Alluvial facies evolution during the Palaeozoic greening of the continents: case studies, conceptual models and modern analogues

The Palaeozoic greening of the continents - the appearance and expansion of embryophytes (land plants) in terrestrial environments - was arguably the most fundamental Phanerozoic change to the Earth system. Thirteen case studies of Cambrian to Devonian fluvial deposits from North America and Europe are documented here to illustrate the evolution of fluvial style during this period. During the Cambro-Ordovician, prior to the advent of terrestrial vegetation, fluvial systems laid down relatively coarse sands with little mud, resulting in self-formed channels and an architecture dominated by broad sheets of trough cross-beds (sheet-braided style). Similar deposits formed across a wide range of latitudes, and passed basinward into sandy coastal deposits. From the mid Ordovician onwards, an increase in floodplain mudstone corresponds broadly with the appearance of embryophytes, which would have progressively enhanced upland weathering, mud production and floodplain storage of fines. During the Late Silurian, small heterolithic channel bodies with lateral-accretion sets provide the first evidence of meandering channels, and floodplain mudstones contain more varied palaeosols, especially calcretes which appear abundantly for the first time. By the Early Devonian, channel deposits comprise sandstone lenses (channelled-braided style), probably due to the increased bank strength and cohesion and reduced potential for sediment sorting imparted by sand-mud mixtures. Muddy coastal deposits are prominent. By the Upper Devonian, fluvial deposits commonly contain fossil trees and large mainstem meandering channels with lateral-accretion sets, indicating that rooted vegetation stabilized channels. The advent of stable floodplains with levees, crevasse splays and organic litter would have encouraged the diversification of terrestrial invertebrates, which left alluvial ichnological signatures from the Late Silurian onwards.