3. JURASSIC LIMESTONE SERIES
Rocks of Jurassic age attain a considerable development both in British Somaliland and in Abyssinia, and there is some reason for believing that these rocks are part of a landmass which once extended eastwards and formed part of the ancient continent of Gondwanaland.[7] In Somaliland, the chief occurrence of the series is in the Gadabursi country, in the west. There it forms the majority of the ridges and scarps, Libaheli Range, Bur Ad Range, Karimo Range, Kara Mut Ridge, Balaat Ridge, Aga Sur Range, Eilo Range, etc. The scarps of the ranges all trend in a N.W.—S.E. direction, and, north of the gneissic Wobleh Range, they follow one another like successive waves of the sea, all parallel and at intervals in some cases of a few hundred yards, in others of two or three miles, the intervening spaces being generally occupied by gneiss. They are all fault-blocks produced by large strike faults. In nearly every scarp, the limestone may be seen resting on gneiss, hornblendic or granitic, directly or with an intervening sandstone bed. The dip of the limestone is to the south-west, at an angle of from 25°-30°, and the dip is extremely regular in all the ranges. Smaller limestone scarps, but apparently with lower angles of dip, exist south-east and south of Borama, and there is evidence that the limestone mass once covered a much larger area than it does at present, for the pebble-sandstone bed which marks the bottom of the series in the scarps occurs over a fairly wide area in the vicinity of Besare, which is now formed chiefly of gneisses, foliated porphyries, and red granites.
Eastwards of the Gadabursi district, outcrops of the Jurassic series become less and less common. Only a few are to be found in the Central district, at Bihendula, at Ida Kabeita, at Bihen Gaha, at Ambal and the ridge of Tar between Ambal and Karam, in a ridge just behind Karam, and in the pass through the Dubriat Range from Bosti to Berbera, on the side of the track. East of a line from Ambal to Karam, no beds of this age have ever been recognized. Moreover, as far as known, no Jurassic outcrop exists in that; part of the Protectorate south of the main watershed.
In Somaliland the outcrop which probably best shows the constitution of the series is at Bihendula, about 25 miles south of Berbera. Fossils from this locality were examined by Prof. J. W. Gregory in 1896, and, from the determinations, the age of the rocks was held to be Bathonian.[8] Collections, however, made by the present writer not only from Bihendula, but from several localities in Western Somaliland, have recently been examined at the British Museum of Natural History, and the result of the determinations, particularly of various Ammonites by Dr. Spath, shows clearly that a large part, if not the whole, of the series is of Kimmeridgian and not of Bathonian age. A full account of the significance of the fossil evidence will be given later in a separate paper on the geology of the Protectorate. As stated by Drs. Wyllie and Smellie, who made a careful study of the Bihendula outcrop, and confirmed by the writer except as regards the thickness of the strata which was not measured, the total thickness of strata exposed is 3,840 feet, made up of the following members, beginning with the oldest:
Top.
Bottom.
A characteristic feature of nearly all the limestone is the presence either of dark grey, nearly black bands or of almost black patches in it. These dark-colored portions give a foetid odour on the fresh fracture. In places in the series, as at Bihen Gaha, are bands of black argillaceous and calcareous rock, which probably contain kerogenous material.
In the west, the succession of the beds is not quite the same as in Bihendula Range. The maximum thickness, as measured by Wyllie and Smellie, is about 3,000 feet. No lavas occur at the base. On the other hand, in Bur Ad Range and in a hill south-east of Hemal, the shales, which are, with the exception of the sandstone, the lowest beds exposed, are intruded and dislocated by a basaltic dolerite dyke. The basal sandstones are only from 10 to 50 feet thick and at the bottom have a pebble bed of red, white, and yellow quartz pebbles about the size of a walnut. The remainder of the series consists of a lower and an upper limestone, composed of bands of compact limestone from 1-3 feet thick, with an intervening zone of shale and mostly yellowish mudstone. In the extreme west, north of Somadu and at least as far as Hensa, there is a great thickness of sandstones which have been hitherto mapped as basaltic lava. Whether these sandstones are a continuation of the basal Jurassic sandstone is not yet known, but it is noteworthy that a little to the south-east of Somadu are two hills of limestone, presumably overlying the sandstone, and, though no fossils were found in this limestone, owing to its proximity to the main Jurassic outcrop at Debraweina, it is at least possible that it is of this age.
The compact limestone bands of the Eilo Range form a good lithographic stone. The shale zone, especially near Meragelleh, contains thin nodular beds, the nodules, of which some are afoot in diameter, containing Ammonites and fossil wood.
At Meragelleh, and both on the side of the Durdur Ad tug at Gardeleh and in the bed of the tug itself, the shale zone contains beds of undoubted kerogenous shale (see below). The shale in a hill on the side of the tug, a little south of the junction of the Durdur Ad with the Gardeleh tug, consists of thin, brownish-black laminae, with selenite films along the joint planes and between the laminae. In the Durdur Ad tug bed, the shale forms a compact brownish-black very fissile mass, with numerous shell impressions on the surface of the laminae. Tests of the shale (the results of which are given below) show it to be undoubtedly kerogenous.
South-east of Bihendula, at Ida Kabeita, is a small outlier of Jurassic limestone faulted against rocks of the Crystalline Series, containing a yellowish calcareous mudstone with shells of Rhynchonella.
At Bihen Gaha, north of Jirba Range, the series is represented by an upper foetid and flaggy and a lower more compact limestone with a brownish-black shale zone between. The shale zone contains bands of dead-black argillaceous and calcareous rock, and numerous belemnites, fossil wood, etc.
At Ambal, the Jurassic beds are exposed in a ridge which trends north and south. The ridge is flanked on the east side by Dubar Sandstone. The series consists of black kerogenous shales 200-300 feet thick, overlain by sandy yellow shales 50 feet thick, and these are in turn overlain by bands of foetid limestone interbedded with more or less marly grey shales. The lower beds of the Bihendula outcrop are absent. The strata of which the hill is composed dip steeply to the north-east, less steeply to the north and south, and as the west side is covered by blown sand, the hill presents the appearance of a dome. As the ridge of Tar to the north, however, and the other outcrops of Jurassic rock as far north as Karam are faulted on the west side, it is fairly certain that Ambal Hill is similarly faulted, and that it is not a true dome. The shale zone contains numerous large calcareous nodules which, as at Meragelleh, contain Ammonites and fossil wood.
The ridge of Tar, north of Ambal, shows the same sequence of beds and the same structure except that it is clearly faulted on the west side, which is not masked by blown sand. As Jurassic rocks are known to occur in great thickness in the Harrar Plateau in Abyssinia, it is evident that the main Jurassic sea existed in Abyssinia and Western Somaliland, and that only an arm extended eastwards into the middle of the Protectorate, an arm which, to judge from the presence of gypsum in some of the Bihendula shales, consisted in part of isolated pools, and which did not extend beyond a line from Karam southwards to Ambal.
4. DUBAR SANDSTONE
Overlying the Jurassic limestone, particularly in the central part of the “Guban” country, and in a notable outcrop on the south-western slopes of the Eilo Range and west of Kabri Bahr in the Gadabursi district, is a succession of fairly fine-grained, predominantly red, brown and yellow sandstones commonly without associated clays, slates or any argillaceous material. These sandstones form a noticeable part of most of the limestone ridges of the Maritime Mountains and Inland Plain and derive their name from the Dubar ridges on which is their nearest outcrop to Berbera. For purposes of description, they maybe considered under two heads: (a) the sandstones of the “Guban” country, (b) those of the main scarp or main watershed:
(a) These, in addition to the outcrops in the west just mentioned, occur in the scarps of Al Wein Range, of Ferrio Range and of isolated hills south-east of Berbera, on the flat south of Bihendula Range, and in great extent and thickness in the extremely dissected country south of Karam and also south of Ankor, and between it and Las Dureh. They appear to be quite unfossiliferous, but as they lie conformably on the Jurassic in Eilo Range and elsewhere, and as they, also, in many places in the “ Guban,” clearly underlie the Eocene limestone, they may reasonably be regarded as of Cretaceous age. Their relation to the Eocene limestone appears to be comformable in some places and uncomformable in others. The strike and dip of the beds are not markedly different from those of the overlying strata in any one outcrop, though they differ considerably in different fault blocks. The sandstones exhibit current-bedding and contain in many places numerous large and small mammillary and botryoidal concretions, some quartzitic, others calcareous. Exceptionally, as in the vicinity of Biyo Dader, they contain thin bands of red mudstone, and on the track between Biyo Gulan and Biyo Dader, some thin bands of yellow shale appear. In the Ambal tug, and in parts of Jirba Range, the sandstones are of a deep-green color. According to Wyllie and Smellie, the beds attain a thickness of at least 5,000 feet in the vicinity of Biyo Dader, though elsewhere in the coastal districts, the thickness appears to be from 2,000-3,000 feet.
(b) The main limestone scarp from the Golis Range eastwards almost to the Italian frontier, consists of a thick cap of Eocene limestone underlain generally by a great thickness of sandstones. The latter are best seen in the scarp at Darass, some twenty miles west of Sheikh. They comprise alternating beds of soft white, grey, yellow, brown, red, purple, and almost black sandstone, all exhibiting marked false bedding, and, so far as could be ascertained, having a slight dip (about 5°-10°) to the south-south-east. Near Gan Libah, at the top of the series, is a purplish red jasperoid slaty stratum, a few inches thick, containing numerous pyritic nodules altered to limonite. The Jurassic Series which underlies the “ Guban ” sandstones is absent in the main scarp, and the beds rest directly on the granitic gneiss. These are the sandstones which previous writers on the geology of Somaliland have referred to as “red unfossiliferous sandstones of unknown age.” They are undoubtedly unfossiliferous, but there is little doubt that, underlying the Eocene limestone as they do, they are the counterpart of the Guban sandstone and of Cretaceous age. In the Golis Range they are about 900 feet thick. Westward from this range the beds occur at Hargeisa and on the track between Hargeisa and Gebilay, where they resemble the Eilo Range sandstones. Near Abudleh, some miles west of Hargeisa, they are in places indurated to quartzite. From the nature of the sandstones— the prevalence in them of current-bedding, the great variety of color, and the total absence of traces of fossils — they would appear to have been laid down under terrestrial conditions.
5. EOCENE LIMESTONE SERIES
With the exception of the few ridges already stated to be of Jurassic age, and a few small limestone hills belonging to the Coastal Limestone Series, to be described later, all the limestone ranges of the Guban from north of Hargeisa eastwards, and the main scarp extending from Gan Libah in the Golis Range eastwards to the Italian frontier, consist of limestone which, as the result of the examination of various fossils by Dr. R. B. Newton in 1905, is definitely of Eocene age. Neither the ranges nor the main scarp, of course, consist wholly of limestone, the latter being generally underlain by Dubar sandstone and overlain in places — particularly in Al Wein and Ferrio Range — by gypsum beds of greater or less thickness, but in all cases, the most noticeable feature of the face of the scarps themselves is this limestone. Especially is this the case in the eastern districts, from Galgal to southeast of Las Gori, where the lofty scarps of Siradli, Dud, Surad Ad, the Afaf, Aroru and Al Hills, consist of a great thickness of massive limestone conspicuous for its reddish color, due to surface stain.
The maximum thickness of the Eocene limestone observed by Wyllie and Smellie in the area examined by them was about 800 feet, but in the eastern districts, from Galgal to behind Las Gori, the thickness is much greater, probably 2,000 feet or more.
The series consists of (a) a lower massive limestone, (b) an upper whitish, chalky, more porous, more thinly bedded limestone associated with which are very thin, yellowish clay shales, in places gypseous. There may be a sandstone stratum under the massive limestone, for, underneath the coal seam at Hedhed in the plateau south of Ankor, which is at the bottom of the Eocene limestone, is a bed of soft, whitish sandstones. At present, it is doubtful whether these are the bottom of the Eocene Series or the top of the Dubar Sandstone.
The massive limestone is mostly hard, yellowish in color, with a surface grey, white or bright red, owing to iron staining. It is fairly fossiliferous, containing corals, gasteropods, lamellibranchs and echinoids, but most of the fossils are more or less silicified, and stained reddish. In the rock also are numerous small patches of iron-stained flint, some of which also contain fossils or fossil impressions.
The upper limestone is best seen in the eastern districts, on the top of the Afaf, Aroru and Al Hills. It is more porous, lighter in color, and more chalky than the massive, and contains more fossils — echinoids and various shells, amongst which, in the Laliskwe Plateau, west of Hargeisa, are some very large gasteropods. Smaller gasteropods shells are also very common on the plateau north of El Afweina. A fairly typical section of this upper limestone division is exposed at Garab Garreh Hill, south of the main scarp of the Afaf Hills. The outcrop consists of yellow gypseous clay-shales about 30 feet thick with intercalated slaty and chalky limestone hands at the bottom of the hill, succeeded by a stratum of eroded grey chalky limestone, the latter containing similar fossils to those in the chalky bands of the shales. The total thickness of beds exposed in the hill is about 300 feet, and the strata dip only very slightly to the south.
There appears to be a gradual passage of this limestone series with its yellow shales into the Gypsum Series which overlies the Eocene beds, which indicates a gradual drying up of the Eocene sea, and a gradual formation of saltwater lagoons or inland seas. The thickness of the upper limestone is about the same as that of the lower in the “Guban” country, about 400-500 feet, but in the east, the massive limestone is very much thicker. In the west, the Eocene Series first makes its appearance some miles west of Hargeisa, where the upper limestone forms the Laliskwe Plateau. It gradually becomes thicker eastwards and reaches a maximum thickness in the plateau south of Hais. Just as, therefore, the Jurassic limestone attains its greatest extent in the west and disappears gradually eastwards until none at all is found east of Ambal, so the Eocene limestone reaches its greatest extent in thickness in the east and gradually thins out and diminishes in extent westwards until none occurs west of Gebilay, between Hargeisa and Borama. The main Jurassic sea was in the western half of the Protectorate, the main Eocene sea in the eastern half.
The actual horizons to which the massive and upper limestones of the Series belong cannot be stated until the completion of the examination of the numerous fossils collected during the tour.
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