Agueni formation
Coordinates: 20 ° 30 ′ N , 13 ° 10 ′ W
The Agueni Formation is the lowest formation in the Taoudenni Basin of West Africa . The continental to shallow marine formation, consisting mainly of fossil sandstones , was deposited on the archaic Reguibat shield around 1200 million years ago BP .
etymology
The formation is named after its type locality Agueni (or Aguenni), a small settlement around 10 kilometers west-southwest of Atar in the Adrar Mauritania .
geography
The Agueni formation forms the southeastern border of the Reguibat shield. Starting from the type locality, it strikes in a band up to 10 kilometers wide to the north-northeast to Zouérat . To the south-southwest it continues 35 kilometers further, and then disappears at Terjit under the Atar Cliffs group , which it covers discordantly . In contrast to the Azougui Formation that follows it, it forms a distinct layer ( Cuesta ) in the terrain .
stratigraphy
The formation, which is a maximum of 160 meters thick, belongs to the Char group and thus to super group 1, lies with an angular discordance on the basement of the Reguibat shield. It is concordantly overlaid by the Azougui Formation , also part of the Char group. In the southern sector of the formation south of the Ntouskes Fault, the Azougui Formation is missing due to erosion; the Agueni formation is therefore concordantly covered by the Foum choir formation of the Atar group .
Facies
Facially, the sandstones of the Agueni Formation belong to a continental, a coastal and several shallow sea facies. The individual facies areas are separated from each other by clear facies boundaries ( English bounding surfaces ).
Lithostratigraphy
Lithostratigraphically , based on these facies boundaries, three units can be distinguished (from hanging to lying ):
- Unit III
- Unit II
- Unit I.
Unit I , which is around 80 meters thick, begins with a coarse-grained to conglomerate continental facies that transgresses over the Reguibat basement . The deposited sediments can be used as a Zopfstromsystem (Engl. Braided stream ) are interpreted properly. This continental facies is not persistent, but rather fills local depressions (in the decameter range) in the erosion surface of the basement with up to 1 meter of sediment - either with lenticular breccia and / or with unsorted, conglomerate arkose sandstone . Incorporated angular clasts of the reclaimed basement made of granite , quartzite or pure quartz can be up to 50 centimeters in size. Above this, when the sea level is gradually falling ( regression ), sandy tidal sediments (tidal sandbanks) , which are deposited, follow the chute and reach a thickness of up to 55 meters. They are replaced in the hanging wall by up to 30 meters thick, also sandy bar sediments , which indicate a gradual return of the muzzle bars to the beach (transition from distal to proximal facies). Unit I ends with a discordance.
Above the facies boundary, Unit II , which is almost 30 meters thick, begins again with a transgressive continental facies that is now 5 to 10 meters thick, which, as in Unit I, is due to a pigtailed current network. A horizontal transgression surface, which indicates the end of the river sedimentation, is followed by a coastal facies up to 5 meters thick after the return of the sea. After the maximum flooding surface ( MFS ) has been reached, the regressive half-cycle begins with only a few meters of distal, structureless, green, mica-rich silts and clays of the outer shelf. Above this, 15 meters of sediments of the inner shelf, dominated by the swell, lay down.
Unit III , which reaches 55 meters, begins similarly to units I and II transgressively with a pigtail facies, which is much more significant at 20 meters. It is also replaced by around 10 meters of heterolithic coastal plains sediments. After almost 5 meters thick lagoon sediments , the Agueni Formation finally ends with tidal sediments of the Schorre measuring just over 20 meters, which culminate in an MFS.
Up to 120 meters of the Azougui Formation, which is built up only from a single tidal facies with a mixed siliciclastic- calcareous character , then lie concordantly on this MFS .
Changes in thickness and facies
Unit I is reduced over a distance of 150 kilometers from 80 meters in the north to 60 meters in the center to just 30 meters in the south. In complete contrast to this, Unit II, which experiences an increase in thickness from 25 meters in the north over 45 meters in the center to 75 meters in the south. Unit III initially remains constant at 55 meters, but then decreases to 35 meters in the south. The total thickness is very similar, which is 160 meters in the north and in the center, but is reduced to 135 meters in the south.
The bar facies of Unit I in the north are still present in the center, but then wedge out to the south. Unit II experiences an interposition of distal and proximal bar facies under the inner shelf sediments in the center. In the south, the inner shelf sediments are even completely replaced by 50 meter thick tidal sediments. Unit III, on the other hand, remains constant and shows no change in facies.
Directions of flow
The evaluation of the flow directions based on inclined stratifications and ripples resulted in a very inconsistent and changeable picture for the individual units. In Unit I, the sand bars were mainly transported roughly parallel to the coastline at that time to the east-northeast, whereas the tidal sediments above were mainly moved along the coast to the north-northeast. The continental facies of Unit II again shows a preferred direction of flow to the east. The following tidal sediments in the southern sector, however, deviate greatly from this - their sand waves indicate a migration to the south and, to a lesser extent, even to the northwest. Unit III's tidal sediments eventually moved north.
The continental sediment input of the Reguibat shield thus generally took place via an interwoven river system from west to west-south-west. On the flat shelf , however, the conditions were sometimes very variable.
Sedimentary structures
The sedimentary structures in the Agueni Formation include sloping layers, various ripple shapes, wadding channel fillings ( gutter casts ), groove marks and dry cracks . Trough, table, bulge and herringbone inclined layers function as inclined layering types. The ripple shapes include flow, wave, adhesion, conductor, and interference ripples (both polygonal and wave types).
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 older, varying between 1109 and 1105 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 Agueni Formation has a minimum age of 1109 million years BP and originates from the Stenium.
meaning
The internal stacking pattern of the three units of the Agueni formation is characterized in particular by a change in the nature of the regressive half-cycles. The latter are increasingly controlled by marine processes towards the hanging wall - if Unit I is still a fluvial-dominated coast and Unit II is a coast dominated by waves, then in Unit III there are already recurring tide ramps. This indicates a generally regressive trend. However, it is of course also conceivable that the sediment supply decreased steadily due to the structural development of the platform arrangement. It is possible that the two processes also worked hand in hand.
The frequent thickness fluctuations and facies changes within the Agueni Formation indicate tectonic instability of the West African platform during the deposition. Disturbances such as the Ntouskes fault evidently had a major influence on the sedimentation process. This instability may be related to the rifting of the Pan-African-Brazilian supercontinent from the south, which occurred between 1100 and 1000 million years ago BP.
swell
- 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
- ↑ Clauer, N .: Rb-Sr and K-Ar dating of Precambrian clays and glauconies . In: Precambrian Research . tape 15 , 1981, p. 331-352 .
- ↑ 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 .
- ^ 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 .
- ^ Trompette, R .: Geology of western Gondwana (2000 - 500 Ma). Pan-African-Brasiliano aggregation of South America and Africa. Balkema, Rotterdam, Brookfield 1994, pp. 1-350 .
- ^ Porada, H .: Pan-African rifting and orogenesis in Southern to Equatorial Africa and Eastern Brazil . In: Precambrian Research . tape 44 , 1989, pp. 103-136 .