THE ACCUMULATION OF ORGANIC DEPOSITS DURING AVULSION: EXAMPLES FROM THE CUMBERLAND MARSHES OF SASKATCHEWAN, CANADA

Since the 1870's, the Saskatchewan River has been undergoing progradational avulsion that has resulted in the development of a 500 km² avulsion belt surrounded by wetlands in an area known as the Cumberland Marshes. The avulsion belt consists of an unstable network of active and abandoned channels, crevasse splays, floodplains and local floodbasins, some containing small shallow lakes. Organic deposits currently accumulate in the uppermost 20 cm of substrate in floodbasins inside the avulsion belt and in shallow lakes and floodplains next to and outside of the avulsion belt. Samples of organic deposits were collected from transects across these environments and analyzed for their organic content. Many of them have organic contents of over 70 % with variation from maxima of over 90% to minima of 10%. Although surface deposits with lowest organic contents are generally found in floodbasins and lakes inside the avulsion belt, some samples display organic contents exceeding 70%. The high organic content of these surface deposits appears to depend on local factors such as channel abandonment and the presence of pre-avulsion features that act as barriers to sediment transport. Regardless of position within or outside the avulsion belt, the organic content of surface substrate increases with distance from an active channel. This channel-to-basin trend reflects greater suspended sediment loads received by near-channel areas than by areas distal to channels. Where the proximal-to- distal trends are not strong, floodplain geography and the influence of multiple channels appear to dominate. A prominent proximal-distal trend of increasing organic content is observed in one lake lying outside the avulsion belt but connected to an active channel via an inlet channel. Conversely, in lakes with no connection to an active channel, there are no discernible trends in organic content.

Although the instability and active siliciclastic deposition normally associated with avulsion seems incompatible with the accumulation of organic-rich deposits, such deposits are commonly associated with the 1870s avulsion belt of the Saskatchewan River. In the geological record, these deposits are likely to appear as thin, laterally continuous beds of carbonaceous mudrock showing lateral trends in organic content within avulsive successions of poorly developed floodplain paleosols.