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Calcrete of the Kalahari

A cache by iPajero Send Message to Owner Message this owner
Hidden : 08/07/2013
Difficulty:
2.5 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:

This earthcache brings you to the Auchterlonie Museum in the Kgalakgadi National Park, which is built of calcrete stone. A daily conservation fee is applicable or free with a valid Wildcard.

If you are driving from Twee Rivieren stop at S26 14.591 E020 34.425 and observe the outcrops of calcrete.


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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) Visitation confirmation: The listed coordinates brings you to a point of interest. What can be seen here? We are looking for the point of interest nearest the coordinates (not everything there is to see in the general area).
2) Describe this calcrete stone - the texture and colour.
3) Take two small samples of rock and knock them together and see if it breaks.

  •      What result do you get?
  •      Explain reason for this.
4) List a few ways how calcrete is formed.
5) Optional: Share your experience with the caching community by log or by uploading photos of special sightings or experience you've had while visiting the park.

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RUINS OF THE AUOB RIVER

Between 1913 and 1917 the present park was surveyed and divided into farms by a Scottish Surveyor Roger “Malkop” Duke Jackson.

With the outbreak of World War 1 in 1914, the Government of the Union of South Africa drilled a series of boreholes along the Auob River to provide their troops with water in case of a South African invasion of Namibia along this route.

Guards recruited mainly from the local community, were hired to protect and maintain the boreholes and these guards were permitted to “settle” next to the boreholes with their families and livestock. They used traditional wattle and daub methods to construct their houses and stock shelters or alternatively used the local calcrete stone. The SA invasion of SWA took place via another route, and the borehole guards stayed on largely forgotten by the authorities.

After World War 1 these farms were made available to White and Coloured People. Only a few families were prepared to settle on these farms and in 1931 the Minister of Lands, Mr. Piet Grobler proclaimed the area a National Park.

Land was purchased south of the park to re-settle these farmers and the borehole guards were given land along the Kuruman River. Their houses and other structures in the Auob River were abandoned.

A BIT OF GEOLOGY
All along the Auob River one finds ridges of calcrete deposits within the Kalahari Group Rocks. The calcretes in the Kgalakgadi National Park (NP) form prominent cliffs between the aeolian sands of the Gordonia formation and the underlying Eden Formation sandstones. Yellow, cream, reddish, brown and even greenish sandstones and siltstones (thin gravel layers) of the Eden Formation outcrop along the Auob and Nossob Rivers in the Kgalakgadi NP. The outcrop is quite evident at S26 14.591 E020 34.425. At this exposure, the bottom 2-3 m consists of thin (~5 cm) beds of pebbles interspersed with layers of sandstone and siltstone with clasts randomly scattered in the matrix. The conglomeratic or gritty layers found in the sandstones consist of red and grey sandstone and mudstone clasts which may represent the reworked remnants of older Karoo Supergroup sandstone, or even reworked Kalahari Group sandstones.

INSIGHT INTO THE FORMATION OF THESE STONES THAT THE RUINS ARE BUILT OF
At some localities along the Auob River as well as directly to the south of the Kgalakgadi NP, the basal pebbly layers are found in large channels, with some cross bedding recognised at Koopan (S27 17.608 E020 19.267 outside the NP). The basal pebbly layer may vary in thickness, and is commonly overlain by a fine- to medium-grained siltstone. The sandstones in the Kgalakgadi NP often exhibit signs of bioturbation (the alteration and disturbance of a site by living organisms; the turning and mixing of sediments by organisms), with an interlocking network of tubes, generally filled with either a calcium carbonate or siliceous matrix, or preserved as hollow tubes. There appears to be a relationship between the weathering characteristics of these tubes and their fill, with the tubes filled with a siliceous matrix weathering in positive relief and those with a calcareous fill generally being left with a hollow tube as the fill dissolves. The tubes are possibly the result of the burrowing activities of organisms as well as having been formed by plant roots, and can be up to 30 cm in length. Irregularly shaped calcareous nodules and powder calcretes generally develop in some of the softer, less consolidated sandstones and siltstones, with thin section work revealing that in some cases up to 50 % of the rock may consist of these calcareous precipitates.

Within the sandstones and siltstones calcretised layers of 20-30 cm thick may be developed where they weather as positive relief features. Well developed nodular calcretes overlying this horizon can reach up to 3-4 m in thickness. Overlying the nodular calcretes, hardpan (dense layer of soil) up to 1.5 m thick may occur and may either be exposed, or covered only by the aeolian sand of the Gordonia Formation. When exposed at the surface, or when groundwater moves through the rocks, a layer of calcrete may erode and dissolve, leaving solution cavities, which may be either filled by rock fragments and re-cemented, or covered and recapped by a hardpan calcrete. Older rocks and calcretised layers may be eroded, redeposited as layers of rock fragments, and re-calcretised, or the process of calcretisation or silcretisation may disrupt and distort an older calcretised sequence. Within the calcretised sediments, silcrete veins and lenses and silcrete fillings of cavities or voids are also observed.

 Silcrete is an indurated soil duricrust (hard layer on or near the surface of soil) formed when surface sand and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete. The silcrete veins are commonly pale brown to grey in colour, and characterised by a conchoidal fracture (describes the way that brittle materials break when they do not follow any natural planes of separation). The silcrete itself is very hard, and may weather out in positive relief. In thin section the silcrete is seen to be composed of equigranular (composed chiefly of crystals of similar orders of magnitude to one another), well-rounded, quartz grains cemented by a silica matrix.

AN IN-DEPTH LOOK AT CALCRETE
Calcretes are the most common duricrust found in the Kalahari basin, with almost all Kalahari Group sediments having undergone some degree of calcretisation in the past.

Calcrete is a sedimentary rock, a hardened deposit of calcium carbonate (calcite). This calcium carbonate cements together other materials, including gravel, sand, clay, and silt. Rainwater saturated with carbon dioxide acts as an acid and also dissolves calcite and then re-deposits it as a precipitate on the surfaces of the soil particles; as the interstitial soil spaces are filled, an impermeable crust is formed. It is also known as hardpan, or duricrust. The term Caliche (calcrete) is Spanish and is originally from the Latin calx, meaning lime. Calcrete occurs worldwide, generally in arid or semi-arid regions. Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. The word "lime" originates with its earliest use as building mortar and has the sense of "sticking or adhering."

Calcrete is generally light-coloured, but can range from white to light pink to reddish-brown, depending on the impurities present. It is generally found on or near the surface, but it can be found in deeper subsoil deposits, as well. The layers can vary from a few centimetres to metres thick, and multiple layers can exist in a single location.

FORMATION
Calcrete generally forms when minerals are leached from the upper layer of the soil (the A horizon – top soil) and accumulate in the next layer (the B horizon – sub soil), at depths of approximately one to 3 metres under the surface. It generally consists of carbonates in semi-arid regions, while in arid regions, less-soluble minerals will form calcrete layers after all the carbonates have been leached from the soil. The calcium carbonate that is deposited accumulates, first forming grains, then small clumps, then a discernible layer, and finally, a thicker, solid bed. As the calcrete layer forms, the layer gradually becomes deeper, eventually moving into the parent material (bedrock), which lies under the upper soil horizons.

However, calcrete can also form in other ways. It can form when water rises through capillary action (the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity). In an arid region, rainwater will sink into the ground very quickly. Later, as the surface dries out, the water below the surface will rise, carrying dissolved minerals from lower layers upward with it. This water movement forms calcrete that tends to grow thinner and branch out as it nears the surface. Plants can contribute to the formation of calcrete, as well. The plant roots take up water through transpiration (the process of water movement through a plant and its evaporation from aerial parts especially from leaves but also from stems and flowers), leaving behind the dissolved calcium carbonate, which precipitates to form calcrete. It can also form on outcrops of porous rocks or in rock fissures where water is trapped and evaporates. In general, calcrete deposition is a slow process, but if enough moisture is present in an otherwise arid site, it can accumulate fast enough to block a drain pipe.

Calcrete formation and preservation is dependant on sometimes complex relationships between precipitation, evaporation, groundwater flow and the composition of solutes.

One of the world's largest deposits of calcrete is in the Makgadikgadi Pans in Botswana, where surface calcretes occur at the location of a now-desiccated prehistoric lake.

ECONOMIC USES

Building applications
  • Calcrete (the calcium carbonate mineral) is used in construction worldwide. The calcrete meets the chemical composition requirements and has been used as a principal raw material in cement production. Where the calcium carbonate content is over 80%, calcrete can also be fired and used as a source of lime, which can then be used for soil stabilization.
  • In many areas, calcrete is also used for road construction, either as a surfacing material or, more commonly, as a base material. It is one of the most common road materials used in Southern Africa. Calcrete is widely used as a base material when it is locally available and cheap. However it does not hold up to moisture (rain) and is never used if a hard rock base material, such as limestone, is available.

Sugar refining
A nearly pure source of calcium carbonate is necessary to refine sugar. It must contain at least 95% calcium carbonate (CaCO3) and have a low magnesium content. In addition, the material must meet certain physical requirements so it does not break down when burned. Although calcrete does not generally meet all of the requirements for sugar refining, it is used in areas where another source of calcium carbonate, such as limestone, is not present. While calcrete requires beneficiation (entails the transformation of a mineral, or a combination of minerals to a higher value product) to meet the requirements, its use can still be significantly cheaper than shipping in limestone.

CALCRETE AND GARDENING

Problems caused by calcrete
Calcrete beds can cause many problems when trying to grow plants. First, an impermeable calcrete layer prevents water from draining properly, which can keep the roots from getting enough oxygen. Salts can also build up in the soil due to the lack of drainage. Both of these situations are detrimental to plant growth. Second, the impermeable nature of calcrete beds also prevents plant roots from going through the bed, which means the roots have a limited supply of nutrients, water, and space, so they cannot develop normally. Third, calcrete beds can also cause the surrounding soil to be basic. The basic soil, along with calcium carbonate from the calcrete, can prevent plants from getting enough nutrients, especially iron. An iron deficiency will cause the plant’s youngest leaves to become yellow. Soil saturation above the calcrete bed can make the condition worse.

 

References
THE SUB-KALAHARI GEOLOGY AND TECTONIC EVOLUTION OF THE KALAHARI BASIN, SOUTHERN AFRICA by Ian Gerald Haddon
Wikipedia

Additional Hints (Decrypt)

Cyrnfr erzrzore gb r-znvy lbhe nafjref gb trbvcnwreb@tznvy.pbz

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)



 

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