The study area is situated on the Tanzanian craton and is a part of the highest highland plateau of Tanzania (Figure 2) (Milne, 1947; Spence, 1951; Fozzard, 1959; Key, 1992; Maboko, 2000; Macheyeki et al., 2008; Salminen et al., 2008).
The craton consists of Precambrian granites with some migmatites, gneisses, amphibolites and quartz veins and it has been intruded by mafic dykes (Milne, 1947; Spence, 1951; Fozzard, 1959; Bianconi and Borshoff, 1984; Key, 1992; Maboko, 2000; Macheyeki et al., 2008;
Salminen et al., 2008). The Chenene Hills are made up of the Bubu Cataclastics which are a deformation of the granitic craton (Maboko, 2000; Macheyeki et al., 2008)). Dating of the granite in the Bubu Cataclastics yield the same date as for the Tanzanian craton, namely 2.5 billion years using Sm-Nd technique while the age obtained from Rb-Sr was 527 million years (Maboko, 2000). The interpretation of this was that the Bubu Cataclastics was originally craton granite highly deformed by the Pan-African orogony which formed the metamorphic Mozambique belt (Maboko, 2000; Macheyeki et al., 2008). The Sanzawa scarp is an old fault trending North West-South East acting as the Southern boundary of the Chenene Hills
separating it from the Bahi depression, as seen in figure 3 (Fozzard, 1961; Macheyeki et al., 2008). It might be of late Pan-African origin (Macheyeki et al., 2008). North West of the Chenene Hills, in the upper part of the study area, an extinct volcano, Hanang is situated (Figure 2) (Foster et al., 1997; Dawson, 2008). Carbonatite lavas and pyroclastics which are highly alkaline were accompanied nephelitic tuffs (Foster et al., 1997; Dawson, 2008;
Macheyeki et al., 2008). From dating of the nephelinite, the active period of Hanang is found
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Figure 2: Topographical map of Tanzania modified after Map of Net (2012). Bahi basin is situated in the marked area. Mount Hanang is found at the top of the Bahi basin drainage area and behind it Lake Balangida.
to be between 1.5 million and 0.9 million years ago (Foster et al., 1997; Dawson, 2008;
Macheyeki et al., 2008). There are also many granitic inselberg cropping up on the plains, sometimes called by the Dutch name kopje in literature (Burtt, 1935; Milne, 1947; Spence, 1951). These are seen as residual hills which stood up in a mature landscape, a peneplain in the Miocene period (Milne, 1947; Spence, 1951). The Killimatinde formation, which is mainly a silcrete but also contain some layers of ferricrete and calcrete, is believed to have been formed in a large swamp, an area with sluggish water movement or shallow evaporative basins on this peneplain (Milne, 1947; Spence, 1951; Fawley, 1956; Fozzard, 1959, Bianconi and Borshoff, 1984; Mancheyeki et al., 2008). The underlying granite basement is thought to have been weathered by percolating water and then cemented by pure silicates or aluminum
25 silicates, sometimes with iron in-situ (Milne, 1947; Spence, 1951; Fozzard, 1959). The
Killimatinde formation has not been interrupted by the Sansawa faulting but all other faults cut it, so the Chenene Hills was already present when the Killimatinde sediments were
deposited (Milne, 1947; Spence, 1951; Fozzard, 1959; Mancheyeki et al., 2008). The younger faults, south of the Chenene Hills surrounding the Bahi Swamp, are part of the East African Rift Structure and seen as an extension of the Manyara-Balangida rift segment and is
illustrated in figures 2 and 3 (Mancheyeki, 2008). The faulting of the Manyara-Balangida area started in Pliocene-Pleistocene and is still going on as is evident by the earthquakes in the study area (Bianconi and Borshoff, 1984; Foster et al., 1997; Mancheyeki et al., 2008). The faulting in the study area consists of half graben structures with uplifting mostly on the west side of the faults and down-faulting of the east side’s (Figure 3) (Bianconi and Borshoff, 1984; Foster et al., 1997; Mancheyeki et al., 2008). In the surroundings of the Bahi depression there are a pattern of south-west trending faults with the south-eastern side down-thrown (Figure 3) (Mancheyeki et al., 2008). The faults are called Saranda, between Killimatinde and Manyoni, Bubu South and North fault, with the North fault separating Bubu River and
Mponde River and Bubu South situated between Killimatinde and Mbwasa (Figure 3) (Fozzard, 1959; Mancheyeki et al., 2008). To the east there is more faults with the same trending (Figure 3). Faults of Dodoma South and North, the Dodoma-Kigwe and Hombolo fault all have trending the same trending and south-east down-throw (Figure 3) (Mancheyeki et al., 2008). The other fault pattern are of faults trending north-northwest with down-throw of the south-eastern block (Figure 3) (Mancheyeki et al., 2008). These are the faults of Bahi, being the southern limit of the drainage area of Bahi Swamp, and Mponde, forming the Mponde valley of which Balangida lake drains into (Figure 2 and 3) (Fozzard, 1959;
Mancheyeki et al., 2008). All the faulting of the earlier peneplain left part of the Killimatinde formation uplifted and are now being eroded, while other parts where down thrown and are today buried underneath younger sediments from Pleistocene to recent, like in the Bahi depression(Figure 1 and 3) (Milne, 1947; Fawley, 1956; Bianconi and Borshoff, 1984;
Mancheyeki et al., 2008). The Bahi depression has the Bahi Swamp situated at the lowermost point (Figure 3). In 1953 there were six drill holes done in the Bahi swamp until fresh
basement rock was found (Fawley, 1956). The reason for the drilling was to see if there was anything for economical production (Fawley, 1956). The depth to basement varied between as little as 33 meters until 109 meters or 358 feet (Fawley, 1956). It was found that the sediments were too alkaline and high on silica to be used for cement but the brines from
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Figure 3: Topographical map over study area. Fault lines modified after Macheyeki et al. (2008).
27 the boreholes might be utilized for salt mining (Fawley, 1956). In the Bahi depression the Killimatinde beds, consisting of grains cemented by silica (Fawley, 1956), are found under approximately 75 meters of lake sediments (Fozzard, 1959) and Fawley (1956) noted it to be between 10 to 70 meters deep. Unfortunately a map with the drilling points has not been located but it is here assumed the shallowest parts are closer to an edge of the Bahi swamp.
The lake sediments deposited on top consists of alternating beds of clays, calcrete and iron-cemented tuff assumed to be of Plio-Pleistocene until recent in age (Fawley, 1956; Fozzard, 1959; Bianconi and Borshoff, 1984).