Browsing by Author "Kaaya, Charles Z."
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Item Depositional Environment of Late Permian Karoo Beds in the Ruhuhu Basin and Mikumi Area of Tanzania(1992) Kaaya, Charles Z.The Middle to Upper Permian Karoo sedimentary rocks in the western part of the Ruhuhu Basin have been stratigraphically redefined. They comprise the Mbuyura, Mhukuru, Ruhuhu and Usili Formations hirtherto known as K3, K4, K5 and K6 units respectively. Mbuyura Formation has further been subdivided into the Scarp Sandstone Member, ..Green Beds" and "Red Beds". Three tectonically controlled depositional cycles are recognized in this succession. The first cycle was initiated by fault movements within the NE - SW trending basin which at that time comprised two half grabens with uneven basin floor. Braided river sands of the Scarp Sandstone Member were deposited from the north and southwest of the basin. They pass upwards and laterally into braid plain mudrocks and sandstones of the "Green Beds". The second cycle was initiated by faulting outside the basin margins which led to overstepping of the "Red Beds" on the Precambrian metamorphics (basement), deposition of braided river sands at the basin margin and formation of mud flats and desiccating shallow ponds towards the basin centre. Amelioration of dry conditions accompanied the deposition of the overlying fluvial-lacustrine sandstones and carbonaceous mudrocks of the Mhukuru Formation. At that time the high grounds in the basin were disappearing under the sediment cover and the depositional area was acquiring a uniform easterly to southeasterly gradient. The overlying Ruhuhu Formation with a total thickness of more than 500 m of calcareous and non-calcareous mudrocks with stromatolite and oolite beds represents an establishment of a broad permanent lake with slightly alkaline waters during most of its history, The 13C isotopic values (+3.457 to -18.251 (PDB)) in the calcareous beds suggest that a great part of the carbon in the carbonates was derived from decomposing plant matter. The third cycle is represented by the Usili Formation. It commenced approximately at the Kazanian/Tatarian boundary as the area with Karoo beds was slowly being uplifted. The uplift coupled with lake level fluctuations resulted in the formation of uplands from which rudaceous sediments (lithic arenites) were eroded and deposited in the low lying mudflats and shallow lakes. The sandstones in the lower three formations are dominated by texturally immature arkoses (up to 70% feldspars) and subarkoses which indicate short transport distances from the basement provenance. Early diagenesis in the sediments as a whole was controlled by climatic conditions and to some extent by depositional environments whereas at the late stages, diagenesis was influenced by temperature, pressure and the type of sediments. A comparison between the Karoo rocks in the Ruhuhu Basin and those in the Mikumi area has been carried out. The available palaeontological data allow a general stratigraphic correlation but are still insufficient for a detailed correlation. A more detailed correlation is nevertheless possible when the palaeontological data are combined with lithostratigraphy. It is concluded that the Karoo succession in the Mikumi area is of the same age as the succession in the Ruhuhu Basin starting from the "Red Beds" to the top of the Usili Formation and both areas were subjected to similar tectonic and climatic patterns, In contrast to the terrestrial depositional environments which prevailed throughout the history of the Ruhuhu Basin. The Mikumi area was for a short period of time within the influence of marginal marine or brackish water conditions.Item Late Quaternary Coastal Stratigraphy on a Platform-Fringed Tropical Coast: A Case Study from Zanzibar, Tanzania(1998-08) Arthurton, Russell; Brampton, A. H.; Kaaya, Charles Z.; Mohamed, S. K.The coasts of Zanzibar's islands, in common with those of the adjacent African mainland coast, are formed largely of Stage 5e Pleistocene limestones of back-reef facies. The limestones form typically undercut cliffs and associated wavecut platforms, commonly more than lkm wide. Where not masked by the deposits of beach ridge plains, the platforms coincide with the contemporary intertidal zone. This coincidence might suggest that the platforms are mid-late Holocene products, formed since the post-late-Weichselian glacial sea level attained its current highstand position. However, the present extremely slow rates of limestone cliff recession due to marine erosion, together with the existence, at the landward margin of the platform, of well lithified beach rocks of lithologies markedly different to those of the contemporary beach sands, indicate that most of the platform erosion occurred pre-Holocene. A sea-level stillstand period, following the peak of the Stage 5e highstand, is suggested. The overwashing of a pre-existing platform as a consequence of Holocene sea-level rise would have significantly and abruptly increased the area of intertidal to shallow subtidal habitats and thus the potential for the increased production of calcium carbonate sediment derived from that biota. The impact on the platform environment of predicted sea-level rise over the next century would be to create an extensive shallow subtidal environment promoting the growth of the calcareous green alga, Halimeda, coral mounds and small patch reefs. The beach ridge plains would become increasingly vulnerable to erosion.Item Stratigraphy and Morphotectonics of Karoo Deposits of the Northern Selous Basin, Tanzania(1991-02) Wopfner, H.; Kaaya, Charles Z.ate Permian Karoo deposits of the northern Selous Basin in south-central Tanzania comprise conglomerates and diamictitic boulder beds of alluvial and scarp-foot fan origin. These merge with grey to greenish sandstones, siltstones and black shales of deltaic and lacustrine environments. Microflora assemblages indicate a late Permian age. Lateral changes and interfingering of various lithofacies units are common. Depositional development was controlled by syndepositional faulting and variations of gradients resulting from fault movements. The position of the fault scarp separating the basin area from the elevated basement horst to the west roughly corresponded with the present boundary between the Selous Basin and the Precambrian metamorphics of the Uluguru Mountains. The late Permian Karoo succession of this part of the Selous Basin apparently overlaps older Karoo deposits contained in the north-northeast trending narrow graben structures. It is therefore regarded as a new depositional event which was initiated by renewed tensional tectonism in late Permian time. During this tectonic episode the narrow early Karoo graben structures were expanded into much broader rift basins. Material eroded from the rift shoulders and associated highlands was literally dumped across the fault scarps, forming debris aprons and scarp-foot fans. Rivers emanating from the highlands formed large alluvial fans and, further afield, deltas issued into freshwater lakes. Some of these late Permian faults were rejuvenated by late Cretaceous to early Tertiary tectonism. Thermal waters circulating along fractures converted feldspars, biotites and hornblendes to prehnite. Further tectonic adjustments in mid Tertiary time led to the present-day morphology.Item Tectono-sedimentary Development of the (Permian) Karoo Sediments in the Kilombero Rift Valley, Tanzania(2000-04) Nilsen, O.; Dypvik, Henning; Kaaya, Charles Z.; Kilembe, E.The Kilombero Rift Valley in Tanzania belongs to the East African Rift System and contains a sedimentary sequence of Karoo age that is more than 6000 m thick. The present Kilombero Rift basin is most likely of Cenozoic origin. Nevertheless, within the present rift, the Karoo sequences were formed in an earlier rift basin characterised by westerly-dipping boundary faults, i.e. with a different polarity than the present one. The Karoo sequences of Permian age comprise three formations in the Kilombero Rift (Ndeke Beds, Calcareous Beds and Ruhembe Beds). These units are made up of terrigenous, mainly siliciclastic sediments. The succession is well correlated with other East African Karoo units, and displays a rift-controlled sedimentation. The Ndeke Beds represent debris flow, braided stream and floodplain deposits within an alluvial fan system, building out from well developed, high relief border faults, situated to the east in a westward-facing half-graben. The present results point towards a half-graben of about 80 km length and at least 40 km width. There seems to have been only one major early rifting phase, which triggered the deposition of the Ndeke Beds. Most of the Ndeke Beds and the succeeding Calcareous Beds represent a post-rift filling phase. In this development, the alluvial fans formed the marginal parts of a basin, which, through a succeeding topographic levelling and the development towards a low relief landscape, were dominated by lacustrine sedimentation. The overlying, mainly fluviodeltaic Ruhembe Beds reflect a late Karoo tectonic activity. This event is displayed in well-developed debris flow zones near the top of the Karoo succession in the Kilombero Rift basin. Our stratigraphical investigation has not discovered any sedimentary units between the Karoo and Quaternary. However, Mesozoic Beds have been described from the western rifts and the coastal areas of Tanzania, and may also be present in the central areas of the Kilombero Rift Basin. The Cenozoic rift phase is clearly reflected in the present topographic relief, where two synthetic, eastwards facing half-grabens are separated by a well developed, high relief accommodation zone (the Gombati Transfer Zone). The accommodation zone displays a complex tectonic signature due to block faulting and rotation of the strata, between prominent dextral transfer faults. This pattern is in great contrast to the minor tectonic disturbances found in the sediments, both to the north and south of the transfer zone. The switch in half-graben polarity and tectonic setting from the Karoo to the Cenozoic situation, clearly demonstrate that even if the tectonic lineaments tend to follow the same zones of weakness, their polarity and internal configuration is far from straightforward.