BASIN CORRELATION OF THE MANDEVA FORMATION
6.2 THE SINQENI FORMATION
Due to the sheet-like nature and lateral extent of the Mandeva Formation, sequence stratigraphic analysis can be applied to aid regional correlation of units. The Sinqeni Formation is the lateral equivalent of the Mandeva within the main Pongola Basin. The type area for the Sinqeni Formation is within the Pongola River valley west of Klipwal Gold Mine, where the sequence attains a maximum thickness of ~650 m (Gold, 1993) and gets its name from the nearby Sinqeni hill. The sequence is dominated by two major quartzite units, the lower Dipka and upper Kwaaiman Members, separated by the Izermijn Member, a thin unit of banded iron formation.
The lower Dipka Member crops out extensively on the farm Gunsteling where it caps a prominent hill in the area (Figure 6.2). At its base the Dipka Member crops out as interbedded coarse-grained sandstone and clast-supported, poorly-sorted, polymictic quartz-pebble conglomerate (Figure 6.3).
Clasts are predominantly vein quartz, sandstone and minor chert; long axes of the clasts range from 5 to 50 mm with a mode of 30 mm. Clasts have random orientations with no imbrication. Both normally-graded and massive beds are evident, with internally massive gritty quartz arenite beds intercalated with the conglomerate lenses (Figure 6.3). Normally-graded conglomerates fine-up from
ounts of sericitic matrix (Figure 6.4). The sericite matrix probably formed due to alteration minerals within pore spaces between the larger
cros
medium-pebble conglomerate to gritty quartz arenite. Sutured grain-grain contacts predominate with minor am
of clay quartz grains. Palaeocurrent orientations show
s-bedded sandstone units.
a unimodal southerly palaeoflow within the trough
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6. Basin Correlation of the Mandeva Formation
Figure 6.2: Google Earth image showing a schematic geological map of the Nsuze-Mozaan unconformity on the farm Gunsteling.
i re 6.3: Interbedded quartz-pebble
k edge shown on the right of the figure marks the entrance to a 6 m prospect adit driven into the conglomerate.
F gu Figure 6.4: Cross-polarised photomicrograph of
content within the matrix. No chert evident, large clasts are quartz. Field of view: 8 mm
conglomerate and coarse-grained sandstone of the Dipka Member on the farm Gunsteling. The sharp
Gunsteling conglomerate showing high quartz roc
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6. Basin Correlation of the Mandeva Formation
On the farm Altona (Figure 6.5), the conglomerates laterally discontinuous ~20 to 50 cm thick lenses conglomerates crop out in a ~30 m thick succession on
of the Bethu Anticline (Figure 6.6) and extend fo m along strike. The conglomerates are generally poorly-sorted, matrix-supported and fine-up to, or are sharply overlain by gritty quartz arenite (Figure 6.7). Clast size varies between 10 and 50 mm with a mode of 25 mm.
Clasts are predominantly subrounded black and bande
rare, but locally developed in some lenses. The matrix is predominantly immature, poorl
coarse-grained quartz, feldspar and sericite and weath ure 6.8). Minor planar cross- bedding is evident within the conglomerates with clasts concentrated on the basal parts of the foresets.
The Dipka Member conglomerates on the farm Altona are ove
bedded quartz arenites. Gold (1993) indicated that the lower portions of the unit have undergone a high degree of quartz recrystallization with original grain shapes being locally destroyed. The quartz arenites become coarser-grained towards the top of the
evident on farm Altona (Figure 6.7)
The quartz arenites are overlain with a sharp contact by ~30 m thick unit of ferruginous shale and banded iron formation of the Izermijn Member. The iron formation is dominated by interlayered jaspilitic chert and magnetite bands (Figure 6.9) which host localized sulphides. This iron formation forms the lateral equivalent of the Vlakhoek Member in the White Umfolozi Inlier and can be traced throughout the Pongola basin.
The banded iron formation is overlain with a sharp contact by ~170 m thick, medium to coarse- grained quartz-wacke of the Kwaaiman Member (Gold, 1993). Gold (1993) identified symmetrical ripple marks and planar and trough crossbedding within the Member which display a prominent, unimodal, southerly palaeocurrent orientation. Ferruginous and non-ferruginous mudstones and siltstones of the Ntombe Formation overlie the quartz-wackes of the Kwaaiman Member with a sharp contact (Gold 1993).
of the Dipka Member form 5 prominent but at the base of the Sinqeni Formation. The the southern side of the main road to the west r a distance of ~200
d chert with minor sandstone. Imbrication is y-sorted, ered pyrite (Fig
rlain by ~200 m thick planar cross-
unit with well developed planar cross-bedding
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6. Basin Correlation of the Mandeva Formation
igure 6.5: Geological map of the Nsuze-Mozaan unconformity on the farm Altona, Bethu anticline and Altona/Sinqeni shear orientations after Gold (1993).
F
Figure 6.8: Plane-polarised photomicrograph of Gunsteling conglomerate showing weathered rounded pyrite grains within quartz matrix. Field of view: 2 mm.
Figure 6.6: Planar cross-bedded conglomerate on farm Altona. Pebbles focussed on foresets dipping from right to left. Lense is ~10 m above Nsuze Mozaan unconformity.
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6. Basin Correlation of the Mandeva Formation
Figure 6.7: Planar cross-bedding within coarse- grained quartz arenites ~10 m below the Izermijn banded iron formation on farm Altona. Foresets dip to the south.
Figure 6.9: Banded chert within the Izermijn banded iron formation at Altona mine, Farm Altona. Hammer handle for scale.
Figure 6.10: Distribution of greenstone remnants and Archaean sedimentary basins on the Kaapvaal Craton.
(modified after Brandl et al., 2006).
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6. Basin Correlation of the Mandeva Formation