Lleweni Game Farm: Wonderkrater

The name Wonderkrater probably derives from the crater-like shape of the large, concave peat mound that rises above the spring eye. The peat is saturated with water at depth, so the ground on the mound tends to oscillate when jumped upon, which probably led to the ‘Wonder’ in the name. A peat mound, so unusual in the semi-arid savannah environment of inland Southern Africa, preserves rare evidence of the palaeoenvironment and climate of the region.

Wonderkrater represents one of 50 warm spring sites known in South Africa, 35 of which occur in Gauteng (Scott 1982). Situated near the Nylsvlei wetland two hours north of Johannesburg, Wonderkrater is a large peat mound formed by an artesian spring that rises along a fault, intersecting rocks of the Karoo Supergroup (200 million years), Bushveld Complex (2 060 million) and Waterberg Group (1 800 million years old).

Significance

Wonderkrater is situated on the fringe of the Nylsvlei floodplain, and is fed from the Waterberg range to its west. This is an area of active sedimentation, and all sediment delivered by the tributaries is retained on the floodplain. Preliminary investigations suggest that both the mound and the surrounding terrain appear to be aggrading at more or less the same rate. The surrounding area receives sediment, mainly coarse-medium grained sand, by stream flow from small rivers. These streams lose discharge as a result of infiltration of water, and gradually become smaller downstream, ultimately disappearing altogether. This loss of water results in sediment deposition. Sediment deposited in this way is further distributed by sheet flooding during heavy storms (McCarthy & Hancox 2000; McCarthy et al. 2002). In contrast, the mound aggrades as a result of the accumulation of peat and the oxidation products of peat, mainly silica bodies called phytoliths. Phytoliths, like pollen, have characteristic forms determined by the host plant, so are useful in reconstructing shifts in palaeo-vegetation (see for example Rossouw 1996). The mound presently rises about 2.5m above the surrounding terrain. Investigations suggest that it consists virtually entirely of plant-derived material (peat and oxidized peat), which extends to a depth of at least 8m below the surface.

Peat is a brownish-black soil that is formed in acidic, anaerobic wetland conditions. It is composed mainly of partially decomposed, loosely compacted organic matter with. Peat consists mainly of reeds, leaves, fruit, dead trees, pollen, phytoliths and animal remains, all in a relatively fresh state because of the slow and halted decomposition rate. Waterlogged soils have little oxygen, which inhibits the decomposition of organic matter by anaerobic micro-organisms, which decompose organic matter slowly and incompletely, and become ineffective at increased depths (Grundling & Dada 1999).

There are two main peatland types: bogs and fens. A bog is a peatland that is fed from rainwater only. A fen is a peatland that is fed from both rainwater and groundwater. The water table may arch upwards into the fen, adding mineral elements from the rocks underground. Wonderkrater, like most of the peatlands on the Highveld, is a fen. The water within the mound is derived from deeply circulating ground water, ultimately derived from rainfall in the region. Siep Talma, of the CSIR in Pretoria, dated the water using radioactive isotopes, and established that it fell as rain about 17 000 years ago. It has a distinctive chemical composition; rich in fluoride, calcium carbonate, sodium and chloride.

Wonderkrater is one of the few ancient spring sites associated with pollen-bearing peat deposits, and so has been the subject of a detailed fossil pollen (palynological) study conducted by Prof. Louis Scott of the University of the Free State (Scott 1982, 1989, 1999; Scott & Vogel 1983; Scott & Thackeray 1987; Scott et al. 2003). Carbon 14 dating indicated that the upper 8 m of peat formed by continuous accumulation over the last 34 thousand years. The pollen in the peat thus preserves a record of the vegetation in the region over that time interval. Using his pollen results, Scott reconstructed the vegetation history and climate of the past 34 000 years, and correlated the vegetation shifts with the pollen record in lake sediments in the Tswaing Crater. This provided insight into the climate in this region over the last 34 000 years, an extremely important time interval because it includes the last ice age, which climaxed 18 000 years ago. This palaeoclimatic record for the region is one of very few available for the interior of the country.

Extensive auger drilling to investigate the subsurface of the site over a 400 x 250m2 area, records interlayered sands, silts, ash and organic peat horizons, some of which contain a relatively high abundance of Middle Stone Age tools (dated to between 30 000 and 200 000 years ago). Besides stone tools, the mineral-rich spring water has preserved bones and teeth, while the peat has preserved a range of organic remains, including seeds, branches, charcoal and insect carapaces. Animals are key environmental and chronological indicators, and analysis of their bones can reveal much about the agent(s) responsible for their modification and accumulation. Flora, besides reflecting the environment, has long been used to make tools, though these are rare at any archaeological site. Very few pre-30 000 year old pieces have been recovered in Africa.

Aims of Current Research

  • We wish to obtain a high resolution sequence of accumulation ages through the peat mound so we can resolve subtle changes in growth rate of the mound
  • From this, we hope to obtain a better understanding of the effect of climate change on the mound
  • We will retrieve, analyse and study artefacts from the site
  • Look for additional traces of human activity at the site
  • Explore deposits for fossil bone
  • Correlate finds (e.g. lithics, fauna, flora) with Scott’s pollen sequence to refine and supplement the record of climate change here and globally
  • Retrieve pre-20 000 BP vegetation types

Preliminary Results

An excavation permit was obtained from the South African Heritage Resource Agency (SAHRA) Permit No. 80/05/05/017/51 in 2005. Controlled excavations in two main areas of the mound (Pits B and C) have provided a high resolution sequence of radiocarbon dates for peat in the first 3 m of the deposit, and yielded bone fragments of a diverse range of large mammals. A sand horizon at the base of the peat deposit contains an in situ and well-preserved Middle Stone Age tools with a minimum age of 30 k. A previously drilled core from this area shows the white sand-MSA horizon to be the first of many such levels represented here.

Acknowledgements

We thank Dr. Walter Ward, land owner, for his support and hospitality during field trips. This research was funded by grants (2005-present) from the National Research Foundation (NRF), University Research Council, University of the Witwatersrand, Palaeontology Scientific Trust (PAST) and Cultural Service of the French Embassy in South Africa.

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