Sedimentology of a wet, pre‐vegetation floodplain assemblage

Abstract

Descriptions 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.

River meandering on Earth and Mars: A comparative study of Aeolis Dorsa meanders, Mars and possible terrestrial analogs of the Usuktuk River, AK, and the Quinn River, NV

“…The appearance of meandering channels in the geologic record coincides with introduction of plants on land during the Devonian period (Schumm, 1968b;Cotter, 1978;Long, 1978;Eriksson et al, 1998;Tirsgaard and Oxnevad, 1998;Davies and Gibling, 2010a,b) and distinctive sandy floodplain assemblages (Fralick and Zaniewski, 2012), in part because of lack of bank cohesion and in part because of fewer fines produced by weathering. This suggests that vegetation is essential for channels to meander rather than braid.…”

Recognition and importance of amalgamated sandy meander belts in the continental rock record

“…Given the problems of recognizing sandy meandering fluvial deposits in outcrop, it will be particularly difficult to recognize these systems in the subsurface (Fralick and Zaniewski, 2012). Core-based studies and borehole imaging techniques are unlikely to be able to identify the large-scale dipping surfaces that would allow recognition of lateral accretion sets.…”

Heterolithic meandering-channel deposits from the Neoproterozoic of NW Scotland: Implications for palaeogeographic reconstructions of Precambrian sedimentary environments

“…The Allt na Béiste Member of the basal Applecross Formation represents an example of a rarely preserved pre-vegetation palaeoenvironment. One comparable example is the 1.4 Ga Sibley Group in Canada (Fralick and Zaniewski, 2012), although that study is not based on three-dimensional analysis. The preservation of such fine-grained alluvial deposits confirms interpretations by Ronov (1964) and Dalrymple et al (1985) that other environmental controls are responsible for the uncommon preservation of fine-grained sediments in pre-vegetation alluvial environments, and that fine-grained sediment was available despite the lower chemical weathering rates due to the absence of vegetation.…”