Z-W2tJO? J/~~ l RHODESIA GEOLOGICAL SURVEY SHORT REPORT No. 40 The Karoo· Rocks in the ~ Mazunga Area, Beitbridge District by A. 0. THOMPSON, M.Se. • ISSUED BY AUTHORITY SALISBURY-- -1975 3 Pri./t' ", ,be Go""" ...., Pri.,", S.lisbury The Karoo Rocks in the Mazunga Area, Beitbridge District INTRODUCTION This account of the Karoo rocks in the Mazunga area is an explana­ tion to accompany three geological maps, the Tull, Mazunga and Gongwe maps, which are published separately. They were compiled during a geological reconnaisance of the outcrop of the Karoo System during 1971 and 1972 using air photographs and 1 : 50000 topographic maps. The mapped area lles between latitudes 21 ° 30' and 22° 12' south and longtitudes 29° 00' and 30° 36' east and covers an area of 6825 km' of which 2125 km' are in the Tull, 2800 km' in the Mazunga, and 1 900 km' in the Gongwe map areas. The area is mainly in the Beitbridge District, but includes parts of the Gwanda and Nuanetsi Districts. It is subdivided into several large ranches and 'African Tribal Trust Lands with no large settlements. Mazunga and Tull are the largest. The main road from Beitbridge to Bulawayo crosses the Mazunga map area, and the main road to Fort Victoria and Salisbury crosses the eastern part of the Gongwe map area. Away from these main roads there are earth roads and tracks, mainly on the ranches, as well as several airstrips. The large rivers affect communications, being wide stretches of sand in the winter and often floods in the summer. The Pioneer road from Tuli to Fort Victoria, and the old coach road from Beitbridge to Fort Victoria, now largely overgrown, both cross the area and, since the survey was made, a rail link from Rutenga to Beitbridge has been completed. The area is one of low rainfall with an average of only 355 to 400 mm per annum falling between October and May. The rainfall is erratic with frequent drought years. The average temperature ranges from around lOoe in June to 29°e in October and November. Very cold nights are experienced in June and July and hot afternoons with temperatures of 30 0 e or more in the summer. In the Tribal Trust Lands, populated mainly by Venda with Kalanga and Ndebele groups, subsistence economy is practised, though on the Tuli and Shashi rivers, where there are irrigation schemes, maize and winter wheat are grown. The European ranches are devoted to cattle with small irrigated areas along the Limpopo and Umzingwane rivers producing citrus and vegetables. 3 2 PREVIOUS GEOLOGICAL WORK PREVIOUS GEOLOGICAL WORK The area, which is the northern continuation of King's (1951, 1952) The basaltic lavas, which covered most of the area, extended east­ Transvaal Lowveld, is a flat one sloping southwards to the Limpopo wards from Botswana past Fort Tuli to the Bubye River. Sedimentary River with a few hills and east-trending ridges. The highest point is rocks outcropped only in the coalfield area near the Umzingwane River Gongwe Hill at 2 450 feet (747 m) elevation in the north-east, and the where they lay unconformably on the metamorphic rocks. All these lowest point is on the Limpopo River in the south at 1 650 feet (493 m). rocks were intruded and metamorphosed by dykes. The coalfield The area is crossed by four major rivers which meander through covered an area of several square miles and contained a 3!-foot seam alluvium-bordered valleys to join the Limpopo. The Shashi, Tuli and of semi-bituminous coal. The only fossil found in the whole area Umzingwane flow S.S.E. and the Bubye E.S.E. Their tributaries flow (Molyneux, 1901) came from the coal seams near Morgan's Hill, and east or west, and a few of the larger ones southwards. Two probable was described by E. A. Arber (Molyneux, 1903, p. 689) as a pith cast examples of river piracy were seen, the Mutshilashokwe from the of a calamite resembling Calamites approximaJa, Brongiart. Umzingwane and the stream 4,5 km S.S.E. of the Shashi Dam. MelIor (1905) wrote that although glacial beds of the Dwyka Series The interfluves between the Shashi and Umzingwane rivers and had been found in the northern Transvaal, none had been found in between the Umzingwane and Mtetengwe rivers slope at about 18 feet Rhodesia. However, Mennell (1905) identified them among specimens per mile (3,5 m per km), and the interfluve between the Mtetengwe and collected by Molyneux. Later (1906, 1907, 1908 and 1910) he wrote Bubye rivers at about 9,6 feet per mile (2,9 m per km) southwards, that the Tuli Coalfield, though not connected with the Karoo rocks showing the flatness of the mapped area. north-west of Bulawayo, resembled them. The coalfield contained basal Dwyka conglomerates, shales and sandstones, a coal-bearing series, PREVIOUS GEOLOGICAL WORK basalt lavas. and later intrusions which were often glomeroporphyritic, A three-foot seam of coal was discovered outcropping in the east never ophitic. and sometimes contained olivine. bank of the Umzingwane River near Morgan's Hill in about 1895 by Zealley (1910) reported that there was a 3!-foot seam of semi­ De Gruyter, and the area became known as the Tuli Coalfield. De bituminous coal, and Molyneux (1909 and 1911) included the coalfield Gruyter's and other coal claims were acquired by Consolidated Gold strata in a correlation of the Karoo rocks of Rhodesia, South Africa Fields of South Africa Ltd. in 1896 and transferred in 1897 to a and Gondwanaland. Maufe. in generalized accounts (1913, 1919. 1924 Bulawayo company, Tuli Consolidated Coal Fields Ltd. (Mehliss, 1951). and 1929), stated that little fieldwork had been done in the Tuli Coal­ The exploratory work was reported upon by F. M. Watson in 1896, field since 1903, but it had been found that discontinuous outcrops of R. Sneddon (1896), by W. H. Chandler in 1897 and R. T. Southwood Karoo rocks extended as far east from it as the Sabi River. Dwyka (1898). The company's holdings were visited in April, 1899, by A. J. C. beds appeared to be limited to the Tuli Coalfield (1919). He gave Molyneux who reported that the coal occurred in three separate, dis­ (1913) the following sequence of strata in the Tuli Coalfield. which was connected basins (a) the Western or Umsingwane basin, (b) the Central estimated to have reserves of about 45 million tons of coal: or Mungesi basin and (c) the Eastern or Letitengwe basin (Mehliss, 1951). By 1903 several boreholes and shafts had been sunk, but as the Tuli Basalts Samkoto Beds=Forest Sandstone railway had by then reached Wankie Colliery interest in the Tuli Coal­ Coal Series field waned. Basal Beds A. J. C. Molyneux (1903) gave the following succession in ascending Anonymous writers (1920 and 1921) gave accounts of the Coalfield order for the strata in the coalfield: and its resources. Renewed interest in the potential of the coalfield Tuli Lavas resulted in Mehliss (1951) examining and reporting to the Gold Fields Umsingwani coal-bearing beds of South Africa Ltd. on their Special Grant No. 15 area, and the Unconformity Samkoto veined sands tones company exploring the coalfield under Exclusive Prospecting Order Uncontormity No. 19 (PelIetier. 1955, and Morrison. 1974). Their work and that of Metamorphic rocks Messina (Rhodesia) Development Co. Ltd. on E.P.O. No. 393 (Johnson, Gneiss and granite KAROO SYSTEM s 4 PRECAMBRIAN ROCKS 1973) provided a great deal of new information on the geology and the Jopempi and Dijana Hills. In the latter. about 7 km south of Tongwe coal resources of the area. Swift (1961) had stated that there were two Dam, there are occurrences of asbestos and magnesite. workable seams in the coalfield with extractable coal reserves of about Quartzite forms a ridge in the Precambrian rocks on River Ranch 40 million tons. and that the coalfield resembled the Malilongwe and and Benfer Estate near the Umzingwane River on the south side of the Sabi coalfields (Swift et al .• 1953) to the east. Karoo rocks. and calc-silicate granulites form hills about 1 km to the east. Garnets are abundant in the sandy shallow watercourses in this The geology of the area is referred to in papers on Karoo stratigraphy area and also about 2 km to the west of Gwawe Dam. by Tyndale-Biscoe (1949) and Bond (1952. 1955 and 1967) and on The Precambrian rocks around the Towla and Jopempi hills to the palaeontology by Lacey (1961) and Bond (1973). The Karoo igneous north of the area of the Mazunga map have been described by Robertson rocks of what is known as the Nuanetsi Igneous Province (mainly adjacent to the mapped area) have been described and discussed by (1973). Tyndale-Biscoe (1949). Worst (1962). Cox et al. (1965). Cox et al. (1967). Jamieson (1966) and Macdonald (1967). KAROO SYSTEM Robertson (1967). with a party of schoolboys. conducted a very INTRODUCTION successful study of the Karoo rocks along the north bank of the The rocks of the Karoo System are widely distributed in southern Limpopo River and the south-west corner of Sentinel Ranch where Africa but in Rhodesia are confined to down-faulted blocks in the they discovered and excavated reptilian fossil remains. Limpopo and Zambezi valleys. From the Zambezi valley Karoo rocks Macgregor (1939) reported upon an abortive search for an alleged extend in tongues across the Rhodesian Craton to near Bulawayo. gold occurrence in the Karoo lavas and dykes that form the ridge at Gwelo and Felixburg. Gokwe and Que Que, and the Charter District Gong's (Gongwe) Poort.