Low sinuosity meandering rivers before vascular plants: Cambrian Tapeats Formation, Arizona, USA

Abstract A longstanding view is that rivers in pre-Silurian landscapes, prior to the colonization of continents by land plants, were braided and that meandering rivers—characterized by lateral migration to the point of bend cutoff—were rare. Evidence for this view includes river deposits dominated by amalgamated sandstone and a lack of muddy floodplain deposits that typify modern meandering river systems. Here, we present detailed analysis of pre-Silurian fluvial deposits from the lower Cambrian Tapeats Formation, Arizona, USA, with low levels of mudstone, but that nonetheless contain evidence for low sinuosity meandering rivers in a sandy floodplain. Channelized granule to pebble conglomerate, pebbly sandstone, and medium-grained sandstone bodies up to 1.2 m thick contain point-bar lateral accretion surfaces, indicating lateral migration of meandering rivers. Low-angle outer-bend channel margins (~10°) may reflect small proportions of floodplain mud at the channel cutbank. Stratal geometries indicate multiple episodes of channel abandonment, many of which occurred when channels migrated only 1–2 channel widths with little or no channel-bed aggradation, suggesting a chute cutoff mechanism. Three intervals record abandonment at times of channel setup (superelevation of bankfull water level above the floodplain) when the channel floor increased in elevation by only 25%–75% bankfull depth, well below values typical of avulsion (~100%). All of this indicates unstable channels and early abandonment compared to modern meandering rivers. Unlike vegetated meandering rivers, for which bends often grow to the point of neck cutoff, bend growth in pre-Silurian rivers was limited by channel abandonment through the rapid development of chutes across the easily erodible, vegetation-free, and predominantly sandy floodplains.