DEPOSITION OF ORGANIC RICH SEDIMENTS IN FLOODPLAIN LAKES IN AN AVULSIVE RIVER SYSTEM (CUMBERLAND MARSHES, SASKATCHEWAN, CANADA)

Organic-rich deposits are common in many modern and ancient alluvial floodplains, some of which are considered to have high potential for coal and hydrocarbons, but little is known about the deposition and accumulation of organic matter on avulsion related river floodplains. The Cumberland Marshes sometimes referred to as the “Saskatchewan Delta” represents a large (5000 km2) floodplain region of the Saskatchewan River in central Canada. The Cumberland Marshes is prone to avulsions. In the 1870's the Saskatchewan River went through its most recent major avulsion in which anastomosed distributary channels prograded into wetlands and shallow lakes and deposited splays and lacustrine deltas over preavulsive organic-rich deposits. Avulsion is the process by which a river channel is diverted to a new course at a lower elevation on the floodplain; it is a consequence of floodplain aggradation. The Saskatchewan Delta is a modern example of ancient avulsive systems and can be used to increase our knowledge about ancient systems, which can help with future coal exploration.

This research focuses on the accumulation of organic matter in shallow floodplain lakes (less then 1 m) located in the Cumberland Marshes. Samples were taken along transects from floodplain lakes inside, adjacent to, and outside the avulsion belt to determine trends in organic deposits and their purity. The analysis of the Total Organic Content (TOC) by combustion from the four floodplain lakes yielded values between 3-82%. The average TOC values for lakes outside or adjacent to the avulsion belt were in the range of 9-82% while the lake inside the avulsion belt had values of 3-7%. This can be explained by the constant movement of water and sediment within the avulsion belt. Avulsion processes, over bank flow, and associated sediment inputs appear to act as controls on organic deposition. This indicates that organic matter is accumulating outside the floodplain at higher concentrations then inside the avulsion belt. It would seem there is a link between avulsion processes and organic deposition and accumulation in lakes.