Ian G. Stanistreet

The Okavango Fan and the classification of subaerial fan systems

Abstract Controversy exists over the classification of fluvially formed subaerial fans. Authors either restrict alluvial fans to debris flow dominated types or extend the spectrum to fans dominated by braided rivers. The Okavango Fan provides an end member which extends this spectrum to fans dominated by meandering and low sinuosity rivers. The fan is a large (150 km radial axis) shallowly sloping (0.00036), highly vegetated subaerial fan, which can be subdivided into four subenvironments. These are: (1) the entry corridor or Panhandle characterized by single and anastomosing meander belts; (2) the upper fan characterized by meander belts diverging from the fan apex, comprising peat-confined meandering channels, with interchannel swamps forming thick peats; (3) the middle fan with highly confined single and anastomosing low sinuosity rivers and less common prograding meander belts whose channels are confined by thick peats with, however, little chance of peat preservation; and (4) the lower fan in which annual floods from relatively unconfined channels spread over the fan surface and interact with pre-existing aeolian and lacustrine deposits. With the recognition of this new fan type, the spectrum of subaerial fan types can be expressed in terms of: (1) debris flow dominated fans of which the Death Valley fans are a member; (2) braided river dominated fans of which the Kosi Fan is a member; and (3) low sinuosity/meandering (losimean) river dominated fans of which the Okavango Fan is a member. This spectrum can be expressed in terms of variation in slope, maximum size and percentage of surface vegetation, but crucial to the evolution of the various fan types is variation in the flashy to continuous nature of the discharge and the degree of channel confinement evident on the fan surface. Comparable ancient examples of the three fan types are recognizable, many of which provide intermediates between the ideal end members. Debris flow dominated and braid dominated fan types are known from throughout earth history. However, the losimean fan type, because of its reliance on confining vegetation, may only have developed after the Devonian Period. The spectrum of subaerial fan types can be expressed on a triangular field of variation with the vertices defined by the relative importance of the processes which shaped a particular fan system be they debris flows, processes associated with braided rivers or processes associated with meandering and low sinuosity rivers.

Cherts of the Barberton Greenstone Belt Interpreted as Products of Submarine Exhalative Activity

Cherts studied from the southern Barberton greenstone belt are identified as replacement products of various pillow lavas, pyroclastic deposits, and sedimentary deposits. Silica-filled fracture systems within the cherts have been found to be associated with this silicification process. These systems include: (1) silica-filled fractures; (2) silica vein stockworks; (3) silica-cemented breccias; and (4) pseudoconglomerate bodies. Some of the silica-filled fracture systems can be shown to have formed soon after sedimentation. Silica vein stockworks can be seen to develop laterally into silica-cemented breccias, the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate. The cherts are interpreted as silicified hemi-pelagic sediments that overlie pillow lavas, some of which are now also cherts. Cells of convective sea water interacted with oceanic crust to produce silica-rich fluids. These fluids silicified lava and sediments on re-entering the near surface environment and produced silica veins and breccias. Silica deposition was mainly confined to the subsurface, as in Phanerozoic exhalative systems. The relatively large volumes and widespread occurrence of replacement cherts in some greenstone belts might result from differing Archean tectonic conditions or crustal compositions. Features used previously to identify shallow water conditions in some cherts of both Phanerozoic and Precambrian age may be products of convective systems of the type described here. The features described in this paper also have implications for greenstone belt mineralization and for the oxygen isotopic composition of Archean cherts.

Sedimentary basinal responses to a Late Precambrian Wilson Cycle: the Damara Orogen and Nama Foreland, Namibia

Abstract In the Damara Orogen sedimentary basinal responses are important in recording the evolution of the fold belt. Here we integrate sedimentological patterns and tectonics to characterise the basin development of both the pre- to syn-orogenic Damara Sequence and the syn- to post-orogenic Nama Group. The evolution of an entire Late Proterozoic Wilson Cycle involved initial rifting, with the opening of two oceanic arms through convergence to collision and foreland basin development. Rift initiation (stage 1) took place along old tectonic weaknesses and extensional rift basins (stage 2) were filled by continental sediments and alkaline/bimodal volcanics. Two oceanic openings occurred: (i) the Adamastor Ocean (stage 3) produced a break-up unconformity and eastward transgression over the Kalahari and Congo Cratons; and (ii) the Khomas Sea gulf subsequently developed betwen the two cratons (stage 4) and is associated with break-up unconformities, and ultimately the development of mature shleves (stage 5). In the latter opening, we envisage an anticlockwise rotation of the Kalahari Craton with respect to the Congo Craton. During convergence the closing Khomas Sea produced an accretionary prism/arc/retro-arc system (stage 6) and the first deformation phase in the Southern Zone. The Khomas Orogeny records the collision between the Kalahari and Congo Cratons (stage 7) including the obduction of oceanic elements onto the Kalahari Craton foreland, and caused the second and third deformation phases in the Southern Zone and first and second deformation phases in the Central Zone. A peripheral foreland basin and peripheral bulge on the Kalahari Craton resulted, which respectively contained and affected the marine and fluvial Nama Group sedimentation. A complementary hinterland basin accepted Mulden Group sediments on the Congo Craton. Ultimately the collision of the South American continent with the newly reconstituted African foreland (stage 8) caused the Adamastor Orogeny and produced a peripheral foreland basin. This basin was divided into two parts by the extant Khomas mountain belt. This event resulted in a dominant westerly source for the fluvial Fish River Subgroup and exclusion of previous marine conditions. We distinguish for the first time, the temporal separation of the Khomas and Khomas and Adamstor Orogenesis. The Khomas Orogeny involved the subduction of hot young oceanic crust associated with the relatively short residence time of the Khomas Sea, and can be dated at just before the Precambrian-Cambrian boundary. In contrast, the Adamastor Ocean had a residence time of about 200 M.a. Convergence therefore involved the subduction of cooled oceanic crust and incorporation of exotic terranes. Collision was associated with relatively major translation of tectonostratigraphic units. The Adamastor Orogeny occurred close to 500 M.a.

Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia

Channel-shaped deposits of well sorted sand with only 2–4% fine-grained material, being either massive and structureless or upward fining with basal lag, are found interbedded with aeolian sand in the Skeleton Coast Erg, Namibia. Detailed analyses of the channel-shaped deposits suggest that they are formed as hyperconcentrated flows within the erg. Grain-size analysis and whole rock geochemical modeling revealed that some of the fluvial sediments contain up to 70% aeolian sand, interpreted as a result of dune collapse into the fluvial system. In certain cases, this instantaneous supply of sand resulted in generation of sandy mass flows with laminar flow behaviour. The presence of smectite as dominant clay mineral proved to be of crucial importance in formation of mass flow deposits. These mass flows had no erosional capacity, and drape the palaeotopography. They are comparable to those generated by catastrophic collapse of dunes, described in the literature. This paper suggests that all these deposits should be termed intra-erg mass flows, as several of them carry little, if any debris. Based on their origin, intra-erg mass flows can be divided into two groups:

A storm surge origin for sandstone beds in an epicontinental platform sequence, Ordovician, Norway

Abstract The upper Ashgillian (Ordovician) sediments of the Oslo area were deposited on an extensive open marine platform. Within the sequence thin sandstones (0.5–10 cm thick) are interbedded with bioturbated silty shales. The beds commonly have sharp lower surfaces and more diffuse tops which suggest sand was rapidly introduced into a tranquil environment. Estimates suggest that the deposition of a sand bed was a very infrequent event, recurring every few thousand years. The sandstones show lateral changes in frequency of occurrence, and total cumulative thickness, which, taken together with palaeocurrent evidence, indicate derivation from the west. Vertical changes in thickness of sandstone per metre, mean bed thickness, bedforms and grain size all reflect an increasing proximality to a westerly sediment source and can be related to a shoreface with eastward longshore drift developed during the upper Ordovician regression. The general reconstruction of Ashgillian palaeogeography in the Oslo region suggests that the sand beds were deposited by currents flowing offshore, obliquely down a low palaeoslope. Most normal currents on modern continental shelves flow parallel to the shoreline, so we have considered other mechanisms which might have produced low-frequency, high-energy currents which flowed offshore and which could have been responsible for the deposition of the thin, discrete sandstones. Three models are discussed for the formation of the thin sandstones, and an ebb current generated by a storm surge is considered the most likely explanation.

Changing tectono-sedimentary scenarios relevant to the development of the Late Archaean Witwatersrand Basin

Abstract Recent studies of the Witwatersrand Basin indicate that contrary to earlier proposals the shape and size of the basin and style of sedimentation were very different early and late in basin history. This reflects a geotectonic evolution which may involve an overall Wilson cycle. Progressive stages prior to, during and after basin development are described in order to map this evolution. Stage 1 saw the structural framework of the Kaapvaal Craton which was to accommodate future basin development. Fault zones representing lineaments and sutures associated with the structural evolution of various greenstone belts of the Kaapvaal Craton were rejuvenated (especially during late stages of Witwatersrand Supergroup deposition) to act as primary synsedimentary fault zones. Stage 2 involved the development of the Dominion Basin which has been interpreted as an extensional basin associated with NNE trending faults, although volcanics show calc-alkaline affinities. Stage 3 saw widespread deposition over the Kaapvaal Craton of a variety of subtidal sediments of the West Rand Group and its correlatives including the Pongola Sequence. This epicontinental style of sedimentation was either associated with a thermal cooling phase related to the Dominion Group extension or with the initiation of a foreland basin. The conformable or disconformable relationship between the West Rand Group and the underlying Dominion Group would point to the former. During stage 4 major changes in sedimentary patterns occurred. Upper Central Rand Group sediments were deposited and draped over compressive basement block-faults, which interacted with the sedimentation causing reverse faults/ monoclines and locally provided sources for coarse gravel. The basin was shrinking considerably in size and fluvial sediments derived from developing pediment areas and uplifted basement interacted with a reduced marine water body. The basin was controlled by NE-SW directed compression which caused a relative tectonic escape of the central part of the Kaapvaal Craton to the SE. The reason for these changes was the initial orogenesis associated with the subduction history preceding and the ultimate A-subduction history during the collision of the Zimbabwe Craton with the Kaapvaal Craton. Stage 5 saw the outpouring of the Klipriviersberg Group flood basalts into the Witwatersrand Basin. Early in this stage the tectonism was similar to that for stage 4, but late in the extrusive history a change in feeder dyke patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the Platberg Group. The latter developed in an extensional rift basin caused by the tectonic inversion of faults from their previous reverse state to a normal movement. This has been interpreted as the result of impactogenal rifting. In the aftermath the collapse of the orogen allowed the deposition of the Pniel sequence and Wolkberg Group in extensional rift basins and the associated thermal cooling led to the development of the Chuniespoort Group chemical sedimentation. At no time did the Vredefort Dome affect this sedimentary history, in fact this was a post-Bushveld event which was followed by the emplacement of the Johannesburg Dome. The Vredefort Dome did, however, have a major structural impact on the Witwatersrand Basin and may have played a major role in its long term preservation up to the present day.

Landscape distribution of Oldowan stone artifact assemblages across the fault compartments of the eastern Olduvai Lake Basin during early lowermost Bed II times.

The density and composition of Oldowan stone artifact assemblages deposited during the first ca. 20,000 years of lowermost Bed II times show a recurrent pattern of variation across recognized synsedimentary faults that compartmentalized landscapes of the eastern Olduvai Lake Basin. When active, the faults created minor topographic relief. The upthrown fault footwalls accumulated assemblages with relatively high densities of artifacts, including types retaining potential usefulness, particularly volcanic flaked pieces, manuports, pounded pieces, and split cobbles. Values for these assemblage characteristics decline toward the lower-lying hangingwall of the fault compartments, accompanied by an increase in the proportionate weight of artifact assemblages comprising quartzite, particularly flaking shatter and potentially useful detached pieces. Values reverse once again at faults, either on the downthrown, hangingwall side or on the upthrown side. The patterns are stronger for the volcanic components of the artifact assemblages than for the quartzite components, reflecting the additional influence of distance from the local source on quartzite assemblage characteristics reported previously. The landscape distributions of artifact assemblages are consistent with a landscape-fault model in which minor fault-induced topographic relief at times created a mosaic of vegetation environments repeated within each of the three fault compartments of the lake margin and distal alluvial fan. The fault-compartmentalized landscape model is currently supported only by sediment thickness and facies changes across synsedimentary faults, but it provides predictions for spatial variation in the cover abundance of trees, freshwater reservoirs and associated distributions of resources and hazards associated with stone artifact use and discard that can be tested if sample sizes of key paleoenvironmental indicators are increased.

Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)

Abstract The Skeleton Coast erg forms a prominent NNW trending dune belt, 6– 22 km wide, comprising dunes up to 50 m high, sub-parallel to the South Atlantic margin of Namibia. To a variable degree along its strike, the dune belt dams west-southwestward flowing ephemeral river systems on their route towards the Atlantic Ocean. The southern rivers, Koigab, Uniab and Hunkab, affecting the erg, are characterized by short, infrequent ephemeral flows from restricted catchments, whereas northern rivers, Hoanib and Hoarusib, are fed by large catchment areas favouring higher discharge and more frequent flows as the intertropical convergence zone and greater likelihood of monsoonal rainfall influence is approached. The potential for river damming increases in the same downwind direction through the erg because of increasing dune belt width and height and the change from low barchanoid dune forms in the south to large composite transverse dunes in the north. Whereas cross-sections of the southern part of the erg show considerable asymmetry with low barchans and sinuous crested dune forms eastwards but a high dune wall at the western edge, cross-sections over the northern erg are more uniformly symmetrical. As a consequence, the northern rivers are effectively dammed at the eastern erg margin, causing flood reservoir basins there. In contrast, southern rivers are dammed within the erg as the most pronounced and commonly encountered barrier is provided by the dunewall at its western border.

Low sinuosity and meandering bedload rivers of the Okavango Fan: channel confinement by vegetated levées without fine sediment

Abstract The river systems of the Okavango Fan negate present fluviological perceptions that fluvial geometry is primarily dependent upon the type of sediment load being carried by the river. In northwest Botswana, meandering and low sinuosity rivers, both of which may show anastomosis, are distinctly bedload in character. This is mainly because of a restricted source of clastic sediment, consisting of aeolian sand from the Tertiary to Recent Kalahari Basin. All that seems to be required, therefore, to control low sinuosity and meandering geometries is adequate confinement of channels. In the study examples this is provided by heavily vegetated levees comprising peat, formed from and colonised by a Cyperus papyrus dominated flora: fine sediment plays almost no role in the confinement process. Active and abandoned examples of low sinuosity river channels were studied. An inversion of topography develops in the latter, caused by the low survival probability of metres thick peat levees. Desiccation and burning of peat ultimately form a degraded ash layer only tens of centimetres thick. The channel sand then stands as a ridge rising above the surrounding fan surface. In both examples studied, no crevasse splays occur, but hippopotami trails breach the levees to form minor distributary channels which become filled with sand. The sand ultimately grades backwards to plug the breach. Meander belts are also developed, particularly in the upper fan and entry corridor. Cut banks incise into the sand substrate and scroll bar topographies can be discerned beneath their peat cover. Fine sediment plays no role in the confinement of these channels, which are maintained by peat levees similar to those encountered in the low sinuosity river channels. The recognition of these bedload low sinuosity and meandering river channels now completes the matrix, whereby any geometry of river channel can be developed in bedload, mixed load and suspended load rivers. Important aspects of the modern channels for the study of ancient river systems are (1) the confining effect of peat levees which have low preservation potential geologically, and (2) in helping to explain river systems which show contradictory evidence of semi-arid and more humid climatic conditions. The latter may be explained as perennial rivers entering an otherwise semi-arid climatic regime from a distant source, which provide a shifting “more humid” overprint to the inherent environmental characteristics.

Lithostratigraphy and depositional environments in the Waterberg-Erongo area, central Namibia, and correlation with the main Karoo Basin, South Africa

Abstract The dissected landscape of the Waterberg-Erongo area, central Namibia, exposes Karoo-equivalent strata deposited in basins that occur throughout sub-Saharan Africa. Although many are of economic interest, including coal-bearing strata, their depositional history is not well understood. This study of the Waterberg-Erongo area provides detailed lithostratigraphical data, which suggest sedimentation from the late Early Triassic to the Early Jurassic in a fault-bounded depository. Subsidence and sediment supply were controlled predominantly by the northeast-southwest trending Waterberg-Omaruru Fault Zone, which defines the northwestern margin of the depository. Facies development and thickness distribution of the Karoo strata in the Waterberg-Erongo area, perhaps the most continuous of any of the Karoo basins, indicate a northeastwardly-migrating depocentre alongside that fault, in response to major extensional movements in the early pre-South Atlantic rift zone. Periodic fault movements repeatedly caused basinward progradation of the alluvial facies, which are reflected by stacked fining-upward cycles in the lithological record. On a broader scale, the results of this study suggest that the northward propagation of the rift zone between Southern Africa and South America, was partially accommodated by transfer lineaments. Local depocentres developed along these lineaments, such as those in the Waterberg-Erongo area, with localised enhanced subsidence greater than that revealed in other Namibian onshore exposures, dominated by the rifting itself.