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... The Richat structure was first highlighted from aerial photographs in the 1950' (Monod 1952). It is located in the northwestern part of the Late Proterozoic-Lower Paleozoic Taoudeni intra-cratonic basin, which rests unconformably over the Reguibat basement in the central part of the Mauritanian Adrar plateaus (Fig. 1). ...
... Two rhyolite outcrops occur around the sebkhas in the SW and NE central parts of the Richat structure (Fig. 2). They rest over an erosion surface in the internal depression (Monod 1952). These rocks are vacuolar and very altered (Fig. 3c). ...
... They are predominantly red to pink in colour, rarely yellow (Fig. 3d). They have been variously interpreted since the pioneer work of Monod (1952) who described them as rhyolite alteration products. According to Bardossy et al. (1963), they are mainly composed of analcime and were thus referred to as "analcimolites" of endogenous origin. ...
The famous circular structure of Richat, sometimes referred to as “the eye of Africa”, is located in the northwestern part of the Taoudeni basin, in the central part of the Mauritanian Adrar plateaus. It is expressed at the surface as a slightly elliptical depression, about 40 km in diameter, marked by concentric ridges of Proterozoic-Lower Paleozoic sediments. Its origin as resulting from either a meteorite impact or a deep magmatic intrusion has been long debated. Modelling of high-resolution airborne magnetic data as well as satellite gravity data reinforces the intrusion hypothesis. Geophysical modelling has been calibrated by determinations of rock properties from various types of magmatic lithologies sampled in the field. The three complementary types of geophysical data allow us to image at various scales and depths the buried structures of the Richat magmatic complex, to determine the areas most affected by hydrothermal alteration and finally to elaborate a kinematic model for its emplacement. We emphasize that (1) the Richat intrusion is characterized by the presence of two important circular magnetic signals that coincide with gabbroic ring dykes partly exposed at the surface, (2) its overall circular structure rests above a deep mafic (gabbroic) body, (3) the upwelling of magma at the surface has been facilitated by the presence of concentric faults and (4) the central zone of the complex recorded intense hydrothermal alteration. This case study aims to provide insights for similar types of magma-induced ring structures observed worldwide.
... Les sites étudiés se situent dans l'Adrar, « montagne » en berbère, qui est principalement constituée de plateaux gréseux paléozoïques constituant l'extrême sud-ouest de la vaste cuvette d'Araouar-Taoudeni. Les formations sédimentaires s'y succèdent du Cambrien au Dévonien supérieur (Monod, 1952 ;Monod et Pomerol, 1973) en une série monoclinale où les couches gréseuses les plus dures constituent des bordures de cuesta superposées en gradins. Dans ces plateaux, un accident tectonique remarquable est constitué par le dôme plus ou moins arasé du Guelb er-Richât. ...
In the Mauritanian Adrar, the Guelb er-Richât and, in particular, the depression of its external ring are the location of rather exceptional Acheulian industry accumulations discovered by Théodore Monod. These sites may have originated from one or more non dated humid phases where seasonal water presence induced faunal and thus predator presence. Piedmont relief, foot slope rock flow and deltaic spreads attest, until the last humid Holocene, the permanence of particularly favourable sectors like upstream of Akerdil and Bamouéré wadis. Outcrops or chaos of Chinguetti quartzite sandstone have supplied the necessary raw material for tool confection; the second factor favouring the presence of these Acheulian sites. The rather difficult aim of the authors is first, to locate lithics in stratigraphic position and second, to establish a chronostratigraphy. Some first examples are presented.
... HUBERT (1933) puis surtout MENCHIKOFF (1946) sont les premiers a identifier la veritable nature de ces structures qu'ils rapprochent des stromatolites. Ces dernieres annees, certains de ces niveaux ont 6te decrits par MONOD (1952, 1954) et MCPHEE et al. (1958. ...
Stromatolites of the Upper Precambrian in the Adrar of Mauritania (western Sahara)
The Adrar of Mauritania is the western part of the Paleozoic (s.l.) Taoudeni Basin. It is composed of three main units separated by two great unconformities which are overlain by glacial deposits. The first paleontologically dated stratigraphic unit is composed of the top of the second formation, with brachiopods sandstone of Late Cambrian to Early Ordovician age, and the base of the third formation, the graptolites shale of Silurian age. The first unit is supposed to be Infracambrian or Upper Precambrian and is characterized, in Adrar, by numerous stromatolitic limestones and dolomites. It is divided in eighteen members themselves subdivided into fifty nine levels.
The stromatolites are of various types: stromatolites with hemispheroidcl laminations (LOGAN et al., 1964), oncolites with spheroidal laminations, cono- phytons and stron-ntolites with flat and continuous laminations (LOGAN,1961). From the study c,f the mor: typical stromatolite formations we can make three main remarks on the taxonom;, the texture and the paleogeographical significance of stromatolites.
The ecology of the Upper Precambrian stromatolites of Adrar is similar to that of the present stromatolites. Their wide distribution, from Atar to the Algerian frontier, involves the existence in western Africa, in Late Precambrian time, of a large shallow-marine basin with very scarce clastic sediments. Such a basin is unknown in present seas.
The term “Pan-African thermotectonic event,” or “orogeny,” has been applied by Kennedy (1964, 1965) to a shield-wide series of metamorphic belts with ages in the range 450–700 m. y., which wrap around and enclose older undisturbed cratonic Precambrian complexes (Fig. 1). This reticulate group of belts has been recognized principally by mineral radiometric ages, which for most part give little information on the earlier history of the belts. In some cases they include folded and metamorphosed equivalents to cratonic sediments and crystalline basement rocks, but in others they consist of metasedimentary, gneissic, and granitic rocks of largely unknown age.
West Africa consists essentially of a Precambrian granitized craton (radiometric ages of 2700 to 1600 m.y.) covered by a thin sedimentary blanket, the oldest sedimentary rocks being as much as 1000 m.y. old. The Precambrian basement has been warped into three major uplifts trending approximately east-northeast (Fig. 1): the Léo uplift (Dorsale de Léo, also known as the Liberia-Upper Volta uplift, and Bouclier éburnéen); the Reguibat uplift; and the Anti-Atlas uplift. Deposition during most of Paleozoic time apparently occurred in epicontinental seas. The principal preserved depositional areas are the Taoudeni basin in Mauritania and Mali (Fig. 2); the Tindouf basin in Morocco, Algeria, Mauritania, and Spanish Sahara; the Volta basin in Ghana and Upper Volta; and the Bové basin in Guinea and Portuguese Guinea.
The Late Precambrian or “Eocambrian” and the lowermost part of Palaeozoic rocks of West Africa and of the Western half of Central Africa include two lithostratigraphic sequences of sediments which we can identify as well on the craton as in the orogenic belts. The lower sequence, argillaceous, green colour, sometimes flyschoid, begins with a tillite or a mixtite. The upper sequence made of reddish, mainly continental sandstones, represents in some areas the molasse of the Pan-African Fold Belt.
These two sequences, and notably at the base of the first one the tillite horizon, seem to be older in the South (Katangan and West-Congolian Chains) than in the North (Adrar of Mauritania). This diachronism is thought to be related to polar wandering, the South pole having shifted in that interval of time from South-Africa to West of the Senegal coast.
Amineralogical-geochronological study has been made of sedimentary levels I 2 and I 5 in the upper Precambrian of the Adrar, Mauritania, showing that clay minerals can be used for geochronology. This is, however, only possible where early diagenesis has homogenised the 87 Sr/ 86 Sr ratio of clay minerals and interstitial waters. Rocks whose clay minerals consist of unaltered mica or illite are not suitable for this approach, as is the case for sediments rich in detrital feldspars: carbonates may however be used. Ages of 964 ± 32 M.Y. (I 2 ) and860± 35 M.Y. (I 5 ) were obtained using a 87 Rb decay constant of 1.47 × 10-11 yr -1 , confirming that the beginning of the late Precambrian sedimentation in the Adrar may be placed at 1000 M .Y.
The northwestern margin of Gondwana was dominated by epicontinental and continental sedimentation from the Ordovician to the Carboniferous. These sediments occur in west Africa, the Florida subsurface, and northern Venezuela. Their character suggests major periods of transgression onto the Gondwana margin occurred in the Middle (?) Ordovician, the Lower Silurian (Llandovery) and the middle Devonian (Emsian). Widespread regressions occurred in the Upper Ordovician (intercontinental glaciation) and the Lower and Upper Devonian (Siegenian and Famennian). Depositional conditions along the northwestern margin of Gondwana changed markedly in the Carboniferous as a result of tectonic activity associated with the assembly of Pangea.
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