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Approximately 200 million years ago, these sediments were capped by volcanic rocks which extruded onto the land surface (the Karoo volcanics). The first to appear were basalts which make up the western lowlying margin of the reserve. These unusual basalts, low in olivine and silica and therefore basic, are terrestrial. They formed as huge sheets of lava extruded by means of fissures over many kilometres north to south. The theory is that at some point, continental material became included within the magma and extruded explosively at high pressures in the form of welded volcanic ash (tuff). This ash originated from the line of volcano stretching north to south from eastern Zimbabwe to northern Natal and now forms the upstanding ridges and high ground of the reserve.
The general description of the Lubombo volcanics was given by du Toit in 1930, but the most recent work has been by Cleverly in the late 1970's (Cleverly 1979). In Cleverly's terminology (1979) he gives the name Twin Ridge Beds to the volcanic geology of the Lubombo. The main Lubombo Escarpment is made up of the Jozini Rhyolite Formation, followed to the east near the Mozambique border by the Umbuluzi Rhyolite Formation and the Oribi Rhyolites (Ndzindza). The basalts formed as a supurating lava subaerially extruded, which oozed across the surface from elongated vents.
The rhyolites, being explosive rocks, are possibly formed by hot rushing clouds of molten rock and gas. They may also include the flows of high temperature magma. The rhyolites are extruded periodically and give rise to various flow banded features which may be representative of several dozen explosive incidents. Each incident begins with the initial explosion and magma flow producing a fine crystalline rock at ground level. The surface of the massive magma flow that follows the surface is in contact with the air and cools more rapidly. It solidifies but then becomes rebroken by the flow beneath it, producing "lava concrete" (autobreccia). A number of outcrops of this can be seen in the reserve.
In addition, the rhyolites also include various air fall debris such as lapilli (small stones), silica raindrops and others siliceous ejectamenta including obsidian (black volcanic glass). Occassionally, the frothy surface of the flows includes gas bubbles which later solidify as geodes. The whole sequence testifies to a period of great geological violence heralding the break up of the supercontinent Gondwanaland and the establishment of Africa as a separate continent. Remarkably, small sedimentary basins of muds and sands were formed in between rhyolitic flows and, where these are preserved, fossils occur in the sediments.
In terms of historical geology, the Karoo volcanics are significant to have occurred as the eastern margin of Africa was being formed. India, Antarctica, and Australasia were being pushed away along a tearing fracture from what today is the eastern edge of the continent. The Lubombo mountains provide insight into the process of continental formation due to the fact that they have been stable since this break up (190 million years). There is evidence of subduction along the plate margin after formation, and rhyolites have been noted at great depth from coastal boreholes in a vertical attitude, off northern Natal.
The different chemical composition of the basic basalts, acid rhyolites, and intermediate dacites are the controlling feature in the dramatic terrain of the reserve. The basalts, being low in silica, are chemically weak and therefore have been susceptible to deep weathering in the Late Tertiary Period. The rhyolites, being rich in silica, are chemically very resilient and therefore have eroded to a much lesser degree. Once continental formation had taken place, the interbedded layers of basalt, dacite, and rhyolite were downwarped to the east forming a monocline. The easily weathered basalts were then etched and eroded out from the sequence, leaving the ridges of rhyolite as the ribs of the strata.
On the western side of the reserve the lowlying relatively flat topography of Mbuluzi Nature Reserve and Simunye Nature Reserve is underlain by basalts. A minor flow of rhyolite produces the ridges adjacent to southern Mbuluzi Nature Reserve along the line of Lusoti hill. A switch from dacite to basalt, when weathered and removed, exists in the south. A further switch back to the massive flows of rhyolite produces the precipitous slopes to the east of the Siphiso and the high ground of Ndzindza.
In terms of the outcrops of rock, the basalts rarely are visible at the surface. They may be seen in a somewhat degraded form along stream beds, particularly the Siphiso near the restcamp, where it has a drab greenish grey colour with white spherules. The rhyolites outcrop is more obvious with ridges covered with angular rhyolite cobbles, varying in colour from maroon brown to reddish grey, with obvious orange crystals of felspar (Urie 1967, Wilson 1980).
Geology | Geomorphology | Soils | Hydrology
Geomorphology
It is believed that after the African continent was left in isolation its surface began to erode. In the course of tens of millions of years, the surface was reduced to a pleneplain known as the African Planation Surface (King 1963). This plain, irrespective of the underlying geology, dipped towards the Indian ocean at an angle of 5 degrees. Across this landscape, mature rivers meandered in broad tracks. However, margin uplift, possibly in the Miocene times of 20 million years ago (Maud and Partridge) led to gradual lowering of these river tracks within their original meandering beds. This caused the etching of the preweathered weak rocks and engorgement of the meanders in the resistant rocks. It is at least during the last 20 million years that the Lowveld has been eroded (Watson and Price Williams 1985) and the Mlawula and Mbulizi gorges have cut down through the Lubombo. This is true of all the rivers flowing through the Lubombo Range. Evidence of remnants of the old African surface can still be seen on the Lubombo crest at Ndzindza where views north and south, particularly towards Mananga, show vestiges of a lightly dipping surface to the east. It may be noted that this is not a function of the bedding planes of the underlying rocks since they are dipping in a generally eastern direction as well, but between 30 and 40 degrees to the east (Cleverly 1979). This surface is picked up again in the extreme western portion of Swaziland (Watson and Price Williams 1985).
On top of the Lubombos are a number of closed depressions or pans, especially along the ridge on Ndzindza. These pans, which retain water and frequently contain iron concrete, are located on remnants of the African planation surface and were probably formed within the last million years. Their origin is associated with the rhyolite which produce zones more susceptible to weathering (Watson 1986).
More recent geomorphological evidence is to be noted along the valleys of the Mlawula and Siphiso rivers. Sloping pediments from the base of the ridges towards the river beds are mantled with colluvium. Colluvium is a weakly sorted mixture of sands, silts, and clays believed to have been emplaced during the terminal Pleistocene some 20 thousand or more years ago (Price Williams; Watson and Goudie 1982; Price Williams and Goudie 1984). This sediment originates from the denudation of the backslope and the formation of colluvial fans resting at angles of about five degrees and reaching thicknesses of several metres. These colluvial sediments, which appear to have clogged drainage lines, were probably formed in climatic conditions quite different from the present day. Subsequent re-establishment of the river beds often exposes sections of colluvium. The colluvium is often underlain by cemented gravels, obviously older, but of indeterminate Pleistocene age.
Overlying the colluvium along parts of the rivers is a large silt river alluvial terrace. The terrace is often many metres thick and forms a flat zone adjacent to the present river course. On the Umbuluzi, it is noticeable along on both sides of the Siphiso river near the rest camp and especially on the inside of the large meander near the airstrip (upstream of its confluence with the Mlawula). The rest camp is built on this terrace. During Cyclone Domoina, the terrace was overbanked but was neither eroded nor added to. This implies that it formed in somewhat different conditions earlier than today during the Holocene epoch (last 12000 years). The most recent geomorphic activity is associated with coarse sands which form part of the present riverine floodplain. These are periodically mobilized and redistributed during flood conditions.
Soils
The soil types found within the reserve vary greatly. Much of the area consists of rock outcrops and stony ground (raw mineral soil) but in the Ndzindza area, the soils found consist of either grey sand or sandy loam on hard rock (Lithosolic). In contrast, the Siphiso Valley area holds mainly brown to black loams or clay (Vertisolic). Grey, black and red clays (Vertisolic) as well as deep red, yellow and orange loams (Fersialitic) have also been recorded in the area. The larger watercourses are flanked by narrow strips of colluvial and alluvial soils.
Soil erosion is a serious problem in places. The Siphiso valley contains large areas that have been subject to sheet, rill and gully erosion as a result of overgrazing. Badly drained roads also cause erosion in some areas.
Hydrology
The Mbuluzi, Mlawula, and Siphiso rivers flow through the reserve. The Siphiso represents the only river whose catchment area is nearly included in the reserve. The Mbuluzi and Mlawula rivers are perennial. The Mlawula only became so since the irrigation of the cane fields in 1979. The remaining rivers and streams dry up during the winter months, except in some stretches such as rhino pools and croc pool. There are a number of small pans in the reserve at Ndzindza and Mbuluzi. There are three dams on the Mlawula that were originally installed by cattle ranchers as well as a weir on the Mlawula River, adjacent to Mlawula Railway Station. The Mlawula and Mbuluzi rivers are polluted by sugar, fertilisers, molasses, and high sediment loads due to commercial agriculture. These pollutants reduce oxygen levels, increase nutrients levels and turbidity. These conditions result in algal blooms and fish kills. The Mlawula and Mbuluzi are also a permanent source of alien plant seeds.
Geology | Geomorphology | Soils | Hydrology
References:
Cleverly, R.W., The Volcanic Geology of the Lebombo Monocline in Swaziland. Trans. geol. Soc. S. Afr. 82(343-348), 1979.
Uric, J.G., Explanatory Notes to Accompany Sheet 2631BD(14)
Geological Survey and Mines Department, Swaziland, 1967.
Watson, A., The Orgin and Geomorphological Significance of Closed Depressions in the Lubombo Mountains of Swaziland, The Geographical Journal, 152:1(65-75), March 1986.
Watson, A. and WILLIAMS, D.P., Early Pleistocene River Gravels in Swaziland and Their Geomorphological and Structural Significance, Z. Geomorph. N.F. 29:1(71-87), Berlin, March 1985.
Wilson, A.C., Explanatory Notes to Accompany Sheets 2631BB(8) and 2632AA, Geological Survey and Mines Department, Swaziland, 1980.
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