The geology of the Republic of the Congo, also known as Congo-Brazzaville, to differentiate from the Democratic Republic of the Congo, formerly Zaire, includes extensive igneous and metamorphic basement rock, some up to two billion years old and sedimentary rocks formed within the past 250 million years. Much of the country's geology is hidden by sediments formed in the past 2.5 million years of the Quaternary.
The Congo Craton is the geologically stable remnant of an early continent. It is the basement rock for much of central Africa and extends through the Republic of Congo and into Gabon, as far as Cameroon, as the granitoid Chaillu Massif. The Massif has two generations of 2.7 billion year old granitoids dating to the Archean and a north–south foliation. The rocks range from gray granodiorite to quartz biotite or biotite and amphibole rich endmembers. Pink-colored potassic migmatite is found in veins within the rock. At a few points, much older greenstone and granitoid schist remain in place and never experienced full granitization.
Republic of Congo has two relict greenstone belts at Mayoko and Zanago. Greenstone belts have sequences of metamorphic and volcanic rock that preserve ancient tectonics from the Archean. The Mayoko greenstone belt has banded iron, amphibolite, biotite gneiss and pyroxene-rich amphibolite sequences. At Zanago, the belt is slightly different, with amphibolite-bearing quartzite, residual pyroxenite in amphibolites and small amounts of dunite. [1]
In the northwest of the country, the Sembe-Ouesso Group spans into Gabon and Cameroon, beneath overlying mixtite formations. The Sembe-Ouesso Group has ascending sequences of quartzite, shale, phyllite, calc-shale, dolomite and another layer of quartzite. Some geologists debate whether the sequence dates to the Paleoproterozoic or Neoproterozoic.
The Neoproterozoic West Congolian Supergroup, is one of three parts of the West Congolian mobile belt, a tectonic feature that runs for 1300 kilometers from Gabon through southwest Republic of Congo to Angola. Geologists subdivide the West Congolian Supergroup into the Haut Shiloango Group, the Schisto-Calcaire Group, Sansikwa Group, Mpioka Group and Inkisi Group. Previously, geologists interpreted two pebbly schist and mixtite layers, that separate the Sansikwa Group, Haut Shiloango Group and the Schisto-Calcaire Group as glacial deposits, but now they are believed to be the remains of mud flows. [2]
As central Africa entered the Phanerozoic, the current geologic eon in which multicellular plant and animal life is commonplace, sedimentary rocks were deposited in what may have been failed rift valleys. These rocks are mainly found in the Congo Basin, a structural tectonic feature that extends into neighboring Democratic Republic of Congo, formed within the Congo Craton. Because of thick layers of sediment atop the craton in central Africa, remoteness and sparse research from boreholes and seismic lines, the exact origins of many of these sedimentary units remains unclear. Geologists currently interpret many of the sedimentary sequences from the Late Permian into the Triassic as the remains of the Karoo rifting period. During the Karoo period, southern Gondwana rifted across a large area encompassing what is now southern South America and South Africa. The rift basin and surrounding regions were cloaked in thick layers of continental sediment and sedimentation accelerated when Gondwana drifted over the South Pole, causing massive glaciation of Africa in the Karoo Ice Age. Ultimately, the glaciers melted at the end of the Permian, creating a swampy delta environment. Rocks directly preserving this sequence are found in the Karoo Supergroup, one of the most extensive stratigraphic units in southern Africa. With slow subsidence in the Cretaceous at the Mesozoic, the Congo Basin experienced slow subsidence and filled with one kilometer of sediments. [3]
Within the past 66 million years, the Congo Basin has continued to sag, surrounded by topographic highs on all sides. In the Miocene, the basin closed due to flank uplift at the Atlantic rifted margin and gorges and rapids formed in the Mayombe mountains. [4]
In the interior, along the Congo River, most groundwater is stored in permeable unconsolidated Congo Basin alluvium, which can be up to 400 metres (1,300 ft) thick. Small areas of this unconsolidated Quaternary alluvium are also found near the Atlantic coast. The Coastal basin aquifer is made up of Cenozoic clay-rich sands, dolomitic sandstones and dolomitic limestones with low hardness and a neutral pH but relatively high salinity and sulfide. The unit is known from exploratory wells drilled in search of potash, water and petroleum. The Stanley Pool series beneath Brazzaville is primarily sandstone with some kaolinite and marl mixed in. In other parts of the center of the country, shallow boreholes into the Bateke Plateau sandstone series are used for local water supplies. Groundwater is also found in the Late Precambrian Inkisi series, Mpioka series and schist-limestone in the Djoue Valley as well as some fractured schist, granite, gneiss and quartzite basement rock. [5]
The Republic of Congo was included in widespread stratigraphic research of the Congo Basin in the late 1950s, with drill cores preserved at the Royal Museum for Central Africa in Belgium. Oil related research continued in the 1970s and up until 1984, spanning into Zaire (the former name of the Democratic Republic of Congo), by Shell, Texaco and Japan National Oil Company. [4]
Natural resources, both minerals and petroleum, are important contributors to the economy of the Republic of Congo. Crude oil makes up 50% percent of the government's revenue and 90% of exports, with almost all drilling offshore. Two metal mines, at Djenguile and Yanga-Koubenza, 290 kilometres (180 mi) west of Brazzaville extract zinc, lead and copper. Unexploited niobium, tantalum, uranium, manganese, titanium, nickel, chromium, thorium, gold and tungsten are known from geochemical analysis of the Mayombe Supergroup. Many of the same minerals may also be found in the Paleoproterozoic Sembe-Ouesso Group.
Placer diamonds are found in the Bikosi Conglomerate, likely dating to the Early Cretaceous. The Archean Caillu Massif hosts itabirite iron ore and gold, associated with amphibolite, although these resources are not extensively mined except by artesanal methods.
The Republic of Congo also has a 50 kilometres (31 mi) belt of phosphate outcrops along the coast, with large deposits at Sintou Kola and Holle, 50 kilometers from Pointe Noire. Phosphate nodules, 110 kilometres (68 mi) west of Brazzaville, close to Comba, are believed to be of Neoproterozoic age. [6]
The Gascoyne Complex is a terrane of Proterozoic granite and metamorphic rock in the central-western part of Western Australia. The complex outcrops at the exposed western end of the Capricorn Orogen, a 1,000 km-long arcuate belt of folded, faulted and metamorphosed rocks between two Archean cratons; the Pilbara craton to the north and the Yilgarn craton to the south. The Gascoyne Complex is thought to record the collision of these two different Archean continental fragments during the Capricorn Orogeny at 1830–1780 Ma.
The Slave Craton is an Archaean craton in the north-western Canadian Shield, in Northwest Territories and Nunavut. The Slave Craton includes the 4.03 Ga-old Acasta Gneiss which is one of the oldest dated rocks on Earth. Covering about 300,000 km2 (120,000 sq mi), it is a relatively small but well-exposed craton dominated by ~2.73–2.63 Ga greenstones and turbidite sequences and ~2.72–2.58 Ga plutonic rocks, with large parts of the craton underlain by older gneiss and granitoid units. The Slave Craton is one of the blocks that compose the Precambrian core of North America, also known as the palaeocontinent Laurentia.
The Aravalli Mountain Range is a northeast-southwest trending orogenic belt in the northwest part of India and is part of the Indian Shield that was formed from a series of cratonic collisions. The Aravalli Mountains consist of the Aravalli and Delhi fold belts, and are collectively known as the Aravalli-Delhi orogenic belt. The whole mountain range is about 700 km long. Unlike the much younger Himalayan section nearby, the Aravalli Mountains are believed much older and can be traced back to the Proterozoic Eon. They are arguably the oldest geological feature on Earth. The collision between the Bundelkhand craton and the Marwar craton is believed to be the primary mechanism for the development of the mountain range.
The geology of Central African Republic (CAR) is part of the broader geology of Africa. CAR occupies a swath of ancient rocks, dating back billions of years that record significant aspects of Earth history and yield minerals vital to the country's small economy.
Gabon is situated at the northwestern margin of the Congo Craton—a region of stable, ancient crust—and preserves very ancient rock units across 75% of the country, with overlying sedimentary units from the Cretaceous and other more recent periods.
The geology of Ghana is primarily very ancient crystalline basement rock, volcanic belts and sedimentary basins, affected by periods of igneous activity and two major orogeny mountain building events. Aside from modern sediments and some rocks formed within the past 541 million years of the Phanerozoic Eon, along the coast, many of the rocks in Ghana formed close to one billion years ago or older leading to five different types of gold deposit formation, which gave the region its former name Gold Coast.
The geology of the Democratic Republic of the Congo is extremely old, on the order of several billion years for many rocks. The country spans the Congo Craton: a stable section of ancient continental crust, deformed and influenced by several different mountain building orogeny events, sedimentation, volcanism and the geologically recent effects of the East Africa Rift System in the east. The country's complicated tectonic past have yielded large deposits of gold, diamonds, coltan and other valuable minerals.
The geology of Ivory Coast is almost entirely extremely ancient metamorphic and igneous crystalline basement rock between 2.1 and more than 3.5 billion years old, comprising part of the stable continental crust of the West African Craton. Near the surface, these ancient rocks have weathered into sediments and soils 20 to 45 meters thick on average, which holds much of Ivory Coast's groundwater. More recent sedimentary rocks are found along the coast. The country has extensive mineral resources such as gold, diamonds, nickel and bauxite as well as offshore oil and gas.
The geology of Malawi formed on extremely ancient crystalline basement rock, which was metamorphosed and intruded by igneous rocks during several orogeny mountain building events in the past one billion years. The rocks of the Karoo Supergroup and newer sedimentary units deposited across much of Malawi in the last 251 million years, in connection with a large rift basin on the supercontinent Gondwana and the more recent rifting that has created the East African Rift, which holds Lake Malawi. The country has extensive mineral reserves, many of them poorly understood or not exploited, including coal, vermiculite, rare earth elements and bauxite.
The geology of Mauritania is built on more than two billion year old Archean crystalline basement rock in the Reguibat Shield of the West African Craton, a section of ancient and stable continental crust. Mobile belts and the large Taoudeni Basin formed and filled with sediments in the connection with the Pan-African orogeny mountain building event 600 million years ago and a subsequent orogeny created the Mauritanide Belt. In the last 251 million years, Mauritania has accumulated additional sedimentary rocks during periods of marine transgression and sea level retreat. The arid country is 50% covered in sand dunes and has extensive mineral resources, although iron plays the most important role in the economy.
The geology of Mozambique is primarily extremely old Precambrian metamorphic and igneous crystalline basement rock, formed in the Archean and Proterozoic, in some cases more than two billion years ago. Mozambique contains greenstone belts and spans the Zimbabwe Craton, a section of ancient stable crust. The region was impacted by major tectonic events, such as the mountain building Irumide orogeny, Pan-African orogeny and the Snowball Earth glaciation. Large basins that formed in the last half-billion years have filled with extensive continental and marine sedimentary rocks, including rocks of the extensive Karoo Supergroup which exist across Southern Africa. In some cases these units are capped by volcanic rocks. As a result of its complex and ancient geology, Mozambique has deposits of iron, coal, gold, mineral sands, bauxite, copper and other natural resources.
The geology of Sierra Leone is primarily very ancient Precambrian Archean and Proterozoic crystalline igneous and metamorphic basement rock, in many cases more than 2.5 billion years old. Throughout Earth history, Sierra Leone was impacted by major tectonic and climatic events, such as the Leonean, Liberian and Pan-African orogeny mountain building events, the Neoproterozoic Snowball Earth and millions of years of weathering, which has produced thick layers of regolith across much of the country's surface.
The geology of Tanzania began to form in the Precambrian, in the Archean and Proterozoic eons, in some cases more than 2.5 billion years ago. Igneous and metamorphic crystalline basement rock forms the Archean Tanzania Craton, which is surrounded by the Proterozoic Ubendian belt, Mozambique Belt and Karagwe-Ankole Belt. The region experienced downwarping of the crust during the Paleozoic and Mesozoic, as the massive Karoo Supergroup deposited. Within the past 100 million years, Tanzania has experienced marine sedimentary rock deposition along the coast and rift formation inland, which has produced large rift lakes. Tanzania has extensive, but poorly explored and exploited natural resources, including coal, gold, diamonds, graphite and clays.
The geology of Uganda extends back to the Archean and Proterozoic eons of the Precambrian, and much of the country is underlain by gneiss, argillite and other metamorphic rocks that are sometimes over 2.5 billion years old. Sedimentary rocks and new igneous and metamorphic units formed throughout the Proterozoic and the region was partially affected by the Pan-African orogeny and Snowball Earth events. Through the Mesozoic and Cenozoic, ancient basement rock has weathered into water-bearing saprolite and the region has experienced periods of volcanism and rift valley formation. The East Africa Rift gives rise to thick, more geologically recent sediment sequences and the country's numerous lakes. Uganda has extensive natural resources, particularly gold.
The geological history of Zambia begins in the Proterozoic eon of the Precambrian. The igneous and metamorphic basement rocks tend to be highly metamorphosed and may have formed earlier in the Archean, but heat and pressure has destroyed evidence of earlier conditions. Major sedimentary and metamorphic groups formed in the mid-Proterozoic, followed by a series of glaciations in the Neoproterozoic and much of the Paleozoic which deposited glacial conglomerate as well as other sediments to form the Katanga Supergroup and rift-related Karoo Supergroup. Basalt eruptions blanketed the Karoo Supergroup in the Mesozoic and Zambia shifted to coal and sandstone formation. Geologically recent windblown sands from the Kalahari Desert and alluvial deposits near rivers play an important role in the modern surficial geology of Zambia. The country has extensive natural resources, particularly copper, but also cobalt, emeralds, other gemstones, uranium and coal.
The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 539 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.
The geology of Senegal formed beginning more than two billion years ago. The Archean greenschist Birimian rocks common throughout West Africa are the oldest in the country, intruded by Proterozoic granites. Basins formed in the interior during the Paleozoic and filled with sedimentary rocks, including tillite from a glaciation. With the rifting apart of the supercontinent Pangaea in the Mesozoic, the large Senegal Basin filled with thick sequences of marine and terrestrial sediments. Sea levels declined in the Eocene forming large phosphate deposits. Senegal is blanketed in thick layers of terrestrial sediments formed in the Quaternary. The country has extensive natural resources, including gold, diamonds, and iron.
The geology of Nigeria formed beginning in the Archean and Proterozoic eons of the Precambrian. The country forms the Nigerian Province and more than half of its surface is igneous and metamorphic crystalline basement rock from the Precambrian. Between 2.9 billion and 500 million years ago, Nigeria was affected by three major orogeny mountain-building events and related igneous intrusions. Following the Pan-African orogeny, in the Cambrian at the time that multi-cellular life proliferated, Nigeria began to experience regional sedimentation and witnessed new igneous intrusions. By the Cretaceous period of the late Mesozoic, massive sedimentation was underway in different basins, due to a large marine transgression. By the Eocene, in the Cenozoic, the region returned to terrestrial conditions.
The geology of Sweden is the regional study of rocks, minerals, tectonics, natural resources and groundwater in the country. The oldest rocks in Sweden date to more than 2.5 billion years ago in the Precambrian. Complex orogeny mountain building events and other tectonic occurrences built up extensive metamorphic crystalline basement rock that often contains valuable metal deposits throughout much of the country. Metamorphism continued into the Paleozoic after the Snowball Earth glaciation as the continent Baltica collided with an island arc and then the continent Laurentia. Sedimentary rocks are most common in southern Sweden with thick sequences from the last 250 million years underlying Malmö and older marine sedimentary rocks forming the surface of Gotland.
The geology of Nunavut began to form nearly three billion years ago in the Archean and the territory preserves some of the world's oldest rock units.
