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Title: ASSESSMENT ON HYDRAULIC PROPERTIES OF THE ETHIOPIA TARMABER FORMATIONS
Authors: ABREHA, G/SLASSIE
Advisors: Seifu Kebede (Ph.D)
Engida Zemedagegnehu (Ms.C)
Keywords: hydrologr
Copyright: Jul-2010
Date Added: 2-May-2012
Publisher: aau
Abstract: The Ethiopian Tarmaber formation represents, Oligocene – Miocene basaltic shield volcanism on the northwestern and southeastern plateaus covering an area of about 47,194Km2 (which is 8% of the total flood basalts aerial coverage in the country). Two types of Tarmaber formations have been mapped. The Tarmaber Megezez formation (Ntb) with an absolute age of (16 – 13 Ma, Kazmine, 1979) is transitional to alkali basalt which covers an area of about 22,058Km2 (which is 3.4% of the flood basalts (or traps) aerial coverage in the country). This formation commonly outcrops in the central highland plateau, the escarpments and the rift floor and at some localities in the south western highland plateau part of the country (sees Fig.-2.1 & 2.3). The Tarmaber Guessa formation (PNtb), with an absolute age of (26 – 16Ma, Kazmine, 1979) is alkaline to transitional basalt, often ridge and cliff forming shield volcanoes with minor trachytic and phonolite flows. It covers a total area of about 25,136Km2 (which is 4.5% of the flood basalts aerial coverage in the country). This formation dominantly outcrops in northwestern highland plateau part of the country (see fig.-2.1). The Tarmaber shield volcanoes become progressively younger to southeastern and south western part of the northwestern Ethiopian plateaus. Well-log data and pump test data’s analyses show that, the Tarmaber formations aquifer system can be categorized as consolidated fractured aquifer category where the dominant aquifer types are, confined, double porosity fractured aquifer system and single plane vertical aquifer systems. The double porosity aquifers are related to deeply drilled wells reflecting presence of large and narrow fracture systems with high permeability but lower storage capacity. It also shows that, Tarmaber Megezez formation (Ntb) has better aquifer productivity than the Tarmaber Guessa formation (PNtb) and yet, both formations show decrease aquifer productivity with respect to increased drilled boreholes depth and increased age of the formation. Besides, boreholes drilled within the Tarmaber formations shows that, the wells have high well loss coefficient values, indicating improper well site location, improper well design and construction factors and well yield deteriorations with time due to clogging, corrosion and incrustation activities of the well screens. Spatially, the Ethiopian Tarmaber formations show an increasing aquifer productivity trend from the highland plateau areas towards the escarpment and the rift floor areas, and yet, from the north, south, northwest, southwest, northeast, southeast and east- west directions of the highland plateau areas toward the Lake Tana basin areas. Generally, the Tarmaber basalts aquifer productivity is highly controlled by the location and geomorphologic setup of the formation outcrop, nature and degree of weathering, hydrothermal processes and nature, extent, frequency and orientation of the associated structural features and yet, weathering, hydrothermal processes and other volcanic activities tend to decrease aquifer permeability while, fracturing, faulting and other tectonic activities tend to increase aquifer productivity of the Tarmaber formations.
URI: http://hdl.handle.net/123456789/2082
Appears in:Hydrogeology

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