| This study explores and examines the geomorphology of a large intracontinental basin,approximately twice the size of Texas,situated in the northern part of Namibia.The focus is directed on how and when this depression,known as Owambo(Etosha)basin,came into being and its stratigraphic sequence,lithofacies were more of particular interest.The investigation was complemented and guided primarily by the application and interpretation of satellite-derived information,seismic,aeromagnetic data and sparsely drilled holes.Owambo basin has attracted scientific investigations for many years,mainly due to its exciting complicated geology and it's a suspect to hold some hydrocarbons of economic interest.The geology of the Owambo Basin,as mentioned above,is known from outcrops along its margins,from interpretation of seismic,aeromagnetic and gravity surveys and from a few widely spaced wells.It is an intracontinental sedimentary basin that is located on the northern border of Namibia and extends into Angola.It is floored by mid-Proterozoic granites,gneisses and infolded supracrustal rocks of the Congo Craton.The basin formed initially as the stable northern platform marginal to the evolving late-Proterozoic Damara Orogen and now contains at least 7600m and possibly 8800 m of sedimentary rocks and semi-to unconsolidated sediments.The Bouguer anomaly map of the basin suggests a deep east-west axis at about 18°30 'S.The oldest fill is fluviatile feldspathic sandstones of the early Damaran Nosib Group which was deposited during intracontinental rifting further south between 900 and 730 Ma.A brief glacial episode may have marked the end of Nosib deposition.During spreading in the Damara Orogen,between 730 and 700 Ma,up to 6000 m of platform carbonates of the Otavi Group were deposited in the basin.Initial,quiet,relatively deep-water conditions resulted in the formation of laminated dolomite with few stromatolites(Berg Aukas Formation).A gradual shallowing,possibly accompanied by the precipitation of evaporitic minerals(Gauss Formation)eventually enabled numerous stromatolites to form(Auros Formation).Changing salinities may have caused deposition of alternating limestones and dolomites.A pre-Otavi topography,particularly along the margins of the basin,was only gradually buried and is responsible for a time-transgressive base to the Otavi Group and for large-scale slumping within the basin.Major facies changes within the lower Otavi Group make it difficult to correlate across the basin.Widespread glacial conditions briefly interrupted carbonate deposition and tills,glacio-marine sediments,low-grade iron formation and thin,rare,clast-free micritic dolomites were deposited along the margins of pre-Damaran basement highs forming the edge of the basin.With the resumption of carbonate deposition during upper Otavi times,more uniform conditions developed right across the basin and many features of the type sections mapped in the east can be recognized in the west.As with the start of Otavi episode,upper Otavi deposition began in quiet,relatively deep water in a restricted basin that shallowed westwards(Maieberg Formation).General shallowing with time led to an increase in the number of stromatolites(Elandshoek Formation)and eventually to the extensive development of oolitic textures and possible local lagoonal conditions(Huttenberg Formation).A bottom topography,possibly still largely influenced by the marginal basement highs was responsible once again for extensive intrabasinal slumping.The spreading that led to the deposition of the Otavi Group and its deeper water equivalents in oceans to the south and west eventually ceased.Reversal of plate motion culminated in subduction and continental collision.The Mulden Group is a northern molasse to the Damara Orogen.It was derived from erosion of the Kaoko Belt to the west following D1 uplift and was deposited unconformably to paraconformably on the Otavi carbonates between 650 and 600 Ma.Proximal in the west and distal in the east and possibly more than 4000 m thick,the Mulden Group consists of a lower arkosic sandstone(Tschudi Formation)that fines upwards through siltstones and shales(Kombat Formation)into an upper shale and carbonate unit that contains gypsum casts(Owamboland Formation).The Otavi and Mulden rocks were folded together during the 600 Ma D2 deformation phase,the last major Damara deformation phase to affect the Owambo Basin.Metamorphic white micas from Mulden rocks near the basin margins formed at about 540 Ma and give cooling ages of 460 Ma.The basal Karoo sediments of the central Owambo Basin rest on a relative fiat post-Damara erosive surface.A major Lower Permian ice sheet to the north of the Owambo Basin and possibly coveting much of the Owambo Basin as well deposited tills in the basin and in westerly flowing,glacial valleys west of the basin(Dwyka Formation).Fluvio-glacial and fluvio-deltaic basin plain shales and peat deposits(Prince Albert Formation)overlie the Dwyka rocks.Upper Triassic Aeolian sandstones of the Etjo Formation appear to be the only other Karoo sedimentary rocks to have been deposited in the Owambo Basin.Basalts which may be equivalent to the Jurassic Kalkrand Formation occur northeast of Tsumeb.The thickest Karoo section intersected in the basin was 360 m thick.Cretaceous to Recent terrestrial deposits of the Kalahari Sequence form the final filling to the Owambo Basin and accumulated under arid to semi-arid conditions.The upper Kalahari sediments blanket all other units with the exception of those forming the folded and elevated margins of the basin.The Ombalantu Formation is a succession of red,well-bedded,friable,partly silicified,semi-consolidated Cretaceous(?)mudstones that forms the base of the Kalahari Sequence in the center of the Owambo Basin.This is followed by a calcrete-cemented conglomeratic sandstone(Beiseb Formation)which,in turn,is overlain by possibly as much as 500 m of aeolian sands with varying amounts of lacustrine clay(Olukonda and Andoni Formations)that were probably introduced by endorheic rivers flowing from the northwest into a large,shallow,semi-permanent lake similar to the present-day Okavango Swamps of Botswana.Evaporite minerals occur in places and the region north of the Etosha Pan is a large underground brine reservoir.The Etosha Pan is a super pan formed by fusion of several pans as a result of pan-edge retreat during periods of pluvial erosion of the present surface.Exploration for petroleum began in 1962 when Hunting surveys Ltd.conducted as aeromagnetic survey over the Namibian part of the Basin for Etosha Petroleum Co.(Pty)Ltd.A gravity survey by Ray Geophysical Division in 1963 covered selected areas in the western and central sectors.From these results,the stratigraphic slim-hole location was selected in a gravity minimum north of Etosha Pan.This test was completed in 1964.The following year,photo geological coverage was obtained over the most of the basin in Namibia.During 1966,fracture density anomalies were mapped from the aerial photos by Geomap S.A.(E.Merchisini,et al.).In 1968,a number of photo and fracture density anomalies were selected for detailed mapping by Geochemical Surveys,Inc.Coincident photo and geochemical anomalies were then selected for Vibroseis survey by Teledyne in 1969.From this program,locations were staked for exploratory wells on 3 large anticlines near the southwestern margin.The wells(5-1A,1-1,2-1)were drilled in late 1969-1970.Over the years,outcrops on the southern and western flanks and exposures in deep mines were studied by a number of geologists,e.g.Rabie(1964),Sohnge(1957),Martin(1965).Rabie measured maximum thicknesses of sedimentary rock in the northern Kaokoveld which,when composited,exceed 12192 meters.The Kaokoveld is on the western flank of the basin.His sections included the lower Mulden,Otavi and Nosib only.Sohnge measured the same units in the Otavi Mountains on the south side of the basin and obtained a maximum composite thickness of 6584meters.The Otavi composite is 5669 meters thick by Rabie's measurements and 5121 meters thick by Sohnge's. |
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