There exist various glacial units in the Neoproterozoic strata of southern Namibia (Kalahari Craton). They were recognised and discussed in the scientific literature for at nearly 100 years (e.g. Coleman, 1926; Gevers, 1931; Schwellnuss, 1941; Martin, 1965). The Snowball Earth theory (Hoffman et al., 1998) had an huge impact on Neoproterozoic geosciences and especially outcrops of the Otavi Group in northern Namibia helped to strengthen and support this idea. Nevertheless, the Neoproterozoic glacial horizons in southern Namibia were difficult to interpret and even more difficult to correlate, due to their tectonic overprint and their scarce outcrops.
In order to correlate and differentiate the various Neoproterozoic glacial units of southern Namibia (western rim of Kalahari Craton) a multi‐method approach based on isotopic analyses on zircon grains, whole rock geochemistry, grain size measurements combined with extensive field work, mapping and sampling was applied. In total, ten sections were mapped and measured from which 33 samples were chosen for further analyses. Two of these samples represent local basement rocks, 19 the siliciclastic Neoproterozoic sedimentary cover including glacial diamictites, and twelve carbonate samples. 3474 single zircon grains were picked and measured for their dimensions (width and length). Of those, 2404 zircons were analysed with LA‐ICP‐MS techniques for their U‐Pb and Th‐U ratios in order to calculate detrital zircon ages and to obtain information about the source magma. 1535 of those gave concordant ages (90‐110 % of concordance). Further, selected zircon grains (in total 346) with concordant U‐Pb ages were analyses for their εHf(t) values. To gather more information and to be able to provide absolute ages for the Neoproterozoic glacial units the new technique of LAICP‐MS U‐Pb dating on carbonate samples was tested and gave reliable results for ten out of twelve samples (representing seven different sample locations).
Field work revealed two sections containing the Sturtian as well as the Marinoan glacial diamictites in relatively undisturbed succession that qualified as reference profiles for Neoproterozoic strata in southern Namibia: the Dreigratberg and the Namuskluft section in the Gariep Belt close to the Orange River.
All analysed samples contain a very similar detrital zircon isotopic record and the whole rock geochemical analyses confirm this interpretation. All siliciclastic samples show a general felsic provenance, with zircon ages mainly divided into two age groups (Mesoproterozoic 1.0 – 105 Ga and Palaeoproterozoic 1.7 – 2.1 Ga), reflecting four different growth and recycling events of Mesoproterozoic to Archaean crustal units. The samples have a geochemical signature of continental island arc and the zircon grain dimensions (width vs. length) are also very similar for all samples. Direct age dating of the samples based on detrital zircons was not possible caused by the lack of ages reflecting deposition times. Nevertheless, the most important differences between the various glacial horizons were found in petrographic features (diamictite pebble contents) and the age peak shift of detrital zircon U‐Pb ages (P/M ratio). Based on these and the two reference profiles correlations to other sections were achievable and the differentiation of four distinct Neoproterozoic glacial horizons for southern Namibia was possible.
Furthermore, these new results provide new insights into the Neoproterozoic Gariep Belt formation comprising Tonian rifting events, Cryogenian formation of the Arachania Terrane and final Ediacaran collision of the Rio de la Plata and Kalahari cratons.
The combination of all results reflects a continuous sedimentary recycling on the western Kalahari Craton. Comparison and statistical similarity tests based on zircon age data bases for possible source areas defined the Namaqua Natal and Gariep belts as the most likely sedimentary source areas, providing the rock material that got recycled for at least 200 Ma from Kaigas glaciation at ca. 750 Ma to Vingerbreek glaciation at ca. 550 Ma.
In addition, the lack of exotic detrital zircon ages within the two Snowball Earth events of this study suggests the interpretation of none or only very minor glacial movement confirming the idea of a completely ice‐covered Earth. The assumed Sturtian and Marinoan ages of Numees Fm and Namaskluft Mbr diamictites were confirmed by the results of U‐Pb cap carbonate dating. Based on these, a minimum duration of ca. 8 Ma for the Sturtian and of ca. 14 Ma for the Marinoan glaciation can be assumed.:Abstract
Kurzfassung
Contents
List of Figures
List of Tables
List of abbreviations
Scientific question and thesis structure
1 Introduction
1.1 The Neoproterozoic era: Supercontinent dispersal and global glaciations
1.1.1 Rodinia supercontinent: Formation, dispersal, and location of Kalahari Craton
1.1.2 Glacial events during the Neoproterozoic era
1.1.2.1 A brief history on the discovery of Snowball Earth events
1.1.2.2 Formation and termination of a Snowball Earth event: The Snowball Earth flow chart
1.1.2.3 Hypotheses for cap carbonate formation
1.1.2.4 Survival of life during a Snowball Earth event
1.2 The Kalahari Craton
1.2.1 Evolution of the Kalahari Craton
1.3 Overview over the Geology of Namibia under special consideration of southern
Namibia (Kalahari Craton)
2 Characteristics of southern Namibian Neoproterozoic glacial samples and sides
3 The problematic correlations of Neoproterozoic glacial deposits of the Kalahari
Craton (southern Namibia)
4 Methods
4.1 Field work
4.2 Whole Rock geochemical analyses
4.3 Heavy mineral separation and SEM analyses on zircon grains of siliciclastic samples
4.4 Zircon grain size analyses
4.5 LA‐ICP‐MS analyses on zircon grains
4.5.1 U‐Pb analyses with LA‐SF‐ICP‐MS
4.5.2 Th‐U ratio determination on zircon grains
4.5.3 Hf‐isotope measurements with LA‐MS‐ICP‐MS
4.6 LA‐ICP‐MS U‐Pb dating on carbonates
4.7 Provenance interpretations and likeness tests based on zircon U‐Pb age data bases
5 Study I: “The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari
Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of
cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana”
5.1 Introduction and geological setting
5.2 Samples and methods
5.3 Results
5.4 Discussion and interpretation
5.5 Summary
6 Study II: “The four Neoproterozoic glaciations of southern Namibia and their
detrital zircon record: The fingerprints of four crustal growth events during two
supercontinent cycles”
6.1 Introduction
6.2 The samples
6.3 Methods
6.4 Results
6.5 Interpretation and discussion
6.6 Conclusion/Summary
7 Study III: “Correlation of Neoproterozoic diamictites in southern Namibia”
7.1 Introduction
7.2 Sample sites
7.2.1. The Kaigas and Sturtian Numees diamictites at the Orange River section
2.1.1. Outcrops of the Kaigas Fm diamictites
7.2.1.2 Outcrop of the Numees Fm diamictites (Sturtian)
7.2.2 The Sturtian diamcitite of the Blaubeker Fm (Witvlei Grp) at the farmgrounds
Blaubeker and Tahiti
7.2.2.1 The Blaubeker diamictite at Blaubeker Farm (type locality)
7.2.2.2 The Blaubeker diamictite at Tahiti Farm (Gobabis‐syncline)
7.2.2.3 Correlation of Blaubeker diamictite at Blaubeker and Tahiti farms
7.2.3 The Sturtian diamictite at the Trekpoort Farm section
7.2.4 The Sturtian and Marinoan diamictites at Namuskluft section (reference profile)
7.2.5 The Sturtian and Marinoan diamictites at Dreigratberg section
7.2.6 Sturtian diamictite and Marinoan‐type cap carbonate at Dreigratberg North
section
7.2.7 The Marinoan diamictite at the Witputs Farm section
7.2.8 The post‐Gaskiers Vingerbreek diamictite
7.2.8.1 The Vingerbreek diamictite along the Orange River
7.2.8.2 The Vingerbreek diamictite at Tierkloof Farm (Klein Karas Mountains)
7.3 Methods
7.4 Data and Results
7.4.1 Results of the U‐Pb detrital zircon data
7.4.2 Results of the U‐Pb carbonate dating
7.4.3 Results of zircon grain width and length measurements
7.4.4 Results of the Th‐U zircon ratios
7.4.5 Results of Lu‐Hf isotopic measurements
7.4.6 Geochemical results of the siliciclastic and basement samples
7.4.7 Geochemical results of the carbonate samples
7.5 Discussion and Conclusion
8 Sediment provenance and Snowball Earth ice dynamics
9 Implications on the evolution of the Gariep Belt
10 Conclusions and outlook
11 References
Supplementary Material / Die neoproterozoischen Einheiten des südlichen Namibias (Kalahari Kraton) umfassen verschiedene glaziale Einheiten, die schon seit fast 100 Jahren bekannt sind und wissenschaftlich beschrieben wurden (z.B. Coleman, 1926; Gevers, 1931; Schwellnuss, 1941; Martin, 1965). Die Schneeball Erde Theorie (Hoffman et al., 1998) hatte einen enormen Einfluss auf die geologischen Studien des Neoproterozoikums, wobei besonders Aufschlüsse der Otavi Gruppe Nordnamibias die Theorie stärken und bestätigen. Im Gegensatz dazu sind neoproterozoische glaziale Horizonte Südnamibias aufgrund ihrer tektonischen Überprägung und der wenigen Aufschlüsse schwer zu interpretieren und zu korrelieren.
Mit dem Ziel, die neoproterozoischen glazialen Einheiten Südnamibias zu unterscheiden und zu korrelieren, wurde ein Multimethodenansatz basierend auf Isotopenanalysen an Zirkonmineralen, Gesamtgesteinsgeochemie, Mineralkorngrößenmessungen und intensiver Feldarbeit angewandt. Insgesamt wurden zehn Profile kartiert und vermessen, von denen 33 Proben zur weiteren Analyse ausgewählt wurden. Zwei dieser Proben stammen vom lokalen Grundgebirge, 19 aus den sedimentären Einheiten darüber (inklusive der glazialen Ablagerungen) und zwölf repräsentieren Karbonatgesteinsproben. 3474 Einzelzirkone wurden hinsichtlich ihrer Breite und Länge vermessen, wovon 2404 Minerale mittels LA‐ICP‐MS nach ihren U‐Pb und Th‐U‐Gehalten analysiert wurden. 1535 dieser Minerale ergaben konkordante Alter (90 – 110% Konkordanz). Darüber hinaus wurden von 346 ausgewählten konkordanten Zirkonen die εHf(t) Werte bestimmt. Um das Datenset zu vervollständigen wurden LA‐ICP‐MS U‐Pb Analysen an Karbonatgesteinen an zehn von zwölf Proben erfolgreich getestet.
Im Zuge der Feldarbeiten kristallisierten sich zwei Profile nahe des Oranje heraus, welche die Sturtian und die Marinoan Vereisung in nahezu ungestörter Lagerung enthalten und sich deshalb als Referenzprofile qualifizieren.
Alle analysierten Proben zeichnen sich durch sehr ähnliche Zirkonisotopenwerte aus, was durch die Gesamtgesteinsgeochemieanalysen weiterhin bestätigt wird. Alle siliziklastischen Proben zeigen eine generelle felsische Provenienz mit Zirkonaltern welche sich hauptsächlich in zwei Altersgruppen unterteilen lassen (mesoproterozoisch 1.0 – 1.5 Mrd Jahre, paläoproterozoisch 1.7 – 2.1 Mrd Jahre).
Diese reflektieren vier verschiedene krustale Entwicklungsstadien vom Mesoproterozoikum bis Archaikum. Die geochemische Signatur aller Proben deutet auf einen kontinentalen Inselbogen hin und auch die Zirkonmineralgrößen sind für alle Proben ähnlich. Eine direkte Altersdatierung auf Grundlage der detritischen Zirkone war aufgrund fehlender junger Alter nicht möglich. Dennoch ist eine Unterscheidung der glazialen Schichten Südnamibias basierend auf den petrographischen Eigenschaften und dem sich verschiebenden Alterstrend der detritischen Zirkone möglich (P/M Verhältnis). In Kombination mit den zwei Referenzprofilen ist eine umfassende Korrelation aller untersuchten Profile möglich und die Unterscheidung von vier Neoproterozoischen glazialen Schichten in Namibia gelungen.
Die Ergebnisse geben weitere Einblicke in die neoproterozoische Entwicklung des Gariep Gürtels, welcher durch Riftvorgänge im Tonium, die Bildung des Arachania Terranes während des Cryogeniums und die ediakarische finale Kollision zwischen den Rio de la Plata und Kalahari Kratonen geprägt ist. Die Kombination aller Ergebnisse zeigt ein kontinuierliches Sedimentrecycling auf dem westlichen Kalahari Kraton. Vergleiche und statistische Ähnlichkeitsanalysen basierend auf U‐Pb Zirkonalterdatenbanken ergaben, dass der Namaqua Natal und der Gariep Gürtel die wahrscheinlichsten Liefergebiete sind. Das Recycling fand für mindestens 200
Millionen Jahre zwischen der Kaigas Vereisung (etwa vor 750 Millionen Jahren) und der Vingerbreek Vereisung (etwa vor 550 Millionen Jahren) statt.
Darüber hinaus zeigt das Fehlen fremder Zirkonalter für die Schneeball Erde Proben, dass sich die Eispanzer kaum oder nur sehr wenig bewegt haben können, was die Theorie einer komplett zugefrorenen Erde unterstützt. Die Ergebnisse der U‐Pb Karbonatgesteinsdatierungen bestätigen des angenommene Sturtian und Marinoan Alter der Numees Fm und des Namaskluft Mbr. Basierend auf diesen Analysen kann eine Mindestlänge von etwa 8 Millionen Jahren für das Sturtian und etwa 14 Millionen Jahren für das Marinoan Schneeball Erde Ereignis angenommen werden.:Abstract
Kurzfassung
Contents
List of Figures
List of Tables
List of abbreviations
Scientific question and thesis structure
1 Introduction
1.1 The Neoproterozoic era: Supercontinent dispersal and global glaciations
1.1.1 Rodinia supercontinent: Formation, dispersal, and location of Kalahari Craton
1.1.2 Glacial events during the Neoproterozoic era
1.1.2.1 A brief history on the discovery of Snowball Earth events
1.1.2.2 Formation and termination of a Snowball Earth event: The Snowball Earth flow chart
1.1.2.3 Hypotheses for cap carbonate formation
1.1.2.4 Survival of life during a Snowball Earth event
1.2 The Kalahari Craton
1.2.1 Evolution of the Kalahari Craton
1.3 Overview over the Geology of Namibia under special consideration of southern
Namibia (Kalahari Craton)
2 Characteristics of southern Namibian Neoproterozoic glacial samples and sides
3 The problematic correlations of Neoproterozoic glacial deposits of the Kalahari
Craton (southern Namibia)
4 Methods
4.1 Field work
4.2 Whole Rock geochemical analyses
4.3 Heavy mineral separation and SEM analyses on zircon grains of siliciclastic samples
4.4 Zircon grain size analyses
4.5 LA‐ICP‐MS analyses on zircon grains
4.5.1 U‐Pb analyses with LA‐SF‐ICP‐MS
4.5.2 Th‐U ratio determination on zircon grains
4.5.3 Hf‐isotope measurements with LA‐MS‐ICP‐MS
4.6 LA‐ICP‐MS U‐Pb dating on carbonates
4.7 Provenance interpretations and likeness tests based on zircon U‐Pb age data bases
5 Study I: “The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari
Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of
cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana”
5.1 Introduction and geological setting
5.2 Samples and methods
5.3 Results
5.4 Discussion and interpretation
5.5 Summary
6 Study II: “The four Neoproterozoic glaciations of southern Namibia and their
detrital zircon record: The fingerprints of four crustal growth events during two
supercontinent cycles”
6.1 Introduction
6.2 The samples
6.3 Methods
6.4 Results
6.5 Interpretation and discussion
6.6 Conclusion/Summary
7 Study III: “Correlation of Neoproterozoic diamictites in southern Namibia”
7.1 Introduction
7.2 Sample sites
7.2.1. The Kaigas and Sturtian Numees diamictites at the Orange River section
2.1.1. Outcrops of the Kaigas Fm diamictites
7.2.1.2 Outcrop of the Numees Fm diamictites (Sturtian)
7.2.2 The Sturtian diamcitite of the Blaubeker Fm (Witvlei Grp) at the farmgrounds
Blaubeker and Tahiti
7.2.2.1 The Blaubeker diamictite at Blaubeker Farm (type locality)
7.2.2.2 The Blaubeker diamictite at Tahiti Farm (Gobabis‐syncline)
7.2.2.3 Correlation of Blaubeker diamictite at Blaubeker and Tahiti farms
7.2.3 The Sturtian diamictite at the Trekpoort Farm section
7.2.4 The Sturtian and Marinoan diamictites at Namuskluft section (reference profile)
7.2.5 The Sturtian and Marinoan diamictites at Dreigratberg section
7.2.6 Sturtian diamictite and Marinoan‐type cap carbonate at Dreigratberg North
section
7.2.7 The Marinoan diamictite at the Witputs Farm section
7.2.8 The post‐Gaskiers Vingerbreek diamictite
7.2.8.1 The Vingerbreek diamictite along the Orange River
7.2.8.2 The Vingerbreek diamictite at Tierkloof Farm (Klein Karas Mountains)
7.3 Methods
7.4 Data and Results
7.4.1 Results of the U‐Pb detrital zircon data
7.4.2 Results of the U‐Pb carbonate dating
7.4.3 Results of zircon grain width and length measurements
7.4.4 Results of the Th‐U zircon ratios
7.4.5 Results of Lu‐Hf isotopic measurements
7.4.6 Geochemical results of the siliciclastic and basement samples
7.4.7 Geochemical results of the carbonate samples
7.5 Discussion and Conclusion
8 Sediment provenance and Snowball Earth ice dynamics
9 Implications on the evolution of the Gariep Belt
10 Conclusions and outlook
11 References
Supplementary Material
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:84016 |
| Date | 08 March 2023 |
| Creators | Zieger-Hofmann, Mandy |
| Contributors | Kleber, Arno, Linnemann, Ulf, Vickers-Rich, Patricia, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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