Organic matter deposition in the Saskatchewan River floodplain (Cumberland Marshes, Canada): effects of progradational avulsions

Abstract

This paper examines organic matter deposition in the avulsion-controlled floodplain of the Saskatchewan River at the Cumberland Marshes, east-central Saskatchewan. Organic-rich deposits were formed throughout the late Holocene in fen/bog (coarse brown peat) and lacustrine (fine dark-brown and black organic-rich mud) environments. The thicknesses of individual organic-rich layers reach 3.2 m but are typically less than 1 m thick. Lateral extents of these beds are limited by elevated alluvial ridges, but they may attain exceptional dimensions of 15 km in transverse and 50 km in longitudinal directions. Conditions for organic deposition on avulsion-controlled floodplains appear to be different from those on other floodplains. In addition to floods, terrigenous sediments are delivered by progradational avulsions into parts of the floodbasin previously accumulating organic-rich sediments. Elevated alluvial ridges resulting from former avulsions, however, block large parts of the floodplain from normal flooding and therefore prevent clastic deposition. Total organic content of floodplain sediments is inversely related to sediment grain size and sedimentation rate. The composition of organic-rich layers reflects a predominantly progradational avulsion style. The center portions of many organic-rich layers are commonly composed of brown fen/bog peat overlain successively by organic-rich and organic-poor lacustrine mud, eventually passing upwards into coarser deposits. The rate of avulsive progradation is roughly reflected in the vertical organic-content gradient within the alluvial sections. Rapid progradation, typical for most portions of the most recent (1870's) avulsion belt, results in relatively abrupt changes of organic content, whereas slower progradation is accompanied by more gradual decreases in organic content.