Discussion on 'Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation' Journal of the Geological Society, https://doi.org/10.1144/jgs2016-063

The first-order importance of tectonic and environmental controls for terrigenous sediment supply has rarely been questioned, but the role of vegetation in the modification of ancient alluvial signatures has been observed since the mid-20th century (Vogt 1941). Studies of sparsely vegetated rivers (Schumm 1968) and alluvial stratigraphic variation (Cotter 1978; Davies & Gibling 2010) led to observations of (1) plant modulation of alluvial signatures and (2) Palaeozoic facies shifts (PFS): unidirectional changes to facies diversity and frequency, in stratigraphic alliance with the plant fossil record. One PFS is the Siluro-Devonian appearance of mudrich, architecturally complex alluvium, traditionally ascribed to meandering rivers, and sedimentologically distinct from prevegetation strata (Davies & Gibling 2010; Long 2011). Using selected secondary data, Santos et al. (2017) dispute the correlation of these observations using three key points, as follows. (1) The mid-Palaeozoic was typified by orogenic assembly of low-gradient equatorial continents and elevated sea level, which led to tropical weathering (abundant fine sediment) and extensive alluvial plains. This drove the PFS by promoting river meandering independently of vegetation. (2) Meandering does not require vegetation; this is shown by examples in Precambrian deposits, on other planets, and in 'non-vegetated' deserts. Meandering rivers were more abundant than the prevegetation rock record suggests, owing to selective bypass and deflation of fine material. (3) Early Siluro-Devonian (meaning Ludlow–Early Devonian) land plants were too small, their biomass and cover too limited, and their wetland habitat too narrow to have stabilized meandering channels, influencing landscape little more than earlier microbial communities. We contest the conclusions and method of the paper, and deal with each point in turn. Cotter, E. 1978. The evolution of fluvial style, with special reference to the central Appalachian Paleozoic. In: Miall, A.D. (ed.) Fluvial Sedimentology. Canadian Society of Petroleum Geologists, Memoir, 5, 361–383.Davies, N.S., Gibling, M.R. & Rygel, M.C. 2011. Alluvial facies during the Palaeozoic greening of the land: case studies, conceptual models and modern analogues. Sedimentology, 58, 220–258.Long, D.G.F. 2011. Architecture and depositional style of fluvial systems before land plants: a comparison of Precambrian, early Paleozoic and modern river deposits. In: Davidson, S., Leleu, S. & North, C.P. (eds) From River to Rock Record: The Preservation of Fluvial Sediments and their Subsequent Interpretation. SEPM, Special Publications, 97, 37–61.Santos, M.G.M., Mountney, N.P. & Peakall, J. 2017. Tectonic and environmental controls on Palaeozoic fluvial environments: reassessing the impacts of early land plants on sedimentation. Journal of the Geological Society, London, first published online December 22, 2016, https://doi.org/10.1144/jgs2016-063Schumm, S.A. 1968. Speculations concerning paleohydraulic controls of terrestrial sedimentation. Geological Society of America Bulletin, 79, 1573–1588.Vogt, T. 1941. Geology of a Middle Devonian cannel coal from Spitsbergen. Norsk Geologisk Tidsskrift, 21, 1–12.