The geology of

Senegal Senegal, officially the Republic of Senegal, is the westernmost country in West Africa, situated on the Atlantic Ocean coastline. It borders Mauritania to Mauritania–Senegal border, the north, Mali to Mali–Senegal border, the east, Guinea t ...

formed beginning more than two billion years ago. The

Archean The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...

greenschist

Birimian The Birimian rocks are major sources of gold and diamonds that extend through Ghana, Côte d'Ivoire, Guinea, Mali and Burkina Faso. They are named after the Birim River, one of the main tributaries of the Pra River in Ghana and the country's mo ...

rocks common throughout West Africa are the oldest in the country, intruded by

Proterozoic The Proterozoic ( ) is the third of the four geologic eons of Earth's history, spanning the time interval from 2500 to 538.8 Mya, and is the longest eon of Earth's geologic time scale. It is preceded by the Archean and followed by the Phanerozo ...

granites. Basins formed in the interior during the

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

and filled with sedimentary rocks, including

tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

from a glaciation. With the rifting apart of the

supercontinent In geology, a supercontinent is the assembly of most or all of Earth's continent, continental blocks or cratons to form a single large landmass. However, some geologists use a different definition, "a grouping of formerly dispersed continents", ...

Pangaea Pangaea or Pangea ( ) was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous period approximately 335 mi ...

in the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

, the large Senegal Basin filled with thick sequences of marine and terrestrial sediments. Sea levels declined in the

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

forming large phosphate deposits. Senegal is blanketed in thick layers of terrestrial sediments formed in the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

. The country has extensive natural resources, including gold, diamonds, and iron.

Stratigraphy, tectonics & geologic history

The oldest rocks in Senegal are

, commonly found throughout much of West Africa. Birimian rock units include the metabasic, meta-andesite,

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

and

greywacke Greywacke or graywacke ( ) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and Sorting (sediment), poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size Lith ...

of the Mako Series, Diale Series meta-basites, red jasper, conglomerate,

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

and

marble Marble is a metamorphic rock consisting of carbonate minerals (most commonly calcite (CaCO3) or Dolomite (mineral), dolomite (CaMg(CO3)2) that have recrystallized under the influence of heat and pressure. It has a crystalline texture, and is ty ...

, as well as the schist, greywacke and conglomerates of the Dalema Series. In the Kedougou inlier, these volcanic and sedimentary rocks were intruded by

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

plutons In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

in the

Paleoproterozoic The Paleoproterozoic Era (also spelled Palaeoproterozoic) is the first of the three sub-divisions ( eras) of the Proterozoic eon, and also the longest era of the Earth's geological history, spanning from (2.5–1.6 Ga). It is further sub ...

, during the

Eburnean orogeny The Eburnean orogeny, or Eburnean cycle, was a series of tectonic, metamorphic and plutonic events in what is now West Africa West Africa, also known as Western Africa, is the westernmost region of Africa. The United Nations geoscheme for A ...

between 2 billion and 1.8 billion years ago. In southeastern Senegal, the

Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...

Madina-Kouta Basin outcrops along the edge of the inlier.

Paleozoic (539-251 million years ago)

The Komba Basin, bounded by the Kedougou inlier, Madina-Kouta Basin and eastern edge of the Bassaris branch contains the horizontal sedimentary rocks of the

Early Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovic ...

Mali Group Mali, officially the Republic of Mali, is a landlocked country in West Africa. It is the eighth-largest country in Africa, with an area of over . The country is bordered to the north by Algeria, to the east by Niger, to the northwest by Mauri ...

. The basin has a northern extension, known as the Faleme Basin is filled with

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

and

potentially related to the glaciations of the Paleozoic.

Mesozoic –Cenozoic (251 million years ago-present)

The Senegal sedimentary basin, also known as the Senegal-Mauritania Basin, formed as the largest marginal basin in Africa during the rifting apart of the center of the

to form the Atlantic Ocean in the

. The basin is 1400 kilometers from north to south and 500 kilometers at its maximum width at Dakar. This Atlantic-type half-basin has gently dipping sedimentary units that get thicker closer to the coast. Due to overlying sediments from the

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

to modern times, its exact structure and origins are unclear. Pre-Mesozoic basement rock slopes gently westward onshore and then drops offshore along a north–south fault. The depth to basement rock is six kilometers off of Dakar and eight kilometers off of Casamance. Two major north–south faults, formed in the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

through the late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

, break up the basin into horsts and grabens. The southern part of the basin has 10 salt domes (also known as diapirs). There are several disconformities, one offshore from the Senonian and the from the

, spanning the entire basin. The sedimentary sequence begins with

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate Hydrate, dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk ...

halite Halite ( ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pi ...

and

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ...

, which coincided with a

tholeiitic magma series The tholeiitic magma series () is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic ...

. Two kilometers of carbonates formed in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

, with large amounts of detrital elements during the

Aptian The Aptian is an age (geology), age in the geologic timescale or a stage (stratigraphy), stage in the stratigraphic column. It is a subdivision of the Early Cretaceous, Early or Lower Cretaceous epoch (geology), Epoch or series (stratigraphy), S ...

and clastic sedimentation between the Aptian and the

Lutetian The Lutetian is, in the geologic timescale, a stage (stratigraphy), stage or age (geology), age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it ...

. The

Albian The Albian is both an age (geology), age of the geologic timescale and a stage (stratigraphy), stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early Cretaceous, Early/Lower Cretaceous epoch (geology), Epoch/s ...

through the

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

are represented by

argillite Argillite () is a fine-grained sedimentary rock composed predominantly of Friability, indurated clay particles. Argillaceous rocks are basically lithified muds and Pelagic sediment, oozes. They contain variable amounts of silt-sized particles. T ...

and organic-rich sediments, followed by biochemical precipitation since the

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

epoch of the

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

. During the

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

, an alkali

syenite Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). It is considered a Casamance Gulf and by the

, the region was experiencing intense, terrestrial weathering. The Cap-Vert experienced two peaks of volcanism, one in the

and the other in the

, producing small occurrences of undersaturated, alkaline lavas. During the Quaternary, volcanism at the head of the Cap-Vert peninsula built the Mamelles volcano and its associated vents and flows.

Hydrogeology

The Senegal Superficial aquifer system is formed of unconsolidated Quaternary sediments, covering the entirety of the Senegal Basin. In some cases, groundwater is hosted beneath basalts, and water depths range considerably between a few meters and up to 72 meters below the surface. The Intermediate aquifer is consolidated limestone karst, beneath the Superficial aquifer. It experiences salt water intrusion near the coast and high iron and fluoride in the center of the country. The deep aquifer system is composed of

calcareous sandstone. This 250 meter thick aquifer is the main water supply for much of Senegal, and experiences comparable salinity and iron contamination issues.

Natural resource geology

Senegal has extensive natural resources and there is high potential for gold in Paleoproterozoic rocks. Artisanal mines extract gold from the

greenschist belts in the southeast. The carbonate-hosted Faleme iron deposit contains 800 million tons of ore, in one of the largest Paleoproterozoic deposits in Africa.

Magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...

and

iron hydroxide An iron oxide is a chemical compound composed of iron and oxygen. Several iron oxides are recognized. Often they are non-stoichiometric. Ferric oxyhydroxides are a related class of compounds, perhaps the best known of which is rust. Iron oxide ...

in the formation is up to 60 percent pure iron. Diamonds are found in the Faleme River and the

Gambia River The Gambia River (formerly known as the River Gambra, French language, French: ''Fleuve Gambie'', Portuguese language, Portuguese: ''Rio Gâmbia'') is a major river in West Africa, running from the Fouta Djallon plateau in north Guinea westward ...

and prospecting for titanium-bearing sands began in the 1980s. In addition to metals, Senegal has large phosphate reserves, formed in Cenozoic, particularly the

. The switch from a marine to a continental environment in the Eocene led to the long-running enrichment of lime phosphates to aluminum phosphates.

References