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Sedimentology and Paleocurrent Study of the Early Triassic Rocks in the Ruhuhu Basin, SW Tanzania

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Sedimentology and Paleocurrent Study of the Early Triassic Rocks in the Ruhuhu Basin, SW Tanzania Tanzania Journal of Science 46(2): 383-396, 2020 ISSN 0856-1761, e-ISSN 2507-7961 © College of Natural and Applied Sciences, University of Dar es Salaam, 2020 Sedimentology and Paleocurrent Study of the Early Triassic Rocks in the Ruhuhu Basin, SW Tanzania Audax Syprian Kiwango* and Diwakar Mishra Department of Geology, College of Earth Sciences and Engineering, The University of Dodoma, P. O. Box 259, Dodoma, Tanzania E-mail addresses: [email protected]; [email protected] *Corresponding author Received 14 November 2019, Revised 9 April 2020, Accepted 27 April 2020, Published June 2020 Abstract The sedimentary succession ranging in age from Permian to Early Triassic in Ruhuhu basin was subdivided into eight informal lithostratigraphic units, identified by the symbols from K1 to K8. Manda Formation (K8) belongs to Early Triassic age and comprises two Members, i.e., Lower Kingori Member and Upper Lifua Member (150-200 m thickness). The implications of present study relate to inter and intra basinal correlations which may provide regional depositional framework from a mass of local details. Present investigation connoting the lithofacies studies in conjunction with palaeocurrent and grain size analysis of the early Triassic strata aims at interpreting the depositional environment of Lifua Member. Based on the present study, five lithofacies have been identified, namely (i) Massive matrix supported paraconglomerate (Gmm), (ii) Massive sandstone (Sm), (iii) Parallel-horizontal laminated sandstone (Sh), (iv) Planar cross- bedded sandstone (Sp) and (v) Fine silt, mud and clay (Fl). Sandstone facies (Sm, Sh, and Sp) exhibit normal grading and unimodal palaeocurrent direction. Grain size analysis indicated that the sandstones were moderately sorted, finely skewed, mesokurtic and most of the grains were silty- sand. Bivariate scatter plots suggest that the Lifua Member sandstone is of riverine environment. Lithofacies, palaeocurrent and grain size studies suggest fluvial environment dominated by sand channel deposits. Keywords: Lifua Member; Early Triassic; Lithofacies, Paleocurrent; Channel deposits; Ruhuhu basin Introduction basin is subdivided into eight informal The Ruhuhu Basin is located in the south lithostratigraphic units, identified by the west Tanzania and contains all symbols from K1 to K8 (Wopfner et al. lithostratigraphic units of Permo- 1991). K8 is termed as Manda Beds that are Carboniferous to Early Triassic (Karoo of Early Triassic age and is the youngest succession). The NE-SW trending Ruhuhu stratigraphic unit of the Karoo sequence of Basin is one of the chain of Karoo basins the Ruhuhu basin. Charig (1957) raised the across southern Africa that formed during the status of Manda Beds as the Manda break up of Gondwana Supercontinent in the Formation and composed of two Members, Late Carboniferous/Early Permian (Kreuser et i.e., Lower Kingori Member and Upper Lifua al. 1990). Member. Ruhuhu basin exhibits excellent The sedimentary succession ranging in outcrops of all units, amounting to a age from Permian to Early Triassic in Ruhuhu cumulative total thickness of up to 3,000 m 383 http://journals.udsm.ac.tz/index.php/tjs www.ajol.info/index.php/tjs/ Kiwango and Mishra - Sedimentology and Paleocurrent Study of the Early Triassic Rocks … (Catuneanu 2005). The study has been fluvial domain. Topographic map of Lifua conducted in the Lifua Member. Most of the showing the location of the studied sections is Lifua Member units are located in the western shown in Figure 1. part of the Ruhuhu basin bordering Lake Nyasa. The study area lies between 34°36ʹE- Geological setting and stratigraphy 34°48ʹ E and 10°27ʹS-10°30ʹS and covering The Karoo basins of Tanzania occur an area of approximately 250 km2. mainly as grabens or half-grabens tilted to the Through the proper study of sedimentary direction E–SE or S. They are faulted into the rock deposits, interpretation of geological Ubendian basement. Boundary faults post- history of the area is feasible. Sedimentary date the deposition and were active most structures such as planar cross bedding and probably during middle Jurassic as a result of trough cross bedding are the most directional the breakup of Gondwana (Kreuser et al. sedimentary structures that are applicable in 1990). Some of the basins that comprise paleocurrent measurements (Selley 2000). Karoo rocks in Tanzania are Tanga basin, The measurement of paleocurrent indicators Ruvuma basin, Ruhuhu basin, Rufiji trough is important in the study of sedimentary and Njuga basin. rocks, since they provide information on the Earlier studies of the Ruhuhu Basin date local/regional palaeoslope, depositional back in 1890’s during which coal was environment, current and wind directions discovered (Stockley and Oates 1937). Later which are useful in facies interpretation on, extensive drilling programs started at (Tucker 2003). Paleocurrent studies along various parts of Ruhuhu basin and other parts with facies/subfacies analysis and grain size of Tanzania such as Ketewaka-Mchuchuma studies could provide a clear picture of and Ngaka coalfields (Semkiwa 1992). The depositional environment. Most of the studies Ruhuhu Basin is not only the basin that still conducted in the Ruhuhu Basin focused comprises a fairly complete succession of mainly on the paleontology and the economic typical Karoo beds, but also due to its potential of coal accumulations in the basin elevated position on the shoulder of the (Kreuser et al. 1990, Nesbitt et al. 2014, Neogene Nyasa Rift it provides excellent Barrett 2015). However, studies on the outcrop of all units, amounting to a depositional environment have been given cumulative total thickness in excess of 3,000 little attention to date. So taking the m (Catuneanu 2005). importance of depositional environment, the The sedimentary succession ranging in present investigation applies integrated age from Permian to Early Triassic in Ruhuhu approach of lithofacies studies in conjunction basin is subdivided into eight informal with palaeocurrent and grain size analysis of lithostratigraphic units, identified by the the Early Triassic strata in interpreting symbols from K1 to K8 (Wopfner et al. 1991, depositional environment of Lifua Member. Figure 2). K1 to K6 represent Permian Analysis of paleocurrent pattern needs to be succession and they are in ascending order in combined with study of lithofacies and grain which K1 represents Glacigene deposits, K2 size for maximum information. In order to represents a lower coal bearing unit, K3 reconstruct depositional environment and represents red and green arkoses, K4 paleogeography, it is essential to establish represents an upper coal bearing unit, K5 regional framework. In this context, the represents lacustrine carbonates, and K6 present study will be useful for inter and intra represents multicolored siliciclastics with basinal correlations which may provide vertebrate remains (Kreuser et al. 1990). K7 regional depositional framework from a mass and K8 are termed as Kingori Sandstone and of local details. It may also provide some Manda Beds, respectively. The beds are of alternative ideas for depositional modelling in early Triassic age and are the youngest 384 Tanz. J. Sci. Vol. 46(2), 2020 stratigraphic units of the Karoo sequence of the Ruhuhu basin. Figure 1: Topographic map of Lifua showing the locations of the studied sections (modified from Douglas Lake Minerals Inc. 2011). 385 Kiwango and Mishra - Sedimentology and Paleocurrent Study of the Early Triassic Rocks … In this aspect, it could be stated that Africa, the Lifua Member is considered to be Charig (1957) was the pioneer in raising the Anisian in age (Nesbitt et al. 2014). status of Manda Beds the youngest unit which A shale pebble conglomerate, consisting was deciphered earlier by Stockley and Oates of clasts derived from the underlying (1937). After this description, the Kingori multicoloured and marls of the K6 unit marks Sandstones have become the Kingori the base of the Kingori Sandstone (Wopfner Sandstone Member of the Manda Formation, et al. 1991). This unconformity between K6 and original Manda Beds has become a and K7 marks the boundary between the subunit of the formation called the Lifua Permian and the Triassic as supported by the Member. On the basis of comparison with the different vertebrate faunas contained in the tetrapod fauna of subzone C of the Lower Bone Bed ‘K6’ and in the Manda Beds Cynognathus Assemblage Zone of South ‘K8’. Figure 2: Stratigraphic table of the Songea Group as established in the Ruhuhu Basin (adopted from Catuneanu 2005). Methodology Lithofacies and their classification The fieldwork was conducted between Each succession was recorded and examined February and March 2016 and involved in detail, thereafter the recorded sediments identification of outcrops of the Lifua attributes were observed. The depositional Member-Manda Beds especially on the flanks structures were considered first since these of rivers and on the sides of road cuts. Each reflect the depositional processes excellently; outcrop was evaluated and interpreted in then followed by texture, lithology and fossil terms of lithology, sedimentary structures, content. Everything was named or numbered texture, and palaeocurrent measurement for future reference. Once the various facies together with collection of samples. Vertical have been differentiated, a table with their profiles and other length related dimensions various features (name, code, typical of beds were also measured.
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