T.S. McCarthy

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.

The mound-building termite Macrotermes michaelseni as an ecosystem engineer

Many organisms create or alter resource flows that affect the com- position and spatial arrangement of current and future organismal diversity. The phenomenon called ecosystem engineering is considered with a case study of the mound building termite Macrotermes michaelseni. It is argued that this species acts as an ecosystem engineer across a range of spatial scales, from alteration of local infiltration rates to the creation of landscape mosaics, and that its impacts accrue because of the initiation of biophysical processes that often include feedback mech- anisms. These changes to resource flows are likely to persist for long periods and constrain the biological structure of the habitat. The value of ecosystem engineer- ing is discussed as a holistic way of understanding the complexity of tropical ecology.

The topography of the Okavango Delta, Botswana, and its tectonic and sedimentological implications

A topographic map of the Okavango Delta and environs has been constructed using a combination of elevation data including trigonometric beacons and spot heights from the government of Botswana, surveys of the navigable channels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. The topography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by uplift and horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear to represent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resulting depression between these arches. The Panhandle has developed along a fault, and may be largely an erosional feature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. There is no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channel location is relatively insensitive to this local topography. Moreover, marked elevation differences exist between adjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arise because of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causing crustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to the west; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge along the Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selinda spillway occupies a marked local depression, which is a graben between the Gumare fault and an extension of the Linyanti fault. It is probable that southwesterly propagation of the uplift zone associated with the incipient rift will ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.

Wetlands in drylands: geomorphological and sedimentological characteristics, with emphasis on examples from southern Africa

Wetlands are poorly documented features of many landscapes, and there is often little understanding of the geomorphological controls on their origin, development and characteristics. This paper addresses the apparent paradox of wetlands in drylands, focusing particularly on the geomorphology and sedimentology of wetlands in southern Africa. Drylands are characterized by high (but variable) levels of aridity, reflecting low ratios between precipitation and potential evapotranspiration, so wetlands can only exist where there are locally positive surface water balances for all or part of the year. Most moderate to large wetlands in drylands are thus maintained by river inflows that combine with other factors that serve to impede drainage or reduce infiltration, including faulting, rock outcrops, swelling soils, and ponding by tributary or aeolian sediments. Together with variations in sediment supply, vegetation communities, and levels of animal activity, this promotes a diverse range of wetlands that span a continuum from permanently inundated, to seasonally inundated, to ephemerally inundated. In detail, every wetland has a unique range of geomorphological and sedimentological characteristics but, at a general level, the dryland setting can be shown to impart some distinctive features. By comparison with humid region (tropical and temperate) wetlands, we propose that many wetlands in drylands are characterized by: 1) more frequent and/or longer periods of desiccation; 2) channels that commonly decrease in size and even disappear downstream; 3) higher levels of chemical sedimentation; 4) more frequent fires that reduce the potential for thick organic accumulations and promote aeolian activity; and 5) longer timescales of development that may extend far back into the Pleistocene. Additional studies of wetlands in different drylands may reveal other distinctive characteristics. Correct identification of the factors giving rise to wetlands, and improved understanding of the geomorphological and sedimentological processes governing their development, is vital for the design of sustainable management guidelines for these diverse yet fragile habitats.

The effect of vegetation on soil and ground water chemistry and hydrology of islands in the seasonal swamps of the Okavango Fan, Botswana

Abstract Evapotranspiration exceeds rainfall by a factor of three in the Okavango swamps of northern Botswana, yet saline surface water is rare. Brines develop in the groundwater beneath islands, however, and very strong lateral concentration gradients develop. These arise as a result of transpiration by trees which grow around the fringes of islands as well as by capillary evaporation of groundwater from the interior of islands. Precipitation of calcite and amorphous silica from the groundwater occurs beneath island fringes. Long-term monitoring of the water table and groundwater chemistry beneath an island in the seasonal swamps has revealed that groundwater rises and falls with the seasonal flood and that the saline groundwater remains centred beneath the island. The study shows that islands act as sinks for dissolved solids during intense evapotranspirational loss from the swamps.

Sills associated with the Bushveld Complex, South Africa: an estimate of the parental magma composition

Abstract Four distinct ages of sills intrude the floor rocks under the Bushveld Complex. The first is a metadolerite and pre-dates the Bushveld Complex. The middle two are generally related to the Complex itself. One has quench-textured olivine and orthopyroxene crystals and chemically equates to the parental magma of the Bushveld Complex. The second suite is a hypersthene microgabbro, representative of the second major injection of magma into the Complex. The last is doleritic and does not obviously correlate with any portion of the layered sequence. The sills genetically related to the Complex display several features 1. -lack of chilled margins, coarse grain size, ability to melt country rock xenoliths, extreme heterogeneity 2. -atypical of normal sills, but explicable in their present context.

Sedimentation on the Distal Reaches of the Okavango Fan, Botswana, and its Bearing on Calcrete and Silcrete (Ganister) Formation

The distal reaches or flood plain of the Okavango alluvial fan of northern Botswana are characterized by gently undulating topography with local relief generally less than two meters. The entire area is blanketed by eolian sand. Although the area is semiarid with evapotranspiration exceeding precipitation, the area is subject to seasonal flooding by annual influx of floodwater from subtropical Angola to the north. Distributary channels on the flood plain are poorly defined and consist of sinuous depressions lacking normal fluvial characteristics such as levees, bars, or incision. The flood water has very little suspended load. Higher ground on the flood plain forms islands during the seasonal flood. Elevated tracts arise by displacive, subsurface crystallization of carbonate and silica, which is induced by trees that grow on the higher ground. Sedimentation on lower-lying areas occurs by a combination of: (1) accumulation of fine clastic material derived from the flood water, and phytolithic silica, both of which are mixed into the sandy substrate by illuviation and bioturbation; and (2) precipitation of fine-grained amorphous silica from the groundwater, induced by transpiration by aquatic grasses and sedges. Accumulation of silica in the soil profile produces a proto-silcrete. This sequence grades laterally into carbonate-dominated island soils. The ultimate cause of this association is an abundance of water with low suspended load in an environment with a high evapotranspiration rate. Ganisters in ancient rocks may have a similar origin.

VEGETATION, HYDROLOGY, AND SEDIMENTATION PATTERNS ON THE MAJOR DISTRIBUTARY SYSTEM OF THE OKAVANGO FAN, BOTSWANA

This study investigated the distribution and determinants of marsh vegetation along the major distributary channel system of southern Africa’s largest wetland, the Okavango Delta, using a large, inter-disciplinary data set. Eight communities were recognized, dominated respectively by Pennisetum glaucocladum, Phragmites mauritianus, Cyperus papyrus, Cyperus papyrus/Miscanthus junceus, Miscanthus junceus, Imperata cylindrica, Pycreus nitidus and a mixed bog community. The Pennisetum glaucocladum community is situated in the Panhandle (a narrow valley reach at the head of the fan) on elevated scroll bars that are flooded seasonally for short periods (days to weeks). The Phragmites mauritianus community occurs largely in the Panhandle where channel margins have high clay contents and where soils are seasonally flooded for moderate periods (months). Fires are widespread in the Panhandle, but both Pennisetum glaucocladum and Phragmites mauritianus are stoloniferous, and meristems occur below the soil surface and are protected from fire. In contrast, the rhizomatous sedge C. papyrus dominates in situations where meristems are permanently submerged, and therefore protected from fire, such as areas of open channel water where current velocities are sufficiently low to enable the extension of C. papyrus into the channel. This situation exists where channel avulsion has recently taken place or where discharge is reduced by water loss from the channel by overspill. The semi-floating habit of C. papyrus in the channel fringe results in high hydraulic conductivities, which promotes water loss from channels and leads to sediment deposition within channels. Miscanthus junceus occurs in areas where the nutrient status of water is low, seasonal changes in water level are small, and the average water level is approximately constant over decadal time scales. It occurs some distance from the channel on the upper reaches of the Delta and progressively closer to the channel downstream such that it is the dominant channel fringe species in the distal reaches. This pattern, where a community occurs progressively closer to the channel downstream is similar for communities dominated by Imperata cylindrica and Pycreus nitidus suggesting that environmental gradients (probably nutrient supply) perpendicular to the channel axis are steep and that they are mirrored by long-range environmental gradients downstream. An analysis of hydraulic characteristics of this distributary river system illustrates that channel width varies most with variation in discharge, while channel depth and current velocity are relatively constant over the range of discharges in the study area. Since channel width is primarily a consequence of vegetation processes in the channel margin, especially the growth of the giant sedge C. papyrus it is clear that channel hydraulics are affected largely by vegetation.

Mineral-chemical and isotopic studies of Namaqualand granulites, South Africa: A grenville analogue

The northwestern part of South Africa and southern South-West Africa/Namibia is amongst the most extensive granulite terranes in Africa. This work reports the results of electron microprobe studies of minerals from two-pyroxene, cordieriteorthopyroxene (-gedrite) (-sapphirine) and garnet and/or cordierite parageneses from Namaqualand, in the N.W. Cape Province of South Africa. Determined PT conditions of prograde metamorphism based on thermodynamic calculations are 800°–900° C and ca. 6–7 Kb; and it is argued that rocks of unusual composition, notably cordierite-orthopyroxene rocks, are restites after the extraction of granitic liquid from former argillites. This interpretation is consistent with previously published data on similar rocks, and with McCarthys (1976) suggestion of extensive partial melting in the quartzofeldspathic rocks in the area.U-Pb isotopic studies of some 50 zircon fractions have been carried out and confirm an age of 1,200 m.y. for the high-grade regional metamorphism; but certain zircon populations record inherited ages greater than 1,700 m.y. Garnet-sillimanite rocks that contain retrograde kyanite reflect PT conditions of 550°–650° C and ca. 7–8 Kb; and constituent biotite has yielded a K-Ar age of ca. 950 m.y. These data, the regional stratigraphy and structure, and the mineralisation are compared with data from the Grenville Province of Canada. Notable similarities are the possible basement-cover relationships, and the calendar of tectonothermal events, while differences include the important stratiform base-metal mineralisation in the supracrustal sequence in Namaqualand, and the Cu-mineralisation in hypersthenebearing intrusives, emplaced some 1,100 m.y. ago, that are areally, and believed to be genetically, related to the granulite facies metamorphic regime.

The role of biota in the initiation and growth of islands on the floodplain of the Okavango alluvial fan, Botswana

A group of islands of varying size on the floodplain of the Okavango alluvial fan, were studied to establish the processes which lead to the initiation and growth of islands. It was found that islands are initiated by the mound-building activities of the termite Macrotermes michaelseni. These termites import fine grained materials to use as a mortar for the construction of epigeal mounds. Their activities create a topographic feature, raised above the level of seasonal flooding, and also change the physical properties and nutrient status of the mound soil. Shrubs and trees are able to colonize these mounds, which results in increased transpiration. As a result, precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds, resulting in vertical and especially lateral growth, causing island expansion.