Geology maps: where is that dolomite?

Apart from the two historic mining maps we have acquired, a substantial amount of our information has come from 1:250,000 maps of the bedrock geology. These provide the geological characteristics, and the location, of the underlying rocks in any area of South Africa. Because the types of caves we are looking for form in dolomite, locating areas where this rock is present is one of the main factors in determining our survey strategy.

The geology maps have been treated in the same way as the historic mining maps, utilising only the information relevant to the project and converting that information into a usable digital format. However, unlike the mining maps, on which the digital information is characterised by a point, the information we require from the geology maps relates to the region (including the size and shape) occupied by a lithological unit of bedrock.

Geology base map. An example section of a 1:250000 geological map showing areas of dolomite in different shades of blue (map published byDepartment of Mineral And Energy Affairs, Republic of South Africa 1981).

 

In order to convert this map information from Raster to Vector formats, we trace around the edges of the bedrock formations we are interested in, save them as a polygon (instead of a point), and then add the lithological information as an attribute of the feature. Not only does this kind of digitisation help in establishing areas to investigate but it also has a number of other advantages.

Geology map with polygons.  In this figure, the blue areas denoting a dolomitic lithological unit have been overlain by a traced polygon using ArcGIS and are now represented by different colours to indicate different attributes the rock unit might have, such as including interbedded chert, quartzite, or shale (map published byDepartment of Mineral And Energy Affairs, Republic of South Africa 1981).

The first advantage is the reduction in size of the file from a large scanned map which may be as much as 390 Megabytes per file to a few Kilobytes of data. This is helpful not only when trying to use the data on a handheld computer but also for storage on the device, since we can fit many more polygons than scanned maps on a 4 Gigabyte memory card. The second advantage of digitising in this way is that we can select the only data that are  relevant to the project and its aims. We can ignore all the other geological formations which do not hold the appropriate characteristics and only digitise those that we need. This again saves space on the memory cards but also saves time, both in the tracing of the geology and in locating regions appropriate to the project in the field.

Geology polygons. This figure shows how the digitisation of the geological formation looks when the area is loaded onto the handheld computers. The pink and orange colours represent the (dolomite) geology and the green background represents the Vector outline of the main provinces of South Africa.

The “Identify” function on the GIS software (ArcPad) we use on the hand-held computers can be used to call up the attributes of any of the Vector layers  once defined. The GPS function on the handheld also enables us to determine our current field position relative to the geological formations (polygons) and to the mines (points). This helps us to avoid spending a lot of time driving around searching in the field!

This is the final step in the digitisation process to prepare the maps for the field. The next blog in this series will deal with some of the hardware and software we are using in more detail.

– A Reid

Malmani Dolomites

To a large degree, our project is a field survey of the geological formation known as the Malmani Dolomite Subgroup, of the Chuniespoort Group of the Transvaal Supergroup. The Transvaal Supergroup is an extensive geological sedimentary rock sequence extending across much of the northern part of South Africa and into Botswana (Ericksson & Altermann, 1998). The Malmani Dolomites are just one rock formation in this sequence, and they are of interest to us because it is in the dolomites that we find the historic lime mines, and thus palaeocave fossil deposits. On the map below, the Transvaal Supergroup, including the Malmani dolomites, is indicated by the light blue-shaded areas in the north & central regions of South Africa. Geological dates for these rocks indicate that they were formed between 2.6-2.5 billion years ago, making them one of the oldest dolomite formations known.

Simplified Geology of South Africa: The Malmani Dolomites in blue in the north and central parts of the country – unfortunately, NOT at the coast!

Dolomite is a type of limestone rock that forms in warm, shallow seas from the slow accumulation of the remains of marine microorganisms and fine-grained sediment. It differs from other limestone rocks in having a higher magnesium content, and these dolomites are also characterised by fossils of algae formations that are known as stromatolites (see picture below). These materials contain high levels of calcium carbonate, and thus such rock formations are often referred to as carbonates.

Photo of stromatolite fossils (the clusters of concentric structures) in the roof of Sudwala Cave, Mpumalanga. These represent ancient algae formations in the sea at the time that the dolomite rock sediments were accumulating, roughly 2.6 billion years ago. 

One important quality of dolomite is that, over long periods of time, this rock is water soluble – ground water is a weak acid that will erode away the calcium carbonate matrix as it percolates through cracks and fissures within dolomite rock layers. This process eventually results in the occurrence of underground solution cavities, and when these break through to the surface due to erosion or collapse they form caves and sinkholes (see schematic below). Further percolation of groundwater through these solution cavities results in the precipitation of calcium carbonate to form various kinds of speleothem – the beautiful cave formations known as stalactites, stalagmites, and other structures. In addition, once the cave or sinkhole is open to the surface, it will begin to fill in with sediment, and if occupied by carnivores the infill will include animal bone. In South Africa, the ancient caves of this kind were open to the surface, and filling in with sediment and bone, between roughly 3-1 million years ago.

Idealised model of cave formation in dolomites (Martini, et al. 2003). The brick-like symbol indicates dolomite, and the differently-stippled symbols represent separate infill deposits over time.

Thus, over a period of millions of years solution cavities will form in the dolomites, break through to the surface as caves and sinkholes, and then fill in again with sediment and bone. The constant percolation of groundwater through the deposits will cement everything together in a calcium-carbonate matrix, and fossilise the bones. Finally, lime miners in the early 20th century blasted and excavated into the dolomite formations to retrieve the extensive cave formations (calcium carbonate, or ‘lime’), and in so doing often exposed the infill deposits containing fossilised bone. This attracted the attention of palaeoanthropologists who were interested in the fossil record of human ancestors and other mammals.

And that is why we are in South Africa surveying the Malmani Dolomite formation, looking for ancient cave deposits!

– KL Kuykendall

Map accuracy: are we there yet?

The project is focused on the relocation of historic lime mines sites across South Africa. Information we have gathered includes high resolution scans of two maps showing the location of every mine in South Africa open at the time of printing. It is from these maps, along with other documentary sources, that we have selected the areas to visit. As you can see below, the location of the mines is clearly marked with a point and a two letter code relating to the mineral being mined with Ls indicating lime (on a side note I have been keeping a beady eye out for those marked as Diamond and Gold, you never know what they might have left behind!).

© Department of Mines, Republic of South Africa 1959

Given that the maps are of a relatively large size it was not feasible to bring them into the field, which even if we did would prove difficult to glean any particularly useful location data from anyway. The solution to this is to make a digital version of the map from the high-resolution scans. Unfortunately the handheld computers we are using do not have a particularly large processing capacity so loading an A0 map scanned at 300dpi and then trying to use it, isn’t efficient or effective. So, what we have to do is to store the data as a much smaller file so it can be used on the handheld computers. This transformation is quite common when using maps like these in an archaeological context, digitising vector data (lines and dots) from raster data (pictures and photos etc.). Vector data files are much smaller than raster data files and they can hold text information about them which is very helpful in the field and saves a lot of time flicking through notebooks.

By using a GIS (Geographical Information System) package it is possible to create a file displaying the locations of the mines as dots, which we can then load onto our handheld computers and use its GPS function to guide us to them. Oh so simple I hear you say, alas no such luck I’m afraid.

A selection of digitised mine locations as displayed in vector format overlaid on a 1:50000 topographical map in raster format.

The main issue with the maps is their accuracy, and furthermore the accuracy of the points we take from the maps. Firstly it is difficult to say how accurately the cartographer plotted the locations. Secondly the scale of the map is so large that the effective accuracy of any point will only be within 1.5 kilometres, in any direction, of the actual location of the mine.  So, even when we are standing right on top of the point stored on the handheld, the mine could be anywhere in a 9 square kilometre area. That’s where we depart from our GPS system to locate the mines and use our eyes, and feet, to find them (always being careful to avoid falling into sinkholes and the occasional thorny bush).

This was only the first step in our preparation for fieldwork, the next will be covered in a subsequent blog post and will deal with how we use and prepare geological maps for the project.

-AR Reid

Taung Heritage Site: 17 June 2012

The early hominid fossil site of Taung is one of the ‘Big Five’ Plio-Pleistocene sites in South Africa, and is historically the first australopithecine site to be recognized. The discovery and recognition of the Taung skull fossil is a fascinating story, and is detailed in Raymond Dart’s Adventures with the Missing Link (Dart and Craig, 1959). In December 1924, a crate of breccias blocks (rocks containing fossils) from the lime mine at Taung was delivered to Raymond Dart at the University of Witswatersrand in Johannesburg. In it, he discovered a small skull of a juvenile hominin, which became nicknamed “the Taung child”. The fossil preserved much of the facial skeleton, mandible, and endocast (an impression of the brain on the inner surface of the skull). Dart quickly published a report on his find in February 1925 and proposed that it represented a new hominin species, Australopithecus africanus – or ”the Southern ape from Africa” (see photo of the Taung skull). More recent research has determined that the fossil is more than 2 million years old.

In his report, Dart stated “the specimen is of importance because it exhibits an extinct race of apes intermediate between living anthropoids [apes] and man [emphasis original]” (1925, p. 125). At the time, many scientists would not accept that humans evolved from an ape-like ancestor and the skull’s significance was largely ignored for several decades. But it was this challenge to Dart’s interpretation of the Taung fossil that led other researchers such as Robert Broom to explore other South African lime mines for further fossil traces of human ancestry.

One of the original published (1925) photos of the Taung Skull in lateral view, showing the facial skeleton to the right, and the brain endocast to the left.

We visited the Taung limeworks near the town of Buxton in the North West province. The site, which was designated a National Heritage Site in 2002 (see plaque photo), is quite large and was an active mine during the 1920s. It was later systematically excavated by paleoanthropologists from the University of California in the 1940s, and from the University of Witswatersrand between 1988 and 1992. Both the mining and excavations resulted in extensive dumps that surround the area of the site from which the skull is thought to derive. However, the exact location at which the skull was found can only be approximately reconstructed from mine records and historical documents – after all, it was only recognized after it arrived in Johannesburg in a wooden crate!

The Taung site is both a National, and World Heritage Site. This plaque provides the essential information about the site, and the Taung Skull fossil.

The Dart and Hrdlička pinnacles, and the monument marking the possible location of the Taung Skull.

How the Taung juvenile died, and became deposited in the fossil record is not certain. One hypothesis is based on taphonomic evidence from the fossil and associated specimens in the deposit, including possible raptor claw marks on the skull. This model suggests that a large bird of prey snatched the child from the air, and the skull was subsequently left below the raptor roost (e.g., Berger and McGraw, 2007). Cases demonstrating this hunting technique by large raptors are documented for other primate species.

Another hypothesis is palaeoenvironmental in nature and is based on deposits of tufa present at the site. Tufa is a calcium-carbonate rock produced when organic matter settles in still or slow-moving water with algal blooms, and builds up into massive deposits over time. Animals may be incorporated into such deposits if they die in or near the water source – thus, it may be that the Taung child died of natural or unknown causes, fell into the stagnant water, settled, and fossilized (McKee, 1993). If the exact location of the deposit from which the Taung fossil was derived were known, these different hypotheses could be tested more rigorously, and a firmer conclusion might be possible.

The Taung site is where the modern field of palaeoanthropology began, because it produced the first early hominid fossils from Africa, a discovery which eventually transformed the field, and our view of our own evolutionary history. For us, it is also significant that the site was located at an old lime mine – and this is one of the reasons that we are currently in the field, attempting to locate other such lime mines in new areas. Who knows what future discoveries these sites might reveal?

REFERENCES

Berger, LR and McGraw, WS. 2007. Further evidence for eagle predation of, and feeding damage on, the Taung child. South African Journal of Science 103:496-498.

Dart, RA. 1925. Australopithecus africanus: the man-ape of South Africa. Nature 115:195-199.

Dart, RA and Craig, D. 1959. Adventures with the Missing Link. New York: Harper & Brothers.

McKee, JK. 1993. Formation and geomorphology of caves in calcareous tufas and implications for the study of the Taung fossil deposits. Transactions of the Royal Society of South Africa 48(2):307-322.

-KL Lewton and KL Kuykendall

Boesmansgat, Mt. Carmel Farm, south of Kuruman: 12 June 2012

As we saw in the Gondolin region, a sinkhole is formed when the roof collapses on an underground solution cavity (or cave) in the dolomites. Sometimes these are very large, and may extend deep enough to fill with water, as at Boesmansgat south of Kuruman. This sinkhole does not hold any known fossil deposits, but it is an  excellent (and impressive!) example of the geographic and geologic features of the palaeocaves for which we are searching. It is thought that the water filling Boesmansgat is connected underground to another large water-filled sinkhole about 60km to the north – the Kuruman Eye – which supplies all the water for the local population.

View from the surface of Boesmansgat (roughly translates to “Bushman’s cave”)

Stagnant water fills the underground chambers of Boesmansgat

At least one diver has died trying to reach the bottom of the Boesmansgat sinkhole, which is 274 meters below the surface (some 337 meters when corrected for altitude) leading to the claim that this is the deepest and largest natural sinkhole in the world.

Our interest in Boesmansgat was to learn more about the occurrence of sinkholes on the farms in the area. With local farmers as our guides, we were able to visit several smaller sinkoles to inspect them for fossil deposits and ancient cave formations. We did find some small speleothem (calcite cave formations) occurrences, and one sinkhole led to a large underground chamber.

Surface of the sinkhole

Kristi photographs the sinkhole

Without proper equipment and safety gear, we could not fully explore the underground caverns on this trip, but in the main chamber at a depth of 10-15m we did find some rock breccia, speleothem formations, and recent micro-mammal bone — as well as one deceased tortoise that fell in to this ”animal deathtrap” and was lodged between the rocks! We could not see how deep the chambers extend underground, but there are at least two deeper tunnels that we will explore when we return on a later trip.

We have now learned that several more farms in this region also have sinkholes, so we will pursue those contacts and plan to return for a more extensive survey – in addition to lime mines, we are now also looking for sinkholes!

-KL Kuykendall

Wonderwerk Cave: 10 June 2012

Wonderwerk Cave is located in the Northern Cape province of South Africa, near Kuruman in the Kalahari Desert. The cave is a National Heritage Site and is open to the public for tours. Wonderwerk Cave has been studied extensively by archaeologists since the 1940s and has yielded quite a bit of material including Early, Middle, and Late Stone Age tools, decorative objects like beads, cave art, and possible evidence of the earliest controlled use of fire. The history of hominin use of the cave spans ~1 million years, with the cave art being relatively recent (~1000-10,000 years) and evidence of use of fire ~1 Ma.

The cave art depicts elephants, giraffes, ostriches, secretary birds, and various bovids. There is even a ‘chongololo’ (millipede)!

The cave has a long, low ceiling and extends back into the hill about 140 meters.

Excavations are still ongoing, and the majority of the cave is marked by an excavation grid:

View of the Kalahari from the hill above the cave:

-KL Lewton

Kuruman: 10 June 2012

Though located in the middle of the Kalahari on the Ghaap Plateau, Kuruman is a thriving mining community in the Northern Cape Province, and was an important missionary post in the early 1800’s. The Scottish missionary Robert Moffat lived here from 1820-1870, and printed the first bible in Africa. Today the mines in the region produce manganese, iron ore, tiger’s eye, and asbestos, and signs of the importance of the mining industry are everywhere.

Wow! we want one of THOSE for next year’s field season!

We drove an approximately 275-km route via Kuruman – Danielskuil – Postmasburg – Kathu and back to Kuruman intending to locate some of the historic lime mines in our database. Frustratingly, we encountered fenced-off fields, locked farm gates and roads-to-nowhere.

The road to nowhere…

Geologically, the hills in this area are predominated by banded iron formations (or BIFs) which is the source of the iron ore in the region – and we saw many iron mines on our drive – but almost no dolomite, and no lime mines at all! The BIF overlays the carbonate platform (our dolomites!), and we did encounter massive slabs of bedrock dolomite along some of the dirt roads in lower-lying areas.

BIFs in the distance; dolomite in the valley floor

We also visited Wonderwerk Cave, which is an archaeological site occupying a massive solution cave in the dolomites – so all hope is not lost. Our next step is to visit the municipal lands office in Kuruman for contact information about the farms on which the historic lime mines are located.

-KL Kuykendall