The reserve lies in the syntaxial region of the Cape Fold Belt (CFB).
The CFB is a deformed sedimentary sequence consisting of the early Cape Supergroup and the later Karoo Supergroup. Table Mountain Group sediments (TMG) of the Cape Supergroup and specifically the Peninsula, Pakhuis and Cedarberg Formations and the Nardouw Supergroup occur in the reserve. The CFB has two trends; an east-west trend along the southern Cape coast and the northerly trend parallel to the Cape west coast. The two trends converge near Worcester, forming the southwest trending syntaxial region.
The stratigraphy consists of the Peninsula, Pakhuis, Cedarberg and Goudini Formations. The Peninsula sandstones are quartz arenites, as are the Goudini sandstones which are interbedded with shales. The Pakhuis is represented by quartz-rich diamicite and the Cedarberg by shales and siltsones.
Folding at the first order scale, with wavelengths varying from 300m to 600m, consists of three synclines and two anticlines. The most southerly syncline and the anticline to the north of it define a monocline. Fold axes plunge gently towards the northeast and the axial planes are moderately inclined towards the southeast. Second order folding has a wavelength of ~10m, and is largely confined to the Upper Peninsula and Pakhuis Formations. It is a product of early soft sediment slumping down a gently inclined slope. The Cedarberg Formation is unconformable on the Pakhuis because of subaqueous or subglacial truncation of slump folds. Subsequent shortening perpendicular to slump fold axes formed the first order folds and modified early slump folds by the flexural-slip mechanism. Second order fold axes and axial plane orientations are consistent with those of the first order folds. Corrugations and microfolds in the Cedarberg shales formed during early shortening of the more competent peninsula and Goudini Formations.
The Cedarberg shale has an early S1 cleavage subparallel to the layering and late crenulation cleavage on the limbs of microfolds. The crenulation cleave is axial planar to corrugations and formed during shear movement of the overlaying sequence relative to the Peninsula Formation during flexural slip folding. A fracture cleavage occurs in the Peninsula and Goudini sandstones but is best developed in the Peninsula Formation. The better-developed cleavage in the Peninsula sandstones is either due to intensification of an early slump cleavage or increased flattening strain in the Peninsula because of folds locking up during flexural slip modification. The cleavage is axial planar to second order olds or fans around first order fold closures. Compared to the Goudini and Pakhuis formations, the Peninsula sandstones show widespread evidence of deformation, including flattened quartz grins parallel to cleavage, deformation lamellae and serrated grain boundaries.
A major northeast-southwest extension fault transects the reserve and has undermined downthrow to the south. Fractures, joints and en echelon crack arrays are products of post folding process.
Deposited unconformably on Precambrian-Cambrian metasediments is the Early Ordovician Table Mountain Group of the southern Cape. The TMG consists of eight formations; Piekenerskloof, Graafwater, Peninsula, Pakhuis, Cedarberg , Goudini, Skurvegberg and Rietvlei. Of these only the Peninsula, Pakhuis, Cedarberg and Goudini Formations are present in the reserve.
The Peninsula Formation, a quartz arenite complex, transgressively overlies the Graafwater and attains a maximum thickness of 1 800m in the Western Cape. The mature, cross bedded sediment is attributed to high energy current and strong winnowing processes on a stable but slowly subsiding shelf.
The Late Ordovician is characterised by a glacial event in the Cape Basin. Diamicitites and tillites of the Pakhuis Formation are products of the glaciation. Sediments of the Pakhuis Formation are regionally associated with soft sediment deformation and erosion of the underlying Peninsula Formation.
Deposition of the Cedarberg Formation followed retreat of the ice sheets. The Cedarberg Formation consists of thinly laminated shales and are rare lenses of siltstones. Both the Pakhuis and Cedarberg Formations, comprising the Winterhoek Subgroup, attain a maximum composite thickness of ~160m in the Western Cape.
Westward thickening quartz arenites of the Nardouw Subgroup transgressively overly the Cedarberg. Fine grained sandstones interbedded with purple shales of tidal origin, make up the lowermost Goudini Formation.
A. Peninsula Formation
There is little variation in the Peninsula Formation sandstones, the rocks essentially being thickly bedded (up to 1m) quartz arenites. In hand specimen the rocks are fine to medium grained, white to pale grey and frequently planar and trough cross-bedded. Fine laminations (< 2mm) occur in places and quartz pebbles up to 1cm diameter are locally present in the sandstone. The rocks are strongly cleaved.
Thin sections of two samples from gently southward dipping fold limbs give an estimated rock composition:
Quartz grain size ranges from <0.1mm to 0.5mm, with an average of 0.2mm. Strained quartz grains are flattened parallel to cleavage and irregular in shape. Serrated grain boundaries and deformation lamellae are common in quartz grains. It is suggested that chlorite and rare muscovite are rotated between quartz grains as they are flattened. The chlorite and to lesser extent muscovite, form thin lath accumulations which anastomose around quartz grains in places.
A glacial period, following Peninsula deposition is represented by the Pakhuis Formation. In the reserve the Pakhuis is quartz-rich, crudely bedded, coarse grained and poorly sorted. Angular to well rounded quartz and quartzite clasts vary in size from 2cm to 5cm, although larger sizes are rare. Cleavage is distinctive but non-penetrative.
In thin section the diamicite has the following approximate composition:
Strained quartz grains (0.1mm to 5mm diameter) are supported in a very fine matrix (0.01mm diameter) of quartz and oriented chlorite and muscovite which define the cleavage. Individual quartz grains rarely touch each other and are minimally flattened in the cleavage plane.
The shaly rocks show limited diversity, depending on grain size and amount of constituent phyllosilicates. In hand specimen they are very fine grained, olive green and frequently finely laminated (<1mm). Cubic pyrite nodules, most likely authigenic, are present throughout the rock. The shale is corrugated except in the more silty layers. Segregated or intruded vein quartz sheets parallel the primary layering and exhibit fibre growth striae.
The quartz sheets are folded with primary layering.
In thin section, a more silty unit has an estimated composition of:
Quartz grains range in size from <0.001mm to 0.2mm and are frequently strained but not flattened.
Recrystallisation is evident, producing very small unstrained quartz grains along larger grain boundaries. Chlorite forms long, thin anastomosing accumulates between quartz grains, parallel to the laminations and defines an early S1 cleavage. In more foliated samples phyllosilicate layers are distinct from fine grained quartz-rich layers and have been microfolded producing a late crenulation cleavage (S2).
Only the Goudini Formation is present in the reserve and conformably overlies the Cedarberg shales. The sandstone beds are generally thinner than Peninsula sandstone beds (<1.0mm thick) and are planar and trough cross-bedded in places. The sandstone is medium to coarse grained, white to dark grey and frequently interbedded with lenticular units of up to 40cm of purple shale. The shale is finely laminated and occasionally develops a corrugated slaty cleavage with quartz veining. Sheet quartz with fibre growth striae sometimes occur on lower bedding surfaces of the sandstone units.
In thin section a typical quartzite has the following approximate composition:
Interlocking quartz grains vary in size from 0.1mm to 0.5mm. Phyllosilicates, including chlorite and muscovite, are generally very fine grained and localised in areas between slightly quartz grains. There is no dimensional preferred orientation of phyllosilicates or quartz grains. Long, thing mica laths are kinked around unflattened quartz grains in places. The opaque minerals are surrounded by reddish coloured haloes.
Dominant broad soil pattern
Erosion rather than deposition has characterised the fynbos landscape in the Cenozoic. Yet in coastal environments and locally inland, substrates which are the product of deep weathering under more humid climates than the present, where not stripped by erosion, may be mantled by sands or younger soils materials. These preweathered substrates could not have formed under present-day conditions and are paleofeatures.
A number of factors have contributed to the evolution of the modern soils.
· Climatic changes throughout the Cenozoic, which influenced the distribution and properties of soil and soil materials found in the region.
· Geomorphological history controlling the evolution of the landscape.
· The Cape Fold Mountains are ancient feature of the landscape and were largely moulded in their present form by erosion in the Cretaceous or earlier times. Transgression and regression of sea levels during the Cenozoic have resulted in changes in stream erosion base levels and landscape dissection with the consequent stripping of older soils, exposure of preweathered materials and remodelling of earlier erosion surfaces. In this way new parent materials have been provided for ongoing soil forming processes.
Weathering and Soil formation in the Cenozoic
The warm humid climates that prevailed at the Cape in the earlier Cenozoic led to weathering and soil formation. Under such conditions and with free drainage on the coastal forelands, there was rapid ferrallitic chemical breakdown of primary silicate with feldspathic minerals weathering to kaolintic clays and micaeous minerals changing to mica degraded clays, such as vermiculites, but also mainly to kaolinite and residual oxides and hydroxides of iron and aluminium. An almost complete loss of basic cations, and to an extent silica, took place under internal drainage and leaching. The end result was uniformly red or yellow-coloured, porous apedal soils with low base status.
Soils in Vogelgat: An overview
The generally steep topography and the slow slow rate of weathering of the quartzitic sandstone resulted in very poor development of soil.The almost clay-free, shallow, coarse, sandy residue characteristic of so much of the area, seldom deserves to be called soil. This residue is rarely deeper than 30cm and usually comprises a high proportion of gravel and small boulders. Its deepth varies over short distances as bedrock is never far from the surface and the landscape is normally punctuated with rocks outcrops or strewn with boulders.
Because of the acidic conditions, organic residues are slow to decompose and so accumulated to a greater degree than would be expected. This black, partly decomposed, peat-like material dispersed in the surface horizon contributes little to th every low fertility status of the soil, but probably plays an important role in improving water holding capacity in the root zone. This effect, reinforced by the coarse sand blanket generally present, serves to minimise moisture loss by drainage and evaporation and so accounts for the surprisingly green appearance of fynbos vegetation in the hot mid-summer of a Mediterranean-type winter rainfall climate
Sandstone weathering resulted in light textured, medium grained light coloured sandy soils, with low water holding capacity and low cat ion exchange capacity. These soils are acidic that normally develop are Cartref form and Podzol variants as Hou Hoek, Witfontein, Lamotte (on deeper alluvial and colluvial material). Lamotte are found on the lower slopes. The other three forms are found on midslopes and grade into sandstone lithosolic soils.
Next to valley beds, stratified sandy alluvium of the Dundee form is found.
Shale bands of finer textured soils, high clay content develops into yellow and reddish colour, are more poor due to the iron in the shale. The higher clay content increases the water holding capacity and cat ion capacity. Dominant soil forms are : Glenrosa, Kulu**?? And Oakleaf forms, and sometimes soils structured with substrate such as Klapmuts and Escourt. Low in pH.
In seepage spots accumulation of organic material occurs and relatively deep black coloured organic horizons develop (Champagne form). This material has a high acid content, low pH and water logged for the greater part of the time of the year. This accumulation is due to anaeromorphic hydro condition, which inhibit total breakdown of humus materials.
Micro relief in landscape has a distinct effect on occurrences of different soil types. Better-drained soils on convex positions will result in yellow, iron rich soils and those on concave positions will get hydromorphic accumulations.
The North facing slopes have drier conditions resulting in soil development been low, due to low water holding capacity. This slow soil development results in shallow, rocky maller.
The deeper soils found in the South facing slopes are due to the rapid chemical weathering which occurs under more moist and cooler conditions.
Number of broad soil types:
One: Quartzitic sandstone.
(See: Whittle-Herbert, 1990 Deformation Structures in the Table Mountain Group centred on Vogelgat Nature Reserve, Hermanus. Thesis prepared in partial fulfilment of the requirements for the degree of B.Sc. Honours in Geology)