Dense shelf water cascades (DSWC) are ubiquitous on continental margins worldwide. They could transform into turbidity currents, shape the seabed physiography, and influence sediment, organic carbon, and pollutants that transfer from the shelf to the basin floor. However, there is still a lack of knowledge regarding how DSWC transforms into turbidity currents, and how DSWC interacts with the seabed. The Central Region of the offshore Gippsland Basin, located on the southeast Australian margin, is seasonally impacted by DSWC (named the Bass Cascade Current; BCC) formed in the Bass Strait. We observed complex seabed morphologies and highly diverse sedimentary processes in this area using high-resolution multibeam bathymetry, seismic reflection, and core description data. Observed sedimentary structures include sediment waves, erosional scours, cyclic steps, submarine channels, longitudinal furrows, submarine landslides and gullies. We ascribe this complexity to a dynamic interaction between BCC, and Westerly wind-associated Ekman transport flow, and strong waves. We found that the along-shelf transported BCC can interact with the submarine landslides and generate supercritical turbidity currents transporting downslope for more than 80 km. We reveal that climate change could significantly impact the seabed morphologies and sedimentation processes, by dictating the strength and pathway of BCC and its generated supercritical turbidity currents. Therefore, the current transformation has critical implications for predicting how seabed geomorphology, sedimentation process, and occurrence of geohazards respond to changing oceanographic and climate conditions.
In this study, a seismic reflection dataset and well-log data were integrated to investigate the geometry and internal configuration of a turbidite channel system within the Late Cretaceous interval of the deep-water Kribi-Campo sub-basin, offshore Cameroon. This interval is characterized by a well-developed submarine channel system consisting of an early and a late-stage channel. Morphologically, the submarine channel system has a northeast-southwest trend and is U-shaped in cross-section with a length of 56 km within the study area. The early-stage channel has a relatively straight morphology and varies in width and depth from 3 to 5 km and 89 to 197 m, respectively. However, the late stage of the channel is characterized by a narrower (1 to 3 km) and shallower (41 to 103 m) incision, with sinuous morphology carved into the early channel infill. The changing interaction of differential tectonic subsidence, relative sea level, source sediment supply and slope gradient change are considered to be the major control on the geometry and internal characteristics of the submarine channel system. Sag subsidence during the Campanian led to basin deepening and the widespread development of basinal sediments as submarine fans and promotion of submarine channel system development. The filling of the channel system occurred during a long-term Maastrichtian relative sea level rise, punctuated by falls in relative sea level. Sand appears to have been fed to the channel system by the palaeo-Sanaga and palaeo-Nyong Rivers, with sand rich aprons developed were these rivers debouched into the study area. The early stage of the submarine channel is dominated by coarse-grained sediments in the southwest and fine-grained sediments in the northeast, while the late-stage channel is mainly filled with fine-grained sediments. The presence of coarse-grained sediments occur within the submarine channel axis downstream represents a potential for hydrocarbon reservoirs with enhanced petrophysical qualities due to a low depositional gradient. The geomorphological analysis of this ancient submarine channel system along the western African margin, as presented in this study, has broad implications in the understanding of the distribution of deep-water sediments with potential for hydrocarbon exploration in the region.
Pockmarks are pervasive geomorphologic features identified along continental margins resulting from fluid expulsion on the seafloor. However, the understanding of the underlying geological mechanism/control in relation to their evolution, distribution, and morphology is limited, especially along data-starved continental margins such as the Northern Orange Basin. Analysis of a high-quality 3D seismic reflection data reveals at least 50 individual pockmarks, two channel-like depressions and several irregular depressions in water depth ranging between 800 m and 2400 m. Morphologically, the pockmarks are circular, elongated, comet-like and crescentic in shape, with diameters and depths ranging between ∼0.2 - 2.8 km and ∼10 - 130 m, respectively. Preferential alignment of these pockmarks on the seafloor in relation to the axis of underlying turbidite channels, erosional morphologies and mass transport complexes portray a genetic relationship. The slope architecture hints at the possibility of both deep and shallow fluid source driving pockmark formation. Under this scenario, deep thermogenic gas derived from Cretaceous source rocks migrated along fault systems associated with the Late Cretaceous Megaslide complex to the overburden. The fluids are stored/redistributed in contourite and turbidite channels and subsequently focused toward the seafloor under an increased pore pressure regime. Yet, the fluids may be either solely biogenic gas or heterogeneous, incorporating biogenic components and pore-water derived from the channels and dewatering of the contourites. Importantly, the discovery of crescentic and elongated end-member pockmark morphologies indicate post-formation sculpting of the initial pockmark morphologies by bottom currents. The discovery of these deep-water pockmarks opens the possibility that such fluid escape features may be more widespread than currently documented in the Northern Orange Basin. This has implications in understanding of the petroleum system here and their potential role in the South Atlantic marine ecosystems and global climate change in terms of the expulsion of climate forcing gases.
The Gippsland Basin is located in the south-eastern continental margin of Australia and hosts a variety of important marine resources (i.e. hydrocarbons, offshore wind, biological diversity, and fishing resources). Recent high-resolution seabed mapping reveals a complex seabed morphology in the Gippsland Basin, containing scarps, cyclic steps, channels, canyons, gullies, and giant submarine landslides. However, previous studies have not yet revealed the dominant sedimentary processes behind this morphological complexity. We combine high-resolution multibeam bathymetric and seismic reflection datasets to investigate the dominant sedimentary processes that are active in shaping these complex seabed morphologies. In the northern region of the study area, slope failures are prevalent on the shelf and slope, which are primed by the deposition of contourites generated by the East Australian Current. In the central and southern regions, the Bass Cascade Current can carry large amounts of sediments, scouring the shelf and slope, and can form supercritical turbidity currents that create a variety of erosional seabed morphologies. We suggest that slope gradient variation, oceanography and a variety of sedimentary processes jointly contributed to the seabed morphological complexity in the Gippsland Basin. The high-resolution seabed morphological analysis within this study provides critical geomorphological and geological information for future submarine constructions (i.e. locating potential wind farms and telecommunication cable installations) along with geohazard prediction and mitigation (i.e. knowing the location of past and predicting future giant landslides).
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