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Karoo-Etendeka Unconformities in NW Namibia and Their Tectonic Implications

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Karoo-Etendeka Unconformities in NW Namibia and Their Tectonic Implications Karoo-Etendeka Unconformities in NW Namibia and their Tectonic Implications Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universität Würzburg vorgelegt von Ansgar Wanke aus Korbach Würzburg 2000 Table of Content ABSTRACT.................................................................................................................................................................1 ZUSAMMENFASSUNG.............................................................................................................................................2 CHAPTER 1: INTRODUCTION..............................................................................................................................3 1.1 PHYSIOGRAPHY...................................................................................................................................................4 1.2 GEOLOGICAL BACKGROUND..........................................................................................................................5 1.3 RESEARCH HISTORY..........................................................................................................................................8 1.4 METHODS..............................................................................................................................................................9 1.5 CO-OPERATION....................................................................................................................................................10 1.6 LOCALITY DEFINITIONS....................................................................................................................................11 CHAPTER 2: THE LATE CARBONIFEROUS-PERMIAN MEGASEQUENCE AND ASSOCIATED UNCONFORMITIES...................................................................................................................................13 2.1 THE LATE CARBONIFEROUS SEQUENCE.......................................................................................................13 2.2 THE PERMIAN SEQUENCE..................................................................................................................................16 2.3 DISCONFORMITIES IN THE PERMIAN SEQUENCE......................................................................................33 2.4 PERMIAN SYNSEDIMENTARY TECTONIC ACTIVITY................................................................................35 2.5 PERMIAN TECTONIC AND CLIMATIC EVOLUTION OF THE HUAB-GOBOBOSEB AREA................38 CHAPTER 3: THE TRIASSIC-JURASSIC MEGASEQUENCE AND ASSOCIATED UNCONFORMITIES....................................................................................................................................40 3.1 STRATIGRAPHY AND FACIES OF THE OMINGONDE FORMATION......................................................40 3.2 STRATIGRAPHY AND FACIES OF THE ETJO FORMATION......................................................................44 3.3 DISCONFORMITIES IN THE TRIASSIC/JURASSIC SEQUENCE..................................................................47 3.4 TECTONIC CONTROL ON THE TRIASSIC/JURASSIC SEQUENCE.............................................................51 3.5 TRIASSIC/JURASSIC TECTONIC EVOLUTION OF NORTH-WESTERN NAMIBIA................................54 CHAPTER 4: ETENDEKA GROUP STRATIGRAPHY........................................................................................57 4.1 STRATIGRAPHY AND FACIES OF THE TWYFELFONTEIN FORMATION..............................................57 4.2 THE ETENDEKA GROUP IGNEOUS ROCKS.....................................................................................................70 CHAPTER 5: ETENDEKA GROUP DISCONFORMITIES AND ASSOCIATED TECTONIC FEATURES....................................................................................................................................................80 5.1 THE KAROO/ETENDEKA UNCONFORMITY IN THE HUAB-ETENDEKA REGION...............................80 5.2 ETENDEKA GROUP DISCONFORMITIES........................................................................................................84 5.3 FAULT ACTIVITY DURING ETENDEKA GROUP DEPOSITION.................................................................86 5.4 THE LOWER CRETACEOUS EVOLUTION OF THE HUAB-ETENDEKA REGION...................................96 CHAPTER 6: CONCLUSIONS IN A GONDWANA CONTEXT.......................................................................98 REFERENCES............................................................................................................................................................102 Chapter 1: Introduction ABSTRACT In north-western Namibia the fills of the Karoo-Etendeka depositories can be subdivided into (1) a Carboniferous-Permian, (2) a Triassic-Jurassic and (3) a Cretaceous megasequence, each recording extensional periods related to successive rifting phases in the evolving South Atlantic. The tectonic environment of the depositories in north-western Namibia changes successively from the coast towards the continental interior, which is reflected by the facies distribution and the position of time-stratigraphic gaps. Close to the present-day coastline synsedimentary listric faults, trending parallel to the South Atlantic rift (N-S), caused the formation of wedge shaped sediment bodies. Here, the Karoo Supergroup is only represented by the Permian succession in the Huab area. A hiatus within the Permian can be recognised by the correlation with the main Karoo Basin in South Africa and the Brazilian Paraná Basin. This stratal gap correlates with a pre-Beaufort Group unconformity in the main Karoo Basin that might be related to an orogenic pulse in the Cape Fold Belt. The Permian succession itself is unconformably overlain by the Lower Cretaceous Etendeka Group. This hiatus extending from the Upper Permian to the Lower Cretaceous has probably been induced by a combination of rift shoulder uplift and additional crustal doming associated with Etendeka flood volcanism. The enhanced tectonism during the Early Cretaceous controlled accommodation space for the alluvial-fluvial and aeolian deposits of the lower Etendeka Group. Disconformities within those deposits and the overlying lava succession attribute to distinct phases of tectonic and volcanic activity heralding the South Atlantic breakup. Towards the south-east, the Karoo succession becomes successively more complete. In the vicinity of Mt. Brandberg Early Triassic strata (Middle Omingonde Formation) follow disconformably above the Upper Permian/Lowermost Triassic Doros Formation. The sedimentation there was essentially controlled by the SW-NE trending Damaraland Uplift. South of the Damaraland Uplift the SW-NE trending Waterberg-Omaruru Fault zone is interpreted as a sinistral oblique-slip fault that compartmentalised the South Atlantic rift. This fault controlled accommodation space of the entire Triassic Omingonde Formation and the Early Jurassic Etjo Formation in its associated pull-apart and transtension structures. A locally well developed angular unconformity defines a hiatus between the two formations. Correlation with the main Karoo Basin in South Africa confirms that this gap is of a regional extent and not only a local, fault induced feature. Furthermore, it might also correlate with an orogenic pulse of the Cape Fold Belt. In general, the Mesozoic megasequences record the long-lived history of the southern Atlantic rift evolution. Rifting has been controlled by orogenic pulses derived from the Samfrau active margin throughout the Mesozoic. The associated intracratonic E-W extension caused the formation of grabens and conjugated oblique-slip zones. The generation of voluminous flood basalts marks the climax of intracratonic extension that was accompanied by enhanced uplift of the rift shoulders. 1 Chapter 1: Introduction Zusammenfassung Das Mesozoikum in Nordwest-Namibia gliedert sich in (1) eine karbonisch-permische, (2) eine triassisch- jurassische und (3) eine kretazische Megasequenz, welche die Entwicklung des südatlantischen Rifts widerspiegeln. Die tektonische Position der karoo-zeitlichen Ablagerungsräume Nordwest-Namibias verändert sich sukzessive von der Küste bis ins Innere des Kontinents, welches sich in der Verbreitung und Größenordnung mehrerer Hiaten ausdrückt. So dominieren in der Küstenregion listrische Störungen, die dem N-S Trend des südatlantischen Rifts folgen. Dabei zeigen keilförmige Geometrien assoziierter Sedimentkörper den syn-sedimentären Charakter dieser Störungen an. In der küstennahen Region ist die Karoo Abfolge nur durch permische Sedimente in der Huab Region überliefert. Eine Diskordanz innerhalb des Perms läßt sich aus der Korrelation mit zeitäquivalenten Ablagerungen im Großen Karoo Becken Südafrikas und dem Paraná Becken Südamerikas herleiten. Wahrscheinlich hängt dieser Hiatus mit einem orogenen Impuls im Kap-Ventana Faltengürtel zusammen. Die permische Abfolge ist wiederum diskordant überlagert von der unterkretazischen Etendeka Gruppe. Dieser Trias und Jura umfassende Hiatus ist vermutlich durch die Anhebung der südatlantischen Riftschulter verursacht worden. Die damit verbundene hohe tektonische Aktivität drückt sich deutlich in der Fazies- und Mächtigkeitsverteilung der unterkretazischen sedimentären Ablagerungen und Laven aus. Dabei spiegeln mehrere Diskonformitäten einzelne Phasen tektonischer und vulkanischer Aktivität wider, welche die bevorstehende Öffnung des Südatlantiks
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