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Neoproterozoic Low Latitude Glaciations: An African Perspective Gijsbert Bastiaan Straathof Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the University of Edinburgh — 2011 Declaration I declare that this thesis has been composed solely by myself and that it has not been sub- mitted, either in whole or in part, in any previous application for a degree. Except where otherwise acknowledged, the work presented is entirely my own. Gijs Straathof April 2011 iv Abstract The Neoproterozoic is one of the most enigmatic periods in Earth history. In the juxtaposi- tion of glacial and tropical deposits the sedimentary record provides evidence for extreme climate change. Various models have tried to explain these apparent contradictions. One of the most popular models is the Snowball Earth Hypothesis which envisages periods of global glaciations. All climatic models are dependent on palaeogeography which as yet re- mains poorly constrained for the Neoproterozoic. This thesis presents a multidisciplinary study of two Neoproterozoic sedimentary basins on the Congo and West Africa cratons including radiometric dating of glacial deposits themselves. In the West Congo Belt, western Congo Craton, a new U-Pb baddeleyite age for the Lower Diamictite provides the first high quality direct age for the older of two glacial inter- vals. This age is significantly different from previously dated glaciogenic deposits on the Congo Craton. This result strongly suggests that the mid-Cryogenian was a period during which several local glaciations occurred, none of which were global. While the palaeo- magnetic results from carbonates around the younger glacial interval are probably remag- netised, detrital zircon and chemostratigraphic results allow correlation with numerous late-Cryogenian glaciogenic deposits worldwide and a Snowball Earth scenario is favoured here. In the Adrar Sub-Basin of the vast Taoudéni Basin, West Africa, the terrigenous Jbeliat glacial horizon has been studied in great detail. Detrital zircon geochronology reveals large changes in provenance through this glacial unit with implications for sedimentological approaches and techniques for provenance characterisations based on one sample alone. Together with recently published U-Pb data these results constrain the age of the Jbeliat v Group to a narrow window providing vital geochronological information for this younger glacial event. Combining provenance geochemistry, chemostratigraphy and U-Pb dating has greatly im- proved our understanding of two of the largest Neoproterozoic sedimentary basins. The dominance of Mesoproterozoic detrital material, for which no source has been reported near either of the field areas, has consequences for the proximity of other cratons at the time of deposition, prior to the final amalgamation of Gondwana. vi Contents Declaration iii Abstract v 1 Introduction 1 1.1 The Neoproterozoic Sedimentary Record and its Apparent Climatic Paradox .2 1.1.1 Palaeogeographic Constraints . .4 1.1.2 Geochronology of Glacial Events . .5 1.1.3 The Occurrence of Warm Water Cap Carbonates . .9 1.1.4 Correlations Based on (Chemo)Stratigraphy . 10 1.2 Models Proposed to Explain Neoproterozoic Low Latitude Glaciation . 11 1.2.1 Snowball Earth Hypothesis . 11 1.2.2 High Obliquity Model . 13 1.2.3 Sedimentological Model: Zipper-rift . 15 1.2.4 The non-GAD Hypothesis: the Magnetic Field in Precambrian Times . 17 1.3 Aims of thesis . 18 2 Proterozoic Palaeogeography and Supercontinents 21 2.1 Introduction . 21 2.2 Rodinia Reconstructions . 24 2.2.1 Congo Craton in Rodinia . 26 2.2.2 West Africa in Rodinia . 31 2.3 Gondwana Configuration . 32 vii CONTENTS 3 Methodology and Procedures 39 3.1 Palaeomagnetism . 40 3.1.1 Palaeomagnetic Procedures . 40 3.1.2 Rock Magnetic Tests . 41 3.2 Geochronology . 42 3.2.1 Baddeleyite Dating . 42 3.2.2 Detrital Zircon Dating . 43 3.2.3 HR-SIMS versus LA-ICPMS . 47 3.3 Stable Isotopes . 48 3.4 Provenance Geochemistry . 49 4 Provenance of the Neoproterozoic West Congolian Group 51 4.1 Introduction . 51 4.2 Geological Setting . 52 4.3 Stratigraphy of the West Congolian Group . 53 4.3.1 Sampling . 54 4.3.2 Metamorphism . 63 4.3.3 Previous Studies . 63 4.4 Sample and Analytical Details . 64 4.5 Stable Isotope Geochemistry . 65 4.6 Baddeleyite Age for the Lower Diamictite Formation . 69 4.7 Provenance Study . 70 4.7.1 Detrital Zircon Geochronology . 70 4.7.2 Provenance Geochemistry . 80 4.7.3 Provenance Trends and Implications . 93 4.8 Discussion and Conclusions . 95 4.8.1 Age of Diamictites and Global Correlations . 95 4.8.2 Palaeotectonic Evolution . 97 5 Palaeomagnetic Constraints for the Congo Craton 101 viii CONTENTS 5.1 Introduction . 101 5.2 Previous Studies . 103 5.2.1 Congo Craton . 104 5.2.2 São Francisco Craton . 106 5.2.3 Palaeogeographic Implications . 107 5.3 Palaeomagnetic Sampling Strategy and Results . 108 5.3.1 Sampling . 108 5.3.2 Results of Palaeomagnetic Experiments . 109 5.3.3 Rock Magnetic Tests . 120 5.4 Discussion and Conclusions . 128 5.4.1 Age of Magnetisation . 128 5.4.2 Geodynamic Implications . 129 5.4.3 Continental Scale Carbonate Remagnetisation . 130 6 The West African Craton: Provenance and Palaeogeography 133 6.1 Introduction . 133 6.2 Geological Setting . 135 6.2.1 Basement Rocks . 136 6.2.2 Mesoproterozoic Evolution of the West African Craton . 140 6.2.3 Neoproterozoic Sedimentation and Tectonic Events: Amalgamation of West Gondwana . 142 6.2.4 Neoproterozoic basins . 147 6.2.5 The Adrar Sub-Basin . 149 6.3 Stratigraphy and Sampling . 150 6.3.1 Hodh Supergroup . 151 6.3.2 Adrar Supergroup . 154 6.3.3 Ahmeyim Great Dyke . 159 6.4 Results . 160 6.4.1 Detrital Zircon Dating . 160 6.4.2 Provenance Geochemistry . 166 ix CONTENTS 6.4.3 Stable Isotopes . 172 6.4.4 Age of the Ahmeyim Great Dyke . 174 6.4.5 Palaeomagnetic Results . 178 6.5 Discussion and Conclusions . 182 6.5.1 Geochronology and Correlation of Diamictites . 182 6.5.2 Sedimentological Implications of Changes in Provenance around the Jbeliat Group . 184 6.5.3 Consequences of the Mesoproterozoic Zircons for Palaeozoic Palaeo- geography . 184 6.5.4 Implications of Newly Dated Ahmeyim Great Dyke . 188 7 Summary 191 7.1 Introduction . 191 7.2 Clues for Neoproterozoic Palaeogeography of the Congo and West African Cratons . 192 7.3 Correlating Diamictites . 194 7.4 Palaeomagnetic Results: Cambrian Remagnetisation . 195 7.5 Concluding Remarks . 196 Bibliography 198 Acknowledgements 231 A Photos 233 Supplementary data on CD S1 Zircon Analytical Results 1 S2 CL Images of Zircon Grains 41 S3 Stable Isotope Analytical Results 57 S4 Whole Rock Geochemistry 61 S5 Stereographic Projections 71 x ”Het is een misverstand dat je het aan de werkelijkheid zou kunnen overlaten zichzelf te vertellen.” ”It’s a mistake to leave it up to the facts to tell themselves.” Tim Krabbé Chapter 1 Introduction The Neoproterozoic (1000 – 542 Ma) was arguably one of the most enigmatic and impor- tant Eras of Earth history. It was the most active period of crust formation and it spans the break-up of supercontinent Rodinia and formation of supercontinent Gondwana (Moores, 1991; Hoffman, 1991; Dalziel, 1991; Meert and van der Voo, 1997; Rino et al., 2008). It records the appearance of metazoan (multicellular) lifeforms (Knoll, 1991) that led into the Cambrian evolutionary explosion (Knoll and Carroll, 1999). On top of that the late Neoproterozoic rock record contains evidence for the existence of marine glaciers at low to near-equatorial palaeolatitudes (Embleton and Williams, 1986; Evans, 2000). The study of these glaciomarine deposits has gripped the mind and imagination of many researchers over the past century and is the main topic of this thesis. In the light of the Pleistocene circum-polar glaciations that are so familiar to many of us, low latitude glaciations seem to be an.