Manjeri Formation Type Section
Submitted by Andrew du Toit on Thu, 06/11/2014 - 09:30 | Last updated: Thu, 06/11/2014 - 10:34
Grid Ref (WGS84 Lat/Long in decimal degrees)
Situated 9.4km SSE of Zvishavane. As a poaching prevention measure, the gate to this exposure is normally locked and arrangements should be made with the ranch manager, whose headquarters are east of the Buchwa Road, some 5km from Zvishavane. The exposure may be reached by a track leading NW from the Buchwa Road, some 1.2km south of the Shavi River; 1.8 km along this track is a road leading to the SW which is followed for 0.8km. A path leading due west passes over a fence and on to the outcrop.
Manjeri Formation, Ngezi Group
This exposure is described in detail by Martin (1978) and Bickle et al. (1975) and only a summary is given here.
The sedimentary succession here is 130m thick with the lower part dominated by argillite and quartzite and ironstone and the other 80m by siltstone and argillite. All lithologies are metamorphosed to lower greenschist grade and non-metamorphic nomenclature is used where convenient.
The lowest unit is a chaotic brecciated argillite which fills a small channel in the granite floor and in part overlies what was assumed to be a mafic xenolith in the tonalite, but is more likely to be an unbedded part of the channel fill. A conglomerate is the lowermost discernible sedimentary rock on the palaeofloor. The pebbles are mostly of vein quartz with lesser tonalite (flattened, pale olive green) clasts and dark (carbonaceous?) argillite. The matrix is sandy and pale brown and grades over 2m into tonalite to the east. The foliation in the tonalite, defined by elongate quartz grains, lies at a shallow discordant angle to the sedimentary banding.
Overlying the basal units are 6m of sandy argillite with conglomerate bands (one containing a chromite pebble) and an argillaceous dolomite. The argillites show two cleavages, the dominant one parallel to bedding. Truncations of bedding give rise to flaser bedding which are generally enhanced by weathering. Within this unit is a small neptunian dyke. Sericite and quartz are the major components with lesser chlorite and accessory tourmaline and hematite. Dolomite with sericite and quartz constitute the carbonate bands.
Above the predominantly lower argillaceous unit are oxide facies iron-formation beds and quartzite with minor conglomerate bands. The quartzites typically consist of angular to sub-rounded grains of quartz as single crystals and granular mosaics to 1.5mm across in a matrix of fine granular quartz with sericite and some siderite and chlorite. Small scale cross bedding is common but best exposed on weathered surfaces. The iron-formation consists of alternating chert and magnetite layers, the latter being partly altered to haematite near surface. The conglomerates differ from those lower in the succession, in that they contain BIF cherts in addition to quartz. Siderite and sericite are common in the silty matrix to the clasts.
Above the well-exposed lower argillites, arenites and BIF is a relatively thick succession of thin bedded siltstone and argillite, with low angle current truncations of bedding and graded units. Capping the Manjeri Formation at this locality is a narrow unit of sulphide facies iron-formation now a gossan. Folded vein quartz within this unit attest to deformation along this contact with the overlying Reliance Formation. However, core drilling across the whole succession shows an interfingering of sediments with the Reliance volcanics, and some sandy units and sulphidic bands sandwiched between mafic flows. Above the sedimentary sequence, komatiitic basalts showing spinifex textures (clinopyroxene) and flow top breccias are exposed. In addition, spherical structures which are thought to result from accumulation of early formed pyroxene around a ball of more viscous magma, can also be observed.
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Chauvel, C., Dupré, B., Todt, W., Arndt, N.T. and Hofmann, A.W. (1983) Pd and Nd isotopic correlation in Archaean and Proterozoic greenstone belts. Eos (American Geophysical Union Transactions), 64, 330. Hall, R. (1983) B.Sc. Dissertation Project, University of Zimbabwe. Martin, A. (1978) The geology of the Belingwe-Shabani schist belt, Geological Survey of Rhodesia, Bulletin, 83. Nisbet, E.G., Bickle, M.J. and Martin, A. (1977) The mafic and ultramafic lavas of the Belingwe greenstone belt, Rhodesia, Journal of Petrology, 18, 521-66. Orpen, J.L., Bickle, M.J., Nisbet, E.G. and Martin, A. (1985) Belingwe Peak (1:100,000), Geological Survey of Zimbabwe. Scholey, S.P. (1989) M.Phil., Ph.D. Transfer Report, University of Southampton, U.K.