Davies, Neil

Gibling, Martin

English

Ghent University Academic Bibliography

Geophysical Research AbstractsVol. 13, EGU2011-11262, 2011EGU General Assembly 2011© Author(s) 2011Pennsylvanian emergence of anabranching fluvial deposits: the parallelrise of arborescent vegetation and fixed-channel floodplainsNeil Davies and Martin GiblingDalhousie University, Halifax, Nova Scotia, Canada (neil.s.davies@dal.ca)Recent research into Palaeozoic fluvial deposits has shed new light on the ways in which ancient river systems werephysically transformed by the expansion of embryophytes from the Cambrian through to Devonian. Extensiveliterature reviews and field studies show that the greening of the continents progressively increased retention ofmud in terrestrial environments, stabilised floodplains (e.g., by roots), and promoted single-channel (meandering)fluvial deposits, which are prominent from the Siluro-Devonian onwards. By the end of the Devonian, a broadspectrum of vegetation-dependent fluvial styles had developed, which persist to the present day. However, in thecourse of original fieldwork and a comprehensive literature review of 144 Cambrian-Devonian fluvial successions,it was observed that no strata had been interpreted as anabranching river deposits.Anabranching (anastomosing) rivers are low-energy fluvial systems consisting of multiple channels sepa-rated by stable islands, which evolve over time through avulsion. Such river systems host a variety of terrestrialsub-environments and habitats that have been relatively common during the Mesozoic and Cenozoic and intothe present day. Anabranching requires bank stability, usually provided by vegetation or cohesive floodplainmuds, both of which were prominent in the fluvial realm by the Siluro-Devonian. However, based on a literaturereview of 188 Carboniferous successions, facies interpreted as anabranching river deposits do not appear until thePennsylvanian (Bashkirian).Original field data from Carboniferous fluvial strata in Atlantic Canada and the SW USA support the as-sertion that a distinct suite of fluvial facies and sandbody geometries, still apparent in recent anabranching fluvialdeposits, makes its first appearance at about this time. The channel deposits are narrow (width:thickness typically<15) with steep margins and aggradational fills, with little evidence for lateral accretion, and encased in floodplainmuds with palaeosols. Although it is rarely possible to demonstrate that the parent channels formed 3D networks,it is probable that some suites were anabranching. Although a few small, narrow channel bodies are known fromDevonian and Mississippian formations, this “fixed-channel” style is conspicuously absent from older fluvialformations but is widespread from the Pennsylvanian onwards.It is argued that the seemingly delayed appearance of this fluvial style in part reflects the infrequency ofrepeated short-term triggers for avulsion prior to a threshold-crossing increase in arborescent floodplain vegeta-tion. The increase in arborescent vegetation through the Carboniferous would have resulted in an increase in thesize, abundance and distribution of large woody debris in fluvial channels. Such debris would have provided, forthe first time, one of the most common triggers for river avulsion: as demonstrated by the fact that large log-jamdeposits also first appear in the rock record during the Bashkirian.The first appearance of the fixed-channel style and, probably, of anabranching systems, during the Pennsyl-vanian thus marks one of the final major developments in the vegetation-dependent biogeomorphology of fluvialsystems. The persistence of such features to the present day re-iterates that the greening of the continents wasarguably the most significant event to have influenced the Phanerozoic evolution of terrestrial landscapes andhabitats.

Pennsylvanian emergence of anabranching fluvial deposits: the parallel rise of arborescent vegetation and fixed-channel floodplains

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Pennsylvanian emergence of anabranching fluvial deposits: the parallel rise of arborescent vegetation and fixed-channel floodplains

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