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Compiled by: Jacques LeBlanc (2022), Stratigraphic Lexicon: The Sedimentary Formations of The Republic of Niger, Africa. Colnes Publishing (Tallin, Estonia), 365 pp. For more information see "About"

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Akakus Formation

Akakus Fm

Period:

Silurian

Age Interval:

Silurian (Wenlock-Ludlow) (9)

Province:
Djado Basin

Type Locality and Naming

Desio, 1936a, 1936b. Origin from Jabal Akakus in the Ghat area, southwest Libya, 100 km N of Takarkhouri Pass. However, this description was incomplete, and a replacement type section was established by Klitzsch (1969) from an area to the south of Ghat where a complete section is present. (Fig. 2)

[Figure 1: Index map of North African Silurian outcrops (black) and subsurface (grey) (Source: Berry et al., 1973)]

[Figure 2: Stratigraphic column of the Akakus Formation at Klitzsch’s Type Locality in Libya (Source: Shalbak, 2015)]

References: Desio, 1936a, 1936b; Plauchut & Faure, 1959; Jacqué, 1963; Freulon, 1964; Klitzsch, 1969; Massa & Jaeger, 1971; Berry et al., 1973; Lababidi et al., 1985; Mergl et al., 2001; Shalbak, 2015; Hallett, 2002, 2016;

Synonym: The former name of the formation was spelled “Acacus”. The spelling was modified to its current form by Hallet (2002) after verifying the official name records of localities in Libya; Equivalent(s): "Formation des Alternances Grèso-Argileuses" [Sandy-shale Formation] in the Ghadames Basin of Libya. Lower part of "Grès Supérieurs de Tassili".

Lithology and Thickness

Mainly composed of fine-grained micaceous sandstones and siltstones, outcropping in both flanks of the Djado Basin. Freulon (1964) describes it as "Generally quite fine sandstones, often with cross-bedded stratification and ferruginous cement. Pelitic levels show numerous Harlania". Freulon (1964, page 105) states a thickness of about 100 meters in the Djado. The total thickness of the Silurian (Tanzuft & Akakus) varies, ranging between 150 to 300 m in the western Djado Basin. Thickness is more reduced along the eastern margin of the Djado Basin, with a maximum of 120 m, but Silurian rocks may be also totally absent.

Lithology Pattern:

Fine-grained sandstone

Relationships and Distribution

Lower contact

Disconformable contact with the underlying Tanzuft Fm

Upper contact

Disconformable contact with the overlying Tadrat Fm

Regional extent

Djado Basin

GeoJSON

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Fossils

Massa and Jaeger (1971) cited the following fossils, primarily ichnofossils, from the formation: Harlania (=Arthrophycus), Cubichnia, Risophycus bilobatus, Palaeophycus, Pelecypodichnis, Cruziana furcifera, Cruziana irregularis, Dimorphichnus, Acutiramus, several mollusks, and trilobite fragments.

Age

Wenlock-Ludlow (Fig. 1)[Figure 1: Age shoreline map of the Akakus Fm]

Age Span:

Beginning stage:

Homerian

Fraction up in beginning stage:

0.0

Beginning date (Ma):

430.62

Ending stage:

Gorstian

Fraction up in the ending stage:

1.0

Ending date (Ma):

425.01

Depositional setting

Shallow water. Massa and Jaeger (1971) suggested that the fossils indicated a marine or lagoonal environment of deposition

Depositional pattern:

Additional Information

Compiler:

Jacques LeBlanc (2022), Stratigraphic Lexicon: The Sedimentary Formations of The Republic of Niger, Africa. Colnes Publishing (Tallin, Estonia), 365 pp. https://doi.org/10.47909/978-9916-9760-6-7 (or via https://sites.google.com/site/leblancjacques)