Communications - Geological Survey of Namibia

Publications
The 1800 km divergent continental margin off southwestern Africa lies between two major orthogonal crustal lineaments: the modern Walvis Ridge abutment and the Agulhas Fracture Zone. The wider conjugate margins of South America suggest that the African side represents the upper-plate fragments of the original, Jurassic South Atlantic rift zone. Oceanic isolation during the early phases of continental separation resulted in a long period of anoxic marine sedimentation in the palaeo-southeast Atlantic. Albian to Oligocene sediments accumulated as huge marine delta/fans adjacent to the Orange River and in the bight of the Walvis Ridge. During the latter half of the Cenozoic the continental margin off southwestern Africa suffered sediment starvation and there was relatively little accumulation on the shelves, and a nett loss from the deep-sea basins because of erosion. -from Author
 
The three branches of the Pan-African (Damaran) orogenic belts in Namibia host numerous sediment-hosted (massive) sulphide (SHMS) deposits and occurrences. Genetically, these occurrences can be grouped as Sedex-type, impregnation (Maubach-Mechernich-Laisvall) type, breccia-pipe-hosted (Tsumeb-) type, and possibly in platformal domains as MVTs sensu lato. For the first group, regional distribution patterns are closely linked to the original, tectonically controlled basin geometry. The principal regional controls are the ba¬sin margins of the initial Damaran rifts and of the subsequently widening basins. High-angle second-order grabens, flanked by basement highs, can be identified along the major basin margins and have acted as funnels for metalliferous brines during sediment compaction and basin dewatering. Transform/transfer faults have provided vertically and laterally extensive pathways for fluid migration. Early Damaran volcanic rocks have provided a local source for Cu and Zn and document an anomalously high heat flow, whereas the sedimentary infill of and locally basement rocks to the basins provided the source for Pb, Zn, and Ba. Saline brine provinces give evidence that Cl-rich, brines have probably leached, stored, transported, and finally precipitated base metals in physical and chemical traps. The positions of the deposit types can be attributed to: i) early rift basin margins; ii) first- and second-order basin margins and high-angle sub-basins during basin subsidence; and iii) platform onlaps onto cratonic regions. It is primarily the pre-collisional basin geometry which has largely determined the distribution and location of the base metal occurrences within the basin, along its margins and on the adjacent platforms.
 
The Mid-Proterozoic Sinclair Sequence in southern Namibia is made up of volcano-sedimentary successions and associated high-level intrusions in a mainly extensional tectonic environment. Granitoids from the marginal Awasib Mountain area have been investigated using precise U-Pb determinations on zircons. A composite regression treatment of all seven zircon fractions indicates that the emplacement ages for these units are indistinguishable with an upper intercept age of 1216.4+1.3/-1.2 Ma (2 sigma). This places a minimum age constraint on the mafic flows of the Haiber Flats Formation and is significantly older than the 1086±44 Ma Rb-Sr whole-rock age reported previously. Tectonothermal activity at ca 1100 Ma in the Namaqua Province could account for the greenschist facies metamorphism of much of the marginal part of the Sinclair Sequence and related disturbance of the Rb-Sr system.
 
Juvenile crustal accretion in the Brasiliano Cycle occurred in the Sao Gabriel Block of southern Brazil, whereas the Dom Feliciano Belt along the Atlantic coast corresponds mainly to Brasiliano granites, gneisses and migmatites mostly derived from a Palaeoproterozoic to Archaean crust. These geotectonic units are separated by a volcano-sedimentary rift-related sequence of probable Mesoproterozoic age, strongly reworked during the Neoproterozoic. The basement rocks are represented by Palaeoproterozoic granulites and the Uruguayan granite-greenstone belts that define an important geotectonic cycle in southern Brazil, the Trans-Amazonian Cycle. However, the Brasiliano cycle was most significant in the region, defined by two main tectonic events, the Sao Gabriel (750-700 Ma) and Dom Feliciano (around 600 Ma) events. W- and NW-verging thrust-related tectonics and later strike-slip deformation are recognised in the region. Granitic and effusive magmatism is abundant as a late manifestation of the Brasiliano Cycle in southern Brazil, along with the generation of foreland sedimentary basins.
 
Gross Brukkaros, Namibia, is a broad (10 km diameter) dome structure in Nama Group country rocks, with a crater-shaped central depression. Formation of the dome structure has recently been attributed to a shallow intrusion, which subsequently became partly depleted during volcanic activity in the Brukkaros Volcanic Field, causing cauldron subsidence. The subsiding depocentre became filled by a sedimentary sequence corresponding to alluvial fan delta/lacustrine environments. This paper gives a detailed petrographical and chemical description of the Brukkaros sediments and their lithic and mineral constitutents. The main sources for the Brukkaros sediments are shales, cherts, sandstones, and quartzites from Karoo Sequence and Nama Group rocks as well as Karoo dolerites. A contribution of aeolian deposits (early Kalahari?) is also identified. Metasomatic-hydrothermal alteration and cementation of the Brukkaros rocks can be shown to have operated already during sedimentation. -Authors
 
The Carboniferous Dwyka Group in southern Namibia can be subdivided into four upward-fining deglaciation sequences, each capped by relatively fine-grained glaciolacustrine or glaciomarine deposits. The uppermost part of the second deglaciation sequence comprises a thick fossiliferous mudstone unit, referred to as the ”Ganigobis Shale Member”. An abundance of marine macro- and ichnofossils as well as extrabasinally derived ashfall tuff horizons characterise the mudstones and provide the basis for an integrated high-resolution biostratigraphic and tephrostratigraphic framework. The marine body and trace fossils record the extent of the first of a series of marine incursions into the disintegrating Gondwanan interior as early as the Carboniferous. Juvenile magmatic zircons separated from tuff horizons interbedded with the Ganigobis Shale Member yield SHRIMP-ages of 300-302 Ma. This dates the uppermost part of the second deglaciation sequence in southern Namibia to the late Carboniferous (Gzelian) and provides a minimum age for the onset of Karoo-equivalent marine deposition prior to the much more widespread Eurydesma transgression.
 
The main phase of the Kombat Cu-Pb(Ag) mineralization is interpreted as being stratabound and syntectonic. The ore deposits are located in Huttenberg Formation carbonates, on the Northern Platform margin of the Damara Province. Mineralization occurs on the contact with the overlying Kombat Formation phyllite. The deposits form a near-linear east-west trend of pendant-shaped ore loci with a strike length of 6 km. Characteristic features are the abundance of sandstone in Damaran-age karsts, Fe-Mn oxide/silicate assemblages, intense faulting, fracturing, shearing and brecciation. A strong calcite alteration halo encompasses the deposits. The calcite alteration is of various ages, and therefore not always related to the mineralizing event. The association of stratiform Fe-Mn oxide/silicate assemblages with the Kombat deposits has led to conflicting ideas regarding the genesis of these deposits. Field evidence, supported by analytical results, has led to the construction of a genetic model for the mineralization and Fe-Mn assemblages.
 
The Okenyenya igneous complex, of Mesozoic age and located in northwestern Namibia, is notable for its extremely wide range of rock types of both tholeiitic and alkaline affinities. The intrusive rocks have been emplaced by a variety of contrasting mechanisms. A detailed geological map of the igneous complex is presented, together with a structural reinterpretation of a number of the component intrusions. The identification of a major peripheral ring fault is fundamental to a new model proposed for the evolution of the intrusive complex. The ring fault is interpreted as having formed during cauldron collapse of an overlying major volcano coincident with voluminous syenitic magmatism. -from Authors
 
The Etendeka volcanic rocks of northwestern Namibia are currently defined as the upper part of the Karoo Sequence in Namibia and have thus been represented as stratigraphically equivalent to the volcanic rocks of the Karoo Sequence in South Africa, Botswana, Lesotho, Swaziland and Zimbabwe. However, the Etendeka volcanic rocks (130-135 Ma) are considerably younger than those of the Karoo Sequence (180-190 Ma) in the areas mentioned above. They are compositionally distinct from Karoo volcanic rocks, and constitute an eastern portion of the Parana Igneous Province of Brazil. -from Authors
 
The Diaz Point Formation is a highly sheared, late Proterozoic diamictite which crops out along the Namibian coast from Lüderitz to Wolf Bay. It lies with a marked structural discontinuity on gneissic basement, which forms part of the Namaqua Metamorphic Complex. Three units can be distinguished from each other by thrust faults. All lithologies are characterised by strong, bedding-parallel foliation and intense north-northwest-trending mineral and stretching lineations. A mixed fluvioglacial origin for these sediments is indicated by locally cross bedded channel forms, quartzite layers and rounding of many clasts. These highly tectonised lithologies are correlated with Nosib Group sediments of the Damara Sequence to the north and basal mixtites of the southern Gariep Group.
 
Stromatolites can be used to study environmental changes in earth's history and have been linked to the oxygenation of earth's atmosphere about 2.4 billion years ago. They have an exceedingly long geological record of no less than 3,500 million years. Their formation is influenced by biological, chemical and physical processes. Namibian specimens are known from the Neoproterozoic and Cambrian. Although the country boasts many stromatolite localities, few detailed studies have been done on stromatolites from Namibia. Well-preserved, diverse Neoproterozoic stromatolites occur on Farm Windpoort No. 428 in the Kunene.
 
Coarsely crystalline metacarbonates exhibiting post-metamorphic exaggerated grain growth fabrics and evidence for hydrothermal alteration have been investigated by means of various methods (e.g. thin section analysis, cathodoluminescence microscopy and the electron microprobe). Each rock type, consisting of graphite-bearing sparry calcite marbles and sparry dolomite marbles exhibit a complete transitional fabric development from host rock to marbles showing exaggerated grain growth. The final stage in the coarsening process leads to the growth of sparry calcite rhombohedral blasts and sparry dolomite porphyroblasts. The dominant mechanism in the sparry calcite marble is grain boundary migration, whereas the sparry dolomite marble formed by a combination of solid state processes (coalescence crystallization) and hydrothermal processes. The driving force behind this process is the reduction of interfacial free energies, leading to an increase in grain size and a reduction of the grain boundary surface area. Grain-scale fluid-rock interaction resulting from the infiltration of Fe-/Mn-rich fluids created a very complex series of cathodoluminescence fabrics in the sparry dolomite marbles. Unique zonation assemblages, formed by diffusion processes and solution/precipitation occur all along the grain boundaries and microcracks. These form an interconnected Fe-rich network and attest to extensive and active material transport. The gradual and abrupt changes in the Fe and Mn contents point to cyclical or episodic fluid influxes.
 
Exploration for aeolian diamond placers within the southern Sperrgebiet requires a thorough understanding of aeolian transport across a broad range of scales from a systemic regional level to a micro-topographic-scale at the individual particle level. Within this arid zone the transport of coarse-grained aeolian bedload, including diamonds, is driven by the impact of saltating sandflow, which produces a uniquely characteristic diamond dispersal pattern as well as a variety of coarse-grained bedload features, textures and fabrics. For the first time the Namibian Aeolian System (NAS : see Annex 1 for abbreviations) which runs along the entire continental margin from the Orange River in the south to southern Angola in the north can be studied within the context of the recently discovered Benguela Low-Level Coastal Jet (BLLCJ). The structures produced in response to the hydraulic behaviour of the BLLCJ flow are shown to influence strongly the location of zones of high-energy erosion and aeolian sand accumulation throughout the system. The boundary conditions of the Namib Aeolian Erosion Basin are redefined to include the influence of the BLLCJ on the architecture and dynamics of sandflow pathways through this high-energy aeolian erosion landscape. Empirical sandflow measurements were previously used to identify narrow, linearly extensive Aeolian Transport Corridors characterised by high sandflow conditions. The corridors are commonly marked by the development of mono-trains of large barchan dunes along their length. The advent of Google Earth Engine time-lapse video provides an observational platform enabling spatial and temporal changes in sandflow and bedforms to be examined over a 32 year period. It thus provides many new insights into sandflow-dune and dune-dune interaction throughout this large-scale system down to the resolution of individual protobarchan genesis. The influence of the hydraulic behaviour of the BLLCJ flow together with localised effects of topography on surface wind flow structure is examined through analysis of a unique 10 km wide regional Airborne Laser Scanner (ALS) dataset stretching 170 km from Chameis Bay in the south to Schmidtfeld, to the north of Lüderitz. Aeolian bedforms and erosional features and patterns of coarse-grained aeolian bedload textural features and fabrics are mapped in detail throughout the Sperrgebiet. Changes in bedform interaction are shown to reflect variation in the pattern of surface flow created by localised topographic blocking and steering effects. The new system-wide perspective on the pattern of sandflow that feeds into, and maintains, the present-day Namib Sand Sea shows clear evidence of the influence of the BLLCJ as well as a variety of different types of vortices, which is a newly recognised element in the boundary conditions of this high-energy system. It is concluded that favourable conditions exist for the formation of horizontal rolls and vortex structures of varying length-scales within the Marine Boundary Layer (MBL) associated with the BLLCJ. It is proposed that these strongly influence both the development of the erosional aeolian landscape and the pattern of bedforms that develop within Aeolian Transport Corridors in response to surface wind flow over complex topography.
 
Interpretive line drawings of Amphechinus cf. rusingensis from Berg Aukas I. A-B) P4/ (BA 63'91, reversed). C-D) m/1 (BA 52'94, reversed). A, C) occlusal, B) lingual, D) buccal views (scale: 1mm).
Updated distribution of Erinaceinae and Galericinae during the Miocene and Pliocene of Africa. Amphechinus is represented by squares, other Erinaceinae by triangles, and Galericinae by circles. 1) Legetet Formation, Hiwegi Formation and Kapurtay Agglomerates; Kenya. 2) Kulu Formation and Karungu; Kenya. 3) Maboko; Kenya. 4) Koru Formation; Kenya. 5) Sperrgebiet localities; Namibia. 6) Beni Mellal; Morocco. 7) Pataniak 6; Morocco. 8) Azdal 1, 3, 6 and 7; Morocco. 9) Berg Aukas I; Namibia. 10) Arrisdrift; Namibia. 11) Amama 1 and 2, Argoub Kamellal 1, Bab el Ahmar, El Hiout, Guergour Ferroudi and Smendou 6; Algeria. 12) Sidi Ounis; Tunisia. 13) Bou Hanifia 5, Koudiet el Tine and Sidi Salem; Algeria. 14) Oued Zra; Morocco. 15) Afoud 6, Asif Assermo and Oued Tabia; Morocco. 16) Amama 3; Algeria. Data are based on Engesser (1980), Ameur (1984), Butler (1984, 2010), Mein & Pickford (2003, 2008), Stoetzel (2013), Zouhri et al. (2017) and this work.
Material of Erinaceidae (Eulipotyphla, Mammalia) from Berg Aukas I (late middle Miocene, Namibia) is described. Originally identified as belonging to the gymnure Galerix, the specimens from Berg Aukas I are herein attributed to the hedgehog Amphechinus cf. rusingensis, and they represent the last known occurence of Amphechinus in Africa. Its persistence in Northern Namibia may have been favoured by its generalist palaeoecology and the heterogeneous aridification of southern Africa during the middle Miocene. In addition, an update of the data acquired on African Erinaceidae is provided: a migration of the Galericinae to southern Africa is no longer supported; all attributions of African middle Miocene to Pliocene material to the genus Galerix are considered to be improbable; at least two migratory waves of Schizogalerix are recognized in northern Africa with S. cf. anatolica in the late middle Miocene (Pataniak 6, Morocco) and S. aff. macedonica in the late Miocene (Sidi Ounis, Tunisia).
 
Tetraconodont suids are in general rather poorly represented in Southern Africa, except at Langebaanweg, Cape Province and the Chiwondo Beds, Malawi. We here describe material from two localities in Namibia and one in South Africa in order to fill out the fossil record of this subfamily of artiodactyls in the subcontinent. The oldest specimens (Nyanzachoerus sp. cf jaegeri) are from Kaukausib Fontein, Namibia (ca 4-6 Ma), the next oldest (Gerontochoerus koobiforaensis) are from Brad Pit ‘A’, Bolt’s Farm, South Africa (ca 3.7 Ma) and the youngest (Gerontochoerus scotti) are from Ekuma Rhino Site, northern Namibia (ca 3.2 Ma).
 
The Naauwpoort quartz porphyry formed during local synsedimentary volcanism towards the end of the initial rifting phase in the central Damara belt of Namibia. SHRIMP U-Pb analysis of zircon from the Naauwpoort rhyolite dates this volcanism at 752+-7 Ma whilst evaporation Pb-Pb analysis on the Kawakeup metarhyolite, a correlative unit from the southern Swakop terrane, suggests a minimum date of 705 Ma. The pre-D2 Okongava Diorite provides a SHRIMP zircon date of 588+-5 Ma which is much younger than previous thought. Inherited zircon cores in the diorite document the presence of ~2 Ga zircons in the source of the diorite
 
U-Pb Single and multi-grain zircon ages have been obtained for volcanic and subvolcanic rocks from three isolated outcrops of the Nückopf Formation in the Rehoboth Basement Inlier of central Namibia. One concordant U-Pb age of 1226.4 ± 10.1 Ma from abraded zircons from the type locality (Farm Nauzerus I I I ) coincides with U-Pb zircon upper discordia intercepts of 1221 ± 29 Ma and 1222 ± 46 Ma from quartzfeldspar porphyries of Farms Alberta 176 and Jan Swart 326, respectively and is interpreted as the time of magmatism. In contrast, •''"Pb/^*Pb modal ages from non-abraded and successively abraded zircons ränge unsystematically between 1219 and 1614 Ma. These surprisingly old ages are best explained by inherited Paleoproterozoic zircon xenocrysts from the assimilation of surrounding country rocks. In a regional context, the results of this study are compatible with those undertaken elsewhere along the westem and northem margin of the Kalahari Craton.
 
Careful preparation released an ammonite specimen from a block of highly indurated sandy limestone from the oyster bed of the Wanderfeld IV occurrence in Namibia. This find is important because Wanderfeld IV is one of two known marine onshore Cretaceous deposits on the west coast of Africa south of Angola. Although ammonites often permit precise dating of the beds in which they occur, the specimens from Wanderfeld IV are placenticeratids, most of which have low stratigraphic value because of the slow evolutionary changes that this family underwent and the provincialism that it shows. This is rather unusual among ammonites and thus, based solely on its ammonite record, the Wanderfeld IV site cannot be dated more precisely than probably Cenomanian to Coniacian. However, the accompanying bivalve fauna consists of rare Protocardia umkwelanensis and abundant Rhynchostreon suborbiculatum, which gives its name to the oyster shell bed. Based on the bivalves, the Wanderfeld IV occurrence is likely to be of Late Cenomanian age. Rare microfossils from the same blocks of rock as the macrofossils did not yield precise stratigraphic information. The Wanderfeld IV deposit is a bioclastic sandy limestone, representing a fully-marine, near-shore, high energy depositional environment as indicated by the presence of immature angular quartz grains in it. There are also a few well-rounded centimetric quartz pebbles in the limestone.
 
The Klein Aub area in SWA/Namibia exposes Late Proterozoic metasediments and metavolcanics of the Sinclair Sequence, which rest on a Lower to Middle Proterozoic basement. The basement is block faulted and, in the vicinity of Klein Aub, forms a major basement high, which supplied sediment into a fault bounded basin. Major faults tapped felsic and minor mafic magma to produce the volcanics of the Nückopf Formation. Continued block faulting resulted in the development of narry, deep graben sub-basins in which the sediments and volcanics of the GRauwater Formation were deposited. Major basemet uplift accompanied deposition of the Doornpoort Formation, a widespread unit of red-bed alluvial fan conglomerates, grading basinwards into sheet flood, braided stream, and aeolian sediments. Cessation of faulting was followed by the deposition of the Kagas Member of the Klein Aub Formation ina shallow lacustrine environment.
 
The Navachab deposit is a reduced gold skarn in the neoproterozoic Damara Orogen in Namibia. Calc-silicate mineralogy differs from typical gold skarns. Navachab garnets are subcalcic with up to 90 mol. % pyralspite and pyroxene contains up to 16 mol. % johannsenite. The metasomatic zonation from marble towards skarn involves an increase in Si, Fe and Mn. Initially, the zonation reflects the protolith, a banded marble, as evidenced by growth of garnet in pelitic layers and clinopyroxene in carbonate layers. Ore mineralization is divided into two parageneses: stage I containing pyrrhotite, chalcopyrite arsenopyrite and sphalerite and stage II with pyrite maldonite, bismuth and gold. The P-T conditions were determined by sphalerite and arsenopyrite geobarometry/-thermometry. Sphalerite yields pressures of 2 – 2.5 kb and a temperature of 500 oC, arsenopyrite yields temperatures of 575 + 15 oC, correlating with the HT–LP - metamorphism in the central Damara Orogen. However, P-T conditions of Navachab are higher compared to gold skarns directly associated with intrusions. This is consistent with deposits like Lucky Draw and Tillicum, that are hybrids between regional metamorphic environment and Phanerozoic plutonism. A barren, camptonitic metalamprophyre is considered to represent a redox-trap for Au-bearing fluids, and its unsual Mn- and Fe-rich composition is reflected in the skarn mineralogy.
 
The Klein Aub Fault Zone, situated 180 km south-west of Windhoek, SWA/Namibia, divides a northern, relatively undeformed tilted block from a gently folded southern block. The fault zone is situated near the boundary between coarse clastic red beds and fine-grainde psammitic to psephitic rocks, all of Middle Proterozoic age. It comprises sub-parallel, en echelon folds and systematicall orientated, steeply-dipping normal and reverse faults. Since such interrelated structures are are characteristic of major wrench fault systems elsewhere in the world, the Klein Aub Fault Zone is interpreted as a right lateral wrench fault system. The stratabound Cu-Ag deposit at Klein Aub is situated immediately adjacent to the main Klein Aub Fault. The geometry and grade of the ore bodies is partly controlled by subordinate faults of the wrench fault system. Sesmic pumping during strike lip faulting probably moved metal bearing fluids upwards through permeable zones leading to epigenetic upgrading or earlier, diagenetic mineralisation. The right lateral Klein Aub Fault can probably be regarded as the northern branch of a positive "flower structure" with its central part probably 6 km further south. The style and geometry of this fault structure is compared with experimental results of shear deformation and field observations at the San Andreas Fault, California. The wrench faul system possibly developed in response to late Damaran convergence of the rigid plates of the Angola and Kalahari Cratons.
 
The Lower Cretaceous deposits in the Huab Basin, NW Namibia, comprise fluvial and aeolian sandstones, lava flows and associated intrusions of the Etendeka Group. The sandstones formed part of a major aeolian sand sea (erg) system that was active across large tracts of the Paraná-Huab Basin during Lower Cretaceous times (133-132 Ma). This erg system was progressively engulfed and subsequently preserved beneath and between lava flows of the Paraná-Etendeka Flood Basalt Province. Burial of this erg by flood basalts has resulted in the preservation of a variety of intact aeolian bed forms. Preserved bed forms vary in type and scale from 1 km wavelength compound transverse draa to isolated barchan dunes with downwind wavelengths of < 100 m. Due to the present-day preferential erosion of the lava flows, preserved aeolian dunes are now exposed in 3-D in the position in which they were migrating ~133 Ma ago. A relatively non-destructive eruption style of inflated pahoehoe flows preserved the bed form geomorphology. These first pahoehoe flow fields, comprising olivine-phyric Tafelkop lavas, define a shallow shield-like volcanic feature. This volcanic feature centres around the Doros igneous centre marking this as the likely source for the lavas. Early lava flows followed low topography between dune build-ups, ponding in the interdune areas. Striations left on the sand surface by the lavas indicate the localised flow directions of the lava. Numerous sediment interlayers, preserved between the lava flows, record a change in palaeowind direction during volcanism. This change in climate may have been driven by the ongoing break-up of the West-Gondwana supercontinent or may be a direct result of the widespread volcanism.
 
Exploration for rare earth element (REE)-deposits hosted in carbonatites and associated rocks is challenging because of the heterogeneous distribution of REE and the variable and often complex mineralogy of such REE mineralisation. The Kieshöhe in southwestern Namibia is a subvolcanic intrusion consisting of calcite-carbonatite, dolomite-carbonatite, ankerite-carbonatite dykes, ring dykes and diatremes, and is regarded to be a part of the Kudu lineament carbonatite complexes. Importantly, it exemplifies many of the problems associated with REE exploration-particularly regarding its heterogeneous REE distribution. It is therefore an excellent site in which to explore the processes related to REE precipitation in detail. Petrography, whole rock geochemistry and microXRF imaging of xenolith-bearing and xenolith-free carbonatites provide insight into the role of silicate xenoliths in the development of a hydrothermal or supergene REE mineralisation in a subvolcanic environment. Xenolith-rich carbonatites are almost barren, whereas xenolith-free carbonatites show REE-contents of up to 10 wt. % total REE (TREE). Therefore, the probability for REE-enrichment in the various Kieshöhe carbonatites is significantly higher in the absence of silicate rock xenoliths. The REE mineralisation is dominated by REEF -carbonates, while monazite is a minor constituent. In contrast to other complexes, where Si assimilation caused REE depletion in the melt during the magmatic stage, this study demonstrates post-magmatic hydrothermal or supergene REE mobilisation and enrichment. REE remobilisation and enrichment is caused by hydrothermal or supergene alteration of silicate xenoliths (Si release) by aqueous fluid(s). In combination with previous studies this study clearly shows that silicate xenoliths have 1) a significant influence on REE-mineralisation in general and 2) may act either positively (precipitation/incorporation) or negatively (mobilisation) on the mineralisation of discrete REE phases depending upon the related syn-magmatic or post-magmatic processes.
 
Cascade tufas are common in Kaokoland, especially in the region north of Oruvandje, and there are impressive examples at Otjitamei and Ojtikondavirongo, yet they have not previously been reported in the literature. The Damaraland tufas further south at Ongongo, near Warmquelle, were described recently, and proved to be of interest on account of their fossil content (plants, gastropods, and a possible frog skeleton). The Kaokoland tufas are more numerous than those in Damaraland and are also highly fossiliferous, containing not only abundant plants and gastropods, but also vertebrates (fish, snakes, lizards, birds, mammals). The mammals are important because they indicate that some of the breccia infilling cavities in the tufas are probably of Late Pliocene and Early Pleistocene age, the first time that the age of Namibian tufas has been reasonably well determined. The geomorphological relationships of the tufa lobes reveal that they span a considerable period of time, some of the older eroded lobes probably being of Late Miocene age, overlain by Pliocene and Pleistocene tufas. Some of the breccias contain large mammal bones and teeth associated with primitive stone tools. The aims of this paper are to document the impressive tufa lobes in Kaokoland, to put on record the discovery of fossil invertebrates and vertebrates within them and to discuss the significance of the fossils for bio-chronology and palaeoecology.
 
Aturia lotzi shell from Priabonian Langental Turritella Beds, Sperrgebiet, Namibia.
Holotype specimen of Hypsorhynchocyon burrelli, from Early Miocene sediments at Grillental-6, Sperrgebiet, Namibia. Note the register number written on 'white-out' consisting of the Locality abbreviation (GT), the specimen number (50) and the year of collection (2000).
Part of the display on fossil vertebrates from Arrisdrift, Orange River Valley, Namibia, including crocodiles (Crocodylus gariepensis), land tortoises (Namibchersus namaquensis), a small-scale model of the giant carnivoran Megistotherium osteothlastes, and the large fossil dassie from the site (Prohyrax hendeyi) compared to much smaller extant dassies from Namibia (Procavia capensis).
Holotype specimen of Sperrgebietomeryx wardi from Elsabethfeld, Sperrgebiet, on display in the National Earth Science Museum, Windhoek, Namibia.
The establishment of the National Earth Science Museum at the Geological Survey of Namibia four years after Namibia's Independence has resulted in the cataloging of a large palaeontological collection. To date the fossil collection consists of more than 12,298 specimens that were collected in the Sperrgebiet since before the First World War until recently. The fossils comprise Cretaceous, Eocene, Miocene and Plio-Pleistocene plants, invertebrates and vertebrates which have been studied and published by many scientists such as Böhm, Stromer, Lang, Hopwood and Lavocat. The collections are frequently visited by researchers from all over the world.
 
Top-cited authors
Clark Isachsen
  • The University of Arizona
Karl Heinz Hoffmann
  • Geological Survey of Namibia
Richard Armstrong
  • Australian National University
Ian Stanistreet
  • University of Liverpool
Harald Stollhofen
  • Friedrich-Alexander-University (FAU) Erlangen-Nürnberg