Tectono-Stratigraphic evolution of the West Africa Margin:Mauritania-Senegal-Guinea Bissau

Abstract

This study utilises a comprehensive dataset consisting of 2D and 3D seismic data and wells located offshore Senegal basin to describe the tectono-stratigraphic evolution of the Mesozoic post-rift sequence in the MSGBC Basin. Renewed tectonic activity linked to the opening of the Equatorial Atlantic during the Aptian disrupted the previously 'passive' post-rift subsidence and played a crucial role in sedimentation and palaeoceanography. This influence extended across the MSGBC Basin, affecting the sedimentary systems, routing, and depositional style. Moreover this opening dominated the contour current circulation, which gave rise to the formation of contour current related features interacting with the Cretaceous turbidity system in the study area. Clear evidence of well-defined scalloped margins has been identified using advanced 3D seismic data along the Cretaceous platform margin. The interpretation highlights four large amphitheatre-like structures, indicating a retreat of the carbonate margin by at least 5 km. Simultaneously, a substantial global drop in sea level during the late Aptian, coupled with tectonic uplift, resulted in a relative sea-level decrease of at least 80 meters. This is evident through an unconformity with signs of karstification. In the basin, five distinct deep-water sediment wave fields have been identified, spanning from the Early to Late Cretaceous (Valanginian to Turonian) and exhibiting unique morphologies and geometric parameters. The recognition of mixed (turbidity and contour) current-related sediment waves during the late Valanginian suggests the initiation of contour currents during that period, indicating interaction between turbidity and contour currents. This observation challenges previous assumptions about the absence of contour currents during the Early Cretaceous. The Cretaceous deposits in the Senegal Basin document the interplay between currents moving along and down the slope. Indications of this interaction include erosional features like channels and depositional features such as mounded drifts, asymmetric levees, and sediment waves, pointing to a combination of contourite and turbidite processes. The formation of this mixed system is significantly influenced by both changes in tectonics and fluctuations in sea level due to eustasy. An increase in the occurrence of mixed turbidite-contourite features becomes evident from the Cenomanian onwards, aligning with the ongoing rise in global sea levels and the opening of the Equatorial Atlantic seaway. In contrast to the prevailing belief that contour currents are solely responsible for the redirection and transportation of fine-grained sediments, the findings and illustrations presented in this study provide evidence that robust contour currents are capable of effectively conveying coarse-grained sediments as well. The implications of this discovery for the future characterization of hydrocarbon reservoirs are substantial.

Date of Award	1 Aug 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Redfern (Supervisor) & Mads Huuse (Supervisor)