Floodplain record of channelized and unconfined flow in a pre-vegetation setting in the Early Cambrian, Southern Brazil

Abstract

Floodplains are areas of unconfined and episodic water flow extensively affected by the first appearance and subsequent evolution of land plants. In this respect, the scarcity of documented examples of pre-Silurian floodplain deposits contrasts with the growing interest in the variability of fluvial channel deposits of the same age. The Cambrian Varzinha Formation (Guaritas Group), of Southern Brazil records coeval floodplain and channel belt deposits, that formed before the first appearance of rooted plants. The deposits are interpreted as forming on an alluvial plain, fed by a braided channel system, bordered by broad areas of sand deposition. These formed as levees with a higher width to thickness ratio than in modern settings. A particular feature of these levee deposits are Froude Transcritical and Supercritical Structures, formed during bank-full flow over bars in the fluvial channel belt, and in regions of local high slope in the floodplain, related to flow from alluvial ridges across levees. Channel belts did not have strong avulsion rates, due to aggradational infilling of the floodbasin aided by transport of sand to the floodbasin through ribbon channels and the wide levee system. Fine grained floodbasin deposits were preserved in the distal reaches of the alluvial plain, with evidence of palaeoweathering, suggesting a well drained floodplain. This record of a coarse grained floodplain in a pre-vegetation setting brings new insight into the identification of floodplain environments in the Precambrian and Early Palaeozoic, and is in agreement with the recent recognition of a greater variety of alluvial deposits prior to the evolution of vascular plants.