Elsevier

Earth-Science Reviews

Volume 233, October 2022, 104159
Earth-Science Reviews

Correlation of Neoproterozoic diamictites in southern Namibia

https://doi.org/10.1016/j.earscirev.2022.104159Get rights and content

Abstract

In order to present a conclusive new correlation model for Neoproterozoic glacial units of southern Namibia, ten different sections and a variety of samples thereof were studied and analysed with respect to field relationships, whole rock geochemistry, zircon U-Pb dating as well as Th-U ratios, Hf isotopic measurements and zircon grain size analyses, combined with the new method of LA-ICP-MS U-Pb dating of cap carbonates. This multi-method approach allowed for the construction of a correlation model for the Cryogenian and Ediacaran units of southern Namibia.

Furthermore it revealed, that (1) sediments deposited during four glacial Neoproterozoic events in Southern Namibia did all show very similar detrital zircon features, allowing the interpretation of continuous recycling of the same material over the most part of the Neoproterozoic, which is also supported by the geochemical whole rock analyses, (2) cap carbonates are worth analysing concerning their U-Pb isotope ratios and can result in valuable age determinations, if reset and overprinted areas are recognised and avoided for laser ablation, (3) proving the Sturtian and the Marinoan age for the Numees Fm and the Namaskluft Mbr by U-Pb dating their overlying carbonate sequences was finally possible, (4) the Witvlei Grp sedimentation ends at 579 ± 52 Ma, which is the age for the stromatolites of the Okambara Mbr, representing the uppermost deposits of the Witvlei Grp, and this leading to (5) the start of Nama Grp sedimentation for southern Namibia was not earlier than 579 ± 52 Ma.

Introduction

Neoproterozoic glacial deposits of southern Namibia (Kalahari Craton) are known from various locations. Hofmann et al. (2014) showed that such deposits represent four different Neoproterozoic glaciations in southern Namibia, comprising the Kaigas, the Sturtian, the Marinoan and the post-Marinoan Vingerbreek successions, although this interpretation is still under discussion and doubted by other authors (e.g. Frimmel, 2018). The problem is the lack of reliable geochronological data for a conclusive correlation that would finally allow distinguishing between the different diamictite units and correlate them with other sections of southern Namibia to prove their existence.

The Sturtian and Marinoan glacial units are Snowball-Earth events as proven by Hoffman et al. (1998) and Macdonald et al. (2010a). Snowball-Earth events are represented by simultaneous glacial deposits on all cratons with erosional base and with overlying cap carbonate sequences. In addition, some of these units were deposited at low latitudes (Kirschvink, 1992). This is essential for a Snowball-Earth event, because it proofs the existence of glaciers from the Polar Regions to the equator. In contrast, the Kaigas and Vingerbreek events were interpreted as representing local glaciations only, because their scarce outcrops don't allow correlations with a variety of other cratons. Furthermore, these glaciations did not (so far) show rock units with defined low latitude palaeomagnetic poles.

An overview map of southern Namibia with the locations of all studied sections is shown in Fig. 1. The sections comprise in total four different glacial horizons, of which only two are assigned to Snowball Earth events: Numees and Namaskluft fms (Fig. 2). The locations are part of different tectonic regimes, causing metamorphic and/or erosional influences on the sections, making correlations difficult. The majority of the Neoproterozoic diamictite sections analysed and discussed in this study is located in the southernmost part of southern Namibia (Fig. 1). Most of those (Orange River, Dreigratberg, Namuskluft, Trekpoort Farm and Witpuetz Farm section) are part of the eastern Gariep Belt (Port Nolloth Zone) where the Neoproterozoic sequences partially overly the Palaeoproterozoic basement of the Richtersveld Province, representing the oldest part of the Namaqua Metamorphic Complex (Miller, 2008a, Miller, 2008b). These sections of southernmost Namibia are compared and set into correlation with analysed sections south of Windhoek (see Fig. 1) belonging to the Neoproterozoic sedimentary units of the Witvlei Grp. Based on stratigraphic correlations, the Witvlei Grp pre-dates the Nama Grp sediments and overlies the units of the Nosib and Tsumis grps (e.g. Hegenberger, 1993). Therefore, the Witvlei Grp deposits can be roughly correlated with units of the Port Nolloth Grp in southernmost Namibia, as shown in Fig. 2. The outcrops of the Vingerbreek glaciation of southern Namibia (Fig. 1) are not part of the Gariep Belt but of sedimentary strata of the lower Nama Grp (uppermost Ediacaran, Fig. 2).

Correlations of diamictite occurrences in southern Namibia are difficult due to their scattered outcrops and their sometimes tectonic overprint. In southern Namibia only two sections showing a complete succession with Sturtian and Marinoan diamictites are known: (1) the Namuskluft section and (2) the Dreigratberg. Both locations were already analysed earlier by Macdonald et al. (2010b), Hofmann et al. (2013) and others. These studies showed that existing geological maps of these areas (McMillan, 1968; Geological Survey of Namibia, 1999, Geological Survey of Namibia, 2010) need improvement. Differentiation between the single diamictite layers is problematic and not always possible solely based on field observations. Therefore, modern analyses like U-Pb detrital zircon dating, zircon grain size studies, geochemical analyses, Hf-isotope analyses on zircon grains as well as U-Pb carbonate dating on cap carbonates are needed. All of these methods were applied to the samples of all (re-)visited sections (see Fig. 1), combined with extensive field work observations and extrapolated sequence-stratigraphic positioning, in an approach to finally and conclusively distinguish all known Neoproterozoic diamictite occurrences in southern Namibia and present a correlation model.

Section snippets

Sample sites

In this chapter a general description including field photographs as well as new geological maps and stratigraphic columns of each sample site are given. The different sections are treated in succession of their stratigraphic age from the oldest to the youngest: Kaigas – Sturtian – Marinoan – Vingerbreek.

In Fig. 1B a more detailed view of the different sections along the Orange River for the Kaigas, Sturtian and Marinoan diamictites is given, showing the relative locations of the sections along

Methods

All sections were visited several times and were subject to intense field work, discussions and interpretations concerning possible correlations among all co-authors of this study. After field mapping key samples were used for unravelling the question of section correlation in southern Namibia. In total two basement samples, 19 siliciclastic sedimentary samples and twelve carbonate samples were taken. Please note, that some of the presented samples and results of this study were already

Data and results

In total ten sections and their surrounding areas in southern Namibia were investigated: Orange River (W), Dreigratberg, Dreigratberg North, Namuskluft, Trekpoort Farm, Witputs Farm, Aar Farm, Orange River (E) with Tierkloof Farm, Blaubeker Farm and Tahiti Farm. Geological overview maps of the Geological Survey of Namibia were used as base maps and modified and corrected where necessary due to own field observations and geological mapping. Sections containing the analysed samples were measured,

Discussion and conclusion

In order to find a way to correlate and date the upper Neoproterozoic sections in southern Namibia containing the well-known glacial diamictite horizons, a variety of methods was used. A combination of extensive field work including mapping, sampling and stratigraphic correlations, geochemical analyses of rock samples, macroscopic characterisation of minerals including optical microscopy as well as SEM, and multiple isotopic analyses and measurements finally helped to unravel the relationships

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This is FIERCE contribution number 106. FIERCE is financially supported by the Wilhelm and Else Heraeus Foundation and by the Deutsche Forschungsgemeinschaft (DFG, INST 161/921-1 FUGG and INST 161/923-1 FUGG), which is gratefully acknowledged. We thank the team of the Geological Survey of Namibia for their support and help over the years. A special thanks goes to the two anonymous reviewers that helped to improve this manuscript.

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