Correlation of Neoproterozoic diamictites in southern Namibia
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.
References (65)
- et al.
Rare earth abundances in chondritic meteorites
Geochim. Cosmochim. Acta
(1978) - et al.
A novel approach to double-spike Pb–Pb dating of carbonate rocks: examples from Neoproterozoic sequences in southern Africa
Chem. Geol.
(2000) - et al.
A new lithostratigraphic subdivision and geodynamic model for the Pan-African western Saldania Belt, South Africa
Precambrian Res.
(2013) - et al.
Stratigraphy, paleontology and geochemistry of the later Neoproterozoic Aar Member, southwest Namibia: reflecting environmental controls on Ediacara fossil preservation during the terminal Proterozoic in African Gondwana
Precambrian Res.
(2013) - et al.
Zircon Th/U ratios in magmatic environs
Lithos
(2015) - et al.
Mixtite deposits of the Damara Sequence, Namibia, problems of interpretation
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(1985) - et al.
Geochemistry of carbonate rocks of the Chilpi Group, Bastar Craton, India: Implications on ocean paleoredox conditions at the late Paleoproterozoic Era
Precambrian Res.
(2021) - et al.
Extremely high Th/U in metamorphic zircon: in-situ dating of the Labwor Hills granulites
Geochim. Cosmochim. Acta
(2006) - et al.
Pushing the boundary: a calibrated Ediacaran-Cambrian stratigraphic record from the Nama Group in northwestern Republic of South Africa
Earth Planet. Sci. Lett.
(2022) The structural and stratigraphic setting of the Rosh Pinah zinc-lead deposit within the Gariep Trough
(1993)
Grundlagen der Geologie, 3
The tectonic history of the Southern Adamastor Ocean based on a correlation of the Kaoko and Dom Feliciano belts
The Sinclair Supergroup of the Rehoboth Volcanic Arc from the Sossusvlei-Gamsberg area to the Gobabis Region
Plate tectonics and geochemical composition of sandstones
J. Geol.
Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins
Contrib. Mineral. Petrol.
Geology of the Skorpion Supergene zinc deposit, southern Namibia
Econ. Geol.
The Baykonurian Glaciohorizon of the Late Vendian
Stratigr. Geol. Correl.
The Interpretation of Igneous Rocks
Geochemistry and tectonic setting of Lewisian clastic metasediments from the early Proterozoic Loch Maree Group of Gairloch, NW Scotland
Precambrian Res.
Neoproterozoic Gariep Orogen
The Kaigas and Numees formations, Port Nolloth Group, in South Africa and Namibia
The Gariep Belt
New Pb-Pb single zircon age constraints on the timing of Neoproterozoic glaciation and continental break-up in Namibia
J. Geol.
Geological Map of Namibia 1:250 000 Sheet 2716 Ai-Ais, Windhoek
Geological Map of Namibia 1:250 000 Sheet 2316 Rehoboth, Windhoek
Geological Map of Namibia 1:250 000 Sheet 2816 Oranjemund (provisional compilation 01/03/2010), Windhoek
Geological Map of South West Africa 1:250 000 Sheet 2218 Gobabis, Pretoria
Nature and extent of a late Ediacaran (ca. 547 Ma) glaciogenic erosion surface in southern Africa
S. Afr. J. Geol.
“Fingerprinting” tectono-magmatic provenance using trace elements in igneous zircon
Contrib. Mineral. Petrol.
Nama Group
Stratigraphy and sedimentology of the late Precambrian Witvlei and Nama Groups, east of Windhoek
Geological Survey of Namibia, Ministry of Mines and Energy, Memoir
Geochemical classification of terrigenous sands and shales from core or log data
J. Sediment. Petrol.
Cited by (3)
Ediacaran Cordilleran-type mountain ice sheets and their erosion effects
2024, Earth-Science ReviewsLithostratigraphy of a portion of the Neoproterozoic Rosh Pinah formation: Host rocks of the Western Orefield, Rosh Pinah mine, southern Namibia
2024, Journal of African Earth SciencesTonian basins record rifting of Kalahari from Rodinia and no evidence of a pre-Sturtian Kaigas glaciation
2023, Earth and Planetary Science Letters