FLOODPLAIN RECORD IN THE EARLY CAMBRIAN: A MOMENT IN THE EVOLUTION OF ALLUVIAL LANDSCAPES

Floodplain deposits older than the Silurian are generally overlooked sedimentary environments, yet they may record a number of transformations of its features due to the evolution of life on land, following the expected transformations in channel-belt deposits. The object of this study is the Varzinha Formation, a sandy and heterolithic interval in the Guaritas Group, the Cambrian sedimentary succession of the Camaquã Basin, a continental rift of the southern Brazil. The unit offers the opportunity to create a model that depicts the relationship between channel-belt and floodplain deposits of Cambrian age, thus allowing a glimpse of the transformation that alluvial systems were facing due to evolution life on land in a key moment of the geological record.

The alluvial environment interpreted for the Varzinha Formation is composed of channel-belt deposits, floodplain deposits and transitional channel-wing deposits. The channel-belt deposits are composed of sandstones with facies and geometry similar to braided fluvial deposits. Floodplain deposits show interbedding of sandstone and heterolithic deposits interpreted as a floodplain basin environment with abundant exposure features, cut by small feed channels and covered in places by the distal reaches of crevasse splays. The channel-wing deposits are sand sheets cut by small channels, which represent the transition between the channel-belt deposits and the floodplain deposits, similar to levees in modern settings, but occupying a broader area in the alluvial landscape.

The record of discrete floodplain, channel-belt and the transitional channel-wing in the alluvial deposits of the Varzinha Formation could be the result of partial stabilization of the landscape due to sediment binding related to biological activity on land. However, such effect would not be enough to allow the bank stabilization required for the formation of channel levees, which gave space to the formation of the more broad, low lying channel-wings. Thus, the Cambrian deposits of the Varzinha Formation could represent one the different stages of development of the alluvial landscape coupled with the evolution of life on land.