The substratum of the Senegalese territory is made up of two major geological domains: the Sedimentary Basin, which occupies more than 75% of the territory, and the Precambrian basement, representing the country’s south east.
The Senegal Basin occupies the central part of the Northwest African Coastal Basin (MSGBC Basin), which extends from the Reguibat ridge at the north end of the Guinean fault. It is typical passive margin opening westward to the Atlantic Ocean and whose eastern limit is represented by the Mauritanides chains. The Senegal Sedimentary Basin is a Mesozoic Basin.It has gone through a complex history in relation to the pre-rift (Upper Proterozoic to Paleozoic), the Syn-rift (Permian to Triassic) and the Post-rift (Central Jurassic to Holocene) at different stages of development of the Basin. Most of the outcrops of the basin are composed of recent sandy covers. Maestrichian and Eocene formations outcrop, however, in the peninsula of Cape Verde while Eocene outcrop in the valley of Senegal River. The description and knowledge of the Basin have been made possible largely thanks to hydraulic and oil drilling data. The Secondary formations include Palaeocene zoogenic limestone exploited at Bandia and Pout by cement plants and aggregates producers.
They include also Maestrichian sands, clays and sandstones. Paleocene and Maestrichian formations are also known to be major aquifers that contribute significantly to the water supply for cities and villages in the basin. Tertiary formations hold into the Eocene compartment, significant resources of phosphates, limestone, attapulgite, clay and ceramics, solid fuels, etc.
A major part of the basin is covered with superficial Quaternary formations, which in the middle and recent parts are characterised by fixed red sand dunes, semi-fixed or alive yellow and white dunes. These dunes, often exploited as building materials around urban centres, constitute also important reservoirs of heavy minerals.
The Precambrian basement formations are constituted at the west by the Mauritanides range bordering the eastern part of the Sedimentary Basin and in the east by the Palaeoproterozoic volcano-sedimentary sequences of the Kedougou- Kenieba inlier.
The formations of the Mauritanides chain are Herycian age and constitute one of the mobile areas of the West African craton. They are known for their numerous copper and chromium occurrences which, in Mauritania, constitute the important copper deposits of the Akjout Region. The Palaeoproterozoic volcano-sedimentary sequences, mostly known as Birimian formations, are of great metallogenic importance, as far as they contain the major ore deposits discovered in the region. The Kedougou-Kenieba inlier is limited to the west by the Mauritanides chain, and on all other sides by the Upper Proterozoic and Cambrian sediments of the Basin of Taoudenni. The Kedougou-Kenieba inlier is interpreted as an accretion of north-easterly trending Birimian age volcanic terrains. It includes two major geological structures, the Senegalomalian Fault and the Main Transcurrent Zone (MTZ) to which gold mineralisation is associated. Recent combination of geological studies including field work, and structural modelling, and of detailed core logging have improved the understanding of the geological structure of the MTZ. Two main zones of mineralisation have been further refined based on the latest geological model. Geological studies suggest that mineralisation in the prospective Sabodala volcano sedimentary belt and the Senegal-Malian shear zone is associated with an altered and sulphidised gabbro, which has intruded along the main structure, and a typical shear zone, hosted, where a structure has developed at the contact between a package of volcaniclastics and sediments. A lapilli tuff acts as a prominent marker horizon in the hanging wall of mineralisation.
The inlier is divided into three main stratigraphic units from west to east: the Mako Supergroup, the Diale Supergroup and the Daléma Supergroup.
- The Mako supergroup hosts Sabodala deposits located in an area of intense shearing and silicification associated with pyrite gold mineralisa- tion. It forms a north-east tectonic structure, turning to north-west near the border with Mali, in the north. Typical lithologies include basalt flows; often carbonate alterations and minor volcaniclastic intercalations, magnesium basalt or komatiites, ultramafic sub-volcanic intrusions (pyroxenites) and numerous massive biotite and amphibole granitoids. These granitoid intrusions are suspected to have been ‘heat engines’ which sparked off the deep mineralised magmatic fluids related to the belated mineralisation in the Kédougou-Kéniéba inlier.
- The Diale Supergroup, located between the Mako Supergroup and the western edge of the Saraya granite is weakly metamorphic. It includes extensively folded formations, deposited after those of the Mako Supergroup and consisting of shale, greywacke, quartzite and volcanodétritic rocks.
- The Dalema Supergroup, located between the Saraya granite and the Faleme river, continues to Mali in its eastern part but disappears in the South under the Segou Madina Kouta the series. It is composed of volcano-sedimentary schist and grauwacke rocks.
These Birimian formations are affected by syn, late and post-tectonic granite intrusions. The Precambrian basement is a metallogenic province of major importance for Senegal, which hosts numerous deposits and anomalies of gold, iron, uranium, lithium, tin,molybdenum and nickel in Birimian formations, and copper and chromium in the Mauritanides range. In addition to these metal resources, there are large marble and other ornamental rocks deposits, but also non metallic indices and deposits of barytes, kaolin,asbestos etc.
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