Taoudenni basin

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The Taoudenni Basin (sometimes also spelled Taoudeni Basin or Taoudeni Basin ) is a huge sedimentary basin in West Africa that has existed since the end of the Mesoproterozoic . It is characterized by an extremely homogeneous, continental to shallow marine, predominantly siliciclastic sediment sequence. Because of its possible oil reserves , it has recently become the focus of international interests.

etymology

Ouadane (Mauritania), located on the north-western edge of the Taoudenni basin, was built from sandstones / quartzites from the Oujelft-Plateaux group CO 9

The Taoudenni Basin was named by Villemur in 1967 after the Taoudenni salt mine in northern Mali . He originally defined the term much more narrowly and referred to the El Hank sub-basin in the northeast of the Syneklise (note: the dimensions of the actual El Hank sub-basin were only determined in 1994 by Villeneuve and Cornée). In the course of time, the term Taoudenni Basin was expanded to include the entire giant depression, which covers more than 2 million square kilometers.

geography

Geographical extension of the Taoudenni basin

The sedimentary basin lying on the Precambrian West Africa craton is the largest basin in Africa . It occupies the main part of central and eastern Mauritania and stretches across northern Mali to southwest Algeria .

The geographical dimensions of the basin, which is elongated in the north-east direction, with dimensions of 1500 × 1000 kilometers are to be outlined by the following locations (starting in the south in Mali):

Ségou - Nara - Néma - Oualata - Tichitt - Tidjikja - Atar - Zouérat - Guettatira - Chegga - Reggane - Ouallene - Tessalit - Gao - Mopti .

The Taoudenni basin is framed by two archaic cratons and a neoproterozoic and a paleozoic orogen belt . In between, however, there are definitely connections to other sedimentary basins in West Africa. The Taoudenni Basin in the west is only poorly separated from the Senegal-Mauritania Basin on the Atlantic Ocean by the narrow and deeply eroded belt of the Mauritanids . In the southeast it is in exchange with the Iullemedden basin (also Iullemmedden basin) by means of the Gourma sub -basin. Access to the Reggane Basin is via the Erg Chech in the north , then further to the northwest to the Tindouf Basin and to the northeast via the heavily eroded threshold of the Ougarta Belt to the Depression of the Western Great Erg (Grand Erg Occidental). To the south, the basin is sealed off by the Léo shield (also called Man or Ivory Coast Shield ) and to the northwest and north by the Reguibat shield , both very old crustal segments of the Archean . In the northeast and east the Taoudenni basin meets the belt of the Pharusids west of the Ahaggar . The southeast section is bounded by the Adrar des Ifoghas , a southern continuation of the Pharusids.

The Niger is the only permanent river that runs through the southern edge of the Taoudenni basin.

The basin can be subdivided into several sub-basins, of which only three are of greater importance, as they have powerful Paleozoic sediment fillings:

The sediments in these sub-basins reach a total thickness of 5 to 6 kilometers. The sub-basins are separated from each other by clump-like threshold areas . For example, between the Adrar Basin and the Hodh Basin, the now sunken Khatt Sill lies, and the El Hank Basin is separated from the Aulakogen of Gourma - a failed Proterozoic rift - by the Achkaikar sill .

geology

introduction

The main sedimentary basins of West Africa

The West Africa craton documents a continuing crustal evolution from the Mesoarchean . The starting point for the development of the Taoudenni Basin was the paleoproterozoic Eburnian orogeny or Birrimic orogeny (around 2000 million years BP), which was completed around 1800 million years BP. The effects of mountain formation can be seen primarily in the Reguibat shield and the Léo shield in the south. Then since 1700 million years BP the craton has stabilized. It was only with the Variscan mountain formation in the Upper Carboniferous and the later opening of the Atlantic in the Upper Triassic that it was again affected by tectonic movements and magmatic intrusions.

The Taoudenni basin was created from the end of the Mesoproterozoic through subsidence of the archaic West Africa shield due to tectonic instability and represents a typical intracratonic sinking basin ( English sag basin ). The subsidence is explained by an above-average dense lower crust, which, in addition to typical crustal gneisses, is composed of heavy, iron-rich quartzites .

Since then, the basin has had a discontinuous history of sedimentation that continues to this day. The initial sinking movements lasted until the middle Paleozoic, the time of the Variscan mountain formation , which raised the basin. However, its sediment content did not undergo any regional metamorphosis, as the temperatures in the basin did not exceed 100 ° C and there was never more than 3 kilometers of load locally. The basin is therefore of great interest for scientific studies of the Meso- and Neoproterozoic because of its barely impaired sequence of layers.

Overall, an average of 3,000 meters of sediment accumulated (with peaks of 5,000 to 6,000 meters), predominantly layers of the Neoproterozoic and Paleozoic Era, with the western half of the basin generally more thick than the eastern half.

stratigraphy

Simplified stratigraphy of the Taoudenni basin. Any oil horizons in blue.

In the Adrar sub-basin (type profile of the Taoudenni basin), the monoclinal layers of which dip slightly to the southeast, four or three supergroups can be distinguished (from hanging to lying ):

Super group 1 or Hodh super group

The Precambrian sediments of the Atar Cliffs Group at the Amogjar Pass between Atar and Chinguetti

Supergroup 1, up to 1500 meters thick (exceptionally up to 3500 meters in the Richat area ), whose sediments were mainly poured from the north-northeast, lies with a basal angular discordance on the archaic and paleoproterozoic basement of the Reguibat shield. Their fluctuations in thickness refer to the existing paleo-relief , reactivation of fractures in the subsurface or changing subsidence rates. It was accommodated by stretching the West African craton rim with the formation of step-like sinking, north-northeast and northeast-trending trenches (nest and trench structure with synsedimentary breccias).

Supergroup 1 can be further subdivided into three groups , which can be separated from one another in packets by erosive discordances. After the sedimentation was complete, the entire supergroup experienced a tipping caused by tectonic uplift before supergroup 2 was deposited and was subject to prolonged erosion - effects of the main phase of Pan-African orogeny .

The beginning of sedimentation is still controversial. According to the rubidium-strontium dating, it is 1000 million years BP at the beginning of the Neoproterozoic ( Tonium ). The rhenium-osmium method , however, yields a much higher age, which brings the beginning of sedimentation into the Mesoproterozoic (around 1200 million years BP) forward by over 200 million years. The sedimentation lasted until the cryogenium around 650 million years ago BP.

Super group 1 is divided into the following three groups (from hanging to lying):

The 280-meter-thick Char group at the base begins as a fairly coarse-grained sandstone sequence that was deposited in a shallow Epeirian sea when the sea level was high. It is overlaid by the limestone and stromatolite- bearing dolomites of the 700 meter mighty Atar group . The Assabet-el-Hassiane group closes with clayey and silty sediments.

Lahondère and colleagues (2003) divided the Char group into two formations , the Agueni Formation in the horizontal and the Azougui Formation in the hanging wall .

The Atar group was broken down into 10 formations by Bertrand-Sarfati and colleagues (1996) (the abbreviations follow trompets):

The equivalent El-Mreïti-Groupe was divided into eight formations:

In the Assabet-el-Hassiane group, Lahondère and colleagues distinguished three formations:

Super group 2 or Adrar super group

Chinguetti , built from sandstones / quartzites from the Oujeft-Plateaux group CO 9
The oasis of Terjit with the Atar Cliffs Group and the Oujeft Plateaux Group

Supergroup 2, which is around 2000 meters thick, is also located on an erosive angular discordance that either affects supergroup 1 or directly on the basement. It begins at 630 million years BP in the late Neoproterozoic at the end of the Cryogenian with glacial deposits of the Marionoic Ice Age and lasts until the Ediacarian . In contrast to supergroup 1, it is characterized by sediment thicknesses and facies distributions that remain fairly constant, which show that the basin is very stable.

It is made up of the following four groups:

Chalky , barium- containing dolomites ( cap dolostones ) of the Jbéliat group were deposited over the basal tillites with scratched debris and dropstones in varven bands . The following 625 to 595 million year old Téniagouri group is characterized by green clay and siltstones by silicified sediments ( chert or silex ). This triad of Tillit-Dolomit-Silex at the base of Supergroup 2 represents a characteristic guiding horizon in the basin. But the green clay stones are also of supraregional importance, as they are not only found in the Taoudenni basin.

The following Atar Cliffs group is made up of quartzite-bearing sandstones and red clayey sandstones with dolomitic interconnections. The final Oujeft Plateaux group consists mainly of red sandstones, which are initially of continental origin (CO 9 to 11) and then take on marine character (CO 12 to 13). The continental red sediments of the last two groups also have supra-regional stratigraphic importance. In addition, they document a change in the direction in which the sediments poured in from the south-east and south. This fundamental change shows a shift in the architecture of the basin, which is probably related to the Pan-African orogeny towards the end of the Neoproterozoic. The sedimentation of supergroup 2 ended due to a general epirogenetic uplift of the West African craton, which can very probably be explained by a takonic phase in the Mauritanids.

Super group 3 or Dahr super group

The locally mostly erosive onset of supergroup 3 (near the Mauritanids but with angular discordance) contains 100 to 200 meters thick glacial sediments from the late Ordovician glaciation of the Hirnantium (around 445 million years BP) at its base . The several hundred meters thick marine Silurian then transgresses over the Tillite with graptolite- bearing shale clays and isolated sandstones. After a hiatus, sedimentation continued in the Devonian with sandstones and limestone. The Devonian deposits, which are up to 500 meters thick (only 150 meters in El-Hank), are usually discordant over layers of the Silurian, but sometimes also over Tillites of the Hirnantium. They begin in the emsium around 400 million years and last up to the frasnium (360 million years). The Devonian has been deposited discontinuously and is only present in the Adrar , Tagant , Hodh and El-Hank.

Super group 3 is made up of the following three groups:

Super group 4 or Dahr super group

The supergroup 4 consists of sediments of carbon, which conform to the layers of the Devonian. They start with sandstones of Lower Carboniferous, which gradually change into limestone. From the Serpukhovian (around 325 million years), however, the sedimentation takes on a continental character with fluvial, lacustrine and lagoon deposits (sandstones, marls and shale clays) that last until the end of the Carboniferous and sometimes even into the Lowest Permian . The carbon is only present in the northeast of the basin, where it reaches a thickness of 600 meters.

The super group 4 contains only two groups:

After supergroup 4 had been deposited, the basin emerged and was only covered by sparse continental deposits from the Mesozoic era , the so-called Continental intercalaire or the Cretaceous Oualata supergroup in Afollé and Hodh. In the Quaternary lacustrine sediments and the now widespread finally followed dunes trains .

Dating

Supergroup 1 was dated by Clauer (1981) on the basis of glauconites in clay-rich layers using the classic rubidium-strontium method to the period 998 to 695 million years BP. For the beginning of the Char group there were 998 ± 34 million years BP and for the final Assabe-el-Hassiane group it was around 695 million years BP. For the Atar group, Clauer found ages between 890 ± 37 million years BP and 775 ± 54 million years BP.

However, a new dating using the rhenium-osmium method by Rooney and colleagues (2010) showed that the Atar group was over 200 million years old, varying between 1105 and 1109 million years BP. This new dating is supported by the course of the chemostratigraphic δ 13 C curve, which corresponds to the values ​​found for the Atar group in the period of the stenium , but not in the tonium.

Ring structures

The Taoudenni Basin with the Richat structure as seen from the ISS

On the western edge of the Taoudenni basin there are two enigmatic ring structures, the large Richat structure and the Aouelloul crater, the ultimate origin of which has not yet been determined with absolute certainty. The Richat structure is likely of endogenous origin and the Aouelloull crater is likely to be due to an impact event.

Fossil content

Overall, the sediments of the Taoudenni Basin are relatively low in fossils. Algae mats and stromatolites are particularly noteworthy in the Atar group. The stromatolites with the taxa Collenia , Conophyton and Jacutophyton are among the best preserved of the Neoproterozoic. Silurian graptolites of the Llandovery can be found in the Oued-Chig group. Brachiopods (with the genera Lingula and Lingulella ) are found in the Oujeft Plateaux group ( Tremadocium ), in the Njakane Abteilli group (Hirnantium) and in the El Aguid group (Lower, Middle and Upper Devonian). The Oujeft-Plateaux group can also Skolithosbauten show. Trilobites can also be found in the Nkajane Abteilli group. In the Téniagouri group (CO 2) sponges have been preserved. Of great importance is the first finding of fossils of the Ediacaran fauna in West Africa, which were discovered in the Cheikhia group of El-Hank.

Icing

As already mentioned, two global glaciations by Tillite are documented in the sediments of the Taoudenni Group - the Marino glaciation at the end of the cryogenium and the brain-antic glaciation at the end of the ordovician. The center of the Marinoic glaciation is likely to have been in the Anti-Atlas , whereas the center of the brain-antic glaciation is assumed to be in the south of the Ahaggar.

raw materials

Exploratory drilling in the 1980s found evidence of oil in layers of the Neoproterozoic, Silurian and Upper Devonian.

Individual evidence

  1. ↑ Imrich Kusnir: Gold in Mali . In: Acta Montanistica Slovaca . 1999.
  2. ^ Villemur, JR: Reconnaissance géologique et structurale du Nord du bassin de Taoudeni . In: Mém. Bur. Rech. Géol.Min. tape 51 , 1967, p. 151 .
  3. ↑ Villeneuve, M. and Cornée, JJ: Structure, evolution and paleogeography of the West African Craton and bordering belts during the Neoproterozoic . In: Precambrian Research . tape 69 , 1994, pp. 307-326 .
  4. ↑ a b Trompette, R .: Le Précambrien supérieur et le Paléozoique inférieur de l'Adrar de Mauritanie (bordure occidentale du bassin de Taoudéni, Afrique de l'Ouest). An example of the sédimentation de craton. Étude stratigraphique et sédimentologique. In: Travaux des Laboratoires des Sciences de la Terre St-Jérôme . B-7. Marseille 1973, p. 702 .
  5. ^ Bronner, G. et al .: Genesis and geodynamic evolution of the Taoudeni Cratonic Basin (Upper Precambrian and Paleozoic), western Africa . In: Bally, AW Dynamics of Plate Interiors (Ed.): Geodynamic Series . vol. 1 AGU-GSA, 1980, p. 81-90 .
  6. ↑ Deynoux, M. et al .: Pan-African tectonic evolution and glacial events registered in Neoproterozoic to Cambrian cratonic and foreland basins of West Africa . In: Journal of African Earth Science . tape 46 , 2006, p. 397-426 .
  7. ↑ Benan, CAA and Deynoux, M .: Facies analysis and sequence stratigraphy of Neoproterozoic platform deposits in Adrar of Mauretania, Taoudeni basin, West Africa . In: Geologische Rundschau . tape 87 , 1998, pp. 283-302 .
  8. ↑ Lahondère ,: Notice explicative des cartes géologiques et gîtologiques à 1/200000 et 1/500000 du Nord de la Mauretanie . Vol 1 DMG. Ministère des Mines et de l'Industrie, Nouakchott 2003.
  9. ^ Bertrand-Sarfati et al.: Subdivisions stratigraphiques nouvelles dans la couverture neoprotérozoique au Nord-Est du bassin de Taoudenni (Algérie) . Ed .: Bitam, L. and Fabre, J. Geodynamique du craton ouest-africain central et oriental: héritage et évolution post-panafricains. Mémoire. Service Géologique de l'Algérie, 1996, p. 63-90 .
  10. ↑ Clauer, N .: Rb-Sr and K-Ar dating of Precambrian clays and glauconies . In: Precambrian Research . tape 15 , 1981, p. 331-352 .
  11. ↑ Rooney, AD et al .: Re-Os geochronology of a Mesoproterozoic sediment succession, Taoudeni basin, Mauretania: Implications for basin-wide correlations and Re-Os organic-rich sediment systematics . In: Earth and Planetary Science Letters . tape 289 , p. 486-496 .
  12. ^ Teal, DAJ and Kah, LC: Using C-isotopes to constrain intrabasinal stratigraphic correlations: Mesoproterozoic Atar Group, Mauretania . In: Geological Society of America Abstracts with Programs . vol. 37, 2005, pp. 45 .