Geological overview of the Angola–Congo margin, the Congo deep-sea fan and its submarine valleys

Abstract

The Congo deep-sea fan is one of the largest fans in the world still affected by presently active turbidity currents. The present activity of deep-sea sedimentary processes is linked to the existence of a direct connection between the Congo River estuary and the Congo canyon head that allows relatively continuous sediment feeding of the deep-sea environment, in spite of a wide continental shelf (150 km). Because of this important activity in terms of sedimentary processes, the deep-sea environment of the Congo-Angola margin presents major interests concerning physical, chemical and biological studies near the sea floor. The main aim of this paper is to present the initial geological context of the BioZaire Program, showing a synthesis of the major results of the ZaïAngo Project including (1) the brief geological setting of the Congo-Angola margin, (2) the structure of the modern Congo deep-sea fan, (3) the sedimentary architecture of the recent Congo turbidite system (from the canyon to the distal lobes), and (4) the recent and present turbidite sedimentation. In order to provide useful information and advice relevant to biological and geochemical studies across the Congo sedimentary system, this article focuses on the present sedimentary processes and the present activity of turbidity current along the Congo canyon and channel.Keywords : Congo-Angola margin, Congo channel, sedimentary deposits, turbidite .1 IntroductionThe accumulation of marine sediment seaward of the Congo River estuary is probably one of the greatest in the world for a submarine system that is still active today. In the continental domain, the Congo sedimentary system is composed by a wide drainage basin, the river and its tributaries. In the nearshore and marine domain: it consists of the estuary, the canyon, which acts mainly as a by-pass zone, and finally an area of preferential sedimentation at the foot of the continental slope and in the deep basin. The Congo submarine system has been geologically studied since Buchanan in 1887 (submarine cable survey) and was the focus of the ZaïAngo project team from 1998 to 2005. Several types of evidence of Holocene and very recent activity in the canyon have been described and linked to sedimentary processes such as turbidity currents, river floods, slides, gas hydrate formation and fluid escape. The aim of this paper is to synthesize the geological knowledge of the area and in particular the results of the ZaïAngo project in order to provide useful information and advice relevant to biological and geochemical studies across the Congo sedimentary system. Marine biologists and oceanographers are usually not familiar with turbidite deposits. These sedimentary deposits were first recognised and described in outcrops of ancient sedimentary series (Bouma, 1962;Walker, 1978;Mutti, 1992). They are generated by submarine gravity processes and they build large sedimentary systems in deep-water environments, termed deep-sea fans. Deep-sea fans are well developed from the foot of the continental slope...

Asphalt mounds and associated biota on the Angolan margin

“…Block 31 has an irregular morphology, as the sea floor is highly deformed by salt diapirism (Evans, 2010;Hill et al, 2010a;Hill et al, 2010b) and compressional structures, including salt domes and reverse faults . Sediment fluid flow features are also present, such as pock marks (Olu-Le Roy et al, 2007;Savoye et al, 2009;Unterseh, 2013) and cold seeps (Cambon-Bonavita et al, 2009;Olu et al, 2009;Sibuet and Vangriesheim, 2009;Unterseh, 2013;Waren and Bouchet, 2009). The extensive hydrocarbon seepage at the seafloor at Block 31 is also indicated by satellite syntheticaperture radar observations of persistent surface oil slicks (NPA Satellite Mapping Global Offshore Seepage Database).…”

“…Angolan licence block 31 covers a 5349 km 2 area of seafloor on the eastern edge of the southern Congo fan (Droz et al, 2003;Savoye et al, 2009). To the north of the block is the Congo submarine canyon, which connects with the Congo River ).…”

An assessment of cloud masking schemes for satellite ocean colour data of marine optical extremes

“…This may indicate that the type and depth of sediment plume from a river is influential, with the shallow deposition inclined environment of the Amazon estuary, and those of the other rivers mentioned, producing higher reflectance values in the near infrared. In contrast, the deeper channel of the Congo limits sediment deposition near its mouth with its course continuing offshore for hundreds of kilometres as a deep-sea fan and underwater canyon system (Savoye et al 2009). …”

Gas hydrate decomposition recorded by authigenic barite at pockmark sites of the northern Congo Fan

“…During the middle Cretaceous (AptianTuronian), black shales and bituminous sandstones were deposited on top of the evaporites, the precursor of the high hydrocarbon potential of this region. Detailed descriptions of the regional sedimentological evolution and the main tectonic features are given in Gay et al (2006a) and Savoye et al (2009). Intensive salt diapirism and raft tectonics characterize the continental margin (e.g., Brice et al 1982), associated with an upward migration of hydrocarbons and fluids.…”