Gai-As Formation

Gai-As Fm

Period:

Permian

Age Interval:

Early-Middle Permian (Kungurian-Wordian)

Province:
SouthAfrica-Namibia-Botswana

Type Locality and Naming

Huab Basin. Named after the Gai-As area (ruins of a colonial German police station located at a spring) in the central Huab Basin of northwestern Namibia.

Synonym:

Lithology and Thickness

Approximately 70 meters thick. Dominated by a characteristic reddish to violet color. It generally forms an upward-coarsening sequence: Lower unit: Distal, shallow lacustrine reddish-violet calcareous shales and mudstones with concretionary calcareous layers and intercalated thin sandstones (tempestites/turbidites). Upper unit: Interbedded sandstones, siltstones, stromatolitic limestones, bone beds of fish scales, and highly distinct white porcelain-like volcanic tuff layers (ash falls).

Lithology Pattern:

Sandstone

Relationships and Distribution

Lower contact

Rests on the underlying Huab Fm (equivalent to the Whitehill Fm). In the west, the contact is gradational; in the east, it is a sharp contact marking a significant depositional hiatus.

Upper contact

Unconformably overlain by the fluviatile Doros Fm or the massive aeolian sandstones of the Early Cretaceous Etjo Fm / Twyfelfontein Fm.

Regional extent

The most widespread depositional unit within the Huab Basin of northwestern Namibia. Formally falls under the Namibian equivalent of the Karoo Supergroup, often correlated chronostratigraphically with the Beaufort Gr and uppermost Ecca Gr (South Africa) and the Passa Dois Gr (Serra Alta and Teresina Formations) in the Paraná Basin of Brazil.

GeoJSON

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Fossils

Age

Kungurian to Wordian stages. The age is tightly constrained by U/Pb SHRIMP radiometric dating of zircons extracted from the volcanic tuff beds in the upper formation, yielding dates of 272 ± 1.8 Ma and 265 ± 2.5 Ma.

Age Span:

Beginning stage:

Kungurian

Fraction up in beginning stage:

0.0

Beginning date (Ma):

283.30

Ending stage:

Wordian

Fraction up in the ending stage:

1.0

Ending date (Ma):

264.34

Depositional setting

A massive, shallow, and highly alkaline intracontinental lake system (the "Gai-As Lake"). The formation records a progressively shallowing and coarsening-upward lacustrine environment.

Depositional pattern:

Additional Information

Compiler:

From the O. Catuneanu et al. / Journal of African Earth Sciences 43 (2005) 211–253; Stollhofen, H., et al. (2000): The Gai-As Lake System, Northern Namibia and Brazil; Wanke, A., et al. (2000): Literature on the stratigraphy and the occurrence of the volcanic tuff horizons used for U-Pb dating.