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Chapter four Geological set up of the study area
CHAPTER FOUR GEOLOGICAL SET UP OF THE
STUDY AREA
In this chapter the general geology, local geology, tectonics and geological structure of the study
area has been presented.
General Geology
The basement Precambrian metamorphic rocks, Paleozoic – Mesozoic sediments and Cenozoic
(Tertiary to Recent) volcanic rock which is directly overlying the Precambrian metamorphic and
Mesozoic sedimentary rocks, are the main rock units found in Ethiopia.
The geological setting of the western Afar margin which is part of the study area can be
described in relation with the origin of Ethiopian rift which is part of the east African rift system.
In the Ethiopian rift system the Somalian plates are pulling away from Nubian plate which
makes much of Africa. These plates are also moving away from the Arabian plates which make a
triple rift junction at the afar depression.
The Ethiopian volcanic province covers an area greater than 600,000 km2 and it is dominated by
fissure fed basaltic lavas forming the volcanic plateau that bound the Afar and Ethiopian Rift.
(Peccerillo et.al, 1997). The period of most voluminous volcanism in Ethiopia was the late
Oligocene – early Miocene (32-21MY), with flood emission of transitional basalt lavas from the
margins of the proto-rift system extending for hundreds of kilometers over the present plateaus
(Mohr and Zanetin, 1988).
The geology of the study area can be grouped in to the Cenozoic (Tertiary to Recent) volcanic
rock units. It falls within the Dessie sheet of NC-37-3 at scale of 1:250,000.
Geological map of Dessie Sheet map, EIGS 2011 (unpublished), classified the general geology
of the area into four main groups of Cenozoic volcanic rocks: - I Eocene-Oligocene, II
Oligocene-Miocene, III Late Miocene and IV Quaternary volcanic rocks and associated
Lacustrine and superficial sediments.
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Chapter four Geological set up of the study area
I. Eocene-Oligocene
The Eocene-Oligocene volcanic rocks comprises of the Ashangie basalt and the Wegel Tena
rhyolitic Ignimbrite.
During the first stage, Oligocene – Pliocene, large fissure eruptions of basalts build up thick
flood lava sequence (Known as Ashange and Aiba basaltic formations) associated with late
ignimbrite sheet (Alaji Rhyolitic formatios).( Peccerillo et al 1997)
The Ashangie basalt is the most dominantly observed volcanic rock near to the study area. It is
exposed along Dessie – Mekelle rout, north central plateaus and in most part of the Afar rift
marginal grabens.
The Ashangie basalts are characterized by strong weathering, different directional tilting,
columnar jointing, intense fracturing and crushing. In many of its exposure it is dominated by
inclined columnar jointed aphanatic basalts. The unit also contains intercalated layers of
agglomerate and volcano - classic sediments and vesicular basalt (Tesfaye et al., 2011)
The Wegel Tena rhyolitic Ignimbrite volcanic rocks exposed near Wuchale, and in the north
central plateau.
II. Oligo – Miocene
The Oligo – Miocene volcanic rocks are the Dessie basalt, Ancharo rhyolite and Tarmaber –
Megezez formation.
The Dessie basalt mostly found in contact with the Ashangie basalt see (Fig 4.1). It forms a chain
of ridge and the contact with the underlying Ashangie basalt is marked by about 50 cm thick
reddish brown paleo soil as seen along Dessie - Kombolcha and Dessie-Hayk road .(Tesfaye et
al., 2011). It is characterized by dark gray to greenish gray, fine to medium grained comprising
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Chapter four Geological set up of the study area
alternating layer of vesicular olivine plagioclase phyric basalt, agglomerated, pyroclastic rocks
and aphanetic basalt.
Fig 4.1 Geological map of Dessie sheet map EIGS (2011, unpublished).
The Ancharo rhyolitic Ignimbrite exposed along Combolcha – Bati Road at the quarry site. And
the Tarmaber- Megezez formation mostly found in contact with Dessie basalt in the north – west
and south- west part of Dessie map sheet. See (fig 4.1).
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Chapter four Geological set up of the study area
According to pik et al (1998), on the north western Ethiopian plateau, Oligocene basalts are
overlain by less voluminous Miocene lavas, erupted from large central vent volcanoes. In
contrast, in southern Ethiopia, Eocene – Oligocene basalts are rare and the volcanic sequence is
dominated by thick Eocene – Miocene acidic lavas and ignimbrites.
III. Late Miocene
The Late Miocene groups are the Kemise Rhyolitic formation and the Delha formation. Kemise
Rhyolite found in the eastern part of the marginal grabenes which is western afar rift margin. It
consists of rhyolite, ignimbrite, tuff and ash. Tesfaye et al (2011). The Delha formation
concentrated near to the escarpment.
IV. Quaternary geological units
The quaternary groups are the Adami basalt, Wederage Basalt, Merto Rhyolite and
undifferentiated alluvial sediments. The alluvial and undifferentiated sediments are accumulated
in the central part of the marginal grabens.
Stratigraphy
Ashange basalt and Wegeltena Rhyolite/ Ignimbrite are the oldest rock unit which is found in
Dessie sheet geological map at a scale of 1: 250,000. The age of these formations can be
recognized as Eocene – Oligocene.
On top of these formations the Dessie basalt, Ancharo Ryolite, and Tarmaber Megezez formation
grouped as Oligo – Miocene formations.
Kemise Rhyolite and Delha formation found on top of oligo – Miocene formations. These
formations grouped at the age of Late Miocene.
The quaternary formations are those like Adami basalt, Wederaje basalt, Merto Rhyolite,
undifferentiated alluvial sediments. The stratigraphy of Dessie sheet map can be seen in the table
4.1.
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Chapter four Geological set up of the study area
Table 4.1Stratigraphy of Dessie sheet map (EIGS 2011).
Age Lithology Description Max.
thickness
(meter)
Quaternary Undifferentaited
alluvial, elluvial &
lacustrine sediments
(Qal)
Black cotton & reddish brown
silty to sandy soil with few
outcrops of diatomite
Merto rhyolite (Qmr) Pink, inclined, coarse grained
and dome forming, inclined,
coarse grained
Wederage basalt
(Qwb)
Dark gray, aphanatic and fine
grained vesicular basalts
Adami basalt (Qab) Greenish gray, coarse grained
plagioclasephyric basalt
Late Miocene Dalha Formation
(Ndb)
Fissural basalt with some
intercalated, Detrial &
lacustrine sediments, with
Rhyolitic flows & ignimbrites at
the upper part
Kemise rhyolite (Tk) Light gray, medium to coarse
grained, intercalated with
ignimbrite layer
Oligo-Miocene Tarmaber-Megezez
Formation (Ttb)
Dark gray, coarse grained,
vesicular, plain and dome
forming pyroxene-
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Chapter four Geological set up of the study area
plagioclasephyric basalt 1000m
Ancharo rhyolite (Tar) Gray and pink, medium grained,
jointed with thin flow banding
Dessie basalt
Formation
Dark gray to greenish grey, fine
to medium grained, comprising
alternate layers of vesicular
alternate layers of vesicular
olivine-plagioclasephyric
basalts, agglomerates,
pyroclastic rocks and aphanatic
basalts
1500m
Eoce-Oligocene Wegel Tena rhyolite /
ignimbrite (Twtr)
Brownish red, pink,
intermediate to white, light to
deep gray, vesicular,
horizontally layered exhibit
glassy appearance with well
developed columnar jointing
500m
Ashangie basalt (Tab) Deeply weathered, greenish
gray, dominated by columnarly
jointed aphanatic basalt,
intercalated with different layers
of vesicular basalt,
volcanoclastic sediment and
agglomerates
1000m
Local Geology
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Chapter four Geological set up of the study area
The geology of the study area has been prepared from the Regional geological map of Mersa
area which is done by EIGS (1998) at a scale of 1:50,000 and the geological map of Wurgessa
area has been done from geological map of Wuchalle – Woldiya area which is done by Amhara
national regional state water works design enterprise (2010) at a scale of 1; 50,000, with some
addition of structural data from field works. The geology of Mersa and wurgessa study area
mainly composed of basic volcanic rocks and associated sediments.
Litho logical units which are found in the study area are porphiritic basalt, Aphanetic basalt,
scoracious basalt and quaternary sediments.
Porphiritic basalt
Porphiritic basalt is the main outcrop of the study area. Microscopically in this unit Pyroxene and
olivine grains are seen as phenocrysts over plagioclase, pyroxene and iron oxide dominated
ground mass. According to Muhaba Jemal and Bisrat Hagos (1998), thin section examination of
this basalt revealed that 35% olivine, 18% plagioclase, 12% opaque minerals and 3% glass
mineral assemblage. Several flows have formed the study area among which pyroxene and
olivine basaltic flows are assumed to be the main one.
The porphiritic basalt in the study area is highly weathered and disintegrated and intruded by
aphanetic basaltic dikes. See (fig 4.2) Spheroidal weathering has affected the rock mass of this
unit.
Aphanetic basalt
Aphanetic basalts are exposed in Mersa study area at the elevated picks of Seblo ridge.
According to Muhaba jemal and Bisrat Hagos (1998) Lava flows, dikes and sills are mode of
occurrences of this unit. Collumnary jointed aphanetic basalts are exposed at Golo River and in
the south-western part of wurgessa study area and at the top of seblo ridge in Mersa.
Quaternery Sediment
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Chapter four Geological set up of the study area
Alluvial and colluvial unconsolidated sediments are deposited in the low-lying areas of the study
area. Very thick alluvial deposits are found in Mersa study area in both sides of Birbissa River
however in the sloppy part of the mountain in both of the study area residual soils and colluvial
material with thickness less than 5m are deposited.
Fig 4.2 Geological map of the study area.
Tectonics
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Chapter four Geological set up of the study area
The study area belongs to the western margin of Afar Rift. Hence, it is affected by the tectonics
of the Ethiopian rift to a great extent. The tensional movements from the afar plumes, (see fig
4.3) which give rise the fissure volcanism, create a local stress in the surrounding area. The
effect of extensive tectonics in the study area can be prevailed by the occurrence of block
faulting, formation of marginal grabens and tilting of blocks which can be seen in the crossection
of fig (4.4), drainage pattern of the streams, the presence of dikes, folds and faults.
According to Beyene and Abduselam, (2005), the western margins of the afar depression, which
includes the study area, separate the Afar depression from the Ethiopian plateau. It is
characterized by N-S trending margional basins, hilly terrain of faulted blocks forming western
margin of the Afar depression. These margional basins are developed at the foot of the Ethiopian
escarpment.
Fig 4.3 The afar plume by Beyene and Abduselam. Fig 4.4 Faulted blocks by Beyene and Abduselam (2005)
Geological Structure
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Chapter four Geological set up of the study area
Since the study area is located at great proximity to the Afar depression which is the most active
zone, the area is highly affected by tectonics. This tectonics event has given rise to the
development of different geological structures in the study area. Some of the structures which
are found in the study area are faults, folds, Dikes, fractures, bedding plane and, joints.
Faults
Faults are geologically where weakness and disintegration of rocks prevailed. In the study area
the presence of faults can be recognized by the drainage pattern and displacement of bed. The
streams in the study area aligned in straight line deviate from the general stream direction
sharply. Some of the major faults are parallel to the major escarpment however some faults are
having a strike direction of west to east. In the study area at locality of Wurgessa the rock mass
has been displaced by 35cm.
Folds
At the locality of Wurgessa on the left bank of Golo stream at geographic coordinates the rock
mass has been folded to a great extent. This may be due to the basaltic dikes intruded in the area.
See fig 4.5.
.
Fold
Fig 4.4 Faults and fracture in Mersa Fig 4.5 fold at the left bank of Golo stream Wurgessa.
Dikes
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Chapter four Geological set up of the study area
Basaltic dikes with strike orientation of NW-SE exposed near seblo ridge and at the crossing of
Birbissa stream of Mersa area and near to Gatira Giorgis of Wurgessa area. See (fig 4.2).These
dikes are aphanetic basalt with thickness less than 1 to 2 meter. These are weak zones which
favors passage of basaltic lavas through easily
Bedding plane
Joints and fractures
Joints are geological structures which are formed as a result of expansion due to cooling, or relief
of pressure as overlying rocks are removed by erosion. If joints and fractures are parallel to the
slope they may serve as a plane of sliding surface. Characteristics of discontinuities like joint
orientation, opening, continuity, nature of filling material and degree of weathering affect slope
instability. Characteristics of structural discontinuities for different facets of the study area have
been presented in appendix.
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