Azougui formation

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The Azougui Formation is the second oldest formation in the Taoudenni Basin of West Africa . The formation, made up of tidal sediments from an evaporite platform, was deposited in the stenium about 1150 million years ago BP on the archaic Reguibat shield .

etymology

The formation is named after its type locality Azougui , an oasis settlement around 8 kilometers northwest of Atar in the Adrar of Mauritania .

geography

The Azougui Formation follows the southeastern boundary of the Reguibat Shield (Amsaga). Starting from the type locality, it strokes in a very thin band only 2 kilometers wide to the north-northeast in the direction of Choum and Zouérat . Before Choum, the formation is veiled by the northeast-southwest- trending sands of Erg Akchar . It continues to the south-southwest for another 20 kilometers until it is suddenly cut off by the Ntouskes Fault .

stratigraphy

The formation, on average 90, in the northern sector also up to 120 meters thick, was defined by Trompette in 1973. It lies concordantly on the Agueni formation . Both formations together form the Char group of Supergroup 1 . The Azougui formation is followed discordantly by the Foum choir formation , which already belongs to the Atar group .

Facies

The Azougui Formation consists of a tide- dominated, mixed siliciclastic-carbonate shallow water facies. In detail, four types of facies can be distinguished:

  • Mudstone facies
  • Dolomitic sandstone facies
  • Sandy dolomite facies
  • Massive dolomite facies

The claystone facies alone make up 60% of the Azougui Formation, the other three types of facies share the remaining 40%.

Mudstone facies

The violet-colored mudstone -Fazies is very bad layered. In the central and southern sectors it is more silty and contains well-rounded quartz grains in the millimeter range. The quartz grains float either in a silty to sandy base mass or in lenticular coarse-grained sand strips in the centimeter range.

The structureless claystone facies can be interpreted as low-energy, subtidal sediment. It is somewhat similar to the sediments in a lagoon , but for which there are no signs of barrier islands or thresholds.

Dolomitic sandstone facies

The dolomitic sandstone facies are made up of medium-grain dolomitic sandstones that appear as 10 to 50 centimeters thick sandstone banks. The coarse-grained, erosive base of the benches contrasts sharply with the person lying down. The benches are characterized by horizontal to wavy parallel layers. But they can also be layered diagonally and contain silt scraps. Their hanging wall surface is covered by wave or current ripples.

The facies can be found in the central and southern sectors, especially in the lying area of ​​the Azougui Formation. In the north sector and in the hanging wall it is increasingly of carbonate and there are typical tidal features, such as herringbone patterns ( English herringbone structures ), sigmoidal bundle and with dry cracks paired clay skins.

Sandy dolomite facies

The sandy dolomite facies appear with 50 to 60 centimeter thick dolomite banks , which typically disintegrate into 1 to 3 centimeter thick plates. As internal structures, it shows undulating parallel stratification and rising wave and flow ripples , which can alternate their sense of direction from position to position. In the southern and central sectors, rock salt models were found on the underside of many slabs of the bank .

The dolomitic sandstone facies and the sandy dolomite facies were sedimented by means of shallow, bimodal currents in the low-energy intertidal area. Bimodal sloping layers and ripple structures indicate low-energy tidal currents. The clay-covered foresets and the rock salt models imply metasaline low water and the dry cracks show that the water has dried out from time to time. The intertidal conditions are also underlined by herringbone inclines, tidal bundles and clay-sand couplets.

Massive dolomite facies

The massive dolomite facies are made up of massive, 10 centimeter thick dolomite bank stacks that form 1 to 2 meter thick horizons. In the structureless benches of the person lying down there are usually chert bulbs with a diameter of 20 centimeters, which are flattened parallel to the stratification. The benches in the hanging wall have wavy and irregular algae mats , which are concealed in places. The algae mats merge into dome-shaped stromatolites on the hanging wall . These measure 10 to 20 centimeters in diameter in the central sector, but form 0.5 to 2 meter tall buildings in the north.

The massive dolomite facies are considered to be supratidal. This is supported by flat-laminated stromatolites with dry cracks, the latter causing the brecciation of algae mats and stromatolite domes. The stratigraphic position of these facies, wedged between intertidal sandy Dolomites and subtidal claystones, allows this conclusion. The large stromatolite structures in the north sector, on the other hand, are interpreted as subtidal reefs in analogy to modern examples . Very similar stromatolite structures were also described as subtidal in the Proterozoic Transvaal Supergroup in South Africa .

Lithostratigraphy

The four facies associations listed above can be organized into different sequences. Three types of sequences can be distinguished:

  • Couplets made of claystone-dolomitic sandstone with grain size increase towards the hanging wall
  • Sequence of claystone-dolomitic sandstone-sandy dolomite-massive dolomite
  • Sequence of claystone-sandy dolomite-massive dolomite.

The couplets are repetitive. They can be found in the southern sector and in the lying area of ​​the Azougui Formation.

The second type of sequence does not always have to be fully developed. There is a steady increase in the carbonate content in it, with the formation of algae mats and dome-shaped stromatolites. These rather complex sequences are restricted to the middle section of the central sector of the formation, whereas incomplete sequences such as e.g. B. claystone-sandy dolomite or claystone-massive dolomite can only be found in the upper section.

The third sequence is characteristic of the northern sector. The dolomite plates of the massive dolomite, which is mainly composed of large stromatolite reefs and dolomite breccias, are structureless.

The first and second types of sequence are interpreted as the deposition environment becoming shallower towards the hanging wall. Sequence type 3 marks an initial shallower followed by lowering.

interpretation

According to Klein (1977), the facies sequences described were deposited in the tidal near-coastal area / coastal area. The shelf platform should have been a ramp with mixed siliciclastic-carbonate sedimentation. Very similar examples from the geological past can be found in the Lower Permian of New Mexico or in the middle Jurassic of the Catalan Basin .

Age

Supergroup 1 of the Taoudenni Basin 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 (Agueni formation) there were 998 ± 34 million years BP and for the final Assabe-el-Hassiane group around 695 million years BP.

However, a new dating using the rhenium-osmium method by Rooney and colleagues (2010) revealed 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.

As a result, the Azougui Formation has a minimum age of 1109 million years BP and originates from the Stenium.

meaning

Overall, the Azougui Formation represents a regressive half-cycle that begins at the base of the formation when the sea level is high ( maximum flooding surface or MFS ). The hanging wall is then followed by repetitive couplets or sequences that document a gradual drop in sea ​​level . The drop in sea level did not occur steadily, but erratically, as evidenced by the aperiodic vertical arrangement of these sequences. This unsettled behavior may be related to the Ntouskes Fault, the movements of which affected the mixed siliciclastic-carbonic shelf ramp.

literature

  • 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 .

Individual evidence

  1. ↑ Trompette, R .: Le Précambrien supérieur et le Paléozoïque inférieur de l'Adrar de Mauretanie (bordure occidentale du bassin de Taoudeni, Afrique de l'Ouest). An example of the sédimentation de craton. Etude stratigraphique et sédimentologique. Thèse Univ. Aix-Marseille III 1973, p. 1-702 .
  2. ↑ Eriksson, KA: Tidal flat and subtidal sedimentation in the 2250 MY Malmani dolomite, Transvaal, South Africa . In: Sedimentary Geology . tape 18 , 1977, p. 223-244 .
  3. ↑ Klein, G. de V .: Tidal circulation model for deposition of clastic sediment in epeiric and mioclinal shelf seas . In: Sedimentary Geology . tape 18 , 1977, p. 1-12 .
  4. ↑ Calvet, F. et al .: Middle Triassic carbonate ramp in the Catalan basin, northeast Spain: facies, system tracts, sequences and controls . In: Int. Assoc. Sediment. Spec. Publ. Volume 9 , 1990, pp. 79-108 .
  5. ↑ Clauer, N .: Rb-Sr and K-Ar dating of Precambrian clays and glauconies . In: Precambrian Research . tape 15 , 1981, p. 331-352 .
  6. ↑ 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 .
  7. ^ 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 .