The Karoo-aged basins evolved from assembly to break-up of the supercontinent Gondwana and were filled by denudating major mountain ranges accompanied by vast sedimentary recycling processes. A succession of rift episodes caused the emergence of a great number of these intra-cratonic basins throughout the Gondwana interior, e.g. the Aranos, Karasburg and Huab basins, which are scattered across today’s Namibia. This evolution may be split into a Permian to early Triassic and a Jurassic phase. The Karoo I phase is confined by ‘passive’ continental rifting and a retro-arc extension at the SW margin of Gondwana. The early Jurassic Karoo II rifting phase of east Africa eventually disintegrated Gondwana and led to the opening of the West Indian Ocean. A terminal early Cretaceous rifting phase led towards the opening of the southern Atlantic Ocean and ended the Gondwana supercontinent sedimentary regime. In the course of this evolution, the Namibian late Paleozoic to Mesozoic sedimentary record yields evidence for changing climates from icehouse towards extreme hothouse conditions. As based on sporadic datings the timeline of this evolution remains mostly unclear. The lack of data is in great contrast to the importance of determining the speed of major climate changes. In addition, sediment fluxes within such a supercontinent regime are not well studied but are key in understanding sediment dynamics during severe ecological and environmental changes. Therefore, this thesis tries to establish a timeframe of the sedimentary deposits for the Namibian Karoo Supergroup sedimentary deposits and furthermore tries to explore the laws of sediment dispersal prevailing in southern Gondwana. In order to answer these research questions a comprehensive dataset comprised of 41 samples with more than 5.700 U-Th-Pb LA-ICP-MS age determinations and over 1.000 Lu-Hf isotopic measurements on single zircon grains of siliciclastic rock material of the vast majority of all Permo- Carboniferous to early Cretaceous Karoo-aged Namibian formations was compiled. All of the investigated zircon crystals were also studied with respect to their grain morphology, including length, width, surface, and roundness, providing valuable information concerning transport distances and energies. In combination with whole-rock geochemical data of a majority of the investigated samples, they help deciphering the sedimentary deposition history during the Gondwana supercontinent cycle. A compiled set of southern African U-Th-Pb zircon age data is of great help interpreting sediment fluxes and inferring provenance areas. The onset of Karoo-aged sedimentation is recorded within the Aranos and Karasburg Basin successions and is represented by glacially induced diamictites of the Dwyka Group partially resting directly on pre-Cambrian basement complexes. In places, two distinct E-W directed ice advances are present. The deposition of these glacial diamictites was prior to 296 Ma, as two ash beds incorporated within the overlying shale successions yield Asselian deposition ages. Further hints concerning ice-induced deposition disappear at the Sakmarian-Artinskian boundary, as the lowermost succession of the Ecca Group yields a maximum deposition age of ca. 290 Ma, documenting the end of the Dwyka ice age in the southern Namibian area. The lowermost Ecca Group deposits of the Huab Basin yield a maximum deposition age of ca. 295 Ma, suggesting an earlier termination of the Dwyka ice age in the north. The uppermost strata of the Aranos and Karasburg Basins were dated ca. 265 Ma and 255 Ma, respectively, revealing a disparate depositional history. Due to a lack of datable ash beds as well as no detrital zircon grain ages near the assumed sedimentation age it was not possible to determine a detailed sedimentation history for the Huab, Kunene River, and Waterberg Basin deposits. Detrital zircon U-Th-Pb ages are routinely used in order to trace siliciclastic sedimentary rocks to their bedrock sources, deriving transport directions. This classic ‘source-to-sink’ approach is most likely obscured by several cycles of sediment homogenization processes. A majority of all investigated samples yield high portions of detrital zircon fractions of late Mesoproterozoic (950-1150 Ma) and Neoproterozoic (440-650 Ma) age. In addition, all Jurassic and Cretaceous samples yield a prominent Permian age fraction of 250-280 Ma, suggesting a Gondwanides orogen provenance. Thus, the investigated siliciclastic rocks consist of already recycled sedimentary material. This observation is supported by a high degree of zircon grain roundness. As of zircon grain hardness long transport distances are necessary to achieve latter. This suggests that one sedimentary sink is source for the next sedimentary cycle. A comparison with the detrital zircon record of other southern Gondwanan Permo-Carboniferous successions shows similar results, strongly pointing towards a supercontinent-wide sedimentary recycling regime. Therefore, detrital zircon age patterns within supercontinent scenarios reflect large-scale sedimentary processes rather than primary provenance information.:1 Introduction 1
1.1 Evolution of the Namibian landscape from Carboniferous to Cretaceous times 2
1.2 Thesis format 5
1.3 References 5
2 Methods 10
2.1 Sample preparation and zircon morphometrics 10
2.2 U-Th-Pb age determination 10
2.3 Lu-Hf model age determination via LA-(MC)-ICP-MS 12
2.4 Geochemical analysis 12
2.5 Comparative statistics 12
2.6 References 13
3 Study I: The Permo-Carboniferous Dwyka Group of the Aranos Basin (Namibia) – How detrital zircons help understanding sedimentary recycling during a major glaciation 15
3.1 Introduction 17
3.2 Regional geological setting 17
3.2.1 Geology of the Permo-Carboniferous Dwyka Group (Aranos Basin) 19
3.2.2 Paleotectonic significance of the Dwyka formations 22
3.3 Methods 24
3.4 Results 26
3.5 Discussion 30
3.5.1 Significance of zircon morphologies for sediment fluxes 32
3.5.2 Potential sedimentation rates and source areas indicated by U-Pb age data 35
3.5.3 Implications for the evolution of the Dwyka Group 42
3.6 Conclusions 46
3.7 References 47
4 Study II: The evolution of the southern Namibian Karoo-aged basins: Implications from detrital zircon geochronologic and geochemistry data 63
4.1 Introduction 64
4.2 Geology of the Aranos and Karasburg basins 66
4.3 Tectonic and structural framework of the southern African Karoo aged basins 70
4.4 Methods 71
4.5 Results 75
4.6 Discussion 80
4.6.1 Timing of the Formation of the Aranos and Karasburg basins 80
4.6.2 Provenance and evolution of the upper Paleozoic Aranos and Karasburg basins 85
4.6.3 Implications for the Karoo-aged basin sedimentary record 93
4.7 Conclusions 95
4.8 References 98
5 Study III: Mesozoic deposits of SW Gondwana (Namibia): Unravelling Gondwanan sedimentary dispersion drivers by detrital zircon 109
5.1 Introduction 110
5.2 Geological background 113
5.2.1 Evolution of the southwestern Gondwanan Mesozoic successions 113
5.2.2 Namibian Mesozoic successions 117
5.3 Methods 120
5.4 Results 123
5.5 Discussion 130
5.5.1 Protosources of the sediments 130
5.5.2 Recycling dynamics of the Mesozoic sediments 133
5.6 Conclusions 139
5.7 References 140
6 Study IV: Tracing southern Gondwanan sedimentary paths: A case study of northern Namibian Karoo-aged sedimentary rocks 153
6.1 Introduction 154
6.2 Geological setting 156
6.2.1 SW Gondwanan rifting history and sediment dispersal 156
6.2.2 The northern Namibian Karoo-aged Huab Basin and Kunene section 157
6.3 Methods 161
6.4 Results 165
6.5 Discussion 175
6.5.1 Timing of deposition of the Huab Basin strata 175
6.5.2 Protosources of the sediments 176
6.5.3 Detrital zircon grain morphology and isotope analysis 178
6.5.4 The northern Namibian Karoo-aged basins within the southern Gondwanan framework 185
6.6 Conclusions 186
6.7 References 187
7 Conclusions and outlook 199
8 Supplements 201
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:75820 |
| Date | 31 August 2021 |
| Creators | Zieger, Johannes |
| Contributors | Kleber, Arno, Linnemann, Ulf, Breitkreuz, Christoph, Technische Universität Dresden, Senckenberg Naturhistorische Sammlungen 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 |
| Relation | 10.1016/j.jafrearsci.2019.103555, 10.1080/00206814.2020.1795732, 10.1007/s00531-020-01864-2 |
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