Sedimentology of a wet, pre‐vegetation floodplain assemblage

AbstractDescriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style; this is probably in part due to the scarcity of well‐developed floodplain successions in fluvial assemblages of these ages. This study describes wet, pre‐vegetation floodplain deposits and processes observed and inferred from a continuous succession of drill core through an extensive, 1·4 Ga, delta‐top channel–floodplain assemblage forming a portion of the Sibley Group, Ontario, Canada. Sub‐aerial deposits are dominated by flaser, wavy and lenticular bedded, fine‐grained sandstones, siltstones and mudstones, with abundant small mudstone rip‐up clasts. Soft‐sediment deformation of these units is ubiquitous, with loading and injection features being the most prominent. Thicker medium‐grained sandstone beds, representing crevasse splays, commonly have poorly developed protosols in their upper portions. Well‐laminated sediments with wave ripples, and only rarely containing rip‐up clasts and soft‐sediment deformation, were deposited in floodplain ponds. These deposits differ from post‐vegetation floodplain sediments in having: (i) better preservation of layering without rootlet bioturbation; (ii) dominance of rippled sand on the floodplain, probably due to lack of vegetation‐induced baffling; (iii) large‐scale generation of small, intraformational clasts; (iv) desiccation crack fills consisting of peds and locally derived intraclasts, probably delivered during Horton overland flow during rainfall events; (v) ubiquitous soft‐sediment deformation features in sub‐aerial deposits; and (vi) well‐laminated, commonly oxidized sediment that accumulated in floodplain ponds. These attributes are the direct result of the lack of macrophyte vegetation, and produce floodplain assemblages that are distinctly different from those presently forming in similar climatic settings.