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tel-00534181, version 1 - 9 Nov 2010 Chapter I micular texture of kaolin crystals with which it is commonly associated. Its identification is based purely on chemical analysis (Table 9) and it was not possible to distinguish it from pore-filling clay minerals. It could be a mixture of amorphous silica, silica rich clay like pyrophyllite, potassium clay and iron oxide. 4.4.2.2. Zone II. This zone is characterised by the presence of abundant hematite cements, the presence of distinct ferruginous crust horizons with very high concentration of hematite and the marked decrease in or the total absence of carbonate and clay mineral cements. This zone has a thickness of approximately 125 m, staring at the 85 m mark and terminating at the 210 m mark on the measured type-section (Fig. 10a). Hematite cements. These are widely distributed within the vertical section studied and range in percentages between trace (less than 3 %) to a maximum of 40 %. On average, all the samples contain between 5 and 10 % hematite cement (visual estimation). It occurs as dark red to brown-black and is commonly a porelining cement, starting out at the fringes of pore space and increasing inwards towards the pore centre. The depth of the colour depends on the thickness of the cement crust, as it was noted to be darker at the pore edges than at the centre of the pore. Where the hematite is very abundant, the rock acquires a reddish colour and locally, these are associated with ferruginous crusts which are common especially near the middle of the formation. In sample LPR 94/04 (Figs. 10 and 22g), the earlier hematite cement is a pore-lining cement with crystals growing towards the center of an open pore space. This cement is reddish-brown in reflected light. That the early cement predates feldspar dissolution is evidenced by the existence of sharp contact between grains of partially dissolved feldspar containing no cement (see arrows Fig. 22g) and hematite-cemented pore space. A later generation of manganiferous cement (MnOx in Fig. 22g) in the sample LPR 94/04 was precipitated in the secondary intragranular pore space of partially dissolved feldspars. This second-generation cement, whose manganiferous nature was identified under SEM analysis remains black under reflected light. 4.4.2.3. Zone III. This zone, which forms the top (285 m) of the LSF and was described in the typesection exposure and completed at the Lokitaung Gorge (Fig. 10b), is characterised by the presence of abundant kaolin cementation and a notable decrease in the amount of hematite and calcite cementation. Kaolin cement. Kaolin is the most widely distributed cement in the LSF and it occurs in virtually all samples throughout the section, varying only in abundance from trace amounts (generally less than 5 %) in the basal parts of the section to a maximum of 30 % (in samples LPR 126/04 and LPR 140/04) towards the top of the formation. SEM-EDS analysis gives the typical structural formula for kaolin Al2Si2O5(OH)4 (sample Figure 23. Petrographic micrographs of the studied sandstones. (a), (b) Kaolin crystals with a booklet shape and various crystallographic orientations. Intercrystalline microporosity remains between the crystals. The general shape of the whole kaolin grains and the presence of a faint brown film of an insoluble residue suggest the kaolinization of a detrital clay clast which suffered mechanical compaction. The kaolin precipitation succeeded an early hematite (Hem) cement. (Sample LOK 12/04, LP). (b) is an enlargement of white box in photograph (a) where the black arrows show the insoluble residue of the kaolinized clay clast. (c) A typical mixture of platy and vermicular crystals in kaolin cement (sample LPR 83/04, SEM photograph). (d) Bent and partially kaolinized muscovite (Ms) grain, kaolinization results in frayed grain edges as it is converted into kaolin (Kln), (LPA; sample LPR 107/04). (e) Three different types of cement are seen in this photograph where the sequence of cementation from the pore edges towards the pore center is a thick pore-lining hematite (Hem) cement followed by a layer of cryptocrystalline quartz and finally in the pore center, kaolin cement (LP, sample LOK 11/04). (f) is an enlargement of the white box in photograph (e). (g) Bioclastic grains are very rare in the LSF; the only occurrence noted are algal grains found in the topmost bed (sample LOK 40/04) of the LSF just below the contact with the Turkana Volcanics (h) Floating grains of detrital quartz in a predominantly carbonate matrix; possibly detrital in origin (LP, sample LPR 136/04). In CL, some dark bioclastic grains are still visible cemented or replaced by a late luminescent calcite cement. (LP, sample LOK 11/04). (LP = Polarised light; LPA = Cross polarized light). Ms = muscovite; Kln = kaolin; Hem = hematite; Qtz = quartz; Cal = calcite (mineral abbreviations from Kretz, 1983). 64 �
tel-00534181, version 1 - 9 Nov 2010 Chapter I (a) (b) 100 μm (c) 20 μm (d) 100 μm (e) 200 μm (g) 100 μm (h) 200 μm 65 (f) Qtz Kln Hem Qtz Kln Qtz Ms Hem Cal Qtz 50 μm Qtz
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