Neogene gravity tectonics and depositional processes on the deep Niger Delta continental margin

Abstract

The continental margin off the Niger Delta is undergoing deformation by gravity tectonism caused by rapid seaward sediment progradation. Three regional structural styles are recognized: (1) an upper extensional zone of listric growth faults beneath the outer shelf; (2) a translational zone of diapirs and shale ridges beneath the upper slope; and (3) a lower compressional zone of imbricated thrust structures (toe thrusts) beneath the lower slope and rise. Linked together on a regional scale, these styles suggest that large portions of this thick sediment prism are slowly moving downslope by gravity gliding or sliding, in a manner analogous to giant mass movements or mega-landslides. This deformation has created local intraslope basin up to 25 km wide, which are generally filled with thick (up to several kilometres) turbidites and mass transport deposits. Submarine canyons cut across these deformed zones and give rise on the lower slope to large, aggradational channel-levee systems which are distributaries for a few very large deep-sea fans. These fans and their channel-levee systems may extend up to hundreds of kilometres down the continental rise and are comparable in size with some of the largest modern fans such as the Amazon and Mississippi. Individual fan boundaries cannot yet be delineated; however, middle versus lower fan environments are recognizable on the basis of (1) channel morphology and size (e.g. large, leveed versus small, unleveed channels), (2) 3.5 kHz seismic facies (echo character) (e.g. well stratified versus prolonged echoes) and (3) sediment facies in piston cores (e.g. clays versus thick silt/sand turbidites). Off the easternmost edge of the Niger Delta, the submarine canyon-channel systems do not feed a deep-sea fan, but rather converge to feed a large, solitary channel-levee system, the Principé Channel, which extends 700 km across the rise to the Guinea Abyssal Plain. Transport and deposition of terrigenous sediments beyond the shelf edge have been accomplished mainly by (1) turbidity currents and (2) mass transport (slumps, debris flows) and have been controlled by fourth and fifth-order glacioeustatic sea-level fluctuations of 100 ka, or less, since at least the Miocene. During the Late Quaternary relatively short (≈10 000 year) periods of maximum sea-level rise during interglacial phases (such as the Holocene) shifted the locus of river sedimentation (i.e. deltas) landward across the wide shelf and temporarily shut off the terrigenous sediment supply to the deep sea. Pelagic sedimentation ensued and thin (<1 m) ‘condensed sections’ formed throughout the region. In contrast, the longer duration (≈90 000 year) periods of sea-level fall caused by glacials exposed the shelf; consequently, river deltas migrated seaward to the shelf edge and submarine canyon heads allowing large amounts of sediment to be continously transported to the slope, rise and deep-sea fans by turbidity currents and related mass flows. Deposition of these potentially sandy deep-water facies, coupled with their subsequent incorporation into intraslope basins and imbricate thrust structures through gravity tectonism, create the potential for significant hydrocarbon reservoirs within the deep-water portions of this continental margin.