressive failure mechanism of mass‐transport complexes has a direct influence on the initiation, distribution and evolution of the canyons. The scarps associated with mass‐transport complexes have greatly facilitated the delivery of sediments and marine plastics from the shelf edge into the deep oceans, especially in areas where fluvial input is missing.