Oolitic Terrain – Secondary Lands of Tigrinyaland and Semien

This is the eleventh part of the geological description of Tigrinyaland¹ and Semien,² which has been adapted from Messrs. Ferret and Galinier’s work published in 1847.³ In this installment, the secondary lands of Tigrinyaland and Semien composed of oolitic terrains will be reviewed.

Under Oolitic land, we have comprehended the lias, the whole series of layers it supports up to the Cretaceous terrains, and the rocks of igneous origin that belong to this geological period.

We saw oolitic terrain in the vicinity of Cheleqot and Hintalo. But it extends east-southeast in the direction of Shoa, so that it may have a large development towards that region.

The deposits we observed belong mainly to the Liassic and lower layers of the Lower Oolitic Limestone. These deposits include limestone, marl, siliceous limestone and sandstone. We found, in these terrains, particularly in the limestone and marl, a large number of fossil shells which have a clearly Liassic or Oolitic physiognomy. However, in general, the samples we brought are not sufficiently characterized to be determined specifically. In order to give an exact idea of the oolitic terrains that we observed, we will describe the section from Dirba to Samré, via Cheleqot and Hintalo.

On the Dirba plateau, the Tertiary terrain is found in more or less horizontal layers, resting on talcschist, which can be seen above all at the bottom of the Mai-Chingua ravine, on the Koen-Cheleqot side. The talcschist, like a part of the Tertiary terrains, is covered, in the bottom of the ravine and on the Dirba side, by a travertine limestone, formed at the expense of the surrounding limestones. From Koen-Cheleqot, we observe the oolitic terrains up to the Mai-Cheleqot ravine, where we find the following section. On both sides of the ravine, i.e. to the north and south, the plateaus are crowned by a grey basalt, which shows through all the layers, including those of the oolitic terrain. Below, and on both sides of the ravine, we observe, as we descend:

1. Sometimes a layer of white or greenish-yellow sintered sandy-clay limestone, sometimes a white sandstone;
2. A layer of yellow shell limestone with isocardia⁴ and cardium;⁵
3. A layer of yellowish-gray clayey limestone with modioli,⁶ with veinlets of spathic limestone;
4. A layer of yellowish-grey jaspoidal clay limestone or marl, intimately bonded at the base with a kind of calcariferous jasper resulting from the extreme modification of the clay limestone under the influence of the basalt at the bottom of the ravine.

Fossils found here include modioli, isocardia, cardium, pleuromeia,⁷ etc. The emergence of the basalt, at the bottom of the ravine, caused the oolitic layers to tilt to the right and left, that is to say that they plunge on one side towards the north and on the other towards the south. At the edge of the basalt, on the southern plateau, the oolitic deposits reappear which pass through Hintalo and which continue to the surroundings of Aragure, where the phyllitic terrain of the country of Samré begins again.

These are the details into which we can enter with regard to the Oolitic land, the land which extends towards the Red Sea and Shoa.

Reference Notes

1. Tigrinyaland was a collective name of the Midri-Bahri (modern-day state of Eritrea) and Tigray (the northernmost region of modern-day Ethiopia). The term employed for Tigrinyaland by Messrs. Ferret and Galinier in their book is “Tigré”, which had been the designation used by the Amhara rulers of Abyssinia to refer both to the Tigrinya people and the Tigrinyaland. ↩︎
2. Semien was historically the frontier province of the Tigrinya with the Amhara. However, since the reign of Emperor Susenyos, the province of Semien had been governed by members of the Amhara royalty and nobility. Following the death of Dejazmatch Sabagadis in 1831, Semien under its Amhara ruler Dejazmatch Wubbe turned from the frontier province of the Tigrinyaland to the power-center of the Tigrinyaland. This continued until the rise of Emperor Tewodros in 1855. ↩︎
3. Ferret, Pierre Victor Adolphe et Galinier, Joseph Germain (1847) Description Géologique du Tigré et du Samen. Voyage en Abyssinie dans les provinces du tigre, du samen et de L’amhara. Tome troisième. Paulin: Paris. ↩︎
4. Isocardia, plural of isocardium, refers to the lamellibranch mollusk, with shells with spirally rolled apices on the side opposite the hinge. ↩︎
5. Cardium, a genus of mussels in the family Cardiidae. ↩︎
6. Modioli, (singular modiolus), horse mussels. ↩︎
7. Pleuromeia, a genus of extinct lycopsid plants from the Triassic Period, and characterized by an unbranched trunk up to 2 metres tall. ↩︎