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Large scale excavation of outer shelf sediments by bottom currents during the Late Miocene in the SE Atlantic

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Abstract

An unusual erosional surface has been identified on seismic data from the outer shelf on the northern Namibian margin. The undulating surface defines a depression extending for 1300 km2 from which fine-grained Miocene sediments have been excavated to a depth of up to 160 m. The depression has a kidney-shaped planform, with three deep pits marking its basinward extent. It is an entirely closed feature, with no evident channels feeding in or leading out. Subsequent sedimentation has completely filled in the depression with the earliest infilling units exhibiting unusual depositional geometries. Well correlation yields a date of 9.9 ± 1.0 Ma for the erosional surface. No analogous features are known, but of all the possible causal mechanisms, it is considered most likely that the erosional depression was created by the extraordinary action of eddying bottom currents. These were probably associated with the well-documented Late Miocene onset of intensification of the Benguela Upwelling System, and were possibly directed by a small local variation in sea bed relief.

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Data availability

Seismic and well data are owned by Namcor, the National Petroleum Corporation of Namibia; DSDP and ODP data are open access.

Code availability

Not applicable.

Notes

  1. An interval velocity of 1850 m/s was derived from check shots in the exploration wells 1911/10–1 and 1911/15–1 for Miocene sediments on the continental slope, giving a depth interval of 35 m. This interval velocity has been used in preference to the unrealistic value of 1500 m/s which Wefer et al (1998) assumed for the entire section between the seafloor and the bottom of the hole.

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Acknowledgements

The authors are most grateful to NAMCOR, the National Petroleum Corporation of Namibia for granting access to seismic and borehole data. Thanks are also due to Schlumberger for the donation of Geoframe software and to IHS Markit for the donation of Kingdom software. The manuscript was greatly improved by comments from an anonymous reviewer and from Roger Swart. AH was partly funded by the American Association of Petroleum Geologists for work which included this study.

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AH received a one off Grant-in Aid from the American Association for Petroleum Geology for work which included this study.

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  1. School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK

    Andrew Hopkins

  2. Department of Science and Technology Studies, University College London, London, WC1E 6BT, UK

    Andrew Hopkins

  3. Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, UK

    Joe Cartwright

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Hopkins, A., Cartwright, J. Large scale excavation of outer shelf sediments by bottom currents during the Late Miocene in the SE Atlantic. Geo-Mar Lett 41, 33 (2021). https://doi.org/10.1007/s00367-021-00708-5

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