ORGANIC DEPOSITS ACCUMULATING IN AN AVULSIVE RIVER SYSTEM: IMPLICATIONS FOR COAL AND CARBONACEOUS SHALE FORMATION

Organic-rich deposits are well known from both modern fluvial environments and ancient fluvial successions. Models for their formation emphasize stable, waterlogged environments that allow plant material to accumulate slowly without degradation. Fluvial systems prone to avulsions are inherently unstable and seem to offer limited potential for the accumulation and preservation of organic deposits. The Cumberland Marshes area of the Saskatchewan River in Canada is possibly the best-studied example of a modern avulsive river system. The current avulsion in this location began in 1870 and the river is still in a state of flux with numerous active channels. Despite this unstable setting organic-rich deposits are accumulating on floodplains within and close to the active avulsion belt; within the avulsion belt total organic carbon (TOC) contents of greater than 60% have been recorded 300 meters from the levee of an active channel. Sampling along transects has revealed general trends in the amount of organic material contained in floodplain deposits. Deposits close to channels are less organic rich than those found in the center of floodplains and there is a correlation between the TOC content of the substrate and distance from channel levees. However, this correlation, and the maximum and minimum values for TOC content of the substrate, does vary among floodplains. These variations can be attributed to local factors such as the location of alluvial ridges and the recent history of nearby channel activity. The ongoing accumulation of organic-rich substrate in the active avulsion belt of the Cumberland Marshes suggests that there are conditions within avulsive systems that are conducive to the formation of organic deposits. This has important implications for our understanding of the type and nature of fluvial environments in which coal and organic-rich shales formed.