South African caves

In South Africa, the famous hominid sites of Sterkfontein, Swartkrans, Krom-draai, and Makapansgat (O Figure 11.3) are formed in a very specific geological setting, compared with the eastern African hominid localities. The fossil hominid--containing sediments accumulated in caves developed in Precambrian dolomite limestones, in Plio-Pleistocene times, through extensive karstification. The cave limestones belong to the Malmani Dolomite, which is part of the Transvaal Supergroup (Eriksson et al. 1976). The age of the Malmani dolostones, deposited in the intracratonic Transvaal Basin, is considered to be between 2.5 and 2.6 billion years (Button 1973). The thickness of the deposits reaches 1,450 m, e.g., in the Sterkfontein area (Eriksson and Truswell 1974). A large sedimentary hiatus from the Precambrian to the Late Tertiary deposits is present. Abundant faulting and folding of the dolomite limestone makes determining the exact stratigraphic position of the basic cave material difficult. The occurrence of stromatoliths in the Makapan Valley led to the idea that at least some of the limestone sediments had been formed in the intertidal zone (Eriksson et al. 1976) near the shore line of the ancient sea.

The initial cave development probably followed the fracture pattern in the dolomite rock, and extensive and deep karstification took place due to ground-water level changes. A typical feature of the Transvaal karst is the occurrence of a three-dimensional hyperphreatic maze of fissure passages (Martini et al. 2003), although large caverns did develop. As a result of erosion, cave ceilings collapsed and opened larger chambers. Carbonate solutions and mineralization built up thick travertine layers on the floors. The rich travertine deposits attracted miners at the end of the nineteenth century because of their pure calcium carbonate content. On the other hand, the carbonate is responsible for the consolidation of clastic sediments that filled up caverns through openings from the land surface. Sand, chert, dolomite, quartzite materials, and also bones were washed down into the caves. Winds probably brought fine grains, following the law of gravity. Possibly several transporting agents led to the high bone accumulation at some localities in the cave systems. Some of the caverns were completely filled up with exogenetic material, and their sediments concreted to cave breccias with a finegrained matrix and carbonatic cement. Differences in color and grain size may indicate different modes and phases of sedimentation. To access the bones, which are baked in the breccias, demands sophisticated physical and chemical preparation techniques.

Gravity and water transport, together with typical processes like collapses, solution, and remineralization, are responsible for the complex geological setting

O Figure 11.3

The caves at Makapansgat belong to a large Precambrian dolomite formation (above) showing profound karstification and sedimentation during Plio-Pleistocene times. Limestone cavities (below) are filled with travertine and characteristic carbonatic bone breccias. Mining for carbonates hollowed the filled cavities and brought thousands of fossils to light. The preservation of the fossils differs from sites in the EARV due to very different processes of site formation

of many of the caves. The extensive lime works in the South African caves brought thousands of bone fragments to light, although blasting operations destroyed many natural features of the caves. Consequently, reconstruction of the diagenesis of the South African cave sites was a real challenge and many aspects still need further investigation. The analysis of the taphonomic history of bone accumulations at the South African cave sites needs detailed on-site observations and also lab work. Bob Brain and his team investigated many years at Swartkrans, before he came up with his last model of cave development, where he proposed nine diagenetic steps (Brain 1993). The formation started with a probably Miocene cavern below the level of standing water. After the opening of the cave, surface sediment began to accumulate inside the cave. The interpretation of the fossilif-erous 'pink breccia' of the Outer Cave, which was shown to be an infilled remnant, the Hanging Remnant Unit of Member 1, proved to be especially time consuming.

Swartkrans Members 1-3 yielded inter alia the remains of Australopithecus robustus, which is likely to have an age of 1.8-1 Ma. Members 4-5 are Middle Stone Age and ca. 11,000-year-old, respectively.

Extensive investigations at Swartkrans and other South African cave sites lead to conclusive models about the development of the deposits and their fossil content in time and space. The experience and strategies used in the past provide the knowledge and tools to further explore the karstified limestones in South Africa and at other localities.

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