Abstract

The Mattagami Formation represents a high-constructive segment of a major large scale river system which drained an extensive tract of the Canadian Shield, eventually debouching into the Albian epeiric seas of western Canada. The deposits consist of poorly consolidated silica sands, clays and lignite. The clays show great variation in colour and organic content, and often exhibit well-developed soil textures. These overbank deposits represent a greater proportion of the total sediment accumulation than the associated channel sands. Evidence of vertical channel stacking, with limited lateral accretion, can be seen in exposures along Adam Creek in Kipling Township. Stacked channel sands pass laterally into organic-rich levee and crevasse splay deposits. These grade into mudrocks with low organic content, representing floodplain soils and pond deposits.

Thin lignites are found along channel margins associated with levee and splay deposits. Thick coal accumulations, such as those at Onakawana, appear to have formed only in those parts of the floodplain adjacent to contemporary topographic highs developed upon underlying Devonian bedrock. These marginal areas were more protected than the interchannel floodplains, with only limited flood inundation, allowing uninterrupted peat accumulation