Fluvial deposits of the middle Kuiseb valley, Namibia

JournalofArid Environments (1983) 6, 333-348

Fluvial deposits of the middle Kuiseb valley, Namibia M. E. Marker *

Accepted 27July 1982 A variety of fluvial depositsof various ages occur in the middleKuisebvalley in the arid Namib. The characteristics, lithology, spatial and altitudinal relationshipsand possible chronology of thesefluvial deposits are considered. A framework of eventsis suggested and theseeventsare shownto havewiderthan localsignificance. It is postulated that onlyone majorand general arid phasehas occurred attendant on the establishment of the Benguela Current in the late Tertiary period. Present conditions are less arid. This conclusion is at variance with earlier chronologies: it postulates that majorfluvial activity is probably mid Tertiary periodat latest. Introduction

Landforms in arid regions are frequently out of phase with present processes, owing their preservation over long time periods to the arid environment. Elucidation of their development may indicate a sequence of changes in that environment. The middle Kuiseb valley in the Namib desert is characterised by valley terrace deposits indicating a long history of cut and fill, the product of fluctuations in the river regime. The relict fluvial landforms of the Kuiseb valley from 16°OO'E to 5 km west of 15°OO'E will be documented, their lithological characteristics considered and their altitudinal and spatial relationships discussed (Fig. I). Although some profiles have previously been described (Marker, 1977a; Marker & Muller, 1978; Wieneke & Rust, 1973), these will be collated with new information and the evidence drawn together to provide a greater perspective. Some attempt will be made to link the Kuiseb sequence to that of other drainage systems. Lastly an .explanatory framework of the phases of cut and fill will be presented and linked to the dating available to act as a working hypothesis for further analysis. The Kuiseb valley divides the central Namib into the southern dune Namib and the northern gravel plains with inselbergs. The valley meanders south-west from the Khomas Highlands out of the Great Escarpment zone, before changing direction westwards, being increasingly deflected to the north as it approaches the coast, by sand encroachment (Marker, 1977a). The discharge is derived from the Khomas Highlands, a summer rainfall regime dominated by 20-year cyclic variations (Dyer & Marker, 1978). In most years flow reaches Gobabeb but the volume of water is diminished by the development of farm dam storage above the escarpment and by extraction for mining in the Namib. The Kuiseb valley under analysis includes part of the headwater sector, the entire canyon sector and the start of the valley sector (Fig. 1). In the headwater sector a valley-in-valley profile is developed. The open rocky upper valley is incised at the base to a depth of under 100 m. Downstream, the upper canyon sector is narrow, 0.5-1 km in width and deeply incised. But in the lower canyon downstream of the Gorob river confluence, the

* Department of Geography, Universityof Fort Hare, Private BagX1314,Alice5700,South Africa. 0140-1963/83/040333+ 16$03.00/0 © 1983 AcademicPress Inc..(London) Limited

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Figure 1. The middle Kuiseb valley; location of the three sectors, the section sites and the area covered by Fig. 7. 1, Carp Cliff; 2, Aruvlei; 3, Gaub plateau; 4, Kamberg; 5, Huduob; 6, Ossewater-Homeb; 7, Gobabeb; 1000m form line inserted.

valley broadens to 1.5 km width and becomes less deep, only 60-100 m incision, since the pedimented sides become lower. Beyond Gobabeb the valley sector is entered. Cliffed rock sides are rare and the valley is less than 50 m in depth. The Kuiseb river bed is everywhere a braided channel occupying a meandering incised valley. The valley length under discussion extends from 16°OO'E west for 190 km and contains remnants ofterraces and associated deposits. These are best preserved in the more open canyon and valley sectors but are never entirely absent even in the upper canyon and headwater sectors. The Kuiseb valley was initiated subsequent to the deposition of the Namib Formation (Besler & Marker, 1979). In this part of the Namib a partially planed Pre-Cambrian Damara System schistose basement with granite intrusions is overlain and infilled by the Tertiary horizontally laid sandstone and limestone Namib Formation. Namib Sandstone is best developed south of the Kuiseb valley although occurrences have been noted at Carp Cliff and Aruvlei and also in the Tumas Valley. The overlying Namib Limestone is more widespread than the Namib Sandstone and extends onto the crystalline basement rocks. The original limestone surface karst is weathered and is thereby easily identifiable. It has a gradient seawards of 1 : 170 (Marker, 1983). This surface is relatively resistant to erosion but once incised, multiple plantation surfaces develop usually along the contact of sandstone and limestone and, at a slightly higher level, in the softer limestone below the recrystalised hardened weathering crust. In most of the headwater sector these three structurally controlled surfaces exist (Fig. 2). The deposits and terraces

General characteristics Older valley fills occupy valleys incised through the Namib Formation into underlying basement rock. These older valley fills reach known thicknesses of 40 m as at Kamberg Cliff and consist of heavily calcified, horizontally bedded, fine-grained sands and silts interbedded with nodular and gritty layers derived from reworking of the Namib Formation. The essentially calm water or fan sediments are capped with up to 9 m of

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Figure 2. Composite scale cross sections to show the relationship between the Namib Formation, Damara System schistsand oldervalley fill. (a)Headwater sectorat CarpCliffand GaubPlateau; (b) Canyonsectorat Huduob. Keyas for Figure4. close-packed, well rounded, dominantly quartzitic cobbles and pebbles in a calcium carbonate matrix. Cobble size diminishes away from the escarpment source area. This conglomerate is strongly lithified, cleavage occurring across matrix and cobble alike. Where the upper surface has weathered, cobble lags have formed and been recemented to form an indurated but unlithified conglomerate. But the original deposit is always strongly lithified, indicating considerable age. The sequence is best preserved in the headwater and upper canyon sectors where the cobble conglomerate now provides an effective caprock to prominent tablelands as at Gaub Plateau, Carp Cliff and Aruvlei (Fig. 2). Downstream the evidence is fragmentary with derived pebbles, probably not far removed from the site of deposition, prominent in the landscape. These older valley fills completely filled the original valleys incised into the Namib Formation. Younger deposits are contained within the incised Kuiseb valley and are banked against rock in most cases. These later deposits consist of four main types of different ages and origin: a conglomerate with sand lenses, fine-grained bedded silts, colluvial wash fans and the present channel deposits. The conglomerate in a calcium carbonate matrix consists of well rounded pebbles with some cobbles, conspicuously brown stained. Coarse sand with grit, as lenses rather than layers, is interbedded. Although occurrences of this conglomerate have been recorded in the upper canyon, the best preserved remnants are found in the lower canyon between Gorob river and Gobabeb. Loose pebbles derived from this deposit are spread as two distinct terraces down valley from Gobabeb. The maximum exposure is of27 m, 14 km upstream of Gobabeb (Marker, 1977a). Elsewhere relict patches indicate that this younger conglomerate fill reached a consistent level above the river but most has since been removed. A greater extent of fine-grained, highly saline, micaceous, light coloured silts has been preserved in gully embayments over a 38 km length of the lower canyon (Marker & Muller, 1978; Rust & Wieneke, 1974). The silts form two distinct terraces dissected into separate deposits. The surface of the upper silt terrace accords approximately and probably foruitously with that of the younger conglomerate. There is some evidence for silt deposition to an altitude of 10 m above the upper silt terrace level of which only traces remain (Marker & Muller, 1978). At Homeb the silts rest on blown sand but little other sand is incorporated. There is insufficient evidence to show whether the two terraces represent erosion into a single deposit or whether the lower terrace represents reworking and redeposition abutting on the earlier deposit. Downstream towards Natab, remnants of the higher silts are preserved under dune sand against the south wall of the valley but the dominant component is the lower silt terrace in many places capped and preserved by angular local colluvial gravel or by pebbles derived from the younger conglomerate [Fig. 3.(c)].

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Figure 3. Deposit relationships fromcomposite crosssections (allaltitudesin metres:Keyas on Fig. S). (a) At Gaub plateaueast; (b) S km downstreamofOssewaterin lowercanyon;(c) At Gobabebin valley sector. Pebblesin infilled channels southofKuisebderivefromtheolderconglomerate; pebble spreads below430m altitude derivefrom the younger conglomerate. Below the level of the lower silt terrace an unconsolidated sand and grit colluvial fan terrace with silt inclusions slopes towards the present flood plain terrace. It abuts against the lower silt terrace and is truncated by the present flood plain terrace (Marker, 1977a). The present bed deposits of the Kuiseb river, both in the channel and on the 1 m flood plain terrace, which is an overbank feature, are micaceous sands. Even small rounded pebbles are absent downstream of Homeb. Cobbles occurring within the canyon are most probably derived from reworking of both conglomerates and appear to be residual since the present fluvial regime seems incapable of transporting them out of the system. The deposits of the middle Kuiseb valley, younger than the Namib Formation, therefore consist of lithified older valley fills of fine-grained sandy sediments overlain by coarse conglomerate, younger valley fills of fine-grained sandy sediments overlain by coarse conglomerate, younger valley fills of a sandy calcified conglomerate, still younger silts of possibly two ages and a more recent colluvial fan deposit. The Namib dune sands are excluded from this analysis as they are considerably younger than the Namib Formation and provide a notable component only for the colluvial fan deposits and the present channel materials. The dune sand spills over into the Kuiseb canyon and is carried away by intermittent flow to be incorporated with micaceous materials derived chiefly from the country rock but also from the silts, as bed and overbank load.

The lithology A characteristic of the Kuiseb terrace deposits is the lithological similarity of deposits of very different ages. It is virtually impossible to distinguish the components of the older

FLUVIAL DEPOSITS

337

and younger conglomerates and the silts of the upper and lower terraces. This is a function of a history of reworking and reincorporation of original materials in younger deposits. The older conglomerate can best be described as an indurated, well-lithified, closely packed conglomerate of well-rounded cobbles and pebbles, dominantly white quartz or brown quartzite. Few cobbles of less resistant material survive. The matrix consists of gritty, silica-rich angular allochems with some plagioclase, cemented with sparitic crystalline calcite. Several generations of calcite recementationcan be seen surrounding the allochems and on some pebbles. At Carp Cliff gypsum and sodium chloride crystal growth has filled joints and planes in the underlying finer sediments. A long history of lithification is apparent. By contrast, the younger conglomerates although consisting dominantly of similar quartz and quartzite pebbles also include rounded Namib Limestone and schist cobbles. A local provenance is indicated for these components. This view is strengthened by the fact that limestone and schist components are scarce on derived pebble sheets as they disintegrate rapidly on release from the matrix. The matrix material of this younger conglomerate is a closely packed angular silica gravel completely infilled by crystalline calcite. Some calcite rim development is apparent but the pebbles are weakly held and drop out of the matrix as weathering proceeds. Thus the younger conglomerate differs from the older by its incomplete lithification, the weaker matrix, the incorporation of more purely local material and the iron-staining. That this stain is impermanent is shown by its loss downstream in the gravel terrace spreads. . Pebble long axis and constitutent analysis at sites in the vicinity of Gobabeb on the older conglomerate of the high terrace and on the younger conglomerate of the lower terraces, demonstrate no statistical difference in constituents nor in size distribution (Fig. 4). There is a greater variation with distance away from the conglomerate source than between the higher and lower deposits. Since the younger conglomerate incorporates materials from the older, this was expected. Nevertheless residual large constituents are proportionally more common in the high terrace. A full description of the sedimentological and lithological characteristics of the silt deposits have been given elsewhere (Marker & Muller, 1978) and will not be repeated here. No attempt was then made to distinguish the two deposits although samples were

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Figure 4. Histograms of size distribution in older and younger conglomerate and lag pebbles at Gobabeb. (a) Older conglomerate in situ;(b) redistributed older conglomerate cobbles; (c) younger conglomerate from upper terrace.

MARGARETE. MARKER

338

drawn from a variety of altitudes. However, observations of scarce sections in the lower terrace seem to indicate a distinction on the grounds of more frequent incorporation of sand lenses. Their development can be envisaged by comparison with the present situation at Tsondab Vlei. After a series of dry seasons, sand encroaches on the silt-covered vlei especially round the margins. Ensuing floods under still water conditions then seal the sand beneath further silt deposits. The silts of the Kuiseb valley represent river end-point deposits from a more arid regime than obtains at present, since the Kuiseb flowsat least as far as Gobabeb in most years. Nevertheless low silt banks can be deposited in backwaters even at the present day.

Vertical sections Detailed vertical sections are exposed in a number of places (Fig. 5). Upstream in the canyon sector are three important sequences. On the north bank both Carp Cliff and Aruvlei demonstrate sequences of fine sedimentary sands and silts of different hues, resting on weathered schist and overlain by lithified conglomerate and its associated weathering lag [Figs S(a),(b)]. On the south bank, Gaub plateau, a residual of the Namib Formation complete with karst dolines, is overlapped by older conglomerate. This outlier has been preserved by an underlying ridge in the basement rocks to the north (Marker, 1982). The north eastern aspect of this plateau reveals valley fill deposits derived from reworked Namib Sandstone resting against in situ Namib Sandstone, recording protoKuiseb incision prior to the deposition of the older valley fill [Fig. 3(a)]. The situation is confirmed at Kamberg where a 40 m deposit rests on Namib Sandstone overlying Damara System schists [Fig. S(c)]. Kamberg also clearly demonstrates the altitudinal relationship between the original Namib Limestone surface with its karst development at 894 m, the secondary limestone surface at 834 m, the sandstone surface and the deposits filling the proto-Kuiseb valley to a conformable altitude of 770-800 m. The gradient of the infill surface was steeper than that of the Namib Formation since valley fill conglomerate overlaps Namib Limestone at Gaub plateau yet reaches only to the sandstone surface 44 km downstream. At Huduob another zone of basement-high deposits are thinner [Figs 5(c),(d)]. Along the south bank at Huduob corner there may be evidence of older fill

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Figure 5. Scale vertical sections through older valley fill. (a) Carp Cliff; (b) Aruvlei; (c) Kamberg; (d) Huduob corner (west); (e) Huduob east.

FLUVIAL DEPOSITS

339

overlying some impure Namib Limestone contaminated by quartz inclusions by virtue of proximity to basement rock. The unconformity is indicated by slight karst piping [Fig. Sed)]. At the same venue the Namib Limestone altitude is 700 m where it overlies Namib Sandstone but the calcified plain at 687 m is the sandstone surface [Fig. 2(b)]. At Ossewater-Homeb and immediately downstream, the younger valley deposits are well demonstrated. The dune Namib surface at an altitude of 540 m consists of calcified Namib Sandstone. Within the Kuiseb valley and reaching altitudes of 500 m are younger conglomerate and the upper silt terrace on both banks. Beloware the lower silt terrace and the colluvial fan [Fig. 3(b)]. These sequences and their relationships have been considered elsewhere (Rust & Wieneke, 1973, 1974; Marker, 1977a; Goudie, 1972). Good exposures of the younger conglomerate are rare. On the north bank 4 km west of Ossewater is a 27 m exposure (Marker, 1977a). There, densely packed conglomerate appears to rest on 5-8 m of un bedded coarse brown sandstone. The constituents of the sandstone are similar to those of the present bedload. Ten kilometres west of Ossewater in an embayment on the south bank, just under 20 m of sloping conglomerate rests on sandstone but the upper part of the deposit has interbedded impure sandstone and sandstone with pebbles. Bedding is present but is far less marked than in the older fills. At both the Ossewater sites the higher silt terrace abuts against eroded conglomerate. At Gobabeb where the more open canyon ends, remnants of the older conglomerate cap the high terrace at altitudes of 470--480 m. Southwards along the dune streets the quantity of cobbles and pebbles decreases. The older cobble conglomerate is restricted to the south bank of the Kuiseb within approximately a 9 km wide belt. Within' the actual Kuiseb valley at a lower altitude than even the derived pebbles from the older conglomerate, there are duplicate terraces of younger conglomerate gravels. They are distributed chiefly along the north bank but are also found on the south bank. Traces of very weathered conglomerate have been found but most of the matrix material has disintegrated and the pebbles lie as lag gravels, at higher altitudes on sites probably not far removed from the position of original deposition but there has also been considerable downslope redistribution [Fig. 3(c)].

Altitudinalrelationships When the altitudes of the exposures are plotted against distance using the course of the Kuiseb river to provide a horizontal axis, the pattern becomes clearer (Fig. 6). The gradient of the Kuiseb river bed is not uniform. Pronounced breaks of slope occur below the Gaub confluence and upstream of Huduob. Below Huduob overdeepening and subsequent infilling has occurred and upstream the Kuiseb occupies a rock bed. Although in the field it is not easy to distinguish the original Namib Limestone surface from secondary planation surfaces without the presence of karst features the vertical relationships demonstrate that the secondary limestone surface diverges from the Namib Limestone surface down-valley from Carp Cliff. It is this secondary surface that is overlain by the older conglomerate, thus explaining the field difficulty in recognition. Since remnants of the older conglomerate have been identified at a number of sites between Huduob and Ossewater-Homeb the projection to Gobabeb to show the coincidence of the high terrace with the secondary limestone surface and with the older conglomerate is strengthened. The plot also demonstrates the altitudinal separation of the older and younger conglomerates. The gradients of the two deposits are similar, suggesting that similar flow regimes were probably responsible for the emplacement of both. No such clear distinction exists for the two silt deposits. Two explanations are possible. The high silt terrace has a gradient comparable with that of the younger conglomerate, one typical of a braided stream such as the Tsondab river at present (Marker, 1979) whereas the lower silt terrace has a lower gradient and may be the product of channel flow. Alternatively, the lower terrace may be erosional in origin, rather than the product of a second period of deposition, then the lower terrace would represent part of the older deposit incised to a lower altitude.

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Figure 6. Altitudinal relationships of depositional components along the middle Kuiseb valley. - - , denotes Kuiseb bed gradient; stippled-bed infill areas outlined are - - - , denote approximate altitude of north bank. Major confluences named are marked by arrows. Letters denote section sites: A, Aruvlei; C, Carp Cliff; G, Gaub plateau; K, Kamberg; 0, Ossewater.

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FLUVIAL DEPOSITS

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Spatial relationships The spatial distribution of the various deposits downstream of Huduob is also instructive (Fig. 7). With the exception of one patch oflag gravels at 583 m altitude on the north bank between the Gorob and Homeb rivers, the older conglomerate is preserved only on the south bank where it rests on calcified sandstone. It seems to be aligned west to east and thus diverges from the present course of the Kuiseb river. The deposits are dissected by a braided channel system which lost altitude to the north as though, during rejuvenation, the channel system was shifting towards the north. The postulation of a course directly east to west is strengthened by the alignment of a granite cliff 8 km east of Gobabeb which records a slightly later period of incision. The presence of a deep channel in the granite basement within the Nara valley provides additional evidence (Blom, Dept. Water Affairs, pers. comm.). The younger conglomerate, however, occurs as patches only within the present incised Kuiseb valley although pebbles are spread on both sides of the channel downstream of Natab below an altitude of 445 m, almost as though they may have originated as a fan deposit at the exit of the lower canyon. In contrast, the silts occupy positions within the rock-cut valley abutting younger valley-fill conglomerate upstream but occurring only in low-altitude embayments between Natab and Salt River. The presence of dissected silt patches overlain by pebbles derived from the younger conglomerate, indicates a period of colluviation prior to a period of more intense runoff sufficient to reactivate the local gully (gramadulla) system in an area where surface runoff is now a long return event (Marker, 1977b). Summary The Namib Limestone surface has been weathered by solution processes sufficient to form dolines and to cause some piping, itself evidence for conditions wetter than at present (Marker, 1982). In the course of erosion a series of structural surfaces were cut. The first major fluvial event was deep dissection of a proto Kuiseb valley followed by slow infilling . with fine-grained sediments and rolled materials from dissection of the Namib Formation. It is likely that the formation of the multiple surfaces was in part contemporaneous with this period of infilling. Contemporaneous calcification occurred, again indicating wetter conditions than at present. The final phase of this older valley infill was the deposition, lapping onto adjacent Namib Limestone or lower surfaces, of rolled cobbles within a calcium carbonate matrix, in a manner similar to the Namib Limestone overlapping the Namib Sandstone onto basement rocks. The cobbles are well rounded, possibly preweathered during the karst phase since constituents other than quartz and quartzite are rare (Martin, pers. comm.), They diminish in size away from the escarpment zone. That they represent a channel deposit is supported by the steep gradient and the limited extent of deposition laterally. A considerable time period then elapsed, judging by the degree of lithification and the formation of derived indurated lag gravels. During the latter part of this period braided channels prolonging the Kuiseb course due west in the direction of Sandwich Harbour, as postulated by Stengel (1970), dissected the conglomerate between Huduob and Salt River. The channels show a gradual shift northwards and downslope as dissection proceeded. Loose cobbles moved downslope to occupy the braided channels. The next event was incision to cut the present Kuiseb valley and canyon. The younger conglomerate with its sand layers marks a period of deposition within that valley to a maximum altitude of 70 m above present river level. Downstream the deposit spread as a narrow fan only 20-30 m above present bed level. The river gradient at the time of deposition was steeper than at present. As the subsequent period of incision and removal of the conglomerate occurred, a lower pebble terrace was formed only 10-20 m above river level.

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The deposition of fine grained, light-toned saline silts along 48 km downstream of the Gorob river confluence was the next major event. It marks a change in the depositional system. The steep surface gradient and the lithology of the deposit are indicative of river end point deposition in a desert environment. It is postulated that the duplication of the deposit into two terraces is evidence for incomplete removal followed by a minor period of infilling. Subsequent to the deposition of the lower younger silt terrace, the Kuiseb river incised once again to remove a large part of the deposit. Colluviation to cover the silts with a pebble lag probably preceded only slightly the dissection of the silts into discrete patches as the gully system was reactivated by local runoff. Since that time there has been further colluviation to form fan deposits of unconsolidated, reworked dune sand interbedded with granitic and schistose angular gravel and finally the formation of the present channel deposits. There are, therefore, five periods of incision and dissection (CUT) indicating greater fluvial efficiency, separated by five periods of deposition (FILL), all prior to the development of the present river bed deposits (Table 1). Discussion

Some problems Any elucidation of the sequential history of landscape evolution>in the Namib faces difficulties which must be kept in mind when postulating the chronology and particularly when using a chronology to infer climatic fluctuations. The first set of problems have been indicated. The very nature of cut and fill and the preservation of materials in an arid environment implies the remobilisation of constituents to form new deposits. Thus the roundness of the pebbles of the younger conglomerate does not indicate marked fluvial activity, since they are derived from the channel load of the older conglomerate. Equally some degree of recalcification, even in a region of extreme aridity, can take place in a year of above average rainfall as in 1976, once the calcium carbonate exists in the system. The distinction between in situ Namib Sandstone and the reworked sandstone of the older valley fill is a fine one. Only when the fill is a rubble of recemented sandstone cobbles is its reworked nature readily apparent. The nature of arid processes facilitates the formation of multiple surfaces both by pedimentation and by the protective action of lag gravels gradually let down (Marker, 1979). The later calcification of such residual gravels adds to problems of correct identification. Relatively recent tectonic deformation along the Escarpment zone sufficient to cause localised rejuvenation has been postulated (Huser, 1976; Rust & Wieneke, 1973). On the other hand no deformation in the gradient of the karst-weathered Namib Limestone surface has been identified although a consistent tilt could have occurred (Marker, 1982). It is therefore unlikely that tectonic deformation is solely responsible for incision in the middle Kuiseb valley subsequent to the Namib Formation. That is not to preclude rejuvenation attendant on general continental eustatic changes. Incision is as likely to result from changes in base level as from changes in the flow regimes. The entire question of the climatic control of processes as evidenced by the landform sequence poses yet other problems. It has been postulated that Namib geomorphology is conditioned by processes acting over a continuum from arid activity through arid stability and humid stability to humid activity (Wieneke & Rust, 1975). At present the situation is one of arid stability. Any climatic changes along this continuum would affect both the fluvial regime and dune sand movement. The features of the middle Kuiseb valley are controlled chiefly by the flow regime of the river and this is dependent on climatic conditions in the Khomas Highlands. The sequence of cut and fill described, must be recognised as a reflection of climatic conditions above the escarpment where there is a summer rainfall regime that equates most closely to Botswana and the Transvaal (Dyer & Marker, 1978; Dyer & Tyson, 1977). Nevertheless, the presence of the

? wet wet wet

wet

wet

FILL 2 CUT 2

FILL I

CUTl

Channel deposition Braided channel deposition Incision

Damara System metamorphosed basement & instrusive Salem granite

Balaeozoic Pre-Cambrian

LITHIFICATION

?

seasonalrunoff

? wet

CUT 3

Incision with localised deposition Channel deposition Incision REJUVENATION

NAMIB FORMATION

Namib Limestone Namib Sandstone

Lithified cobble conglomerate Fine grained calcified sandstone, sandstone rubble and silts Valleys cut by proto Kuiseb & other rivers Formation of multiple surfaces

Dissection & removal of conglomerate Formation of lower pebble terrace Formation of younger conglomerate Kuiseb valley & canyon dissection including gully system Kuiseb channel shift to northwest Dissection by braided channel system

dry

? slightly wetter dry much wetter

dry dry

CUT 4 FILL 3

dry

wet

CUTS

FILL4

wet ?

Present Day CUT 6 FILLS

Sequence

wetter drier

Redeposition Incision River endpoint deposition Deflation

Deposition Incision Colluviation Deflation Reactivation of gullys by local runoff Colluviation

Present river bed & flood plain terrace Slight incision and dissection Colluvial fan terrace (glacis) Dune sand north of Kuiseb Dissection of silt terraces

Pebble & colluvial wash over lower silts Reworking of silts: formation of lower silt terrace Formation of upper silt terrace Dune sand north of Kuiseb at Homeb

Process

Fluvial landforms

Teritary

Older valley fill

Younger valley fill

Designation

Postulated climate Khomas Highlands Namib

Table 1. The deposits ofthemiddleKuiseb valley and theirsigniji£ance. A framework for discussion

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gramadulla gully system originally cut during the Kuiseb canyon incision and its reactivation to dissect the silt terraces presupposes conditions wet enough locally to cause surface runoff. It is important to distinguish between wetter conditions inland that still occur and affect the Kuiseb river regime and local wetter conditions which are very rare. The formation of the low level colluvial fan terrace requires local surface runoff, but not necessarily so wet nor so prolonged as to reactivate tributary channel flow. A further problem relates to the lack of positive evidence for arid activity, chiefly because the materials under discussion are fluvial in origin. Dune sand, however, underlies the higher silt terrace at Homeb on the north bank. Dune sand was also available on the north bank for incorporation into the colluvial fan terrace and it has been postulated that the deposition of the silts themselves are a function ofa dune barrier (Marker & Muller, 1978; Wieneke & Rust, 1973) although this is not necessarily conclusive since deposition of silt will inevitably occur at the point where evaporation exceeds discharge, the situation that obtains at Tsondab Vlei today. A framework ofevents The evidence from an 190 km length of the middle Kuiseb valley is nonetheless sufficient to postulate a framework of cut and fill events that can be linked to changes in the flow regime and indirectly to climatic fluctuations (Table 1). The analysis demonstrates that five major alternations of cut and fill have occurred between the deposition of the Namib Forniation and the present phase of dissection and river channel deposition. The early history of the Kuiseb valley indicates climatic conditions wetter than at present, at least until after the deposition of the younger valley-fill conglomerate. Torrential flowconditions intervened during the deposition of both conglomerates. Channel flow cut the protoKuiseb valley and the later Kuiseb canyon system. Braided flow would appear to have been the mode of deposition of the lower sediments of the older valley fill and of their later dissection. The climate until completion of the induration of the younger conglomerate must have been considerably wetter locally than at present since the deposit indicates contemporaneous calcification implying karst solution upstream to provide runoff high in carbonates. The upper silt terrace provides the first unequivocal evidence for drier conditions. It appears to have been preceded by a phrase of arid activity when dune sand could cross the Kuiseb valley. Dry conditions with a slightly wetter intervening period persisted until the end of the silt deposition phases. A much wetter period locallycaused dissection of the silts and probably during its wane favoured the formation of the colluvial fan terrace. The middle Kuiseb deposits consist of an older lithified valley fill capped by conglomerate, a younger valley fill of calcified sandy conglomerate and yet younger silt deposits. This sequence suggests only one major arid period between two longer periods wetter than at present. The remainder of the variations are on a lesser scale and can be attributed to climatic fluctuations restricted to the Khomas Highlands or to the effects of rejuvenation. This interpretation is at variance with previous interpretations (Rust & Wieneke, 1973, 1974).

Corroborative evidence Some events documented in the middle Kuiseb sequence also occur elsewhere indicating that the climatic conditions that triggered geomorphological change were of more than local significance. Valley incision with subsequent deposition of reworked sandstone to form older valley fills has been identified in the Koichab and Kaukasib valleys (Besler & Marker, 1979) and in the Tumas valley. A high levellithified conglomerate terrace rests on Namib Sandstone below the level of the Namib Limestone in the upper Gaub and Tsondab valleys. Duplicate conglomerate terrace deposits are to be found in many Namib

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valleys such as the Gaub, Tsondab and Tsauchab. At Sesriem in the Tsauchab valley, the higher terrace has been incised to expose at least 20 m of cobble and limestone fill. Martin (1950) reports the existence in most valleys in Namibia and Botswana of three cobble terraces. The highest is lithified equivalent to the older valley fill in this study, the intermediate one is calcified and the lowest is not indurated. The middle terrace is considered by him as the equivalent of the younger conglomerate (Martin, pers. comm.). Incision into the Namib Formation and basement rocks followed by deposition of the older valley fill thus equates with similar events over a wide area. Subsequent deep incision followed by the formation of a calcified younger conglomerate was also widespread. The early history of the middle Kuiseb valley is not of purely local significance; it records widespread conditions.

The dating possibilities To provide a dated sequence is another matter. Even the age of the Namib Formation, the datum taken for this discussion, is not established (Belser & Marker, 1979). At best it can be dated as mid-Tertiary an equivalent of the lower Kalahari Formation. On lithological evidence the older valley fill is also of considerable age and probably lies beyond the limits of current dating techniques. The younger conglomerate equates with the middle calcified terrace that contains rolled Chellean and fresh Acheulian stone tools (Martin, 1950). But archaeological tool typologies may span a considerable age range depending on their provenance. More firmly established 2:I4U 2:11'Th dates of 210,000 years B.P. and 240,000 years B.P. are quoted for a lake at Narabeb some 40 km beyond the present Tsondab Vlei endpoint (Selby, Hendy etal., 1979). This is believed to have been a temporary lake and is associated with an Early Stone Age site. The relationship of the Narabeb lake to the Kuiseb valley sequence is speculative. The pronounced Tsondab valley upstream of the vlei exhibits the karst-affected Namib Limestone, a lithified conglomerate a few metres lower and a calcified younger conglomerate terrace only a few metres above the present valley floor. Downstream the two terraces continue to within 10 km of Narabeb, considerably modified by subsequent erosion, itself an indication of age (Marker, 1979). Narabeb lake occupied part of the abandoned floor after the river had ceased to flow as far. It, therefore, records a final locally wet phase, provided Selby is correct in his assumption that no major wet phase has occurred since the drying up ofthe lake since 210,000 years B.P. It might equate with the phase of gully reactivation by local runoff in the Kuiseb valley (CUT 5). Such an interpretation would suggest that the published dates for the Homeb silts are incorrect, probably as a result of contamination by reworking of calcium carbonate. The onset of extreme aridity in the Namib has been the subject of considerable discussion. The aridity is largely a function of the effect of the cold Benguela Current in conjunction with the action of the subtropical high pressure system. The establishment of an effective Benguela Current is now believed to be late Pliocene (Siesser, 1980), to cause a late Tertiary general aridification. Stratigraphic evidence along the west coast indicates that throughout the Miocene period seasonally wet conditions prevailed with widespread grasslands and local bush cover along major drainage lines (Siesser, 1978). On this basis the entire early history of the Kuiseb valley is Tertiary. Evidence from the Namib Formation and the older valley fill sequence accords with seasonality of rainfall such as affects much of southern Africa today. Changes in river regime superimposed on periodic rejuvenation would explain the alternation of cut and fill and changes in sedimentation. The Kuiseb Canyon incision is so pronounced that it must be due to rejuvenation. It may be postulated that the younger conglomerate was emplaced during the onset of aridity. Flash floods mobilised and deposited material available from previous erosion. The silt terraces record the onset of true aridity. The minor oscillations from wetter to dry since then can probably be attributed to Plio-Pleistocene climatic fluctuations. This interpretation is at variance with the view that the middle Kuiseb valley records a complete and complex sequence that can be directly equated with the Pleistocene record and that

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both conglomerates are evidence of wet periods within the Pleistocene period. It accords with the view held increasingly by southern African geomorphologists that many landscape components are of extreme age and that in low latitudes, the effects of only the most marked of climatic fluctuations left a lasting record. Conclusion A framework setting out the sequence of major landform events in the middle Kuiseb valley has been presented, based on detailed evidence from sections and surface distributions over a 190 km zone. Previous records have been set in the context of new information to provide a perspective impossible to achieve on a local scale. The events have been classified as 'cut' or 'fill' and demonstrate a series of five cycles. A tentative indication of the associated climatic conditions has been made. Evidence has been presented to show that at least the earlier Kuiseb events also occurred over a wide area of southern Africa. An attempt has been made to indicate the age range involved. The conclusion that only one major arid period can be identified on the basis of the fluvial record is, however, at variance with earlier views but may be a function of the scale involved. The field work was carried out during visits to D.E.R.U., Gobabeb between 1976 and 1980. Field assistance from staff of Nature Conservation, S.W.A. and of the Desert Ecological Research Unit is grarefully acknowledged. The research wasfunded by C.S.I.R.

References Besler, H. & Marker, M. E. (1979). Namib Sandstone; a distinct lithological unit. Transactions of South African Geological Society, 82: 155-160. Dyer, T. G. & Marker, M. E. (1978). On the variation of rainfall over South West Africa. South African Geographical f oumal, 60: 144-149. Dyer, T. G. & Tyson, P. D. (1977). Extended forecasts of above and below normal rainfall over South Africa. Journal ofAppliedMeteorology, 16: 144-149. Goudie, A. S. (1972). Climate, weathering, crust formation, dunes and fluvialfeatures ofthe Central Namib desert near Gobabeb. Madoqua, 11(1): 15-31. Huser, K. (1976). Kalkkrusten im Namibrand bereich des Mittleren Sudwestafrika. Mitteilungen der BaslerAfrika Bibliographie, 15: 15-81. Marker, M. E. (1977a). Aspects of the geomorphology of the Kuiseb river, S. W. A. Madoqua 10: 199-206. Marker, M. E. (1977a). A long return geomorphic event in the Namib desert, S. W. A. Area, 9: 209-213. Marker, M. E. (1979). Relict fluvial terraces on the Tsondab Flats, Namibia. Journal of Arid Environments, 2: 113-117. Marker, M. E. (1983). Aspects of Namib geomorphology: a doline karst. Annals ofSouth African Museum, (in press). Marker, M. E. & Muller, D. (1978). Relict vlei silts of the middle Kuiseb river valley S. W. A. Madoqua 11(2): 151-162. Martin, H. (1950). Sudwest Afrika. Geologische Rundschau, 38: 6-14. Rust, U. & Wieneke, F. (1973). Grundzuge der quartaren Reliefentwicklung der Zentralen Namib, Sudwestafrika. Journal ofSouth WestAfricanScientific Society, 27: 5-28. Rust, U. & Wieneke, F. (1974). Studies on gramadulla formation in the middle part of the Kuiseb river, S. W. A. Madoqua, 11((3):5-15. Selby, M. J., Hendy, C. H. & Seely, M. K. (1979). A late quaternary lake in the central Namib desert, Southern Africa and some implications. Palaeo-geography, Climatology, Ecology, 26: 37-41. Siesser, W. G. (1978). Aridification of the Namib desert: evidence from ocean cores. In Zinderen Bakker (Ed.), Antarctic Glacial History and Palaeoenvironments, pp. 105-113. Rotterdam: E. Balkema.

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Siesser, W. G. (1980). Late Pliocene origin of Benguela Current upwelling of northern Namibia.

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Stengel, H. W. (1970). Die Riviere der Namib und ihre Zulaufzum Atlantic. NamibDesert Scientific Research Paper, 22.50 pp. Wieneke, F. & Rust, U. (1975). Zur relativen und absoluten Geochronologieder Reliefentwicklung und der Kuste der Mittleren Sudwestafrika. Eiseitalter under Gegenwart, 26: 241-251.