[1] Based on acquired geophysical, geological and geotechnical data and modeling, we suggest hydrate dissolution to cause sediment collapse and pockmark formation in the Niger delta. Very high-resolution bathymetry data acquired from the Niger delta reveal the morphology of pockmarks with different shapes and sizes going from a small ring depression surrounding an irregular floor to more typical pockmarks with uniform depression. Geophysical data, in situ piezocone measurements, piezometer measurements and sediment cores demonstrate the presence of a common internal architecture of the studied pockmarks: inner sediments rich in gas hydrates surrounded by overpressured sediments. The temperature, pressure and salinity conditions of the studied area have allowed us to exclude the process of gas-hydrate dissociation (gas hydrate turns into free gas/water mixture) as a trigger of the observed pockmarks. Based on numerical modeling, we demonstrate that gas-hydrate dissolution (gas hydrate becomes mixture of water and dissolved gas) under a local decrease of the gas concentration at the base of the gas-hydrate occurrence zone (GHOZ) can explain the excess pore pressure and fluid flow surrounding the central hydrated area and the sediment collapse at the border of the GHOZ. The different deformation (or development) stages of the detected pockmarks confirm that a local process such as the amount of gas flow through faults rather than a regional one is at the origin of those depressions. Citation: Sultan, N., et al. (2010), Hydrate dissolution as a potential mechanism for pockmark formation in the Niger delta,
In previous works, it has been suggested that dissolution of gas hydrate can be responsible for pockmark formation and evolution in deep water Nigeria. It was shown that those pockmarks which are at different stages of maturation are characterized by a common internal architecture associated to gas hydrate dynamics. New results obtained by drilling into gas hydrate-bearing sediments with the MeBo seafloor drill rig in concert with geotechnical in situ measurements and pore water analyses indicate that pockmark formation and evolution in the study area are mainly controlled by rapid hydrate growth opposed to slow hydrate dissolution. On one hand, positive temperature anomalies, free gas trapped in shallow microfractures near the seafloor and coexistence of free gas and gas hydrate indicate rapid hydrate growth. On the other hand, slow hydrate dissolution is evident by low methane concentrations and almost constant sulfate values 2 m above the Gas Hydrate Occurrence Zone. Study Area and Main ObjectiveThe investigated area is located in deep water of Nigeria. Bathymetry in the area ranges from 1100 to 1250 m ( Figure 1). This area was previously shown to host a field of (sub) circular pockmarks [Georges and Cauquil, 2007]. These range in shape from a slightly depressed, hummocky seafloor to a much more pronounced depression and each of them is several tens to a few hundreds of meters wide (Figure 1). The various morphologies of the pockmarks suggest either distinct modes of formation or different evolutionary stages [Sultan et al., 2010]. Most of the pockmarks are located in an area bounded by two NW-SE trending deeprooted normal faults, which delineate a graben linked to the axis of anticline in the subsurface. Several deep and shallow faults and three N-S trending buried channels were recognized with high-resolution 3-D seismic data (Figure 1). The buried channels, which are situated between 80 ms and 180 ms (two-way travel time, TWTT) below the seabed, may have the potential of accumulating amounts of free gas and play therefore an important role for the gas hydrate distributions.Based on geophysical and sedimentological data, and in situ piezocone measurements, Sultan et al. [2007] have shown that pockmark-associated gas hydrate accumulated within a few meters thick sediment layers at shallow depth. In addition, Sultan et al. [2010] proposed that the formation of a circular depression around the gas hydrate occurrence zone (GHOZ) is related to multiple steps in the pockmark evolution. The sequence is starting with hydrate formation induced by upward migration of fluids oversaturated in gas through fracture systems followed by decrease of fluid flow resulting in gas undersaturation, hydrate dissolution, generation of excess pore pressure, and by concurrent collapse of the gas hydrate-bearing sediment structures. Respective analyses were mainly based on subseabed approaches, using piston cores and in situ piezocone geotechnical measurements with a maximum penetration of 30 m below seafloor (mbsf). Howe...
We report on a reconnaissance analysis of the geochemical composition of authigenic carbonates and sediment samples collected from various seepage sites on the Niger deep-sea fan. Our aim has been to investigate whether evidence for the presence of authigenic carbonates and gas hydrates within sediments is discernible from solid-phase sediment geochemistry. We show that sedimentary Sr/Ca and Mg/Ca ratios can be used to infer the presence of authigenic aragonite (Sr-rich) and Mg-rich carbonate phases (high-Mg calcite, dolomite) in cold seep settings. Using Sr/Ca and Mg/Ca ratios, the proportion (wt.%) of authigenic carbonates in Niger Fan sediments can be calculated from a mixing model between sediment fractions of terrigenous material, biogenic calcite, aragonite and high-Mg calcite. This approach was applied to high-resolution geochemical profiles along sediment cores recovered from various cold seep settings (mud volcano, diapirs, pockmarks, gas-hydrate bearing sediments). Our data reveal that authigenic carbonates occur as discrete phases in sediments from gas-hydrate-bearing areas, suggesting that such carbonate-rich sediment layers may represent paleoindicators for ancient methane seepage in marine sediments, possibly associated to gas-hydrate reservoirs.
The role of internal and external forcing of sedimentation in turbidite systems remains a subject of debate. Here we propose new insights from the quantitative analysis of architectural parameters of the Congo Axial Fan.Fifty-two channel-levee-lobe systems, spanning the last ca. 200 ka, are visible on the seafloor, most of them having slightly elongated lobe complexes at their termination. Volumes of lobe complexes (usually 3 to 196 km3) are highly variable in time and space. The cumulative volume of the lobe complexes represents approximately 30% of the volume of the Axial Fan.The Axial Fan is sequentially divided into periods of increasing/decreasing channel lengths and basinward/landward migrations of avulsion points, representing successive prograding/retrograding architectural patterns called architectural cycles. These cycles are either symmetrical saw toothed and bell-shaped with progressive progradation and retrogradation phases, or asymmetrical, with long-lasting progradation phases and abrupt retrogradation phases that correspond to channel avulsions occurring high up on the fan.Our study points to the interplay between internal and external factors controlling the architecture of the Congo Axial Fan. The local topographic constraint is a major factor in the fan's stacking pattern. However, cyclic evolution of the architecture reveals major shifts in the deposition site that are linked to very upfan avulsion events. These events are interpreted to be driven by external factors (e.g. climate and/or eustatic sea-level change) that were able to drastically increase and/or coarsen the sediment supply to the fan. Highlights ► Exhaustive architectural evolution of the Congo turbidite system since 200 ka. ► Shifts of channellevee-lobe systems reveal cyclic patterns at different time scales. ► Dimensions of terminal lobe complexes do not exhibit cyclic evolution. ► Pointing to permanent internal control by inherited topography. ► Revealing possible exceptional periods of increased or coarsening Congo River inputs. At a system scale, for instance on the Amazon Fan, the chrono-stratigraphic framework provided by Leg 155 ODP drillings (Flood and Piper, 1997) delivered clues that uncovered a link between sedimentation and architectural evolution to climate and eustasy. Maslin et al. (2006) assumed that channel avulsions in the Amazon Fan could be triggered by pulses of sediment flux, and therefore be externally forced by factors such as sea level and/or climate fluctuations. Lopez (2001) also suggests that sea level variations influence the occurrence of the avulsion process, which is more frequent during periods of sea level lowering. Additionally, at the levee scale it was demonstrated that external forcing mechanisms such as sediment flux pulses or sea level fluctuations control the growth of the levees (Bonneau et al., 2014; Jorry et al., 2011; Toucanne et al., 2012) therefore potentially playing a role in triggering channel avulsions and thus influencing the distribution pattern of the channel-levee-lobe syste...
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