Chapter 5Geology and Hydrogeology - 45 - Chapter 5 Geology and Hydrogeology 5.1 Geology of the Study Area Geology in Mtwara and Lindi Region has been studied reviewing earlier reports (Mtwara-Lindi Water Master Plan, Finnwater 1977 and Mtwara -Lindi Revised Water Master Plan, Finnwater 1986) and through the field survey during the stage 1 of the Study. The formations in the area are classified into following four typical geological units: - Pre-Cambrian basement rocks - Paleozoic and Mesozoic continental sedimentary rocks (Karoo) - Mesozoic and Tertiary marine sedimentary rocks (Coastal Sediments) - Recent river, marine and terrace deposits (alluvium) The summary of the hydrogeological characteristics of each geological unit is shown in Table 5-1,and geological map with location of village is shown in Fig.5-1. 5.1.1 Pre-Cambrian Basement Rocks The basement complex (X) consists mainly of various metamorphic rocks such as marble, schist and gneiss. Basic intrusive rocks and dike rocks, as well as quartzite and pegmatite are also included in the basement rocks. Basement rock exposure is widespread in the districts of Masasi, Newala, Liwale and Nachingwea. Since these rocks are generally hard, massive and with scare porosity, the potential for groundwater development in this formation is very low except on the fissure-abundant zones related to the faults and weathering. Outcrops of the fresh basement rock are generally limited to the monadnocks (island mountain). Surface portion of these rocks are weathered to soil in most of the area to a depth of several meters (20 meters in average), therefore, the extraction of stored groundwater in the weathered portion is possible by construction of shallow wells. Most existing shallow wells in this area, however, are dried up in the dry season. The survey for groundwater development in the basement rock areas should focus on locating the fissured zones. 5.1.2 Karoo Formations The Karoo (K) formation consists mainly of sandstone and limestone. The formation is covered by lateritic sandstone at the surface portion, which is predominated by sand and gravel. Since the fresh rocks in this formation are generally compact and massive, the groundwater development potential is not high except the deep weathered portion or on and around the faults as in basement rocks.
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Chapter 5 Geology and Hydrogeology
- 45 -
Chapter 5 Geology and Hydrogeology
5.1 Geology of the Study Area Geology in Mtwara and Lindi Region has been studied reviewing earlier reports
(Mtwara-Lindi Water Master Plan, Finnwater 1977 and Mtwara -Lindi Revised Water Master
Plan, Finnwater 1986) and through the field survey during the stage 1 of the Study. The
formations in the area are classified into following four typical geological units:
- Pre-Cambrian basement rocks
- Paleozoic and Mesozoic continental sedimentary rocks (Karoo)
- Mesozoic and Tertiary marine sedimentary rocks (Coastal Sediments)
- Recent river, marine and terrace deposits (alluvium)
The summary of the hydrogeological characteristics of each geological unit is shown in
Table 5-1,and geological map with location of village is shown in Fig.5-1.
5.1.1 Pre-Cambrian Basement Rocks The basement complex (X) consists mainly of various metamorphic rocks such as marble,
schist and gneiss. Basic intrusive rocks and dike rocks, as well as quartzite and pegmatite are
also included in the basement rocks. Basement rock exposure is widespread in the districts of
Masasi, Newala, Liwale and Nachingwea.
Since these rocks are generally hard, massive and with scare porosity, the potential for
groundwater development in this formation is very low except on the fissure-abundant zones
related to the faults and weathering. Outcrops of the fresh basement rock are generally limited to
the monadnocks (island mountain).
Surface portion of these rocks are weathered to soil in most of the area to a depth of several
meters (20 meters in average), therefore, the extraction of stored groundwater in the weathered
portion is possible by construction of shallow wells. Most existing shallow wells in this area,
however, are dried up in the dry season.
The survey for groundwater development in the basement rock areas should focus on
locating the fissured zones.
5.1.2 Karoo Formations The Karoo (K) formation consists mainly of sandstone and limestone. The formation is
covered by lateritic sandstone at the surface portion, which is predominated by sand and gravel.
Since the fresh rocks in this formation are generally compact and massive, the groundwater
development potential is not high except the deep weathered portion or on and around the faults
as in basement rocks.
Chapter 5 Geology and Hydrogeology
- 46 -
The Karoo formation outcrops in the districts of Liwale, Ruangwa and Tandahimba. The
difficult areas for groundwater development (Karoo and Basement) occupy about 70% of total
land of the two regions.
5.1.3 Mesozoic and Tertiary Sedimentary Formations The other sedimentary formations are of Mesozoic (MS) to Tertiary (TS). The Sediments
of Cretaceous Epoc are composed of sandstone, siltstone, conglomerate and alternating beds of
clay, sand and sandstone, while Tertiary Sediments consist mainly of silt, clay, sand and loose
sandstone (half-consolidated sandstone). The sandstone in these formations is almost
horizontally stratified. Sandy bed and semi-consolidated high permeable sandstone bed may
have the highest potential for groundwater development in the formations.
The sedimentary formations are wide spread in the eastern coastal area of the two regions,
i.e., Newala, Tandahimba and Mtwara Rural districts in Mtwara region, and Kilwa and Lindi
districts, and eastern part of Ruangwa district in Lindi region.
5.1.4 Recent Deposit Recent deposits (ND), which consist of sand, clay and gravel, are distributed along the
coastal line and rivers (especially at rivers of Ruvuma, Mbwemkuru, Matandu and Lumesule).
Sandy and gravelly beds of these deposits are normally good aquifers for shallow groundwater
development.
Geological Classification(rock facies) Distribution (Districts) Major Aquifer Remarks Solution
Anxiety of highconductivity and high SO4content groundwaterexistence.
Further study is required to clarifythe relation between location ofsaline well and the direction offissure.
Mesozoic SedimentaryRocks (C)
(mudstone, shale, sandstone,conglomerate, alteration of
sandstone andconglomerate)
Plateau of the eastern partof the area
(east of Ruangua, east ofMasasi, Newala,
Tandahimba, MtwaraRural, Lindi Rural, Kilwa)
sandstone and fissureabundant zone in shale
Horizontal distribution anddepth of the aquifer variesat plateau area. The layerbecome soft by themoisture (very dry lateriticsediments).
Further study is required to clarifythe distribution of aquifer at thedrilling location. Drilling with mudrotary should take care of thenature of those rock (whichbecome clayey when wet).