An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
Send your answers to us in an email via our profile page.
Any logs not accompanied by an email will be deleted.
Logging Tasks:
1. In your own words describe how a sinkhole is formed.
2. At the listed coordinates look around at the smaller boulders. Describe the rock formation in terms of colour, grain, weathering and texture.
3. How was this dolomite formed?
4. At the listed coordinates there is a board. What is written in red on this board?
PLEASE NOTE: Although there is an entrance fee, try asking at the gate to pop in for a few minutes to find the caches (and take a few photos) as there is a possibility that you may be allowed in without charge, but this cannot be guaranteed (especially in busy time periods and over weekends).
General Overview
Thousands of years ago, Wondergat was a water-filled cavern until the roof caved in and left a magnificent hole filled with water. In the early 70's the deepest point at this dive site was more than 70 meters and today it's only 58 meters.
The western section of the Transvaal Sequence occurs in an area bounded by Vryburg, Kuruman and Douglas known as the Ghaap Plateau. The eastern section extends from the south in the Potchefstroom district to the north of Ventersdorp, west of Mafikeng and to the Rustenburg-Brits section.
Lower strata of the Transvaal sequence comprise mostly of dolomite (with some chert and shales interspersed in places) while the upper strata appear to be more varied in constituents. Dolomite consists largely of calcium carbonate and is hence vulnerable to solution, especially by the carbonic acid found in rainwater percolating downwards. The dissolution of dolomite can lead to the formation of underground caverns and horizontal chambers often filled with large volumes of groundwater. Malmane Dolomite appears to be one of the main elements of the Transvaal sequence. It is believed that the dolomites were laid down in shallow inter-tidal or sub-tidal zone of open water seas.
Simplified Geology of SA: The Malmani Dolomites in blue in the north and central parts of the country
Although the North West Province has few surface water resources it has a large reservoir of subterranean water in the form of fractured aquifers and dolomitic compartments.
Climatic conditions in the North West Province vary significantly from west to east. The far western region is arid, encompassing the eastern range of the Kalahari Desert.
Dolomitic Eyes and Sinkholes
The dolomite area covers approximately 4022km2 of the North West Province and forms the main watershed of the east-flowing Limpopo River system and the west-flowing Molopo River. The sources of many rivers, which rise in the Province, are located in this dolomite area. These include the Molopo, Marico, Malmani, Mooi, Schoonspruit and Harts Rivers. The actual source of these rivers are known as dolomitic eyes, which are water bodies fed by groundwater originating from fractures in the underlying dolomite. The water from these dolomitic features is typically alkaline (pH 7.5-9.3) having picked up magnesium and calcium carbonates through solution from the parent dolomite. By their very nature, such dolomitic features are partial to both factors affecting surface water and those affecting groundwater. For this reason, dolomitic eyes are extremely sensitive and often possess unique ecosystems characterised by a high degree of endemicity (species which are found only there). The North West Province has a number of these unique natural features most of which are classified as springs while one, known as Wondergat, is considered to be a sinkhole. The features of a sinkhole is generally circular, up to 50m in diameter, steep sided and deep
How do Sinkholes form?
Weathering of Dolomite and Ground Instability
Rain water takes up carbon dioxide in the atmosphere and soil (where the concentration of this gas may be up to 90 times greater than in the atmosphere) to form a weak carbonic acid. The weakly-acidic groundwater circulating along tension fractures, faults and joints in the dolomitic succession causes leaching of the carbonate minerals. The solubility of dolomite is high in comparison to other rocks, but significant solution cannot be observed over short periods (months and years).
The process of dissolution progresses slowly in the slightly acidic groundwater (above and at the groundwater level). The resultant bicarbonate-rich water emerges at springs and is carried away. This process of dissolution has resulted in a vertically zoned succession of residual products, which in turn are generally overlain by geologically younger formations or soils. Hard, unweathered dolomitic bedrock is overlain by slightly weathered jointed bedrock and thereafter, through a sudden, dramatic transition, to totally weathered and low strength, insoluble residual material consisting of mainly manganese oxides (wad), chert and iron oxides that reflect the original insoluble matrix structure. Depending upon the local subsurface structure, this very low strength, porous and permeable horizon may in certain locations be up to several tens of meters thick but is generally less than 10 meters thick. With the passage of geological time, concurrently with the downward progression of the intense weathering of the dolomitic bedrock, compaction by the mass of the overlying materials has resulted in a progressive densification of these low strength materials. Consequently, the vertical succession of the residual products of weathering reflect an upward increase in strength and a decrease in porosity and permeability. This process results in a decrease in overburden quality with depth, which in turn leads to higher rates of penetration, so often noted in drilling investigations, when the dolomitic bedrock is approached. Infiltrating water from leaking services or surface accumulations acting on this low-density material results in a loss of support through slumping or subsurface erosion.
Given sufficient time and the correct triggering mechanisms, instability may occur naturally but is expedited many orders of magnitude by man’s activities. The primary triggering mechanisms in such instances include the ingress of water from leaking water-bearing services, poorly managed surface water drainage and groundwater level drawdown. Instability can occur in the form of sinkholes and dolines. Topography and drainage, the natural thickness and origin of the transported soils and residuum, the nature and topography of the underlying strata, the depth and expected fluctuations of the groundwater level, and the presence of structural features such as faults, fractures and dykes are all factors which influence the risk of subsidence taking place.
The mechanism of sinkhole formation is briefly summarised as follows:
- Cavities exist within bedrock or the overburden, which may be in a state of equilibrium.
- Active subsurface erosion caused by concentrated ingress water will result in transportation (mobilisation) of materials downwards into the nearest cavity (receptacle).
- Headward erosion leads to successive arch collapse. The last arch may be stable for a considerable length of time and is sometimes supported by a near-surface layer of hardpan ferricrete.
- A triggering mechanism leads to the breaching of the last arch. Particularly in the case of small sinkholes, the cross-section resembles a bottleneck (narrow opening at surface), a shape that may be maintained for some time.
A number of independent conditions are necessary before a sinkhole can form:
- There must be adjacent rigid material to form abutments for the roof of the void.
- A condition of arching must develop in the residuum.
- A void must develop below the arch in the residuum.
- A receptacle must exist below the arch to accept mobilised material.
- Some disturbing agency must arise to cause the roof to collapse.
The Geology of Dolomite
Ancient carbonate rocks are predominantly composed of two minerals; calcite or dolomite. When a carbonate rock is dominated by calcite (more than 95% with less than 5% dolomite), it is called limestone, when it is dominated by dolomite (the mineral) it is called dolomite (the rock). Limestone is a chemical or biochemical sediment consisting essentially of calcium carbonate primarily in the form of calcite, and minor constituents such as silica, feldspar, pyrite and siderite. Dolomite, as a rock, contains more than 90% dolomite and less than 10% calcite as well as detrital minerals and secondary silica (chert). Very few, if any, sedimentary dolomites are truly stoichiometric, encompassing the spectrum from calcium to magnesian dolomites. Researchers seem to agree that the carbonate rocks in the Malmani Subgroup are essentially dolomitic limestones (a limestone that has been incompletely dolomitized) with few limestone bands, whereas the Campbell Rand Subgroup consists of dolomite and limestone.
The onset of limestone deposition has been estimated at 2 550 Ma. The limestone precipitated in favourable environmental conditions by algal photosynthesis and inorganic precipitation. The precursor mineral to dolomite was aragonite. The limestone then underwent dolomitization in an environment where meteoric and marine water mixed and the saline brine became supersaturated with respect to magnesium and silica and undersaturated with respect to calcite thereby increasing the potential for dolomitization and chertification. Sufficient evidence exists to indicate that dolomite is syndepositional, the minerals forming contemporaneously with sedimentation.
Dolomite and limestone, with little or no impurities, are valuable industrial commodities and are used in the making of a wide range of products such as cement and glass.
Aquatic life in Wondergat
The Banded Tilapia (Tilapia sparrmanii) is endemic to Wondergat and won't be found at any other fresh water hole in the world. Other fish to lookout for are (if you have come to dive) the Sharptooth Catfish (Clarias garipinus), Southern Mautbrooder (Pseudocrenilabrus philander) as well as fresh water shrimps and crabs.
This is the only place in South Africa where Stromatolites have been found under water and are still in very good condition. (The term stromatolite refers to lithified, frequently laminated, organo-sedimentary structure built by microbial communities through the processes of sediment trapping and binding and/or of carbonate precipitation.) It contains abundant algal stromatolites, evidence of an aquatic environment in ancient times. The algal stromatolites have a number of distinctive shapes such as domes, columns and spheres, their shape being governed by the environment in which they were formed. The only other area where similar Stromatolites were found is in Sharkbay, Australia.
References