THE DEVELOPMENT AND DISTRIBUTION OF ORGANIC DEPOSITS ON A MODERN AVULSIVE FLOODPLAIN: EXAMPLES FROM THE CUMBERLAND MARSHES, SASKATCHEWAN, CANADA

Coal and carbonaceous shale are common components of fluvially deposited successions. They have frequently been described as accumulating in back swamp environments, distal to active channels. Accumulation and preservation of organic deposits in such environments is dependent on minimal sediment influx and a waterlogged substrate. Limited input of sediment maintains the high organic content of the accumulating deposit whilst preservation is facilitated by high water tables that prevent organic decay by oxidation. Coals and carbonaceous shale have been reported from fluvial avulsion sequences. However, the instability of an avulsion belt appears to be incompatible with conditions required for organic deposits to accumulate and it is unclear where in an avulsion belt conditions conducive to the accumulation of organic deposits exist. The Cumberland Marshes of Saskatchewan, Canada, have developed in response to a well-documented avulsion of the Saskatchewan River that began in 1870. Organic-rich clay and peat (precursors to carbonaceous shale and coal) appear to be accumulating on floodplains and in shallow lakes within and outside the present avulsion belt. Cores from transects across these floodplains and lakes contain clay, organic-rich mud and peat as well as surface accumulations of modern roots and vegetation. Organic–rich accumulations in shallow lakes contain significant proportions of clay but limited amounts of peat. Peat is the predominant organic deposit found in floodplain cores. In general the highest levels of total organic carbon (TOC) content in organic deposits occurs near the centre of floodplains whilst lowest levels are close to the floodplain levees. However on some floodplains peat with little clastic material is observed within a few hundred meters of active channels. As yet no trends in TOC content have been found for lake transects Organic-rich deposits that are the precursors to both carbonaceous shale and coal do form on floodplains and in shallow lakes close to active channels within and close to an avulsion belt. The waterlogged nature of these environments exerts a control on the preservation of organic deposits by limiting degradation of organic material. Controls on the limitation of sediment influx are harder to discern but distance from the channel does appear to be a factor in the purity of floodplain peat.