Fluvial Avulsions and the Mudrock Sandwich: Interpreting Successions of Mudrock and Coal in Alluvial Settings

Successions of mudrock sandwiched between beds of coal or carbonaceous shale are common in ancient alluvial successions. Recent work from a modern avulsion belt indicates that variations in organic content of fine-grained deposits in alluvial successions can be interpreted in relation to avulsion cycles. The Saskatchewan River at the Cumberland Marshes in Saskatchewan has been undergoing avulsion since the 1870's. The resulting avulsion belt consists of an unstable network of active and abandoned channels, splays, and floodbasins with mud accumulating within the floodbasins. The total organic content (TOC) of the mud depends on the amount of siliciclastic sediment received by the floodbasins with avulsion stage the most important control. The avulsion is currently waning, and the avulsion belt stabilizing, resulting in decreased floodplain sedimentation and a corresponding increase in the TOC of floodplain mud. Increased TOC of mud suggests a return to conditions of floodplain sedimentation prior to the current avulsion, when organic mud and peat accumulated. If preserved in the geological record, the fine-grained deposits of the Saskatchewan River avulsion belt would be recognized as mudrock sandwiched between thin beds of coal or carbonaceous shale. Similar successions of mudrock and carbonaceous shale from the geological record are found in the Lower Eocene Willwood Formation of the Bighorn Basin, Wyoming. It has been suggested that the Willwood deposits formed in association with avulsion, with limited pedogenic alteration of mudrock indicating rapid floodplain sedimentation on an active avulsion belt and carbonaceous shale reflective of limited sedimentation between avulsions. The modern example appears to confirm that packages of mudrock sandwiched between layers of carbonaceous shale or coal may be thought of as hiatus-avulsion-hiatus deposits in which organic deposits form during hiatuses in sedimentation between avulsion events and mud-rich deposits form during avulsion.