Cenozoic deposits of South Africa
The Cenozoic Era comprises the last 65 million years of Earth's history, which is divided into the Tertiary and Quaternary Periods. The deposits of the Cenozoic Era are reflected in many surface features covering South Africa including; 1) palaeosols; 2) clastic sedimentary deposits such as cave sediments, gravel deposits, the Pebble Marker; periglacial deposits, redistributed sand deposits and drainage depressions; 3) pedogenic deposits such as calcrete, silcrete, dorbanks, ferricrete, manganocrete, phoscrete, gypcrete and intergrade pedocretes. Each feature linked to the Cenozoic Era reflects certain characteristics of specific palaeoenvironmental conditions or palaeoclimatic change. The extent and the characteristics of the respective Cenozoic features differ considerably. The Cenozoic deposits cover vast surface areas over South Africa therefore modern society frequently interacts with these materials. This said the objectives were to assimilate information regarding the Cenozoic sediments and pedogenic material with respect to its geotechnical, economical, agricultural and tourism potential. The aims were to compile a distribution map of the South African terrestrial Cenozoic deposits as well as a basic chronostratigraphic timeline. Physical analyses included angle of repose, atterberg limits, particle size distribution, water retention and loss on ignition, amongst others. Geochemical analyses included, but were not limited to, pH, electrical conductivity, cation exchange capacity and X-ray fluorescence. Mineralogical analyses included scanning electron microscopy and X-ray diffraction. These methods were used to comply with the aims and objectives of the study. The selected palaeosol localities at Florisbad and Cornelia were mainly used to gain more information on the horizon characteristics. The determined geo-chemical results were used to compare with previous literature, regarding the palaeo-environmental conditions which were linked to these deposits. The geochemical analyses supported the palaeoenvironments discussed in literature. The fauna evolutionary stages linked to these sites e.g. the Cornelian Land Mammal Age and Florisian Land Mammal Age were used in the chronostratigraphic timeline. The clastic sediment results illuminated the variation that occurs in different caves with regards to geochemistry and microbial activity. The main geochemical components were phosphate, nitrate and ammonium and the microbial activity were ascribed to the presence of bats. The bat guano can contribute to the economic potential of the Cenozoic deposits in the form of fertilizers. Information obtained from literature regarding known caves, such as Sterkfontein and Makepansgat, were used in the chronostratigraphic timeline. The gravel deposits from Windsorton were used as an example of gravel terraces associates with palaeodrainage systems. These gravel deposits were linked to the Riverton and Rietsputs alluvial gravel deposits obtained from literature. The gravel deposits indicated fluvial episodes linked to the Pleistocene Epoch, and can indirectly refer to wetter palaeoenvironmental conditions that existed. The Pebble Marker was investigated at two selected sites and indicated that the sediments in this „gravel layer‟ were not uniform with respect to the gradation and composition. The origin of the Pebble Marker was stated to be associated with ancient river systems or formed by termites as hypothesised by Brink (1985). An alternative hypothesis is used in this research as being formed during periglacial environmental conditions. The age of approximately 19 000 years was proposed for the Pebble Marker from dated fossilised giraffe bones present in this layer. A periglacial deposit was investigated close to Groot Marico in the North-West province and was linked to a Period between 300 000 years - 1.7 million ago, as Acheulean stone tools were found in the deposit. This indicated that colder periglacial palaeoenvironmental conditions existed during the Late Pliocene and Pleistocene Epochs. The terrestrial sand deposits were divided into the Kalahari sand deposit and the redistributed coastal sands. The Kalahari Group stratigraphy observed from three borehole logs were compared with the stratigraphy by Thomas (1981) and correlated well. The geochemical analysis of the Kalahari and coastal deposits mainly indicated that SiO2 was the dominant mineral. The Scanning Electron Microscope interpretation of selected samples indicated that wind was the mode of transportation. The geotechnical analysis indicated that the sand deposits may have the potential to collapse when used as base foundation. The agricultural potential was low due to a low water retention potential and cation exchange capacity. The drainage depressions indicated a variation in mineral compositions and in some occurrences were relatively saline. This may be due to the drainage depressions being contaminated or the salt being concentrated after evaporation takes place. The geotechnical evaluation indicated that the drainage depressions sediments have a high shrink and swell potential and are not suitable to build on. The drainage depressions were not suited for agricultural purposes due to the high water retention potential resulting in an insufficient amount of water available for plants. The pedogenic deposits were linked to certain climatic conditions, and were compared to the Climatic N-value map of Weinert (1980). Calcrete, silcrete and ferricrete correlated well and indicated that calcrete and silcrete formed under semi-arid to arid conditions and ferricrete under humid conditions. Literature obtained stated gypcrete formed under very arid conditions. Intergrade pedocretes are mixtures of different dominant geochemical components such as silica and iron-oxides, and were interpreted as being formed during rapid environmental change or microenvironmental change. The distribution of the calcrete, silcrete and dorbanks, and ferricrete were also compared to the distribution of calcic, silicic, and oxidic soils in South Africa, respectively (Fey, 2010). Calcrete correlated well to the distribution of calcic soils, silcrete correlated poorly to silicic soils, but dorbanks correlated well with silicic soils and ferricrete correlated well with the distribution of oxidic soils. The distribution and geochemical analyses of phoscrete and gypcrete deposits correlated well with literature. The intergrade pedocretes correlated well with the Cenozoic deposit distribution map. The geochemical compositions were determined for selected samples of all the pedogenic material and overall correlated well with the minimum requirements stated by literature. The geotechnical implications of the pedogenic deposits were mainly dependent on the stage of development and therefore are very inconsistent due to variability in the deposit. It was found that the Cenozoic deposits have high economic potential such as the alluvial diamond bearing gravel deposits; calcrete, silcrete and ferricrete used for road construction material; and phoscrete as fertilizer, amongst others. These deposits also contribute to the tourism industry by allowing the public to access selected caves sites, such as the Sterkfontein and Cango cave and the Langebaan Fossil Park, to name a few. The compilation map of the Cenozoic deposits obtained from this research was compared to a Geological Map form the CGS (2004). The sample localities overlap the Cenozoic deposits from the Geoscience map but also extended the distribution indicated in the CGS map. This implies that the terrestrial Cenozoic deposits cover wider areas of South Africa in comparison to the deposits indicated in the CGS map. The basic chronostratigraphic timeline indicated that various climatic changes existed in the last 65 million years and is reflected in the different Cenozoic deposits. The conclusion was made that the standard Cenozoic deposit map of South Africa was an underestimation of the extent of the Cenozoic deposits, and that further research is needed to compile a detailed map and chronostratigraphic timeline.