The Tra-Tra Formation is a predominantly argillaceous, shallow marine to paralic sedimentary succession of Eifelian (Middle Devonian) age within the Bokkeveld Group (Cape Supergroup) that crops out extensively within the Cape Fold Belt of South Africa. It comprises three discrete lithofacies associations (termed E-G) which are interpreted as deposits of channelised tidal flat-lagoons, transgressive beach-barriers and wave-influenced prodeltas to distal delta-fronts. They accumulated within a series of incised coastal-plain valley-fill system along the palaeoshoreline of the Cape Basin following a protracted forced regressive phase during sedimentation of the underlying Hex River Formation. A discrete, geographically-extensive, 25 to 30 m thick, single or double, positive-weathering tabular sandstone within the Tra-Tra Formation is recognised herein as the Grootrivier Member. Palaeontologically, the Tra-Tra Formation comprises a restricted fauna of Malvinokaffric Realm invertebrates, fish and plant fossils that are of biostratigraphic importance in inferring its Eifelian age.

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