CHAPTER FOUR GEOLOGY-1

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Geological map of Dessie Sheet map, EIGS 2011 (unpublished), classified the general geology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.

Landslide studies Page 1


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



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).



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.



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-



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



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



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



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



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



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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CHAPTER FOUR GEOLOGY-1