Abstract

The Neoproterozoic Kuujjua Formation is an up to 120 m thick, texturally submature quartzarenite typified by laterally persistent tabular planar cosets of simple and compound planar crossbedding, intercalated with rare, thin dolomitic siltstone lenses up to tens of kilometres wide. It is interpreted as the deposit of a big river, occupying a braid plain, at least 150 km wide, which flowed into the Amundsen Basin from the southeast. The dominant elements of this deposit are stacked tabular and laterally continuous compound crossbeds, interpreted as very large channel forms, which migrated mainly by lateral accretion of superposed small- to moderate-scale two-dimensional dunes. Simple planar crossbedding represents moderate to large two-dimensional periodic bedforms deposited in channels. Rare trough crossbedding represents three-dimensional dunes, which probably were deposited in narrow low-stage chutes that cut across the larger bedforms. Dolomitic siltstone lenses are interpreted as deposits of large flood basin playa lakes that were periodically rejuvenated by river floods. Unrestricted migration of the channels back and forth across the braid plain reworked many of the thin lake deposits and produced the observed multistoried sandstone sheet geometry. A prevailing arid climate is indicated by the occurrence of evaporite casts and pseudomorphs in the flood-basin deposits. The Kuujjua Formation shares features with deposits of the Brahmaputra River; however, there appear to be no modern analogues for the thick, large-scale braided-stream deposits that characterized many Proterozoic cratonic basins.

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