The "impact" craters of Mauritania
Aouelloul, Tenoumer, Temimichat and El Mrayer

 Norbert Brügge, Germany
Dipl. - Geol.

Geology of Mauritania

Mauritania comprises four major geological domains. The central part of the Reguibat Shield - an uplifted part of the West African Craton which has been stable since 1700 Ma and dominates the northern third of the country's surface geology.
The N-S Mauritanides Orogenic Belt. The western margin of this orogen is concealed beneath coastal basin sediments.
The western part of the Taoudeni Basin infilled by Neoproterozoic to Devonian sedimentary strata unconformably overlying crystalline rocks of the Palaeoproterozoic to Archaean West African Craton. The continental to shallow marine Taoudeni Basin does not exceed 5000 m in thickness, and though it varies somewhat, comprises a remarkably homogenous lithological sequence.
A western Mauritanian Coastal Basin infilled by Mesozoic-Cenozoic sediments.

  

Reguibat Shield

The West African Craton is primarily composed of two shields, the Reguibat Shield in the north of Mauritania and the Leo Shield in Liberia and Sierra Leone, separated by the sedimentary (Upper Proterozoic to Palaeozoic) Taoudeni Basin.
The Reguibat Shield dominates the northern third of the country's surface geology. It constitutes a very extensive portion of Precambrian crust, bounded on its western side by the Mauritanides Belt and to the south it is overlain by late Precambrian sedimentary rocks of the Cambro-Ordovician and Devonian series.
The Reguibat Shield is divided into two main parts:
The eastern shield or 'Eburnean terrane', with ages of 2000 ±200 Ma corresponding with the Birimian tectonothermal events and predominantly comprising intrusive granites in the west and volcanic formations in the east.
The western shield or 'Archaean terrane' mainly with ages of ≥2500 Ma and predominantly comprising migmatites gneisses, granitoids, ferruginous quartzites and Banded Iron Formation (BIF), amphibolites and feldspathic gneisses. The highest grade gneissic and granulite terrain is the Archaean basement located in the western part of the shield in the Akjoujt area where it outcrops as poorly granular, migmatic felsic gneiss and coarsely foliated orthogneisses.




Taoudeni Basin

The Taoudeni Basin has experienced only one period of modern oil exploration. Agip and Texaco were each awarded blocks in 1970 with seismic and in 1974 with two drillings.
The Taoudeni Basin is a broad intracratonic sag which appears to have formed in response to the Pan-African Orogeny. The basin initiated in the Infracambrian, and continued to develop throughout the Palaeozoic until the Carboniferous. Despite the proximity of the Pan-African, Caledonian and Hercynian Orogenic belt, there is no evidence of significant deformation in the basin. For example, the Hercynian Orogeny, which formed the Mauritanide Fold Belt, is pinned by a fore-thrust in the eastern part of the fold belt, and significant deformation does not appear to have propagated eastwards into the present day Taoudeni Basin.
There are some broad, long-lived structural highs through the basin: The northeast striking Abolag–Ouasa High compartmentalises the basin into two depocentres: The Maqteir Depression to the west, and the main Taoudeni Depocentre to the east. These are broad, relatively unstructured depocentres, although there are some large features evident in the 2D seismic data (El Mrayer High).

In southernmost Mauritania, a Cretaceous rift basin is superimposed on the Taoudeni Basin. This poorly known basin is only inferred from gravity data. The Nara Rift is a northeast-striking feature straddling the southern Mauritanian border.

Crater Aouelloul


Crater Aouelloul: Latitude 20° 15' N; Longitude 12° 41' W


The crater Aouelloul is located in the Adrar region of the western Sahara Desert, Mauritania. The crater is one of the smaller known craters, having a diameter of 390 m. The rim is well-defined and rises 15-25 m above the local topography, and 53 m above the crater floor. The crater is located in Ordovician sandstones and quartzite, and is filled with sandy silt and Aeolian sand. Estimates of the thickness of this sedimentary fill are around 23 m.

While breccias are not found at Aouelloul, impact glass can be found on the south, southeast, and north outer part of the crater rim. Because the glass is enriched in siderophile elements, has a low water content, and contains lechatelierite, it has been interpreted as an impact glass.
Proving Allouelloul’s impact origin has proved extremely difficult. Because the impact occurred in sandstone, the shock wave energy that normally forms planar deformation features was largely dissipated within the porous rock. Therefore, different stages of shock metamorphism occur, and PDF's rarely form. Petrographic study of samples from Aouelloul show shattered and fractured quartz, but no distinct PDF's.
Instead of relying on petrography, scientists have used chemical analysis to prove an impact origin here. While most of the glass is compositionally similar to the local sandstone, the glass does have high concentrations of Fe, Co, Ni, and Ir. All of these elements are extremely common in stony meteorites, and then ratios in the impact glass are consistent with ratios seen in meteorites. In addition, the Re-OS isotope ratios of the glass were measured. This method examines the admixture of small amounts of recondensed material with low Os ratios to target rocks with high Os ratios.
At Aouelloul, the Os ratios of the glass are extremely similar to those of both chondritic and iron meteorites. This suggests that the Os present in the glass came from a meteorite, as the amount present is much too high to have had a terrestrial origin. An absolute age of 3.1±0.3 million years was obtained by fission track and K-Ar dating of the impact glass.








Glasses with included differently clasts
Source: www.carionmineraux.com



Demolished and impregnated sandstone


Baked sedimentary debris


 


Glasses with included differently clasts


 
Sources: www.star-bits.com & www.meteoritearticles.com

 

Cambridge Conference Correspondence:
http://abob.libs.uga.edu/bobk/ccc/cc101598.html

THE PROBLEM OF CONFIRMING THE IMPACT ORIGIN OF SMALL CRATERS
C. Koeberl, W.U. Reimold, S.B. Shirey: The Aouelloul crater, Mauritania: On the problem of confirming the impact origin of a small crater
METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.3, pp.513-517

The impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features. A case in point is the 3.1 Ma Aouelloul crater (Mauritania), 390 m in diameter, which is exposed in an area of Ordovician Oujeft and Zli sandstone. We studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects. In thin section, a large fraction of the quartz grains show abundant subplanar and planar fractures. Many of the fractures are healed and are evident only as fluid inclusion trails. A few grains showed sets of narrow and densely spaced fluid inclusions >trails in one (rarely two) orientations per grain, which could be possible remnants of planar deformation features (PDFs), although such an interpretation is not unambiguous. In contrast, an impact origin of the crater is confirmed by Re-Os isotope studies of the target sandstone and glass found around the crater rim, which show the presence of a distinct extraterrestrial component in the glass (?).

Crater Tenoumer

The Tenoumer crater is about 1,900 m in diameter and is located in the western Sahara desert, Mauritania. The crater is almost perfectly round in shape, and the rim is 110 m high from bottom to top.
Tenoumer is located on a peneplain consisting of Precambrian gneisses and granites. A thin layer of sediments that are Pliocene or younger covers the Precambrian rocks. Tenoumer is located on top of the Pliocene sediments.
Due to the rare occurrence of fused igneous materials outside the crater, (basalt and rhyodacite) a volcanic origin was favored.
The origin of the crater Tenoumer by an impact is currently preferred. Basement rocks have been found in outcrops outside of the crater, located in individual bodies up to 20 m long. These rocks are dark grey, vesicular, and contain altered gneiss and granite clasts. Small fractured glass inclusions are also present. Several deformation features have been found at Tenoumer. The most prominent of these is planar deformation features in quartz grains; the presence of lechatelierite (glass), and deformation of biotite, feldspar, and other minerals.
 

"Like at Temimichat were found structures, which could be mighty shatter-cones. (probably not valid). Breccias are frequently and diversified. Glasses form thick layers on thrown out blocks at the periphery of the crater. In the northwest and in the northeast of the crater were found as basalt (dolerite ?) identified blocks." (Christian Laroubine, 2005)

Chemical compositions of impact melt breccias and target rocks from the Tenoumer impact crater, Mauritania
Authors: Pratesi, Giovanni; Morelli, Marco; Rossi, Angelo Pio; Ori, Gian Gabriele
Source: Meteoritics & Planetary Science, Volume 40, Issue 11, Pages 1563-1752 (November 2005), pp. 1653-1672(20)
Result:
The impact melt breccias from the Tenoumer crater (consisting of a fine-grained intergrowth of plagioclase laths, pyroxene crystals, oxides, and glass) display a wide range of porosity and contain a large amount of target rock clasts.
Analyses of major elements in impact melt rocks show lower contents of SiO2, Al2O3, and Na2O, and higher contents of MgO, Fe2O3, and CaO, than the felsic rocks (i.e., granites and gneisses) of the basement. In comparison with the bulk analyses of the impact melt, the glass is strongly enriched in Si-Al, whereas it is depleted both in Mg and Fe; moreover, the impact melt rocks are variably enriched or depleted in some REE with respect to the felsic and mafic bedrock types. Gold is slightly enriched in the impact melt, and Co, Cr, and Ni abundances are possibly due to a contribution from mafic bedrock.
Evidences of silicate-carbonate liquid immiscibility, mainly as spherules and globules of calcite within the silicate glass, have been highlighted.
HMX mixing calculation confirm that the impact melt rocks are derived from a mixing of at least six different target lithologies outcropping in the area of the crater. A large contribution is derived from granitoids (50%) and mica schist (17–19%), although amphibolites (15%), cherty limestones (10%), and ultrabasites (6%) components are also present.
The very low abundances of PGE in the melt rock seem to come mainly from some ultrabasic target rocks; therefore, the contamination from the meteoritic projectile appears to have been negligible.

Petrographic Investigation of Ejecta from the Tenoumer Impact Crater, Mauritania
Authors: Jaret, S.J. and  Kah L.C., Dep. of Earth and Planetary Sciences, University of Tennessee, French,  B.M., Dep. of Paleobiology, Smithsonian Inst.
Source: http://www.lpi.usra.edu/meetings/lpsc2009/pdf/1281.pdf
Result:
In 2003, a suite of 8 crystalline rocks was collected from Tenoumer ejecta. Specifically, the suite includes 4 samples collected from the crater rim, 2 samples from immediately outside the crater rim, 1 sample from the upper ejecta blanket, and 1 sample from the lower ejecta blanket.
Rim and near rim ejecta samples show no evidence of shock metamorphism (i.e., shocked quartz) and are indistinguishable in both hand sample and thin section from basement samples collected inside the crater .
The crystalline samples are amphibolite to green-schist facies metamorphosed granitic geneisses. Major minerals include K-feldspar, plagioclase, quartz, and biotite, with minor amounts of amphibole, apatite, and opaque minerals. Pre-impact dynamic recrystallization fabrics dominate the rocks. Quartz forms clean, small smooth crystals in narrow bands between larger feld-spar grains indicating metamorphic conditions between the melting points of quartz and feldspars.
Shock indicators occur primarily in the more distant ejecta samples and the melt rocks. Tenoumer melt rocks can be described as vesicular melt-matrix breccias, containing clasts of granitic basement in a plagioclase microlitic glassy matrix. Within the melt rocks, PDF’s oc-cur primarily within granitic clasts entrained in the matrix and rarely within individual quartz grains. Additional shock related features found include PDF’s in feldspars, lechatelierite, and ballen textures.
Flow structures within melt phases indicate rapid movement during molten stage. Within the shocked quartz grains, up to 5 sets of PDF’s were found, but grains most commonly exhibited 2-3 sets. Decorated PDF’s and heavily toasted quartz in the majority of samples indicates at least minor post shock alteration. Ballen quartz and lechatelierite are extremely common in the melt rocks and often occur together. The mean shock pressures of the Tenoumer melt rocks was found to be much lower than that required to melt quartz, supporting the notion of preferential melting of grains (or parts of grains) resulting from heterogeneities within the host rock.




Crater Tenoumer: Latitude 22° 55' N; Longitude 10° 24' W



Tenoumer  3D-picture


            
Crater: Uplifted granitoide basement in the picture on the right

   
  Source: http://www.saharamet.com/expedition/2003/crater.html
 



Volcanic gas bubbles in a vesicular matrix



Source: www.carionmineraux.com



 
Glassy crust



Volcanic melt with gas bubbles



 



Volcanic matrix



Sedimentary breccia



No impact-breccia



"Clasts of gneiss and granite embedded in a vesicular matrix,
 are found around the crater's rim."  Source: SaharaMet.com



Crater rim




In the foreground is visible a dyke on the bottom of the crater

Crater Temimichat


Crater Temimichat: Latitude 24°15' N; Longitude 9° 39' W

The Temimichat crater is located in northern Mauritania
(24° 15' N, 9° 39' W). The bedrock is made up of crystalline basement rocks, with no sedimentary cover. The structure has been listed together with other Mauritanian craters or crater-like features. According to POMEROL (1967), mafic rocks have been found in the area.
Basement rocks include granitoid gneisses and gabbros. The crater appears moderately eroded, with a rim height ranging from few meters to few tens of meters. Its diameter is about 700 m.

  

The crater rim is not completely preserved. Large portions are eroded and dissected, or masked by eolian deposits. The rim is mostly formed by granitoid gneisses. The low-lying portions of the rim correspond to the occurrence of gabbroic dikes, which seem to be more easily erodible, with respect to the dominant granitoid bedrock.The crater interior is covered by recent eolian Sediments. Below this surficial cover, sedimentary deposits are likely filling the crater.
Inside the granitoid gneisses of the rim, structures that strongly resemble pseudotachilite veins crop out discontinuously, and do not occur in the surrounding undisturbed basement. The term pseudotachilite is used with a descriptive meaning only, not implying any genetic process.
The pseudotachilites are dark to greenish veins of glassy material with fluidal texture at places. Inside these veins Small (less than 1 mm size) fragments of the host rock are present. Pseudotachilite veins mainly appear along small-scale faults, with offsets up to few centimeters. Shear zones are also present, with a brittle to brittle-ductile style. Inside the shear zones, appar-ently re-melted granitic clasts (with dimension of few millimeters up to few centimeters) are visible and their shape varies from angular to highly rounded.
The granitoid gneisses have an ipidiomorphic texture with a mineral assemblage formed by quartz, K-feldspar, plagioclase, biotite and some opaque minerals. The gabbroic bodies are composed mainly of plagioclase and amphiboles.
No ejecta blanket appears to be preserved outside the crater. All around the structure only eolian and fluvial deposits are present, with sporadic large rocky blocks that are also visible farer outside the crater, in the surrounding plain.
The time of formation of the structure is still unconstrained, but the present erosional level suggests a relatively old age of formation.
 

Conclusion: Temimichat crater has been poorly studied during recent times. No ejecta blanket appears to be preserved. The bedrock is formed of crystalline basement rocks, mainly granitoid gneisses, with locally gabbroic bodies. Granitoid gneisses locally show cataclastic deformation effects and are crosscut by dark veins, that show a striking resemblance with pseudotachilites. Veins are often pervasive. They are linked to small scale faults, with centimetric to decimetric offset. Brittle or brittle-ductile shear zones are associated with these veins, in which rounded granitic clasts also occur. Both veins and shear planes appear fresh, suggesting a relatively young age of formation. No regional brittle structures have been observed. The basement deformational style is ductile and no recent tectonic structures can be observed in the area. These observation could be consistent with the hypothesis of an impact origin for these pseudotachilitic veins. No cataclasites or vein appear in the gabbros.

Source: http://www.lpi.usra.edu/meetings/lpsc2003/pdf/1882.pdf

On the edges of the crater are distributes large blocks with shatter cones (probably not valid) and such with glassy cover. Some blocks are pasted with a characteristic glassy mass. Blocks of granite have been thrown on the western crater - edge. In this area are numerous breccias visible. (Christian Laroubine, 2005)



Source: http://www.saharamet.com/expedition/2003/crater.html
 


Crater rim of Temimichat


Uplifted and crushed granitoid basement material


"Granitic rocks with dark glassy veins are to be found at Temimichat"
Source: SaharaMet.com



"Pseudotachilite veins along shear planes in a faulted granitic block"



Rock with marks of hydrovolcanic metasomatism



Granitoid rock with marks of hydrovolcanic metasomatism


Crater El Mrayer (syn. El Mreiti, Mejaouda)
 

      
Crater El Mrayer: Latitude 22° 43' N; Longitude 7° 19' W

The crater has a diameter of about 3 km and it is located on Cambro-Ordovician substrate. The geometry appears rather complex: a subcircular inner ring is bound by an apparently concentric deformed region. Nearby (2 km toward N) a possible secondary small crater is visible.

Source: http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1309.pdf


Crater Gogui
 

      
Crater Gogui : Latitude 15° 50' N; Longitude 11° 40' W

The crater is about 500-600 m wide and shows a relatively
pristine morphology. The bedrock is constituted by Paleozoic metamorphic rocks.
It has a distinct circular rim and a rather flat bottom, apparently. The spatial resolution of the image is at the limit for this observations.
The age of formation of the crater could be very recent, looking
at its pristine aspect.

Source: http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1309.pdf

 

Some remarks: I have not studied the craters at the place. However it is very uncertain, whether the craters of Mauritania have emerged by an impact. There are no clear proofs for an impact. An endogenic origin is probable. There are to find clear proofs for hydrovolcanic metasomatism. The craters are similar of endogenic structures of Libya (Oasis, BP, Arkenu) and Egypt (Gilf Kebir). The Mauritanian structures however are builded by an climbed mafic magma. The crater rimes are pushed up. The material is composed of crushed granitoid and metamorphic basement material on Reguibat Shield (Temimichat, Tenoumer) or Paleozoic sandstone material (Aouelloul). The craters are in the detail clearly atypically for an impact event. They have no cut edges.
The pushed up material of Basement was changed by high pressure and heat. Magmatic processes are also responsible for the glassy injections in veins. Clear shock metamorphic features are not found in the granitoid rocks of the Basement. Polymicte breccia, glasses and PF's are no clear proofs for an impact event. Glasses and the other forms can emerge by high pressure and heat due to endogenic-magmatic processes.
I believe, the mafic magma plug (Dolerite ?) hidden at the bottom of the craters. On rims of the craters is evident to find mafic material. A volcanic explosion has not occurred. The craters are plugs. They were lifted up contemporaneously along an active tectonic line. At the region of Sfariat along such tectonic disturbances mafic dykes and plugs are spread. On the top of these structures the rock layers were erected in the vertical. That is typically for all structures, which are caused by a uplifted magma (see below). All mafic intrusions (dykes and plugs/craters) have emerged probably in the tertiary period.

 

Dyke (Sfariat)                                                          Diorite (?) (Sfariat)                                                                            Erected layers on a rim (Sfariat)                      

last update: 04.09.2010
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