Geometry and chronology of late Quaternary depositional sequences in the Eastern Niger Submarine Delta

On seismic profiles, the Eastern Niger Submarine Delta displays spectacular prograding wedges separated by discontinuities on the continental shelf and correlative conformities seawards. In spite of the numerous studies focused on deep oil reservoirs in this area, and because of the presence of pronounced syndepositional deformation, the detailed geometry and chronology of the Quaternary succession has been neglected, while it contains relevant information about sedimentary processes and the factors controlling sediment depocenters, presence of fluids and sediment deformation.This study provides the first comprehensive view of the Late Quaternary stratigraphic architecture of the Eastern Submarine Niger Delta. The stratigraphic interpretation is based on: (1) a seismic dataset composed of 2D seismic profiles with variable resolution (Sparker, Chirp) and four 3D seismic blocks (short offset processing); and (2) in situ measurements and samples (3 Calypso and 2 Stacor long piston cores). The correlation of seismic reflectors with chronological information from long piston cores (δ18O values and XRF-derived Ca profiles), shows that the upper-most 5 depositional sequences formed during the last ca. 500 kyr BP, in response to glacial/interglacial fluctuations driven by 100-kyr Milankovitch cycles and the subsidence rate for the late Quaternary period are around 300 m/Myr. The prograding wedges at the shelf break have distinct seismic facies with high-angle clinoforms building regional scale depositional sequences locally disrupted by growth faults. Although highly influenced by syndepositional deformation with thick accumulation in the hanging walls of growth faults, the main depositional sequence pattern is tuned with glacio-eustatic oscillations and the high-angle clinoforms near the shelf edge correspond to upper shorefaces/delta fronts or subaqueous deltas formed during glacial lowstand periods. The largest Mass Transport Complex in this zone is interpreted as resulting from oversteepening of the shelf edge during MIS 10.

► Five Late Quaternary sequences offshore Niger reflect 100 kyr glacio-eustatic cycles.
► Integration of 3D and 2D seismics is necessary to obtain isopach maps of sequences.
► Growth faults at the shelf edge and shale anticlines control thickness distribution.
► Two types of lowstand prisms indicate the position of the paleo-shoreline.
► Estimated subsidence rate at the shelf edge for the upper 4 sequences is 340 m/Myr.