THE PALEOZOIC RISE OF AVULSIVE FLUVIAL SYSTEMS AS TERRESTRIAL VEGETATION EVOLVED

Avulsion is the permanent relocation of river channels on an aggrading alluvial plain through the generation of new reaches or the reoccupation of pre-existing reaches. A fundamental aspect of modern alluvial plains, avulsion was much less frequent and varied in Precambrian and early Paleozoic fluvial systems. A range of avulsive river strategies arose over about 150 million years from the Ordovician to the Pennsylvanian, as rivers diversified in parallel with the evolution of terrestrial vegetation. Cambrian and Ordovician rivers were wide, shallow sandbed systems, in which avulsion may have been largely nodal as rivers fanned out below upland exit points. In these low-sinuosity systems, local avulsion (the channel rejoins its course downstream) would have been uncommon, and most avulsions were probably regional (forming new courses). Meandering single-thread channels appeared in the latest Silurian, as indicated by heterolithic lateral-accretion deposits in > 30% of preserved fluvial rock units by the late Devonian. This change accords with the incoming of rooted vegetation, which stabilized river banks and promoted systematic channel migration. Within such sinuous channels, local neck and chute cutoff would have been prominent. From the Pennsylvanian onwards, strategies for dryland colonization, especially by early conifers, lead to increased vegetation cover, enhanced root systems, and abundant large woody debris. These changes probably account for the rise of narrow channels with stable banks and vertical aggradation that constituted > 10% of fluvial units by the end of the Pennsylvanian. Some of these avulsive systems were probably anabranching. Within Pennsylvanian braided-river deposits, evidence of deep channels with abundant wood and log jams suggest that island-braided or wandering systems had become common. In contrast to many modern rivers from which woody debris has been removed, log jams may have greatly influenced avulsion in these systems. By the end of the Pennsylvanian, alluvial plains would have had far more active and abandoned channels per unit area, a greater length of riparian corridors, a wide range of avulsive strategies, and avulsion belts similar to those of modern plains. This analysis underscores the importance of vegetation in promoting avulsion.