Geology, Geohydrology and Development

Home , Beaufort Group, Bergville, Clarens Formation, Council for Geoscience, Drakensberg Group, Ecca Group

Geology, geohydrology and Development Potential Zonation of the uThukela District Municipality; specialist contribution towards the Environmental Management Framework

Client: Nemai Consulting

Consultants: G.A. Botha (PhD, Pr.Sci.Nat), R. Singh (MSc, Pr.Sci.Nat) Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200

1. Geology, lithostratigraphy and terrain morphology ...... 3 1.2 Beaufort Group ...... 5 1.2.1 Normandien Formation ...... 5 1.2.2 Tarkastad Subgroup ...... 8 1.3 Molteno Formation ...... 8 1.4 Elliot Formation (formerly Red Beds) ...... 9 1.5 Clarens Formation (formerly Cave Sandstone) ...... 9 1.6 Drakensberg Group lavas ...... 10 1.7 Mesozoic to Quaternary erosion and deposition...... 12 1.8 Quaternary deposits, geomorphology and drainage development ...... 12 1.8.1 Masotcheni Formation and alluvium ...... 13 1.9 Structure and faulting ...... 13 1.10 Terrain morphology ...... 14 2. Geohydrology ...... 15 3. Development Potential Zonation (DPZ) ...... 18 3.1 Low Development Potential Zones ...... 19 3.2 Medium Development Zones ...... 25 3.3 High Development Potential Zones ...... 28 4. References ...... 30

List of Figures

Fig. 1 Geological map of the uThukela District Municipality region.

Fig. 2 Typical terrain expression of the upper Karoo Supergroup rocks in the uKhahlamba- Drakensberg park area of uThukela District Municipality (after Botha, 2000).

Fig. 3 Typical topography of the western boundary of uThukela District Municipality showing the ‘Little ‘Berg” escarpment in the foreground and the main Drakensberg escarpment. In these areas very steep slopes, slope instability due to mass movement and flood risk in steep gradient streams is the most significant environmental and cost risk to development.

Fig. 4 Hydrogeological map of the uThukela municipality (after DWA, 1998 and 2000)

Fig. 5 Development Potential Zonation (DPZ) map of the uThukela District Municipality region.

Fig. 6 Upper Karoo Supergroup rocks forming unstable slopes in the “Landslide” and “Trail” zones.

Fig. 7 Slope class map showing the distribution of zones of high slope instability (>18o) and the irregular topography created by dolerite and sandstone hills (12-18o) as well as extensive low gradient areas.

Fig. 8 Development close to the boundary of the “Trail” zone, Cathkin Park.

Fig. 9 Very steep slopes defining the Thukela valley east of Colenso. Vryheid Formation and dolerite sills create the irregular topography where very steep slopes and slope instability through high mass movement potential constrain development. Photo source, GA Botha.

Fig. 10 View of the DPZ 5 terrain east of Estcourt showing dolerite hills and low gradient slopes underlain by Normandien Formation argillites. Masotcheni Formation colluvium on lower slopes and valley bottom alluvium is eroded to form gullies. Photo source; GA Botha.

Fig. 11 Municipal area map showing channel, floodplain and dam inundations zones as well as the distribution of wetlands. Large areas of low gradient slopes <2o also pose a risk of sheet flooding after high intensity rainfall.

Fig. 12 Aerial view over the Drakensberg foothills in the Bell Park Dam area, Cathkin showing the typical DPZ 6, low gradient, undulating topography underlain by Normandien Formation. Photo source; GA Botha.

List of Tables

Table 1 Geotechnical factors associated with Development Potential Zones (X = critical geotechnical factor, x = subcritical geotechnical factors).

Geology, geohydrology and Development Potential Zonation of the uThukela District Municipality; specialist contribution towards the Environmental Management Framework

G.A. Botha (PhD, Pr.Sci.Nat.) and R.G. Singh (MSc, Pr.Sci.Nat.)

Council for Geoscience, KwaZulu-Natal unit, P.O. Box 900, Pietermaritzburg, 3200.

1. Geology, lithostratigraphy and terrain morphology

The uThukela District Municipal area includes the headwaters of the Ukhahlamba-Drakensberg escarpment from Giants Castle to the Biggarsberg in the north. The headwaters of the Thukela River and its tributaries have incised deep valleys that expose the upper units of the Karoo Supergroup. The strong lithological and structural control of this succession influences the typical topographic expression of the lithostratigraphic sequence of the upper Karoo Supergroup in the UDP, shown in Fig. 1. The Ukhahlamba Drakensberg Park (UDP), World Heritage Site preserves outstanding examples of long, relatively continuous periods of geological history during which there is abundant fossil faunal material and a rock record which preserves evidence of evolutionary processes in response to dramatic climatic changes during the Triassic and Jurassic. This description of the geological history and lithostratigraphy of the municipal region is based on the report by Botha (2000). The geological map (Fig. 1) is based on parts of the published 1:250,000 geological series maps 2828 Harrismith (Council for Geoscience, 1998), 2830 Dundee (Geological Survey, 1988) and 2928 Drakensberg (Geological Survey, 1981).

The Drakensberg foothills record life after the mass global extinction at the Permian-Triassic boundary. The floral diversity during Late Permian (260 - 251 Ma) in the Normandien Formation and Late Triassic during Molteno Formation is preserved at sites in the municipal area.

This report provides background information concerning the diverse geological units, their stratigraphic relationships, evidence of palaeoenvironmental change and interprets their characteristics in the landscape in the context of financial and environmental constraints to development.

The Molteno-Elliot-Clarens transition gives very good insight into a palaeoclimatic transformation during the latter part of a ~150 million year latitudinal drift from a subpolar position in the Carboniferous towards the current subtropical location. From cool upland conditions inferred for the Molteno distal fans and fluvial braidplains, passing through the semi-arid Elliot Formation conditions with fluvial activity in the south and ephemeral, fluvially reworked loessic deposits in the north. This succession is unique in that it preserves an almost complete fossilized record of reptile 3 evolution (MacRae, 1999).

The limit of public access roads to many resorts and the eastern boundaries of the ‘Park are situated around the 1 600 m asl elevation on the upper part of the Triassic Tarkastad Subgroup, part of the Beaufort Group succession deposited after 258 Ma (million years ago). During a subsequent tectonic event dated at 230 Ma sediment was deposited on a broad fluvial braidplain represented by the Molteno Formation. The overlying Elliot Formation represents reduced fluvial energy and comprises “red bed” floodplain argillites and minor channel-fill sands. Progressive aridification is represented by deposition of the predominantly aeolian Clarens Formation after 215 Ma. The Karoo succession culminated in a trans-supercontinental scale desert which was buried by continental flood basalts which heralded an extinction event before the breakup and dispersal of remnants of the supercontinent Gondwana. The sharp contact with the overlying Drakensberg Group lavas defines a pronounced topographic break along the entire Ukhahlamba Drakensberg mountain watershed. Further east the Ecca Group rocks are exposed in the deeply incised valleys of the Thukela River and its tributaries.

1.1 Ecca Group

1.1.1 Vryheid Formation

The Vryheid Formation is exposed by the deeply incised Thukela River valley east of Colenso where it is intruded by numerous dolerite sills. Upthrown along the Tugela Fault in this zone, the Estcourt/Normandien Formation contact with the underlying Vryheid Formation has been downthrown some 300m south of the Thukela River and is exposed in the Bushman’s River gorge on Darkest Africa, about 25 km to the east near Weenen. The Vryheid Formation comprises thick coarse-grained sandstone beds and carbonaceous shale with thin coal seams in the Colenso area.

1.1.2 Volksrust Formation

The uppermost unit of the Ecca Group, the Volksrust Formation comprising blue-grey or black siltstone and shale is exposed at the base of the Normandien Formation along the Drakensberg escarpment foothills in the Klip River catchment northwest of Ladysmith (Linström, 1987, Geological Survey, 1988). Towards the west these rocks form the low lying parts of the Sand River catchment. These rocks have been upthrown against the Normandien /Adelaide rocks by the Tugela Fault which runs east-west along the river valley through Colenso.

In the Ladysmith area these rocks are highly intruded by dolerite sills and colluvial hillslope sediments mantle the slopes. The soils are commonly clay-rich, structured profiles and the underlying Masotcheni Formation colluvium is highly erodible, forming deep dendritic gullies (dongas).

1.2 Beaufort Group

In the northeastern Drakensberg and eastern Free State region the Beaufort Group thins and some of the lower formations that occur in the Karoo region pinch out. Here recent sedimentological research by Groenewald (1989) and Muntingh (1997) has subdivided the succession into the lower Normandien Formation (comprising alternating green shales and sandstone units), overlain by the Tarkastad Subgroup.

The cultural heritage significance of the Permo-Triassic Beaufort Group in uThukela DM is that it contains the oldest land-living reptiles. Together with the overlying Molteno, Elliot and Clarens Formations, this upper Karoo Supergroup succession is unique worldwide in that an almost complete 80 million year record of vertebrate evolution is preserved (MacRae, 1999). The thick Beaufort Group succession has been subdivided into eight biozones based on specific fossil assemblage zones. The Drakensberg foothills expose the uppermost Lystrosaurus and Cynognathus Assemblage Zones and many specimens have been collected from the eroded areas east of the UDP.

The Beaufort Group accumulated as fluvio-lacustrine sediments laid down on gently subsiding alluvial plains in a semi-arid climate with highly seasonal rainfall. Riparian vegetation along the river meander belts and semi-permanent lakes supported a diverse reptilian fauna dominated by “mammal-like reptiles” (Smith et al, 1993). Exposures in the Colenso area reveal rocks representing floodplain environments that supported a rich diversity of fossil plant groups including moss, ferns, sphenophytes, lycopods, equisetalians, pteridosperms (glossopterids), gymnosperms and insects (MacRae, 1999, Prevec et al., 2009). Fossil wood from these rocks has distinct growth rings suggesting a seasonal climate.

1.2.1 Normandien Formation

Various generations of mapping have characterised the transitional successions between the Ecca and Beaufort Groups in different ways. On the 2830 Dundee map (Geological Survey, 1988) the Volksrust Formation is overlain by the Estcourt Formation siltstone, shale and sandstone. The more recent geological map 2828 Harrismith sheet (Council for Geoscience, 1988) maps these rocks as the Adelaide Subgroup.

No specialist lithostratigraphic study has been conducted in the uThukela DM area to tie specific lithological units exposed in the UDP foothills with the Normandien Formation type area (Grundling, 1989; Muntingh, 1997). This is further complicated by the displacement of the Adelaide/ Normandien Formation by the western end of the Tugela Fault.

Fig. 1 Geological map of the uThukela District Municipality region.

Fig. 2 Typical terrain expression of the upper Karoo Supergroup rocks in the uKhahlamba- Drakensberg park area of uThukela District Municipality (after Botha, 2000).

The fluvio-deltaic Normandien Formation comprises a 400-600m m thick series of sandstone members with interbedded green mudrock units (Groenewald, 1989, Johnson and Verster, 1998). The influence of this succession of sandstone / mudstone units varies due to the differing structural stability and topographic expression of the overlying lithological unit. This current lithostratigraphic subdivision of the Beaufort Group in northwestern KwaZulu-Natal into two subgroups results in difficulty in direct correlation with earlier subdivisions of Lower-, Middle- and Upper Beaufort Series status. Groenewald (1989) summarized the correlation of the Verkykerskop Formation sandstone units with earlier stratigraphic subdivisions which refer to the “Upper sandstone unit” of the “Middle Beaufort Stage” (Visser and Bishopp, 1976).

In the Colenso area, Prevec et al., 2009 have described 51 distinct plant fossils morphotypes, including glossopterids, sphenopsids, and ferns, collectively represented as foliage, axes, fructiﬁcations, and dispersed seeds. A specimen of ‘Oudenodon,’ found in a nearby stratigraphically equivalent outcrop, is attributable to the Dicynodon Assemblage Zone. The Normandien Formation in this area provides a glimpse into a late Permian ecosystem of primary producers, herbivores, and insectivores—a prelude to the crisis that engulfed life at the end of the period.

The different interpretations between lithological maps and lithostratigraphic maps has some bearing on development in the UDP foothills where development planning zonation is constrained by the relationship between topographic features and zonation (see section 3).

1.2.2 Tarkastad Subgroup

The river headwaters incise deep valleys traversed by the approaches to the UDP exposing the sandstone and mudstone succession of the Late Triassic, Tarkastad Subgroup. This unit is differentiated from the underlying Normandien Formation (cf Adelaide Subgroup) by the higher proportion of sandstone relative to the green and reddish brown mudstone beds (70:30, Johnson and Verster, 1994). The lower part of the 250m thick succession is a package of three to five sandstone units, each up to 10m thick with interbedded mudstone. The Tarkastad Subgroup can be subdivided into the basal Verkykerskop sandstone Formation and the overlying red mudstone of the Driekoppen Formation.

1.3 Molteno Formation

The conspicuous Molteno Formation scarp comprises thick beds of pebbly feldspathic sandstone with thin interbedded shale and mudstone units. This alternation forms ‘terraced’ hillslope morphology with large, flat slabs of the coarse-grained sandstone scattered on slopes below outcrop. The basal contact erosively overlies the Beaufort Group lithologies and the gradational upper boundary is difficult to define in the field due to the prevalence of red or mauve mudstone similar to the overlying Elliot Formation (Erikkson, 1984). The thickness is variable, from 200m in the south to 45m thick in the Giants Castle area, thinning markedly towards the north where it is represented by one 15m thick sandstone unit (Lindström, 1981). In the UDP foothills this unit is characterized by a thin basal conglomerate, laterally persistent coarse-grained ‘sparkling sandstone’ (due to crystal faces of secondary overgrowths on quartz grains), carbonaceous shale and mudstone. Grey and grey- green mudstone dominates the thin inter-sandstone units. Thin coal seams and oil shales (actually carbonaceous shale with bituminous beds) occur locally. Fossils of Dicroidium ferns and Dadaxylon tree leaves are preserved locally (MacRae, 1999). This succession preserves evidence of terrestrial life forms after the Permo-Triassic global extinction event, representing a unique record of Late Triassic plant and insect communities which is unrivalled elsewhere in the world (MacRae, 1999). Biodiversity levels at that time may have rivalled those of today with a particularly rich gymnosperm flora. The excellent fossil leaf and insect preservation at a number of sites permit the reconstruction of seven different habitats with the Molteno Biome (MacRae, 1999). Diverse plant fossil assemblages were found at a site near Little Switzerland (38 species). A diverse insect fauna is found within the plant fossil zones.

The Molteno Formation rocks overlie the Cynognathus Assemblage Zone but have not yielded significant animal fossils although the oldest three-toed dinosaur tracks in the world were discovered in these strata (Macrae, 1999).

1.4 Elliot Formation (formerly Red Beds)

Above the gradational contact zone with the Molteno Formation is a succession of red and purple mudstone and subordinate, lensoid medium-grained feldspathic sandstone beds known colloquially as “red beds’, now termed the Elliot Formation of Upper Triassic to Lower Jurassic age. The grassy slopes below the Clarens sandstone cliffs are divided by thin sandstone beds and interbedded red mudstone. The succession thins northwards from 370m to about 210m thick near Giants Castle (de Decker, 1981; Lindström, 1981).

The predominant red mudstone colour points to an oxidising environment and subaerial exposure interpreted as drier conditions than the Molteno environment (de Decker, 1981). The lower part of the Elliot Formation accumulated in a meandering stream channel and paludal floodplain environment with the middle section representing drier ephemeral stream and semi-arid floodplains (Smith and Kitching, 1997) and accumulated in the distal areas of the Molteno braidplain and alluvial fans. The upper units represent further aridification with loessic floodplain and playa environments being replaced by flood-fan and aeolian dunes close to the contact with the overlying Clarens Formation aeolian dune sandstone.

This unit is of cultural heritage significance due to the large herbivorous dinosaurs (Euskelosaurus; 3m high and 6m long) at the base, changing upwards to more gracile, light-limbed forms (Fabrosauros, Massospondylus) (Smith et al, 1993; Smith and Kitching, 1997). The basal Euskelosaurus Range Zone has a low diversity of animals whereas the overlying Massospondylus Assemblage Zone preserves abundant fossils representing a higher faunal diversity. A regionally extensive bone bed which concentrates Tritylodon (small advanced cynodont) and reworked pedogenic carbonate nodules represents an erosion hiatus marking the Triassic/Jurassic boundary (Smith and Kitching, 1997). True mammal fossils are encountered for the first time in the African palaeontological record near the top of the Elliot Formation, which also contains some of the oldest dinosaurs discovered (MacRae, 1999).

1.5 Clarens Formation (formerly Cave Sandstone)

The sheer cliffs defining almost every steep river valley in the Little ‘Berg are incised into fine- grained sandstone comprising the Clarens Formation (Fig 3) which is characterised by light yellowish-brown or red, fine- to medium-grained, quartz-rich sandstone, cemented by calcium and locally containing calcareous concretions associated with reddened palaeo-weathering profiles or palaeosols. The thickness of the unit is variable due to local palaeotopography and erosional features (Smith et al., 1993).

This Late Triassic unit (~215 Ma) represents emplacement of aeolian sediments into the Karoo Basin after a progressive aridification of this region within Gondwana. The gradational change from the underlying Elliot Formation comprises red mudstone and minor channel-fill sandstone deposited in 9 meandering streams and floodplain environments (Cole, 1992). The basal transition from the underlying Elliot Formation may reach 40m in thickness, and comprises massive, thickly bedded, light brown and light red sandstone with thin water-lain, horizontally laminated sandstone and mudstone displaying small-scale cross stratification, ripple marks, desiccation cracks, dinosaur footprints and trace fossils representing life in periodically flooded wadis and playa lakes (Eriksson, 1981a, b). The lowermost units of the Clarens Formation comprise coarse- to fine-grained sandstone and siltstone deposited in wadi and playa lake systems debouching from mountainous areas. In areas to the south of Lesotho there is evidence that some early volcanic activity commenced before deposition of the Clarens Formation (Beukes, 1970).

The Massospondylus Assemblage Zone extends through the Clarens Formation and preserved shoals of fish have been discovered in sediments representing desiccated lakes. Vertebrate footprints are preserved on bedding planes or within thin interbedded clay beds forming cave roofs at Giants Castle. At least 10 different types of quadrupedal and bipedal dinosaur tracks ranging in size from the equivalent of the modern sparrow to an ostrich have been discovered across the basin (Smith et al., 1993; Erikkson, 1981a, 1884; Tankard et al., 1982).

The tensional tectonic regime which preceded Gondwana rupture and dispersal led to outpouring of flood basalts which locally preserved palaeodune topography. Thin interbedded sandstone beds and cracks infilled with fine-grained sandstone in the basal lava flows suggest a lava field environment with localised shifting sand dunes. Karoo vulcanism did not entirely eliminate life from the southern African region of the basin. Fragmentary dinosaur remains and a fairly diverse fossil pollen and spore assemblage documents refugia from which the region was recolonized during breaks in lava extrusion (MacRae, 1999).

1.6 Drakensberg Group lavas

The high mountain plateau of central Lesotho preserves a 1370m-thick (possibly up to 1 800m) succession of extrusive igneous rocks, part of the Karoo Igneous Province, which record an immense volcanic event which covered large regions of the Gondwana supercontinent during the early Jurassic (Smith et al., 1993; Duncan et al., 1997). The continental flood basalt (CFB’s) succession of tholeiitic (sub-alkaline) lava flows and subvolcanic plexus of intrusive dolerite dykes and sills preserved and exposed along the Drakensberg escarpment is the thickest exposed section of this remnant of this widespread event. It has been estimated that the Lesotho basalt pile was originally 1600-1800m thick prior to erosion (Dunlevey et al, 1993). The majority of the igneous activity has been dated to a narrow time frame during the Toarcian centred on 183±1 Ma (Marsh et al, 1997; White, 1997) over much of the former Gondwana region.

Emplacement of the continental flood basalts was the precursor to the fragmentation of Gondwana, preceding breakup and creation of the Indian Ocean by about 10 my (White, 1997). A mantle plume created updoming and a tensional tectonic regime which facilitated the emplacement of large

10 volumes of magma into higher levels of the crust by convective upwelling (Smith et al., 1993; White, 1997). The intrusion of magma as dykes exploited pre-existing basement structural weaknesses resulting in a dominant west northwest - east southeast (WNW-ESE) orientation in the Drakensberg.

The low viscosity flood basalts flowed large distances from dyke-fed fissures without a great deal of associated explosive activity and the lack of significantly weathered contacts implies fast accumulation (Haskins and Bell, 1995; White, 1997). The basalt succession is intruded by numerous dolerite dykes and sills which were conduits to magma feeding high level fissures from which younger lava flows erupted on the accretionary continental flood basalt surface. The homogeneous or composite dolerite dykes and sills are of similar geochemical composition to the basaltic lava flows but are coarser grained due to slower crystallisation at hypabyssal depths below the surface. Some of the dykes intrude the highest basalt flows suggesting that they were feeders to the highest- lying flows which have been removed by erosion. The different weathering patterns of the dykes/sills are due to their textural homogeneity and the lack of vesicles/amygdales which were formed by concentration of volatile phases in the extrusive phases. An important aspect of the dolerite dykes is the preferential weathering which occurs along the brecciated margins and

11 exploitation of these weathered zones by stream incision. The structural control of dykes on tributary stream valleys is a distinctive aspect of the deeply incised river valleys in the region.

A late volcanic event is represented by the emplacement of kimberlite pipes and dykes which are only exposed in Lesotho.

1.7 Mesozoic to Quaternary erosion and deposition

Since emplacement of the Late Cretaceous-age, kimberlite pipes approximately 300m of rock has been eroded from the crest of the Lesotho mountain (Partridge, 1997). Although the geomorphology of the Lesotho region had largely been created by the start of the Cenozoic era (~65 Ma), subsequent uplift events during the Miocene (~20Ma) and late Pliocene (5-3Ma) resulted in renewed incision of river systems to new base levels. There are no sedimentary deposits preserving this lengthy, ~80 Ma period of degradation since the Late Cretaceous during which the long-lived “African” and “Post- African” landsurfaces were successively created and progressively truncated. Drainage systems have commonly exploited the sheared, brecciated or preferentially weathered zones along fractures and subvertical dolerite dykes intruding the basalt succession.

There are no long-term records of climatic change from the high plateau region of Lesotho. Grab (1996) has implicated low temperatures during the “Last Glacial Maximum (LGM)” period at ±18 000 years BP as being responsible for the possible preservation of glacial ice at insolation-protected sites. Analysis of fossil pollen from swamp deposits in similar altitudinal and topographic settings near Clarens in the Free State has shown significant changes in the species composition and structure of vegetation during the stepwise cooling leading up to the cooler drier conditions of the Late Pleistocene hypothermal (Scott, 1989).

1.8 Quaternary deposits, geomorphology and drainage development

A unique geomorphological aspect of the upper Tugela River basin, in the northern UDP, is the step- like descent of tributary river elevations when viewed transverse to the catchment divides, towards lower levels at the upper part of the basin near the Amphitheatre (Matthews, 1969). It has been implied by Matthews (1969) that the Tugela River basin was back-tilted towards the west during Cenozoic uplift although the gradient of the Tugela remained steep enough to maintain active channel and valley side slope erosion. Prominent valley side slope asymmetry is exhibited in many tributary sub-catchments (Matthews, 1969). The southeastern slope tributaries are generally several times longer than the northwestern side slope tributaries on most of the northeast flowing tributary basins. The asymmetry of valley slopes was interpreted as being due to the westward tilting of the bedrock incised by the basin (Matthews, 1969).

An alternative explanation for this unique physiography is based on slope aspect. Slopes with a

12 south-facing aspect are cooler, moister and dominated by slow mass movement processes whereas the opposite, north-facing valley flanks have lower gradients and are relatively warmer and drier with dominance of sheetwash transport processes (Boelhouwers, 1988). Moderately to well-drained, red and yellowish soils commonly form due to the enhanced chemical weathering on slopes with a northerly aspect whereas shallower, organic enriched soils occur on slopes with a southerly aspect. The effect of reduced temperatures during cold phases of the Pleistocene and possible influence of cryogenic processes under periglacial conditions has also been invoked to explain the valley asymmetry at high altitudes (Meiklejohn, 1992).

1.8.1 Masotcheni Formation and alluvium

Hillslope deposits are widespread in the topography on lower hillslopes within the Thukela River basin. The stratified, sandy hillwash and colluvial deposits are interbedded with buried palaeosols, forming the Masotcheni Formation. These deposits occur on low gradient footslopes, generally underlain by mudrock or shale, below hills where the steep upper slopes are formed of dolerite or Karoo sandstones. Over large areas of the valleys north of Estcourt, the Middelrus and Weenen, the colluvial hillslope deposits are preferentially eroded to form dendritic gullies or dongas. In the Klip and Sundays River valleys near Ladysmith, extending towards the Sundays River valleys in the Thukela River headwaters, these deposits are incised. The hillslope deposits interfinger with alluvium along the margins of low order stream channels and floodplains.

The cyclical hillslope erosional history of the interior over the past ~130,000 years is preserved within the Masotcheni Formation colluvial sediment and interbedded palaeosol succession (Botha et al., 1990, 1992; Botha, 2006; Botha and Partridge, 2000; Clarke et al., 2003). The deposits reveal periodic, widespread gully erosion of hillslope sediment cover in response to global climatic change cycles since the Last Interglacial period during the late Pleistocene.

In the areas north of Ladysmith, extensive alluvium is mapped on the published geological map 2828 Harrismith. This indicates a subtle change in the topography associated with the Volksrust Formation relative to the Normandien Formation landscape south of Ladysmith where Masotcheni Formation hillslope deposits are the dominant unconsolidated surficial deposits.

1.9 Structure and faulting

The course of the Thukela River has been strongly influenced by the lithological structural control along the Tugela Fault. This ‘scissor fault’ can be traced towards the east from near Colenso and has a displacement of around 300 m with localized tilting of strata at 20o-50o towards the south on the downthrown southern side due to fault drag. A prominent fault north of Bergville has a displacement of about 120m towards the north. Other faults aligned N-S or E-W occur in the area south of Bergville. A fault with 100m displacement to the east occurs in the iThonyelana valley at Cathedral

Peak and in the Mlambonja valley near Zunkels there is a fault with 30m displacement to the west. The Beaufort strata are also juxtaposed along an ENE-WSW trending normal fault cutting across the stratigraphy from Bergvlei through Bell Park and onto Dreifontein near Cathkin. This fault downthrows the upper Tarkastad Formation sandstone unit on the southwestern side to lower elevations.

There is a strong association between dolerite dykes and N-S or E-W structural lineaments suggesting that dolerite intrusion was influenced by fault tectonics associated with the initial fracturing and rifting of Gondwanaland. There is no evidence of fault reactivation since the Cretaceous.

1.10 Terrain morphology

The Drakensberg escarpment or the “Great Escarpment” forms the western boundary of uThukela District Municipality and the border with Lesotho. Reference to the jagged escarpment skyline is captured in the Zulu name “Ukhahlamba” (“barrier of upturned spears”) or the Afrikaans name “Drakensberg” which likens the silhouetted profile to the proverbial dragon’s back. During the Mesozoic this elevated mountain range also gave rise to the largest drainage systems traversing the country which were also responsible for the high erosion rates and rapid retreat of the escarpment from the coastline of the newly born African continent.

The 1500m high Drakensberg escarpment can be divided into distinct physiographic regions where the topography and slope characteristics are directly controlled by the underlying geology and erosional processes. The “Little ‘Berg” or foothills below 2000m asl lies in the shadow of the towering “Great Escarpment” which rises to 3482m asl in this area. A schematic representation of the typical geological succession exposed in the Little ‘Berg and escarpment, shown in relation to the typical topographic expression of each lithostratigraphic unit is presented in Figure 2.

Delineation of the “Landslide Zone” has been correlated with the outcrop pattern of the alternating sandstone / mudrock units of the Elliot Formation, and the sandstone cliffs formed by the Molteno Formation. The topographic expression of the upper Driekoppen mudstone unit of the Beaufort Group is strongly related to the structural resistance to weathering of the overlying Molteno Formation sandstones. Lower slopes in this “Landslide Zone” are underlain by the less resistant. Apart from numerous shallow-based landslides generated off the steep slopes underlain by red mudrock, the Elliot Formation outcrop is littered with large Clarens Formation sandstone blocks that have toppled/rolled/slid onto the lower slopes. Downslope of Molteno Formation sandstone outcrop forming low cliffs the hillside is typically characterised by tabular blocks deposited by mass movement onto Driekoppen Formation mudrocks.

The “Trail Zone” in the Drakensberg Approaches Policy (DAP) extends down the topography onto slopes underlain by the upper Beaufort Group rocks including the thin Driekoppen Formation unit

(Burgersdorp Formation correlative), above the Verkykerskop Formation (Katberg Formation correlative). The lower “Trail Zone” boundary has been delineated as the upper boundary of the “Middle Beaufort”, a very outdated or informal classification. The alternation of sandstone and shale units defining this zone are influenced by dolerite sill and faults that displace these units to lower elevations extend the distribution of this planning zonation. This demonstrates that the formal delineation of a regional mapping unit boundary used for planning, based on the combination of specific lithological units within a thicker lithostratigraphic unit, and the generalized topographic expression of this combination of variable lithological units can be subjective given local geological conditions. This can result in subjective interpretation by developers and authorities alike.

2. Geohydrology

The principal groundwater occurrences of uThukela Municipality region are associated with the porosity of the diverse rocks and sediments and as well as the fracturing and weathering they have undergone. The Department of Water Affairs and Forestry (DWAF) has published 1:500,000 groundwater maps of the region including the uThukela municipality which falls within the Durban, Vryheid and Kroonstad hydrogeological maps sheets (DWA, 1998 and 2000).

According to the hydrogeological mapping, all the litholological units in the uThukela municipality represent “Intergranular and fractured” aquifers (Fig 4). The argillaceous and arenaceous Permo- Triassic sedimentary rock of the Clarens, Elliot and Molteno Formations as well as the Beaufort and Ecca Groups are characterised by the d1 to d4 classes where the d4 class has high borehole yields of between 2.0 to 5.0 median l/s. The d4 class is however limited in the municipal region occurring in areas north of Estcourt town and Spioenkop Dam. The Jurassic Drakensberg basalts characterising the escarpment areas in the Okhahalamba and Imbabzane local municipalities are associated with the d1 and d3 borehole yield classes. The poorer borehole yielding d1 class is associated with much of the basalt areas in the Okhahalamba local municipality and has borehole yields ranging from 0.0 to 0.1 median l/s. The contact between the d1 and d3 class associated with the basalt areas occurs <5km north of the Xeni River. The basalt areas to the south of the contact are better yielding (0.5 to 2.0 median l/s). According to Vegter (1995a) in regions of Drakensberg basalt the probability of drilling a successful borehole is <40% with a 20-30% probability of a successful borehole yielding >2 l/s (Vegter, 1995a) The widespread intrusive post Karoo dolerites have borehole yield expectancies that range from poor to moderate and are of the d1 to d3 classes. The quaternary alluvial deposits approximately 10km west and south of Estcourt are of the higher yielding d3 and d5 classes, where the d5 class characterises areas with >5.0 median l/s borehole yields.

The municipal region is characterised by predicted storage coefficients ranging from of <0,001 to 0.01 and drilling depths of between <20 to 30m below groundwater level (Vegter, 1995b). Typical depths to groundwater level are 10 to 30 metres below surface, and the standard deviation range by which the groundwater level fluctuates about the mean is 8 to 30 m. According to Vegter, (1995b), in

15 the uThukela district there is a predominance of the hydrochemical class “Type B” where the 2+ +2 - groundwater is cation-dominated by Ca and/or Mg and anion-dominated by HCO3 . The study region also is associated with a paired type dominance of “Types A and B”. The hydrochemical class “Type A” has the same cation dominance as “Type B” but is anion-dominated by Cl- and/or 2- SO4 (Vegter, 1995b).

The Department of Water Affairs (DWA) Groundwater Resource Information Project (GRIP) (http://www.dwa.gov.za/Groundwater/GroundwaterOffices/KZN/GRIP_Kwa-ZuluNatal.pdf) addressed the groundwater data gap in the management of groundwater resources. The GRIP project collated groundwater data from various statutory-funded water supply projects, consultants and developers, into a centralized database. Groundwater data collected identified high yielding aquifers that can support larger municipal schemes in the augmentation of existing water supplies. The location of groundwater borehole data points incorporated in the GRIP for the uThukela Municipality is seen in Fig. xx.

DWA is also responsible for the compilation of Regional Groundwater Master Plans and Water Use License Application: Groundwater Abstraction. The groundwater quality is also indicated by the amount of total dissolved solids (TDS), and within the uThukela municipality figures range from < 300 mg/l to 1000-1500 mg/l (Vegter, 1995b). The KwaZulu-Natal Groundwater Plan (DWAF, 2008) describes the groundwater resources of the province and groundwater monitoring. The monitoring of groundwater abstractions, water level fluctuations and chemical quality. About 48 groundwater points are currently being monitored actively for water level fluctuations and/or chemical parameters.

The uThukela DM region falls within “North-western Middleveld Hydrogeological Region” (DWAF, 2008) that extends from Ixopo in the south through Vryheid and Paulpietersberg and along the Drakensberg drainage divide that forms the western boundary of uThukela DM. Economic activities include summer grain crop production in the west that benefits from irrigation and agriculture along the Thukela River floodplain and valley slopes. The extent of groundwater use for irrigation and the impact of commercial plantations are not known. Large and small-stock grazing is practiced extensively and much of the northern and eastern valley areas are communal land used for grazing. Acid mine drainage is described as a potential risk although there is very little coal mining within the uThukela DM region.

Figure 4 Hydrogeological map of the UThukela municipality (after DWAF, 1998 and 2000).

3. Development Potential Zonation (DPZ)

Geomorphic processes and geology have long been recognized as significant controlling factors in the development of the Drakensberg. Altitudinal zones formulated as part of the Drakensberg Policy Statement (DPS) (Phelan, 1976) were derived from the lithologically influenced terrain morphology that can be delineated on the basis of the upper Karoo Supergroup lithologies. This planning scheme delineated zones parallel to the escarpment that define decreasing gradients that are associated with landuse potential. The DPS was subsequently reviewed and the zonation retained within the framework of the Drakensberg Approaches Policy (DAP) (Martin, 1990). The DPZ 1 includes the basalt escarpment (“Wilderness Zone”), the “Landslide Zone” associated with the Clarens Formation cliffs, the lower “Trail Zone” that covers the underlying Elliot, Molteno and Upper Tarkastad Subgroup lithologies. In these areas very steep slopes and mass movement potential impose the most severe development risk.

Provincial spatial planning authorities have argued for the designation of “Special Case Area” status for the Drakensberg mountains (Metroplan, 2001). Part of this area has since been granted World Heritage Site status. The scenic value of the area is closely related to the geological history and geomorphic development of the Drakensberg SCA area. Criteria used for the definition of the SCA include altitude and geomorphology which are directly related to bedrock geology, geomorphological processes and slope gradients.

The delineation of zones with similar geological and geomorphological characteristics and geotechnical conditions is a parallel approach to the designation of development nodes and will overlap other development zonations. At the District Municipal scale, development is influenced by geotechnical and environmental constraints that are closely related to bedrock lithologies, slope gradients, regolith and soils and drainage systems. For strategic planning it is counter-productive to try and incorporate the full complexity of potential geotechnical conditions or geohazards for each bedrock type. Simplification of geotechnical conditions for the benefit of regional planning follows the Development Potential Zonation (DPZ) approach. This engineering geological approach towards development planning addresses the full range of geological, slope, regolith and geomorphological aspects in the area. The highest risk to development is assigned top priority in which case this supercedes the range of other development constraints that may also influence planning or infrastructure construction.

It is important to note that this approach cannot replace more detailed desktop assessments, followed by field work and site specific investigations that are required at the smaller scales of specific developments, infrastructure or sites.

Fig. 5 Development Potential Zonation (DPZ) map of the uThukela District Municipality region.

3.1 Low Development Potential Zones

Low Development Zones are differentiated from High and Medium Development Zones on the basis of areas at high risk of flooding or seasonal inundation and generally steeper gradient slopes >12°, which create constraints to development. This includes rocky areas and thin soil cover characteristic of steep slopes or thick hillslope deposits, talus or palaeo-landslide deposits that are at risk of secondary movement through soil creep, weathering and soil piping.

3.1.1 DPZ 1: Inundation; flood risk in rivers, floodplains, dams and pans

All of the rivers and their tributaries, as well as floodplains and low level terraces have been mapped as this most predictable and high level risk. The channels and boundaries have been buffered using the 32 m riparian zone buffer stipulated by the NEMA EIA regulations 2010.

Table 1 Geotechnical factors associated with Development Potential Zones (X = critical geotechnical factor, x = subcritical geotechnical factors).

soil

Inundation Active/expansive soils Poorly consolidated soil collapsing Erodible dispersive Excavatibility of Permeability soil Shallow water table instability Slope DPZ Description 1 Inundation and flood risk; X x x x x channels, floodplains and dams 2 Very steep slopes; >18o and mass x x x x X movement risk 3 Steep slopes on dolerite hills (12o - X x x x X >18o) 4 Dolerite uplands, low to medium X x gradient slopes 5 Normandien Fm with Masotcheni x x x X x x Fm colluvium and dongas 6 Low to medium gradient slopes on x x x x x x x Volksrust/Vryheid Formation + dolerite, Masotcheni Formation and alluvium 7 Low gradient slopes; x x x x Tarkastad/Normandien Formations

However, other planning guidelines or regulations require developments to adhere to setback distances imposed by the 1:50- or 1:100-year return flood level. Deep gully systems incised into thick colluvium have been delineated within the lower gradient slopes of DPZ 3, 4 and 6.

Ladysmith CBD flooding and counter measures

The Ladysmith CBD and suburbs are laid out within the incised meanders of the Klip River which rises in the Drakensberg foothills near De Beers Pass to the northeast and is joined by the Sand Spruit upstream of the town. Downstream of the town the Klip River meanders through a broad floodplain created due to the decrease in gradient imposed by the base level created by a dolerite sill. An assessment of the flood history and counter measures was undertaken by Botha and Haycock (1998) in the context of sand mining from the Klip River channel.

Since 1884, 31 floods have affected the Ladysmith town lands along the Klip River (Directorate: Hydrology, 1994). During the process of evaluation of flood control measures several alternative

20 physical measures were considered. A system of raised levees was rejected as breaching of raised levees would contain floodwaters in the town resulting in more damage. Channel improvements such as shortening of meanders, removal of bridges and sandbanks or concrete-lining of the channel were considered. Concrete-lining was estimated to cost as much as the proposed dam at that stage. The existing levees along the river were adapted to improve storm water discharge although floods larger than the design capacity of the dam will still inundate the lower lying parts of the town. Backflow up storm water discharge pipes is controlled by non-return valves. The operation and maintenance of the dam is managed in terms of an agreement between the Government (DWAF) and Ladysmith / Emnambithi Local Council who consult with a community group, the Ladysmith Flood Liaison Committee, on all matters concerning flooding.

In order to control floods with a return periodicity of up to 100 years, a flood attenuation dam was constructed at Mount Pleasant upstream of the town. The past risk of flooding of the low-lying areas of Ladysmith, which was approximately 20% in any year, has been reduced to ~1% since the construction of the Qedusizi Dam (Director General: Water Affairs, 1991). The flood attenuation dam is constructed to contain a flood of a particular magnitude (set at a specific risk of occurrence) and release water at a controlled rate. A maximum discharge capacity of 300m3/s (~1:100 year flood) will flow from the uncontrolled bottom outlet of the dam resulting in a flood of 450m3/s in the town area. Flood prediction is not an exact science so the 1:100 year flood could be between 300m3/s and 600m3/s (Stewart Scott Inc, 1998) and a flood of 450m3/s is regarded as being bankfull discharge with the levees in place. The greater flood level will be on average 0.75m to 1m above the level of the lowest ground in the CBD, Mosque area and Leonardsville (Stewart Scott Inc, 1998). The dam has a capacity of 205 million m3 and area of 2 600 ha and can cope with a 1:200 year return (2 % annual risk) design flood with a peak outflow of 720m3/s causing a flood of 800m3/s in Ladysmith.

The predicted effects on the downstream channel, the area from which sand is excavated within the town, will be a reduction of the silt load to 20% of the pre-dam quantities. The Klip River flow will seldom exceed 300m3/s and flood durations will lengthen (Director General: Water Affairs, 1991). The changed flow conditions will result in gradual geomorphic changes to the river channel geometry through scouring and sedimentation which cannot be accurately determined in advance. The reduction in large floods could lead to a reduced capacity of the river channel, particularly if vegetation becomes established in the channel (Stewart Scott Inc, 1998). Sediment may be scoured from the channel downstream of the dam due to altered (concentrated) flow. It is probable that 50- 78% of suspended sediment in floodwaters will accumulate in the dam and during the tail end of a flood, incision of the deposited sediment could result in re-deposition and restriction of the channel downstream (Stewart Scott Inc, 1998). The channel and possible changes of river course will have to be constantly monitored to avoid possible damage and channel improvements may be necessary (Director General: Water Affairs, 1991; Stewart Scott Inc, 1998).

3.1.2 DPZ 2: Very steep slopes (>18o) and mass movement risk

Areas characterised by slopes exceeding 18° are a major constraint to development. Slope instability may result in rock falls, landslides and possible debris flows during periods of intense rainfall. Along the western boundary of the municipality within the Ukhahlamba-Drakensberg Park the very steep slopes are associated with the main basalt escarpment, the prominent Clarens Formation sandstone cliffs (Little ‘Berg), the underlying Elliot Formation slopes upon which large sandstone blocks dislodged from the cliffs above have come to rest. The red mudrocks within this zone are erodible and jointing within the sandstone or along intruded dolerite dykes results in localised spring seepage that can trigger debris flows on the very steep slopes. The Molteno Formation sandstone cliffs are relatively low in this area although the Tarkastad Subgroup sandstone units do form cliffs capping low ridges or hills with the mudrock underlying lower slopes covered by rockfall and landslide debris. These risks can be traced along the entire western and northwestern margin of the municipal area.

Very steep slopes are not limited to the Drakensberg. The resistance of dolerite to weathering or the formation of boulder fields associated with spheroidally weathered dolerite sills that cap many high hills in the area. Dolerite sill intrusions are concentrated within the lithological context of the sandstone/shale successions of the Normandien Formation and the underlying Volksrust and Vryheid Formations (Ecca Group). The deeply incised valleys of the Thukela River, the Sundays, Bloukrans, and Klip Rivers have all created very steep, lithologically influenced slopes defined by interbedded sandstones within more weatherable argillaceous rocks or dolerite intrusions. Close to the eastern boundary within the Thukela valley, steep slopes with relief of over 1000m between prominent mountain crests and the river valley are dominated by stepped topography including very steep, potentially unstable slopes.

Fig. 6 Upper Karoo Supergroup rocks forming unstable slopes in the “Landslide” and “Trail” zones. Photo source, GA Botha.

Fig. 8 Development close to the boundary of the “Trail” zone, Cathkin Park. Photo source; GA Botha.

3.2 Medium Development Zones

3.2.1 DPZ 3; steep slopes associated with dolerite hills

Differential weathering between dolerite and the more erodible argillaceous host rocks such as Normandien/Volksrust/Vryheid Formations results in irregular topography that includes steep slopes, exposed sheet dolerite or dolerite boulder fields on slopes or hill crests.

Dolerite intrusions in the Tarkastad Subgroup outcrop area along the Drakensberg foothills have been included within the DPZ 2 area characterised by very steep slopes.

Steep dolerite capped hills commonly have patches of sheet dolerite exposed on their crests. Eroded spheroidal weathering profiles developed in dolerite results in accumulation of rounded or angular hard corestones as the softer weathered material is removed. Apart from slope instability and a high risk of rockfalls, excavatibility is an issue for foundation installations, particularly for large structures such as powerline pylons and tower piles.

In the lower rainfall parts of the eastern valleys within the municipal area, structured clay soils are associated with weathered dolerite and colluvium derived off these slopes. Active melanic and vertic soil horizons pose a problem for small structure foundations.

3.2.2 DPZ 4; dolerite uplands and low to medium gradient slopes

Low hills formed by dolerite sills can be effectively developed, particularly if the soils are moderately or well drained (red structured or red apedal horizons). The elevated topography defines relatively small, irregular areas with variable slopes which limits large scale developments. In these areas there is commonly sufficient soil thickness above the spheroidally weathered bedrock to accommodate civil infrastructure such as roads, sewage systems and pipelines in shallow trenches. Foundations for small structures can be strip foundations.

3.3 High Development Potential Zones

3.3.1 DPZ 5; Normandien Formation topography with Masotcheni Formation colluvium

East of the Drakensberg foothills are extensive low gradient plains surrounding low mountains or hills formed of Estcourt/Normandien Formation rocks and dolerite sills. This topography extends from Estcourt through the Colenso area and Ladysmith (Fig. 10). The steep hillslopes formed of sandstone or dolerite have low gradient pediment footslopes incised into more erodible argillaceous rocks. Low order tributary valleys or depressions on hillslopes are infilled with stratified Masotcheni Formation colluvium. Soils associated with the shales, mudstone or siltstones are commonly of duplex character and contain active smectite clays. These soils and the underlying colluvium are preferentially susceptible to soil piping, rill- and gully erosion that forms deep dongas. Apart from flash flood risk, the vertical donga walls are liable to frequent collapse and even failure along arcuate, rotational failure planes. Gullies can extend rapidly along drainage lines or sites where runoff is discharged from contour banks or road culverts.

Much of this landscape is low gradient slopes with deep soils that are not directly linked to gully systems. However, the distribution of colluvial sediments within infilled bedrock depressions or even shallow valleys is commonly not evident from the surface. The distribution of Masotcheni Formation deposits as shown on the Harrismith and Dundee geological maps (Geological Survey, 1988, Council for Geoscience, 1998) is commonly associated with the extent of gully erosion and does not adequately delineate the extent of the thick sandy clay deposits.

Fig 11 shows the distribution of low slope gradients of less than 2o where sheet flow of storm runoff, combined with discharge from shallow rills and gullies can impose a high risk of short term flooding, particularly on large scale housing developments.

Development in these areas requires detailed planning of surface water runoff control and foundation design to address the potential for soil pipes, subsurface drainage features with collapse potential. These are not necessarily a major constraint to small infrastructure development but can inhibit large township development schemes.

3.3.2 DPZ 6; Low to medium gradient slopes on Volksrust/Vryheid Formation with Masotcheni colluvium and alluvium

Large parts of the Emnambithi and Indaka Municipal areas north of the Thukela River are underlain by Volksrust and Vryheid Formation rocks intruded by dolerite sills. The valley bottoms are characterised by extensive alluvial deposits along stream channels and floodplains. Erodible colluvial deposits are also common. Hillslopes have relatively thin soils and scree deposits of dolerite boulders are extensive on mid to lower slopes. Shale or dolerite lies at a shallow depth beloow the surface and 28 excavatibility is difficult in many parts.

3.3.3 DPZ 7; low gradient slopes, Tarkastad/Normandien Formations

The interbedded sandstone and shale successions of the Karoo Supergroup produce low gradient slopes extending over large areas. These zones are subdivided into broad valleys by the low hills formed from dolerite sills. Areas where the hills are low or widely spaced are not characterised by extensive, thick colluvial deposits. In the west the soil profiles are likely to be thicker than the profiles developed on similar rock types in the drier eastern valleys.

Fig. 12 Aerial view over the Drakensberg foothills in the Bell Park Dam area, Cathkin showing the typical DPZ 5 and 7, low gradient, undulating topography underlain by Normandien Formation and dolerite sills. Photo source, GA Botha.

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