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Bathymetry, Microphysiography and Reflectivity Characteristics of the West African Margin Between Sierra Leone and Mauritania

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Geology of the Northwest African Continental Margin

Abstract

This report maps and analyzes the bathymetry and seafloor reflectivity characteristics along the northwest African margin between 3° and 23°N. The bathymetric trends strongly suggest that the morphology associated with the mid-ocean ridge (MOR) spreading system strikes north throughout the area mapped (north of ~10°N).

The system of equatorial fracture zones defines a dominant E-W morphologic fabric in the southwestern portion of the map area, and appears to extend through the northeasttrending Sierra Leone Rise in the southeastern part of the area. The Sierra Leone Rise is an aseismic system of plateaus probably formed at the MOR crest during a phase of excess basalt generation.

The reflectivity characteristics and microphysiography show that both downslope processes (turbidity flows and sediment slides) and along-slope processes (geostrophic bottom currents) have been important in shaping the present morphology of the continental rise. Turbidity flows and sediment slides are sparse south of 10°N; north of 10°N numerous turbidity flow pathways occur on the continental slope and rise. The Cayar Canyon System defines the largest pathway which extends from Senegal to the Gambia Abyssal Plain (900 km). Sediment slides are extremely common and occur on the continental slope and rise and at the base of isolated seamounts, islands, and ridges. The longest and largest sediment slide extends from the shelf break near Senegal to southeast of the Cape Verde Islands (570+ km). The largest slide complex is adjacent to the largest turbidity flow system. We believe rapid deposition by turbidity flows may provide an important mechanism for generating unstable slope and rise deposits that later fail as sediment slides.

Strong geostrophic bottom currents are not presently active in much of the map area. However, in the recent past, such currents probably flowed in the basins along the continental margin. In the basins north of the Kane Gap, a north-flowing bottom current operated that had two likely sources: Antarctic bottom water (AABW) flowing eastward through fracture zones, and bottom water flowing north through the Kane Gap. In the Sierra Leone Basin, a counter–clockwise gyre of bottom water was probably sustained by eastward-flowing AABW in the south.

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Authors and Affiliations

  1. Department of Geological Sciences, State Univ. New York at Buffalo, 4240 Ridge Lea Road, Amherst, NY, 14226, USA

    Robert D. Jacobi

  2. Lamont-Doherty Geological Observatory, Palisades, NY, 10964, USA

    Dennis E. Hayes

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  1. Robert D. Jacobi
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Editors and Affiliations

  1. Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 3000, Hannover 51, Germany

    Ulrich von Rad  & Karl Hinz  & 

  2. Geologisch-Paläontologisches Institut der Universität, Olshausenstraße 40/60, 2300, Kiel, Germany

    Michael Sarnthein  & Eugen Seibold  & 

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Jacobi, R.D., Hayes, D.E. (1982). Bathymetry, Microphysiography and Reflectivity Characteristics of the West African Margin Between Sierra Leone and Mauritania. In: von Rad, U., Hinz, K., Sarnthein, M., Seibold, E. (eds) Geology of the Northwest African Continental Margin. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-68409-8_10

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  • DOI: https://doi.org/10.1007/978-3-642-68409-8_10

  • Publisher Name: Springer, Berlin, Heidelberg

  • Print ISBN: 978-3-642-68411-1

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