TECNICAS ACONSEJABLES PARA EL ESTUDIO DE LAS ACTITUDES POSDELINCUENCIALES

The Kilombero Valley is a SW-NE trending trough-like depression, covered with Cenozoic sediments be- tween crystalline rocks of the Precambrian Mozambique Belt (Beck 1964, Mruma 2002). This depression is a southern extension of the EARS (McConnell 1972). Mruma (2002) described the Kilombero Valley as a part of the northeast-trending Kilombero Rift, where the Udzungwa scarp forms the uplifted and the Kilombero Valley the down-thrown block of the rift fault. Ne e theless, the te Kilo e o Valle o Kilo e o Rift is often used for the north-trending rift to the east of the study site (Nilsen et al. 1999, Dypvik et al. 2001, Dypvik and Nilsen 2002, Le Gall et al. 2004). The Kilombero Valley as described by Beck (1964), Jätzold and Baum (1968), and Mruma (2002) comprising the study site, is a southwestern extension of this north-trending rift. In terms of consistency, the te s Kilo e o Valle a d Kil- o e o Rift will henceforth refer to the northeast-trending valley, while the north-trending rift struc- ture described by Nilsen et al. (1999), Dypvik et al. (2001), Dypvik and Nilsen (2002), and Le Gall et al. (2004) will be called Kidodi-Mkata Rift following Mruma (2002).

As the eastern branch of the EARS crosses the Tanzanian Craton, it splits up into ill-defined arms (Mruma 2002). One of these arms forms the Kilombero Rift, which is a northeastern continuation of the Ruhuhu Rift and joins the Kidodi-Mkata Rift in the north (Mruma 2002). To the northwest, the Kil-

ombero Valley is bounded by the so-called Udzungwa scarp, the escarpment of the Udzungwa Moun- tains that was formed by large-scale faulting and tilting of the Kilombero Rift System (Geological Survey of Tanganyika 1962). To the southeast, the valley is bounded by the northern foothills of the Mahenge Mountain block (Geological Survey of Tanganyika 1962). In the northeast, it ends at the Haldemann sills, rocky outcrops crossing the river bed, which act as weirs (Jätzold and Baum 1968). The valley itself is filled with a sedimentary basin infilling (Beck 1964).

The Udzungwa Mountains belong to the Neoproterozoic Mozambique Belt, which is located to the east of the Tanzanian Craton (Schlüter, 1997). It was strongly affected by the Neoproterozoic (Pan-African) metamorphic event from 650 to 615 Ma (Möller et al. 1998). In former studies, some authors assigned the rocks of the Udzungwa Mountains to the Usagaran System (Geological Survey of Tanganyika 1962), maybe due to the ambiguous usage of the term Usagaran, which was sometimes applied to both Palae- oproterozoic and Neoproterozoic rocks (Schlüter 1997). However, this belt represents a zone of Neopro- terozoic orogenies associated with the collision of East and West Gondwana (Reddy et al. 2003). The rocks were formed and metamorphosed during Archaean and Palaeoproterozoic times and have been actively reworked during the Pan-African tectonothermal event in the Neoproterozoic (Schlüter 1997). The belt exhibits mainly paragneisses and orthogneisses representing amphibolite to granulite facies (Schlüter 1997). The part of the Mozambique Belt being exposed at the northwestern flank of the Kil- ombero Valley is referred to as Western Granulites and comprises gneisses with greenschist to lower amphibolite facies in the west and granulite facies rocks in the east (Tenczer et al. 2007).

The existence of a weathering profile (section 2.2.2) overlying crystalline rocks in Tanzania was proven in several studies (e.g. Little and Lee 2006, Kashaigili 2010). Ministry of Water URT (2012a) mentioned this weathering profile to occur in the Rufiji Catchment as well. However, no information about depths and characteristics of this profile exist so far and no publication mentions the weathering profile to exist within the Kilombero Catchment. Nevertheless, it is most likely that the crystalline rocks surrounding Kilombero Valley have been exposed to weathering.

Only little information is available regarding the age of the normal fault structure, which separates the Udzungwa Mountains from the sedimentary basin infilling of the Kilombero Valley (Le Gall et al. 2004). Fracture investigations of Mruma (2002) revealed three different sets of fractures trending in different directions (WNW-ESE, E-W, NE-SW, NW-SE). All fractures showed relatively steep dips and most of them showed a reverse sense. While the striking directions of fractures fit with those of the fault system of the Udzungwa scarp, the reverse sense contradicts with the normal sense of the scarp faults. Mruma (2002) gives two possible explanations. Firstly, it could be that the reverse faults are older than the normal faults, with the latter being reactivated structures of Permo-Triassic age. Secondly, both faults could be of the same age either Permo-Triassic (with reactivations in Cenozoic) or Cenozoic. For the Kidodi-Mkata Rift located north of the study site, Le Gall et al. (2004) assumed a first rifting phase during Karoo times and a subsequent reactivation of the fault system in Neogene-Holocene times. Consequent- ly, it is most likely that the Kilombero Rift belongs to the EARS, but first movements already took place in Karoo or even earlier times.

During Karoo times, huge formations of continental sediments were deposited in the tectonic depres- sion. These formations are mainly made up of sandstones and conglomerates (Geological Survey of Tan- ganyika 1962). Le Gall et al. (2004) described 4,000 to 5,000 m thick Karoo rocks within the Kidodi-Mkata rift north of Kilombero Valley. After Geological Survey of Tanganyika (1962), it is most likely that these rocks underlie huge parts of the Kilombero Valley as well.

Comparatively little information about the unconsolidated sediments deposited in the Kilombero Valley exists. No geological publications are available regarding the sedimentary basin infilling. Following Beck (1964), the Kilo e o Valle is o e ed ith Plio e e a d Pleisto e e deposits a d allu ial ate ial of e e t age . Bo a ius 975) classified the sediments in terms of soil forming processes into i) braided and meandering river zone in the widespread central alluvial plain, ii) alluvial fans laid down by streams entering the valley, and iii) marginal colluvial sand-flats or miombo plains.

Information about geological units and structures within the Ifakara study site is solely available from the geological map by Geological Survey of Tanganyika (1962). The Precambrian metamorphic rocks of the southern hills of the Udzungwa Mountains are exposed in the north of the study site (Fig. 2.6). These hills, building the mountain fringe of the Udzungwa Mountains, are referred to as Itula range of hills. They are made up of Neoproterozoic migmatitic, quartzo-feldspathic gneisses and pyroxene granulites. In the very northwest of the study site, garnetiferous biotite gneisses occur. These metamorphic rocks of the Mozambique Belt are separated from a sedimentary basin infilling via the normal fault system, which developed during rifting. Two major faults cross the study site in west-eastern direction building a tilted block faulting structure with tilting in northern direction.

The sedimentary basin infilling is divided into non alluvial sediments, covering the mountain fringe and the tilted block between the two major faults, and alluvial sediments in the south. The non-alluvial sed- iments in the central and western part of the basin between the two normal faults are made up of pale locally ferruginous sands. Red-brown and light sandy earths are found in the east of the basin and as overlays of the hard rocks in the depressions of the mountain fringe. Ferruginized cemented sands and gravels occur in small parts along the southern fault. These non-alluvial sediments are of Neogene age and once covered the whole Kilombero Valley, but have been eroded away in huge parts. They might represent the zone of marginal colluvial sand-flats described by Bonarius (1975). The Quaternary alluvial sediments are described as alluvial sands, silts, and clays, without any further description. However, based on the suggested classification of Bonarius (1975), it is most likely that these sediments are made up of river deposits, alluvial fans, and maybe colluvial sediments.

The alluvial fan within the study site was described by Jätzold and Baum (1968) and is clearly visible in topographic elevation. The fan is located around Lumemo River (Fig. 2.6). The higher elevated areas of this fan underlie the city of Ifakara. These areas do not become flooded by over-bank flow of Kilombero River and thus result in a bottleneck structure of the floodplain south of the fan. River and floodplain deposits are located near Kilombero River, which has a meandering character in the study site, and in potentially former river or floodplain locations. Colluvial sediments were probably deposited by smaller rivers along the southern normal fault, east and west of the alluvial fan. Nevertheless, a differentiation of these depositional environments is not possible based on the geological map. The delineation of the alluvial fan was adopted from Jätzold and Baum (1968). However, it is not clear which criteria they used for the delineation. As all of these sediments were deposited in Quaternary to recent times, it is most likely that they interfinger with each other. Recently, Kilombero River is cutting into the alluvial sedi- ments, maybe due to further fault movement or tilting (Geological Survey of Tanganyika 1962). Alt- hough it is most likely that sedimentary rocks of Karoo age underlie parts of the Kilombero Valley, Geo- logical Survey of Tanganyika (1962) supposed no Karoo rocks around Ifakara. They rather described a ridge of Proterozoic gneisses between Ifakara and Kivukoni, which is located south of Kilombero River.

Fig. 2.6: Geological map of the Ifakara study site. Data sources: Geological Survey of Tanganyika (1962) (geology, streams), Jätzold and Baum (1968) (alluvial fan) (IF).