2 Previous studies
Coal has been known to occur in Somaliland since it was first reported by Farquharson (1924) who described the analyses of coal samples collected from Hed-Hed in 1915. During British colonial occupation, the geological investigation of the country increased, and as a result, several more coal and lignite deposits have been discovered (e.g., MacFadyen, 1933; Hunt, 1951; Pallister, 1959; Gellatly, 1960; Hunt, 1960).
The few studies that have been carried out since independence include a United Nations Development Programme (UNDP) investigation in 1980 of the Tertiary coal and lignite occurrences in the Daban Basin, southeast of Berbera (Peeters, 1980). Further UNDP coal survey has been carried out in 1980/1981 to investigate Cretaceous outcrops north of Erigavo (Aden and Peeters, 1981). In 1983, the Bundesanstalt Fur Geowissenschaften und Rohstoffe (BGR) had carried out feasibility studies of the coal deposits of northern Somalia as part of technical cooperation between the governments of Germany and Somalia. The fieldwork in Somalia was conducted by the consulting company of Grundstofftechnik (1983) on behalf of BGR. Grundstofftechnik (1983) conducted detailed geological and geochemical investigations on the Hed-Hed coal site. Five boreholes were drilled with a maximum depth of 27 meters and trenches were excavated. Geologic investigations revealed the existence of four main coal units, designated as A, B, C, and D. These units were traced over a strike length of 360 meters but probably extend over the entire Hodmo Basin, which occupies an area of 60–80 km2.
3 Stratigraphy and depositional setting
3.1 Jurassic sequences
The Jurassic sequences of Somaliland have been investigated by several authors and a brief summary of Jurassic sedimentary rocks is given here. Detailed descriptions of Jurassic deposits can be found in MacFadyen (1933), SOEC (1954), Luger et al. (1990) and Bosellini (1992).
Jurassic sedimentary rocks of Somaliland consist of a thick sequence of conglomeratic continental deposits (basal Adigrat Sandstone) resting directly on the peneplain basement rocks and overlain by a succession of limestones (Bihendula Group), generally with some marl and shale intervals. Jurassic sediments were deposited in NW-SE trending grabens (e.g., Bihendula graben) that were formed as a result of tensional forces associated with the rifting of Africa from Madagascar-India-Seychelles (Bosellini, 1992; Bott et al., 1992). However, the Jurassic sequence is absent in the areas east of Hed-Hed and northeast of Erigavo due to either its non-deposition or Early Cretaceous erosion.
Jurassic strata outcrop in the following areas: the Borama-Zeila area of western Somaliland; the Bihendula area of north-central Somaliland; and Al Mado region of northeastern Somaliland. However, the Jurassic outcrop at Bihendula, 35 kilometers south of Berbera, was the first to be recognized in the country and has since been the most extensively studied (e.g., MacFadyen, 1933; Luger et al., 1990; Bosellini, 1992). It is where the greatest thickness (more than 1,200 meters) of fossiliferous marine Jurassic beds is exposed in the country. The Adigrat Sandstone rests unconformably on Precambrian rocks and consists predominantly of medium to coarse-grained quartz sandstone (Figure 4). The unit is transgressive from a lower fluvial facies to an upper littoral marine facies (Luger et al., 1990). The Adigrat Sandstone is overlain by marine sediments (Bihendula Group) containing various limestone and shale sequences of Middle and Upper Jurassic (MacFadyen, 1933; Bosellini, 1992).
3.2 Cretaceous sequences
A major crustal uplift and block faulting has affected the area prior to the Cretaceous sedimentation. This has been related to stress system imposed on eastern Africa due to the northward rifting of the adjacent Indian plate (Bosellini, 1986). As a result, the Cretaceous rocks of Somaliland unconformably overlie Jurassic or Precambrian rocks. Neocomian-Barremian interval is generally absent and Aptian sedimentation is characterized by lateral lithologic variability resulting from transgressing seas from the east interfingering with coarser terrigenous sands derived from the west.
As a consequence, thick carbonate and shale sections were deposited in the eastern part of the basin and equally thick sequences of sandstones were deposited in the west. However, the Aptian transgression did not cover the entire country. For example, the Las Anod arch reminded exposed and transgressed by the sea during the Turonian (Bosellini, 1992).
The name ‘Nubian sandstone’ has been applied to describe the entire Cretaceous clastic sequences seen in outcrop (e.g., MacFadyen, 1933). However, the Cretaceous sequence has been divided into continental fluvial of Yesomma (also spelt Jesomma) Sandstone and marine Tisje carbonates (e.g., Bosellini, 1992). It is difficult to distinguish the different formations due to the transitional nature of the interfingering clastic and carbonate facies, although the area east of Erigavo is dominated by Tisje Formation. The best exposed section of Yesomma Sandstone in the country is at Bihendula with a total thickness of 1,708 meters (MacFadyen, 1933). The succession consists of highly weathered course grain quartz-rich sandstones.
The contact with the underlying Upper Jurassic Gawan Limestone is unconformable and an angular unconformity has been documented at the base of the Yesomma succession (MacFadyen, 1933). In the Dagah Shabel area (south of Daban Basin), the base of the succession consist of a shaly facies, the Shabel beds, of about 300 meters thick suggesting a possible coastal plain or deltaic environment (SOEC, 1954).
3.3 Tertiary sequences
Tertiary rocks lie conformably over the Cretaceous formation and form most of the surface exposures of the country. The Tertiary sedimentary section can be divided into two parts (Figure 2).
3.3.1 Eocene sequences
Eocene deposits of Somaliland consist of massive limestones of Auradu and Karkar Formations separated by massive to banded gypsum and anhydrites of the Taleh Formation. This is the result of Eocene seas transgressing from the east depositing Auradu limestones. Periodic regression in the Middle Eocene resulted in evaporatic conditions, which led to the deposition of the Taleh Formation (Anhydrite Series). Further transgression in the Upper Eocene caused the deposition of marine cherty limestone of the Karkar Formation (SOEC, 1954).
3.3.2 Oligocene-Miocene rift sequences
Oligocene and Miocene sediments are mostly restricted to narrow and isolated sub-basins along the coastal belt bordering the Gulf of Aden, occasionally extending inland in low-lying regions. They were deposited in localized grabens caused by the rifting of the Gulf of Aden and consist of a thick (up to 2,700 meters) syn-rift sequence of red-brown, green sand, siltstone and gypsiferous sandstone (Bosellini, 1992; Abbate et al., 1993a). These sedimentary rocks are almost entirely terrigenous and were deposited in lagoon, delta, and alluvial environments (MacFadyen, 1933; Sagri et al., 1989; Abbate et al., 1993a). The best outcrops of Oligocene-Miocene sediments occur in the Daban Basin (southeast of Berbera) bordering the Somaliland plateau (Figure 2). The sediments were deposited in fast subsiding faulted rotated blocks during rifting and opening of the Gulf of Aden (Bosellini, 1992; Abbate et al., 1993a).
The basin is bounded to the south by the Dagah Shabel fault and to the north and east by Eocene strata of the Taleh Formation in which the Daban Group rests conformably. The Daban sequence has an east-west strike and dip gently to the south. However, close to the Dagah Shabel fault, a local syncline is recognized (Bruni et al., 1987). The sequence is overlain unconformably by alluvial conglomerates (boulder beds) of Pliocene age which is related to the uplift of the Somaliland plateau (MacFadyen, 1933; Abbate et al., 1993a).
Figure 3 General stratigraphic column of Somaliland showing stratigraphic occurrences of lignite and coal deposits (see online version for colors)
[su_button url=”https://saxafimedia.com/geology-coal-potential-somaliland/4/” style=”soft” size=”12″ wide=”yes” center=”yes” text_shadow=”0px 0px 0px #FFFFFF” rel=”lightbox”]CONTINUE READING ON THE NEXT PAGE >[/su_button]
