Factors affecting the accumulation of organic-rich deposits in a modern avulsive floodplain: Examples from the Cumberland Marshes, Saskatchewan, Canada

Since the 1870s, the Saskatchewan River at the Cumberland Marshes (east-central Saskatchewan, Canada) has been undergoing progradational avulsion, resulting in development of a 500 km(2) avulsion belt. The avulsion belt consists of an unstable network of active and abandoned channels, splays, and floodbasins, some containing small shallow lakes. Fine-grained deposits of varying total organic content (TOC) are currently accumulating within floodbasins inside, adjacent to, and outside the avulsion belt. The TOC of these deposits depends on factors that control the amount of siliciclastic sediment received by floodbasins. Five factors that affect TOC content are identified: distance from an active channel, position relative to the avulsion belt, presence of alluvial ridges, changes in local channel activity, and developmental stage of avulsion. A channel-to-basin trend of increasing TOC is present at many locations, reflecting greater suspended sediment loads received by near-channel areas, and locations within the avulsion belt generally accumulate sediment with lower TOC than those adjacent to or outside the belt. Alluvial ridges locally act as barriers to floodbasin deposition, and channel abandonment reduces suspended sediment to adjacent floodbasins, both of which increase TOC of sediment. Finally, as avulsion wanes, less siliciclastic sediment is distributed throughout the avulsion belt, resulting in increased TOC. In the geological record, such fine-grained avulsion deposits are likely to appear as intervals of mudrock between thin beds of coal or carbonaceous shale formed from organic-rich sediment deposited between avulsion events, a succession that is common in many ancient alluvial strata.