FLOODPLAIN ENVIRONMENTS BEFORE THE EVOLUTION OF LAND PLANTS: COMMON GROUNDS TO IDENTIFY FLOODPLAIN DEPOSITS IN PRE-VEGETATION SETTINGS

Floodplains are areas of unconfined and episodic water flow that, given the availability of water, nutrients and stable substrate, have been widely affected by the appearance and evolution of land plants. The scarcity of documented examples of pre-Silurian floodplains contrasts with the continuous debate on the effects of land plants evolution on fluvial channel styles. Given the importance of floodplains as sites of sediment storage, the lack of data on ancient floodplains has great implications for our understanding of the source-to-sink systems and the climate record of the Precambrian and the Early Paleozoic.

Three examples of alluvial environments developed prior to the evolution of land plants offer the opportunity to assess the main characteristics of pre-vegetation floodplains. The Mesoproterozoic Tombador Formation (Chapada Diamantina, Northeastern Brazil) is part of thick sedimentary successions deposited in an intracratonic basin; the Neoproterozoic Santa Bárbara Group and the Cambrian Guaritas Group (Camaquã Basin, Southern Brazil) both represent alluvial successions in a continental rift system. There were found some common features of pre-vegetation floodplain deposits, such as relatively coarser grained deposits in comparison to modern examples, better preservation of sedimentary structures and abundance of sandstone facies representative of sedimentation in unconfined flow settings, as well as evidence of paleosol formation. Steeper longitudinal fluvial profiles than those on modern systems, coupled with lower flow resistance in the absence of vegetation can explain the coarser grained floodplains and why they would be a more common feature of these ancient deposits.

The pre-vegetation floodplain deposits used to draw this simplified model are very similar to ephemeral river deposits, often evoked to explain the formation of thick successions of sheet-like sandstone. The re-evaluation of pre-vegetation fluvial deposits considering the possibility of coarse grained floodplains suggests that this environment has been greatly overlooked, and climate inferred from the alternation of contrasting fluvial styles might be misleading, since alternation of perennial and ephemeral fluvial deposits can be part of the same system (e.g. channel-belts and sandy floodplains).