Geological controls on focused fluid flow associated with seafloor seeps in the Lower Congo Basin
Introduction
Focused fluid migration in marine sediments is a widespread phenomenon which is increasingly gaining attention in the context of environmental discussions, even though it still is not well understood (Berndt, 2005). However, increased data coverage and the advent of new tools in oceanic exploration, such as backscatter imagery, multibeam swath bathymetry maps and 3D seismic data, can provide new evidence of relatively small-scale fluid seep structures on modern continental margins, and can help towards improving our understanding of the underlying processes. Fluid migration in sedimentary basins is an important process because 1) the input of greenhouse gases into the ocean/atmosphere system may be an important component of the atmospheric carbon budget (Judd et al., 2002), 2) the fluid expulsion at the seafloor may play a role in potential instabilities on slopes (Prior and Coleman, 1984, Evans et al., 1996, Yun et al., 1999, Cochonat et al., 2002), representing a risk for human activities (Sultan et al., 2001, Elverhøi et al., 2002), and 3) fluid expulsion sites form the basis for a plethora of chemosynthetic benthic ecosystems that play an important role in the deep marine communities (Sibuet, 2003). Fluids can follow different pathways but the occurrence of fluids and gas seepage is manifested in the Lower Congo Basin by the presence of pockmarks on the seafloor. Since their initial identification on the Scotian Shelf by King and MacLean (King and MacLean, 1970), pockmarks have been reported repeatedly during offshore hydrocarbon exploration and scientific surveys in various depositional systems at water depths ranging from 30 m to over 3000 m (for a detailed review see (Josenhans et al., 1978, Werner, 1978, Hovland, 1981, Whiticar and Werner, 1981, Hovland and Judd, 1988, Solheim and Elverhoi, 1993, Baraza and Ercilla, 1996, Rollet et al., 2006). They generally appear in unconsolidated, fine-grained sediments as cone-shaped circular or elliptical depressions, ranging from a few meters to 300 m or more in diameter and from 1 m to 80 m in depth, and they concentrate in fields extending over several square kilometers. In some cases, they have been identified along straight or circular lines correlated with glaciomarine tills (Josenhans et al., 1978, Whiticar and Werner, 1981, Kelley et al., 1994) suggesting a geological control on focused fluid flow (Eichhubl et al., 2000, Cifci et al., 2003). In particular, structural surfaces along bedrock (Shaw et al., 1997), salt diapirs (Taylor et al., 2000, Satyavani et al., 2005), and faults and faulted anticlines (Boe et al., 1998, Soter, 1999, Vogt et al., 1999, Eichhubl et al., 2000, Dimitrov and Woodside, 2003) create pathways for fluid migration. These observations suggest that discontinuities or unconformities are much more effective for fluid migration than a simple diffusive seepage through the sedimentary column (Abrams, 1992, Brown, 2000) and are responsible for focused fluid flow, fluid escape at the seafloor and pockmark development (Abrams, 1992, Orange et al., 1999). The crater-like nature of pockmarks suggests an erosional power of fluid venting (Hovland and Judd, 1988), commonly related to an overpressured buried reservoir of biogenic gases, thermogenic gases, or oil, interstitial water, or a combination of the three. However, time varying fluxes may be recorded into seafloor fluid seeps. An integrated study conducted on a giant pockmark of the Lower Congo Basin at 3200 m water depth has shown that the mineralogical, chemical, and biological facies are clearly related to upward fluid intensity (Gay et al., 2006c).
The objectives of this study are: (1) to identify the main flow pathways for shallow and deep fluids through the sedimentary column of the Congo/Angola passive margin. We integrate high-resolution 3D seismic datasets and multibeam imagery within the Lower Congo Basin to determine the main fluid pathways and to characterize the expression of fluid expulsion on the seafloor; (2) to understand the underlying structural control on focused fluid flow structures. Drawing on the extensive database, we document how different structures such as normal and reverse faults, salt diapirs, shallow and deep turbiditic channels, and erosional surfaces can lead to fluid flow focusing.
Section snippets
Geological settings
Rifting of the Congo/Angola margin started in the early Cretaceous (144–140 Ma) and culminated in the opening of the South Atlantic Ocean around 127–117 Ma (Brice et al., 1982, Jansen et al., 1984, Guiraud and Maurin, 1992, Karner and Driscoll, 1998, Marton et al., 2000, Karner et al., 2003). Two major post-rift stratigraphic units overlie a thick Aptian salt layer. They reflect a major change in ocean circulation and climate (Séranne et al., 1992). From Late Cretaceous to Early Oligocene time,
Data and methods
The bathymetry of the Congo–Angola basin was surveyed during the 1992–1993 GUINESS and 1998–2000 ZAIANGO (ZAIre–ANGOla) projects. The main objective was to map and investigate the Pliocene/Quaternary sedimentary history of the slope and deep-sea fan (Savoye et al., 2000). The bathymetric/multibeam map was acquired with a Simrad EM12 dual bathymetric and acoustic imagery. At 3000 m water depth, the processing of EM12 data provides a final terrain numerical grid with a 100 × 100 m horizontal grid
Seafloor backscatter analysis
The multibeam backscatter imagery, which extends from approximately 10° to 12° E and from 5° to 6° S in the study area, shows high backscatter anomalies that are oblique or overlap the ship tracks, indicating they are not data acquisition noise (Fig. 3). These high backscatter features fall into several categories by appearance: large sub-circular to ovoid areas, linear to curvilinear bands, and irregular to rounded patches (Fig. 3).
Wide sub-circular and ovoid areas are almost always associated
Pockmarks, fluid pipes and BSR on 3D seismic data
The morphology of the pockmarks, their distribution and orientation are clearly revealed by a dip map of the seafloor based on the 3D seismic dataset (Fig. 4; see Fig. 3 for location). They range from 100 m up to 800 m in diameter, and from a few meters to 40 m in depth. Most of them have a circular shape in map view, but the largest pockmarks are elongated in one main direction. Detailed displays of 3D seismic data show that these large pockmarks are irregular and sometimes link to several
Normal faults
In the north-eastern part of the study area, the seabed is affected by curved depressions, 5 to 8 km long and a few meters deep. They are oriented NW–SE, parallel to the Cretaceous carbonate platform (Anderson et al., 2000). The dip map of Area 1 (see Fig. 4 for location) shows a continuous curved depression, 300 m wide and 20 m deep, at about 800 m water depth. The seismic profile EF (Fig. 6C) shows that this depression is related to a basinward, i.e. SW-dipping, normal fault. Such normal
Pockmarks related to erosional surfaces
The ZAIANGO 1 and 2 cruises surveyed a large part of the Zaire Fan (Savoye et al., 2000, Droz et al., 2003) and showed that the fan surface is characterized by a multitude of inactive channel–levee systems (Fig. 3). Most of the channels and palaeochannels are meandering with variable sinuosity. The youngest and still active channel–levee system is located in the axial part of the fan and has been entirely surveyed down to the distal fan at 5000 m water depth. The shelf and the slope are deeply
Shallow buried palaeochannel
On the northern side of the Zaire Canyon (Area 7: see Fig. 4 for location), a large number of pockmarks are aligned along a sinuous belt from the southeast to the northwest (Fig. 12A). They are 100 to 800 m wide with a maximum depth of about 40 m. They are regularly spaced about 300 m apart, but some of them have formed from the coalescence of several smaller pockmarks. Despite the number of pockmarks visible on the seafloor dip map, relatively few of them appear as high backscatter anomalies
Discussion
The evaluation of seafloor fluid seeps in a sedimentary basin requires a thorough understanding of the parameters that control the migration and the accumulation of fluids before redistribution to the seabed. The origin of shallow fluids (0–3000 m water depth) is commonly attributed to biogenic and/or thermogenic processes (Tissot and Welte, 1984). In both cases, the fluids are derived from organic matter either as the result of bacterial activity or of thermogenic processes, which are
Conclusion
During the ZAIANGO project, we have collected bottom video records using ROV dives, sediment and water for geochemical analysis, and gravity cores above some high backscatter (black) patches, 100 to 800 m wide, previously identified on the extensive EM12 reflectivity map acquired in the Congo/Angola Basin (Savoye et al., 2000). Although all these circular backscatter anomalies were not explored, one of the main results of this collaborative project is that these anomalies are commonly
Acknowledgements
This work has been done as part of the ZAIANGO project that is a collaborative project funded by IFREMER and TOTAL and the participating French universities, in particular the University of Lille 1 and the University of Montpellier 2. We are very grateful to technicians and crew of Genavir who operated ships, the ROV, and the sampling tools during the ZAIANGO cruises. We also would like to thank TOTAL for providing wonderful 3D seismic data. We would like to thank both reviewers for the time
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