Lithostratigraphy of the Area Between the Swakop, Khan and Lower Omaruru Rivers, Namib Desert

Communs geol. Surv. Namibia, 11 (1996), 71-82 Report: lithostratigraphy of the area between the Swakop, Khan and lower Omaruru Rivers, Namib Desert M.I. Lehtonen1, T.E.T. Manninen2 and U.M. Schreiber3 1Geological Survey of Finland, Betonimiehenkuja 4, 02150 Espoo, Finland; 2Geological Survey of Finland, P.B. 77, 96101 Rovaniemi, Finland; 3Geological Survey of Namibia, P.O. Box 2168, Windhoek, Namibia The study area, situated between the Khan, Swakop and lower Omaruru Rivers, comprises rocks of the Abbabis Metamorphic Com- plex overlain by the Damara Sequence, as well as a variety of Cambrian and Mesozoic intrusives. Field studies assisted by LANDSAT image interpretation have led to a revision of the stratigraphy of the Central Zone of the Damara Orogen, which has been subject to controversy. A schist - calc silicate rock - marble sequence present between the Chuos and Karibib Formations is introduced into the stratigraphic nomenclature as the Arandis Formation, and correlated with the Oberwasser, Okawayo and Spes Bona Formations of the Karibib area. However, distinct lithostratigraphic successions are recognised in the northern and southern parts of the Central Zone, separated by the Omaruru Lineament. Introduction ous northeast trending dolerites and subordinate acidic dykes of Jurassic and Cretaceous age. The study area is situated along the central coast of Parts of the area were previously mapped and stud- Namibia between the towns of Swakopmund and Hen- ied by Smith (1965), Jacob (1974), Marlow (1981) ties Bay, and extends inland to the Khan River (Fig. and Brandt (1985). The present mapping was under- 1, inset). Tectonically it lies within the western Cen- taken within the framework of the Finnish-Namibian tral Zone of the Damara orogenic belt. The oldest rocks Geological Mapping and Maps Project with the aim of present comprise highly metamorphosed and deformed providing a more detailed lithostratigraphic description gneisses of the Mokolian Abbabis Metamorphic Com- of a previously poorly known area, and thus contribute plex, which outcrop in domal and anticlinal cores along towards clarifying the ambiguous stratigraphy of the the Khan River (Fig. 1). They are unconformably over- Central Zone of the Damara Orogen (Table 1). In ac- lain by folded and metamorphosed sediments of the cordance with the recognition of a schist and marble Neoproterozoic coarse to fine clastic Nosib Group and unit below the Karibib Formation in the study area, the the carbonates and fine-grained clastics of the Swakop subdivision of the middle Swakop Group, established Group (Damara Sequence). The Nosib and Swakop by Botha (1978) along the lower Omaruru River and Groups are separated by a paraconformable contact by Badenhorst (1987, 1992) in the Karibib area, has (Henry, 1992). On the plains northwest of the Omaruru been revised. The newly introduced Arandis Forma- Lineament extensive sand and calcrete cover prevails tion (Table 1) conformably overlies the Chuos Forma- and outcrops are scarce. tion mixtites and comprises the Oberwasser, Okawayo, Both the Abbabis Metamorphic Complex and the Spes Bona and Karub Members, of which the first three overlying Damara Sequence exhibit complex small and are correlatives of the Oberwasser, Okawayo and Spes large scale fabrics, while on a regional scale northeast Bona Formations of the Karibib District (Badenhorst, striking folds are the predominant deformational fea- 1992). The basal marble layer (Karub Member) appears tures in the mapped area (Brandt, 1985). The northeast to be restricted to the area between the Khan River and striking Omaruru Lineament transects the area (Fig. 1) the Omaruru Lineament. The Arandis Formation is ex- and separates the southern and northern Central Zone posed south of Rössing Mountain and in narrow belts of the Damara Orogen. Facies changes across it and the surrounding the domes and anticlines along the Khan preservation of higher stratigraphic levels in the north- River. Isolated outcrops are also found among the large- ern Central Zone are ascribed to vertical movement ly sand covered areas north of the lower Swakop River along this lineament (Klein, 1980). The style of folding and east of Henties Bay. appears to be tighter in the southern Central Zone. Widespread granitic to granodioritic melts intruded Lithostratigraphy of the area mapped the sedimentary sequence during the Neoproterozoic (late Namibian) and Cambrian in association with the Abbabis Metamorphic Complex various deformational events of the Damara Orogeny. In the study area porphyritic biotite “Salem” granites The highly metamorphosed rocks of the pre-Damara and red granites are common, while leucocratic granites Abbabis Complex outcrop in the cores of domes and an- are relatively scarce. Pegmatites are associated with all ticlines adjacent to the Khan River (Fig. 1). This hetero- of these suites. Younger intrusions comprise the Creta- geneous sequence, in which primary features have been ceous Henties Bay Syenite (Fig. 1), as well as numer- almost entirely obliterated, comprises augen, migmatit- 71 Lehtonen, Manninen and Schreiber 72 Lithostratigraphy of western Central Zone, Damara Belt 73 Lehtonen, Manninen and Schreiber ic, biotite sillimanite and granitic gneisses, biotite schist forms hills and ranges east of the Khan River. Contra- and amphibolite. The predominant augen gneiss con- ry to previous mapping, the Rössing Dome (Fig. 1) is sists of large (5 to 20 mm) microcline porphyroblasts in now thought to consist entirely of Etusis gneisses and a foliated quartz-biotite-feldspar matrix. Local granitic quartzites exhibiting various stages of metamorphism bodies and pegmatite dykes probably were generated and deformation (Oliver and Kinnaird, 1996). at various stages in the Damara Orogeny. Amphibolite The predominant, light-brown to pink, fine-to me- inclusions in granitic gneisses in the Khan River area dium-grained quartzites and meta-arkoses vary in ap- are up to tens of metres in thickness. They represent pearance according to metamorphic grade. Parallel either boudinaged pre-Damara basic dykes or volcanic bedding and low angle crossbedding are well preserved flows similar to metabasalts of the Naob Formation in in the Khan River area (e.g. on the farm Namibfontein the Abbabis Inlier (Marlow, 1981; Brandt, 1987). Dark 91), where the grade is comparatively low. Within the magnetite and sillimanite bearing quartzofeldspathic more intensely deformed and metamorphosed domes gneisses around the western part of the “Twin Dome” parallel bedding and cross-laminae are defined by thin (Kröner, 1984) possibly can be correlated with the bi- heavy mineral layers. The microscopic texture of the otite sillimanite gneisses and dark biotite schists of the feldspathic quartzites is predominantly blastoclastic lower Naob Formation (Smith, 1965; Brandt, 1987). (recognisable relict lithic clasts) to granoblastic. The rare rocks in which primary clastic grains remain rec- Damara Sequence ognisable consist of rounded to subangular quartz and feldspar, the latter commonly exhibiting perthitic and Nosib Group microperthitic intergrowths. Garnet is usually present Etusis Formation The up to 3500 m thick Etusis as an accessory. Formation commonly grades down into the increas- A 10 to 15 m thick intraformational conglomerate, ingly metamorphosed rocks of the Abbabis Complex. consisting of 5 to 15 cm deformed quartz and quartz- Recognition of the contact between the units is further ite pebbles in an arkosic matrix, is present on the farm complicated by the general conformity of the foliation. Stinkbank 62 (Fig. 1, inset). On the farm Sukses 90, one The arkosic and micaceous gneisses and feldspathic to 3 m thick graded beds of quartz pebble conglomerate, quartzites of the Etusis Formation outcrop in narrow fining upward into medium grained meta-arkose, occur zones around the granitic or gneissic cores of domes in the upper third of the Etusis Formation. Elsewhere, and anticlines (Fig. 2). Red syntectonic granites present the uppermost part of the formation often consists of a in some dome cores (Fig. 1) were possibly derived from some tens of metres thick red arkosic gneiss with ori- these rocks and the underlying gneisses during orogenic ented white quartz sillimanite nodules. Mica schist and events. Commonly characterised by low relief and poor calc silicate quartzite are encountered within the unit at exposure in the mapping area, the Etusis Formation various stratigraphic levels. 74 Lithostratigraphy of western Central Zone, Damara Belt Khan Formation Generally the pyroxene-, amphibole- and feldspar-bearing quartzites and gneisses of the Khan Formation form a sharp contact with the underlying Etusis Formation. In places, however (e.g. on the farms Namibfontein 91 and Vergenoeg 92), the Khan Formation is represented by a calc silicate-rich inter- layer of variable thickness within the upper part of the Etusis quartzites, indicating an interfingering relation- ship. The prominent inselberg of Rössing Mountain consists of greenish-grey scapolite-bearing calc-silicate quartzites of the Khan Formation, which in the west of the area reach up to 1000 m in thickness (Fig. 4). At Rössing Mountain they are thickly bedded, whereas thin bedding, emphasised by alternating light-coloured arkosic and greenish pyroxene-amphibole-bearing layers, is predominant in the Khan River area. Intraforma- tional conglomerates, consisting of rounded quartzite pebbles (< 10 cm) in a coarse-clastic matrix, are ex- posed southeast of Ebonyberg,

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