Sedimentary Rocks

Sedimentary rocks are formed from sediment accumulated over long periods, usually under water. They are distinguished from the other rock types by their characteristic layer formation and are termed stratified rocks. The strata may vary in thickness from a few centimetres to many metres. The rocks may be coarse or fine-grained, soft or hard. The materials that form sedimentary rocks may be brought by streams, glaciers, winds or even animals. They are non-crystalline and often contain fossils of animals, plants and other micro-organisms. Sedimentary rocks are thus the most varied in their formation of all rocks. Sedimentary rocks are classified according to their age and different kinds of rocks formed during the same period are grouped together. It is
more useful to know the characteristics of the various kinds of rocks.
Sedimentary rocks may be classified under three major categories in accordance with their origin and composition.
1.     Mechanically Formed Sedimentary Rocks. These rocks have been formed from the accumulation of materials derived from other rocks which have been cemented together. In West Africa, examples include the continental sedimentary rocks formed from the sand deposits in the arid regions of north-western Nigeria, south-western Niger Republic, Mali and Senegambia. Sandstones are probably the most familiar sedimentary rocks. They are made from sand grains, often  quartz fragments derived from granites. Their texture, composition and colour vary tremendously. Sandstones are widespread in West Africa, but their main areas of occurrence include the rolling terrain of the Western Plains of Senegal where they are associated with continental and lacustrine deposits, and the varied sandstone mountains of Mali underlaid by schists and gneisses. The entire Niger-Benue trough is underlain by Cretaceous sandstones. Sandstones are also important among the Lake Chad, Sokoto basin and coastal formations of Nigeria. Many types of sandstones have been quarried for building purposes or for making grindstones. A coarser type of sandstone is known as grit. When larger pebbles are firmly cemented to form a rock it is called conglomerate when the pebbles are rounded, or breccia when the fragments are angular. The finer sedimentary materials form clay, widely used for brick making, shale or mudstone, Sand and gravel may occur in uncemented form.

2.     Organically Formed Sedimentary Rocks. These rocks are formed from the remains of living organisms such as corals or shellfish, whose fleshy parts have been decomposed, leaving behind the hard shells. The most common rocks formed in this way are of the calcareous type. They include limestones and chalk. Nigeria is the only country in Africa where substantial amount of limestone have been found, and a number of cement works have been set up based on this mineral. Some limestone also occurs in Senegal, east of the Cape Verde Peninsula, in Mali in the Timetrine-Terrecht region north of Gao and west of Adrar des Iforas, in the plains of Talak (Niger) where it is covered by desert sand, and in the south-western plains of Ghana.
The carbonaceous rocks are also organically formed but from vegetative matter - swamps and forests. The pressure of overlying sediments has compressed the plant remains into compact masses of carbon which eventually become peat, lignite or coal, all of which bear great economic value. Africa is generally poor in coal. The only coal mines in West Africa, for instance, are Enugu and Okaba in Nigeria. They belong to the cretaceous age, are sub-bituminous; and, therefore, are of relatively poor economic value. There are beds of lignite in the southern sedimentary rocks north-west of Asaba on the Niger. Other coal deposits in Africa occur in the Republic of South Africa and in Wankie, Zimbabwe.

3.     Chemically Formed Sedimentary Rocks. Such rocks are precipitated chemically from solutions of one kind or another. Rock salts are derived from strata which once formed the beds of seas or lakes. Gypsum or calcium sulphate is obtained from the evaporation of salt lakes, such as the Dead Sea, which have a very high salinity. In similar ways, potash and nitrates may be formed.