Evolution of fixed-channel alluvial plains in response to Carboniferous vegetation

The establishment of terrestrial plants during the Palaeozoic era was one of the most significant changes to the Earth system during the Phanerozoic eon. The continuing evolution and expansion of land plants irrevocably altered the alluvial landscape: the broad, unconfined and unconsolidated blankets of coarse sediment found at the start of the Cambrian period were replaced by a diverse array of braided and meandering channel styles and stable floodplains by the end of the Devonian period. Here we show that the first appearance of a sedimentary facies suite attributed to low-energy, organic-rich river systems with multiple channels and stable alluvial islands, known as anabranching or anastomosing rivers, occurred during the Carboniferous period. Our field studies and literature review demonstrate that the appearance of these rivers coincides with the continuing evolution of tree-like plants. We suggest that increased floodplain stability and new triggers for channel avulsion were provided by the increase in complexity and diversity of root assemblages, density of floodplain forests and production of woody debris associated with expansion of arborescence. We conclude that the expansion of tree habitats led to the crossing of a threshold in vegetative control of floodplain and river morphology during the Carboniferous.