Das Chausib-Turbiditbecken am Südrand des Damara-Orogens, Südwest-Afrika

Am Südrand des Damara-Orogens ist innerhalb der Hakos Subgroup eine Turbiditserie von 1500 m Mächtigkeit entwickelt. Sie stellt als neudefiniertes Chausib Member eine Faziesvertretung der „Hakos-Quarzite“ (Chaibis Member) dar. Die Zusammensetzung der Turbitbänke ist quarzitisch. Infolge metamorpher und tektonischer Überprägung ist eine Gradierung nach der Korngröße nicht mehr vorhanden; sie kann aber aus einer typischen Änderung des Stoffbestandes vom Liegenden zum Hangenden einer Bank abgeleitet werden. Die an sich hochfaltbare Serie liegt in einer einfach gebauten Großstruktur von 10 km Wellenlänge vor; die Faltung wurde durch eine mächtige unterlagernde Quarzitfolge gesteuert. Einige für eine Wechselfolge kompetenter und inkompetenter Lagen charakteristische kleintektonische Phänomene werden beschrieben. Für die paläogeographische Situation wird ein Modell entwickelt, nach dem die Chausib-Turbidite als Beckenablagerungen den Chaibis-Quarziten als gleichalte Schwellensedimente gegenüberstehen.A 1500 m thick turbidite series, developed within the Hakos Subgroup at the southern margin of the Damara Orogen, is defined as the Chausib Member. It represents a timeequivalent of the “Hakos-Quartzite” (Chaibis Member) in a different facies. The turbidites are quartzitic. As a result of the metamorphic and tectonic overprint, they no longer show a grading of grain size. However, a former graded bedding can be inferred from a typical compositional change from the base to the top within each bed. The turbidite series is potentially very susceptible to folding, but occurs in a simple structure with a wave-length of about 10 km. The style of folding was determined by an underlying, thick, quartzite unit. Several small-scale tectonic phenomena are described, which are characteristic for such an alternation of competent and incompetent layers. A palaeogeographic model is envisaged in which the Chausib turbidites are basinal deposits bordering the time-equivalent sediments of a submarine rise, represented by the Chaibis quartzites.A la bordure sud de l'orogène de Damara, on trouve une série de turbidites d'une épaisseur de 1500 m intercalée dans le Hakos Subgroup. En tant que Chausib Member, nouvellement défini, elle représente un faciès équivalent des quartzites de Hakos (Chaibis Member). Les bancs de turbidite ont une composition quartzitique. Mais la superposition métamorphique et tectonique ne permet plus de discerner le granoclassement qui peut cependant être déduit du changement typique dans la composition chimique et minéralogique du matériau d'un banc, du bas vers le haut. La série dont la capacité de plissement est certaine, se présente avec une structure majeure simple, d'une longueur d'onde de 10 km, le plissement ayant été handicapé par une puissante série sous-jacente de quartzite. Quelques phénomènes microtectoniques caractéristiques d'une série interstratifiée de couches compétentes et incompétentes, sont décrits. Un modèle de situation paléogéographique est développé, d'après lequel les turbidites de Chausib s'opposent comme dépôts sédimentaires de bassin contemporains, aux quartzites de Chaibis, considérés comme sédiments de seuil de même âge.На южном краю орогена Дамара внутри подгру ппы Hakos развилась серия Turbidite в 1500 м мощностью. Она представлена, как наново описанный член Chausib, фаций «кварцит ов Hakos». Банки Turbit состоят из ква рцита. В следствие пер екрытия метаморфных и тектонических проц ессов распределение по зернистости не наблю дается. Но её можно вывести из типичного изменения состава от подошвы до кровли банки. Сильное смятие в скла дки этих серий создал о крупные структурные единицы до 10 км. На характер скл адчатости повлияли м ощные слои кварцитовых пород, подстилающих е е. Описаны некоторые х арактерные чередования мелкотектонических образований. Установлено, что Chausib-Turbidite от ложились в глубине ба ссейна, а кварциты Chaibis оказались пороговыми отложени ями того же возраста. Реконструирована па леогеография данног о бассейна

Setting and sedimentary facies of late Proterozoic alkali lake (playa) deposits in the southern Damara belt of Namibia

The late Proterozoic Damara Belt of Namibia has evolved from an elaborate system of continental rifts in which the basal portion (Nosib Group) of the Damara Sequence was deposited. In the southern rift, situated at the southern margin of the Damara Belt, the Nosib Group is represented by coarse elastic sediments (Kamtsas Formation) and fine-grained partly dolomitic deposits (Duruchaus Formation). Both formations occur, with interfingering relationships over nearly the total length of the rift. In the Geelkop Dome area the pelitic-dolomitic Duruchaus Formation includes, in its upper part, a sequence of sediments that are characterized by cyclical deposition, high sodium contents, abundant albite pseudomorphs after primary evaporite minerals, and concordant solution and collapse breccias. This 300 m thick sequence has been interpreted as deposits of an alkali lake or playa complex. Based on the playa lake models of Eugster and Hardie (1975) and Rowlands et al. (1980) four distinct facies have been distinguished in the upper part of the evaporitic Duruchaus Formation: (1) the submergent lake facies, represented by laminated shales containing layers of laminated siltstone; (2) the submergent/emergent mud flat facies, represented by laminated siltstone and calcareous shale with layers of dolomitic mudstone, calcareous shaly siltstone with scapolite and laminated sandy albitic dolomite with abundant pseudomorphs of albite after primary evaporite minerals (e.g. shortite, thermonatrite, borax); (3) the exposed saline crust facies, characterized by solution breccias, “albitolite” and “albitolite” breccias, all originating from former salt crusts; and (4) the elastic marginal facies, represented by quartzites of fluviatile and partly aeolian origin. From identified primary and diagenetic to metamorphic minerals, and from the composition of several generations of fluid inclusions, it is concluded that the brines of the alkali lake were of a NaHCO3(KBClS) type. Material creating the extremely alkaline environment was derived from eroded acid-to-basic magmatic basement rocks and from coeval alkali-rich volcanism. During tectogenesis the evaporite sequence was overridden by nappes approaching from the closing Damara geosyncline. Saline crust horizons were thereby partly mobilized and the resulting mush of dolomite and crystal fragments was squeezed into the thrust planes of the nappes where they acted as lubricants which enhanced late-orogenic thrusting along the southern margin of the Damara Belt

Geodynamic Model for the Geosynclinal Development of the Damara Orogen, Namibia, South West Africa1

The Damara Orogen forms part of the Upper Proterozoic-Early Palaeozoic Pan-African belt system, parts of which have passed through a geosynclinal stage. The Damara Orogen comprising a N-S trending (“coastal”) and SW-NE trending (“intracratonic”) branch belongs to this category. Under geodynamic and sedimentary aspects, the depositional development of the Damara Orogen may be subdivided into four successive stages, namely the graben, the downwarping, the synorogenic, and the lateorogenic stage. The development of the Damara geosyncline is controlled largely by the specific evolutions of the non-contemporaneous active and inactive rift systems. Rifting started about 900 to 1000 Ma ago, when the “northern graben” was formed. This rift system remained active, as indicated by volcanic activity, until about 840 to 750 Ma ago. Due to a subsequent cooling and thermal contraction of the assumed asthenolith situated below the rift, the graben subsided differentially. This led to the development of a trough in which a thick turbidite sequence was deposited (Okonguarri turbidites). When the rate of subsidence decreased the trough was filled up by sediments and eventually overstepped by carbonates which gradually formed an extensive platform throughout the central and northern Damara Orogen (“Karibib Platform”). Rifting in the southern Damara Orogen started somewhat later than farther north. A hypothetic rift (“Khomas rift”) was formed in the crest of a major rift dome, while two subsidiary half-grabens developed at the northern and southern flanks of the dome (“central” and “southern” rifts”). The Khomas rift remained active throughout the entire geosynclinal development. Continued rifting and extension in the southern Damara Orogen was accompanied by mafic igneous activity and considerable crustal thinning and may have led to continental separation and formation of an ocean. Crustal thinning in the southern Damara Orogen initiated the development of a major basin (“Khomas Trough”). Sediments were supplied by turbidity currents, irom both the northern and the southern margins (Tinkas and Chausib turbidites) and later mainly from the east where the basin probably shallowed and perhaps closed (Kuiseb schists). The Khomas Trough may have been connected with the Gariep Belt in southern Namibia/SWA where ocean opening appears to have occurred. It is assumed that this ocean gradually narrowed towards the Khomas Trough finally to end in a continental rift at the eastern closure of the Khomas Trough. The Khomas rift may thus be interpreted as a mid-ocean rift propagating into and ending in a continent. After crustal separation in the Khomas Trough or due to a thermal relaxation of the stretched lithosphere, the entire Damara Orogen was affected by a final phase of regional subsidence. At this stage the Karibib Platform of the central and northern Damara Orogen subsided and the Kuiseb schists attained their maximum distribution in an extensive depository. The development of the coastal branch corresponds largely to that of the intracratonic branch, with the “Sesfontein graben” corresponding to the northern graben and a hypothetic rift west of the present Atlantic coast corresponding to the Khomas rift

The Join of the Coastal and Intracontinental Branches of the Damara Orogen, Namibia, South West Africa

The Brandberg West area is characterised by N-S trending and west-facing open to isoclinal folds and by a low-grade to very low-grade regional metamorphism. The regional style of deformation indicates a high structural level and, possibly, the presence of a surface of detachment which separated a comparatively thin sedimentary cover (Khomas Subgroup) from underlying less deformable pre-Damara basement rocks. Both the westerly vergence of the main folding and the low-grade to very low-grade metamorphism of the sediments are in agreement with the assumption of a basement ridge (“Huab Ridge”) extending from the large Kamanjab Inlier towards the southwest. The Huab Ridge separated, during a major part of the Damaran depositional history, the intracratonic from the coastal branch of the orogen. It is thus a major structural feature of the Damara Orogen. Syn- to late-tectonic granitic intrusions led to extensive post-s1 contact metamorphism which was superimposed on the very low-grade regional metamorphism. K/Ar datings carried out on pelites and subgreywackes on the fraction 2 μm yielded ages of 490 ± 10, 418 ± 10, 422 ± 10, and 431 ± 10 Ma. The age of 490 ± 11 Ma may represent the peak of low-grade regional metamorphism, while the younger ages may correspond to a local rejuvenation caused by late- to post-tectonic granitic intrusions