The East Arm graben contains a thick sequence of well-preserved, unmetamorphosed sedimentary rocks. Three units have been recognized: the Wilson Island Group (c. 2400–2300 Ma); the Great Slave Supergroup (c. 2300–1790 Ma), and the Et-Then Group (c. 1700–1600 Ma). The Wilson Island Group contains red beds and overlies an apparently normal regolith formed on Archaean granites. The Great Slave Supergroup contains a normal regolith, numerous red beds, glauconitic horizions, biogenic hydrocarbons and sedimentary uranium deposits. There is also abundant evidence for the former presence of evaporites in that part of the sequence that was deposited within 30° of the palaeoequator.

The uranium deposits originated during low temperature diagenesis by precipitation from oxidizing waters in areas of early accumulated hydrocarbons. These observations suggest that the Lower Proterozoic climate, atmosphere and hydrosphere were similar to modern ones and that uranium prospectors in such old terrains should look for chemical deposits as well as the classic placer types.

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