Antecedent bedrock control on the sediment-starved continental shelf of south/central Namibia

Andrew N. Green; W Senna; Andrew Cooper; T. Heeralal; H Labuschagne

doi:10.1016/j.margeo.2024.107242

Antecedent bedrock control on the sediment-starved continental shelf of south/central Namibia  

Andrew N. Green
, W Senna
, Andrew Cooper
, T. Heeralal
, H Labuschagne

University Milano-Bicocca

Research output: Contribution to journal › Article › peer-review

2 Citations (Scopus)

474 Downloads (Pure)

Abstract

The Namibian inner continental shelf immediately north of the Orange River delta is sediment-poor, dominated by outcrop of metamorphic basement with a scarcity of sandy cover. Using a combination of high and very-high resolution geophysical tools and drill cores, an examination of the evolution of the inner shelf stratigraphy and geomorphology shows persistent antecedent bedrock controls throughout the Pleistocene.

The bedrock of the inner shelf comprises schists, in places overlain by Eocene sandstones, over which a subaerial unconformity (H1) formed in the late Oligocene. H1 was repeatedly re-occupied during succeeding transgressive/regressive cycles, creating a backstop for erosion/sedimentation and creating a repeating topography on which transgressions took place. Apart from localised development of Miocene phosphatic limestones, little sediment has since accumulated. Large-scale karst weathering of the limestones reflects several periods of subaerial exposure most recently during the Quaternary lowstands of sea level including MIS 2 and 1.

Thin Quaternary cover, housed within depressions on the H1 surface is predominantly preserved in the lee of bedrock ridges, behind which shallow and occasionally hypersaline lagoonal conditions developed. These preserved back-barrier sedimentary sequences developed post-MIS 5 (previous phases were either eroded/weathered or not deposited). Dolomite precipitation occurred when sea-level stabilised at −75 m 37,000 cal BP and restricted lagoonal conditions prevailed.

Currently, there are very limited MIS 2/1 deposits preserved above H1. The absence of thick sand cover, adjacent to and downdrift of the Orange River delta, suggests that the onshore diversion of sand that currently supplies the Namib Sand Sea also operated during lower sea levels, related to the bedrock control and lack of accommodation in surface H1. This persistent onshore diversion of deltaic sediment throughout the Quaternary has implications for sediment source as well as budget calculations for the Namib Sand Sea.

Original language	English
Article number	107242
Pages (from-to)	1-19
Number of pages	19
Journal	Marine Geology
Volume	469
Early online date	15 Feb 2024
DOIs	https://doi.org/10.1016/j.margeo.2024.107242
Publication status	Published (in print/issue) - 1 Mar 2024

Bibliographical note

Publisher Copyright:
© 2024 Elsevier B.V.

Funding

We are grateful to Trans Hex Marine Pty Ltd. for their logistical and financial support, especially for their permission to publish the data herein. We thank the master, crew and colleagues aboard the MV DP Star during the data collection, especially Kurt Dennis who collected the data and assisted in multibeam processing. Prof. John Compton is thanked for his friendly and helpful discussions surrounding the core set published in 2001. John Pether further assisted with discussions on fossil ages. We especially thank Drs. John Goff, Samuel Johnson and two anonymous reviewers for their helpful and constructive feedback which helped benefit the paper significantly. WS acknowledges an Erasmus Mundus mobility grant to South Africa for his MSc, of which this paper forms part of. This paper forms a contribution to the EU-H2020 funded project iAtlantic Grant agreement No. 818123.

Funder number
818123

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

SDG 13 Climate Action
SDG 14 Life Below Water

Keywords

Antecedent setting
Geological control
Inner shelf morphology
Seismic stratigraphy
Namib Sand Sea
Orange Delta

Access to Document

10.1016/j.margeo.2024.107242

Green_et_al._2024Author Accepted Manuscript, 4.87 MBLicence: CC BY-NC-ND

Cite this

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title = "Antecedent bedrock control on the sediment-starved continental shelf of south/central Namibia  ",

abstract = "The Namibian inner continental shelf immediately north of the Orange River delta is sediment-poor, dominated by outcrop of metamorphic basement with a scarcity of sandy cover. Using a combination of high and very-high resolution geophysical tools and drill cores, an examination of the evolution of the inner shelf stratigraphy and geomorphology shows persistent antecedent bedrock controls throughout the Pleistocene. The bedrock of the inner shelf comprises schists, in places overlain by Eocene sandstones, over which a subaerial unconformity (H1) formed in the late Oligocene. H1 was repeatedly re-occupied during succeeding transgressive/regressive cycles, creating a backstop for erosion/sedimentation and creating a repeating topography on which transgressions took place. Apart from localised development of Miocene phosphatic limestones, little sediment has since accumulated. Large-scale karst weathering of the limestones reflects several periods of subaerial exposure most recently during the Quaternary lowstands of sea level including MIS 2 and 1. Thin Quaternary cover, housed within depressions on the H1 surface is predominantly preserved in the lee of bedrock ridges, behind which shallow and occasionally hypersaline lagoonal conditions developed. These preserved back-barrier sedimentary sequences developed post-MIS 5 (previous phases were either eroded/weathered or not deposited). Dolomite precipitation occurred when sea-level stabilised at −75 m 37,000 cal BP and restricted lagoonal conditions prevailed. Currently, there are very limited MIS 2/1 deposits preserved above H1. The absence of thick sand cover, adjacent to and downdrift of the Orange River delta, suggests that the onshore diversion of sand that currently supplies the Namib Sand Sea also operated during lower sea levels, related to the bedrock control and lack of accommodation in surface H1. This persistent onshore diversion of deltaic sediment throughout the Quaternary has implications for sediment source as well as budget calculations for the Namib Sand Sea.",

keywords = "Antecedent setting, Geological control, Inner shelf morphology, Seismic stratigraphy, Namib Sand Sea, Orange Delta",

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T1 - Antecedent bedrock control on the sediment-starved continental shelf of south/central Namibia  

AU - Green, Andrew N.

AU - Senna, W

AU - Cooper, Andrew

AU - Heeralal, T.

AU - Labuschagne, H

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N2 - The Namibian inner continental shelf immediately north of the Orange River delta is sediment-poor, dominated by outcrop of metamorphic basement with a scarcity of sandy cover. Using a combination of high and very-high resolution geophysical tools and drill cores, an examination of the evolution of the inner shelf stratigraphy and geomorphology shows persistent antecedent bedrock controls throughout the Pleistocene. The bedrock of the inner shelf comprises schists, in places overlain by Eocene sandstones, over which a subaerial unconformity (H1) formed in the late Oligocene. H1 was repeatedly re-occupied during succeeding transgressive/regressive cycles, creating a backstop for erosion/sedimentation and creating a repeating topography on which transgressions took place. Apart from localised development of Miocene phosphatic limestones, little sediment has since accumulated. Large-scale karst weathering of the limestones reflects several periods of subaerial exposure most recently during the Quaternary lowstands of sea level including MIS 2 and 1. Thin Quaternary cover, housed within depressions on the H1 surface is predominantly preserved in the lee of bedrock ridges, behind which shallow and occasionally hypersaline lagoonal conditions developed. These preserved back-barrier sedimentary sequences developed post-MIS 5 (previous phases were either eroded/weathered or not deposited). Dolomite precipitation occurred when sea-level stabilised at −75 m 37,000 cal BP and restricted lagoonal conditions prevailed. Currently, there are very limited MIS 2/1 deposits preserved above H1. The absence of thick sand cover, adjacent to and downdrift of the Orange River delta, suggests that the onshore diversion of sand that currently supplies the Namib Sand Sea also operated during lower sea levels, related to the bedrock control and lack of accommodation in surface H1. This persistent onshore diversion of deltaic sediment throughout the Quaternary has implications for sediment source as well as budget calculations for the Namib Sand Sea.

AB - The Namibian inner continental shelf immediately north of the Orange River delta is sediment-poor, dominated by outcrop of metamorphic basement with a scarcity of sandy cover. Using a combination of high and very-high resolution geophysical tools and drill cores, an examination of the evolution of the inner shelf stratigraphy and geomorphology shows persistent antecedent bedrock controls throughout the Pleistocene. The bedrock of the inner shelf comprises schists, in places overlain by Eocene sandstones, over which a subaerial unconformity (H1) formed in the late Oligocene. H1 was repeatedly re-occupied during succeeding transgressive/regressive cycles, creating a backstop for erosion/sedimentation and creating a repeating topography on which transgressions took place. Apart from localised development of Miocene phosphatic limestones, little sediment has since accumulated. Large-scale karst weathering of the limestones reflects several periods of subaerial exposure most recently during the Quaternary lowstands of sea level including MIS 2 and 1. Thin Quaternary cover, housed within depressions on the H1 surface is predominantly preserved in the lee of bedrock ridges, behind which shallow and occasionally hypersaline lagoonal conditions developed. These preserved back-barrier sedimentary sequences developed post-MIS 5 (previous phases were either eroded/weathered or not deposited). Dolomite precipitation occurred when sea-level stabilised at −75 m 37,000 cal BP and restricted lagoonal conditions prevailed. Currently, there are very limited MIS 2/1 deposits preserved above H1. The absence of thick sand cover, adjacent to and downdrift of the Orange River delta, suggests that the onshore diversion of sand that currently supplies the Namib Sand Sea also operated during lower sea levels, related to the bedrock control and lack of accommodation in surface H1. This persistent onshore diversion of deltaic sediment throughout the Quaternary has implications for sediment source as well as budget calculations for the Namib Sand Sea.

KW - Antecedent setting

KW - Geological control

KW - Inner shelf morphology

KW - Seismic stratigraphy

KW - Namib Sand Sea

KW - Orange Delta

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U2 - 10.1016/j.margeo.2024.107242

DO - 10.1016/j.margeo.2024.107242

M3 - Article

SN - 0025-3227

VL - 469

SP - 1

EP - 19

JO - Marine Geology

JF - Marine Geology

M1 - 107242

ER -

Antecedent bedrock control on the sediment-starved continental shelf of south/central Namibia

Abstract

Bibliographical note

Funding

UN SDGs

Keywords

Access to Document

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Cite this