A Mesoproterozoic hybrid dry-wet aeolian system: Galho do Miguel Formation, SE Brazil

Abstract

Based on Phanerozoic and present-day cases, aeolian systems are categorised into dry, wet and stabilising types. It is questioned here whether these models are applicable to Proterozoic systems when environmental conditions on the Earth's surface were markedly different. Facies and architectural-element analyses have been applied to the Mesoproterozoic aeolian succession of the Galho do Miguel Formation, SE Brazil. The aim is to identify and discuss what controlling factors govern the construction and preservation of Proterozoic aeolian systems, and to explain how these differ from Phanerozoic models. In the metaquartzarenites of the Galho do Miguel Formation four aeolian subenvironments - megadunes (draas), large-scale isolated dunes with dry interdunes, small-scale isolated dunes with damp or wet interdunes and salt flats - coexisted and alternated temporally and spatially. The construction of megadunes, large-scale dunes and dry interdunes occurred in topographically elevated areas, usually above the water table, but that were occasionally flooded; isolated dunes with damp and wet interdunes, and salt flats formed in low-lying areas with water table at or close to the surface.A long-lived sediment supply combined with ongoing tectonic subsidence enabled the accumulation of a thick aeolian succession (1000–1500 m) that covered a large area (4000 km2). The water table controlled the accumulation of this unit. Where it was close to the accumulation surface, it acted to limit the availability of the wind-blown sand, hampering the construction of large and compound bedforms and allowing the deposition of damp and wet interdunes and salt flats as a wet aeolian system. Where large and compound bedforms with dry interdunes developed as a dry aeolian system, slow but progressive subsidence-driven water-table rise provided accumulation space that enabled system preservation. The Galho do Miguel Formation constitutes a hybrid aeolian system, in which both dry and wet environmental conditions were coeval.In the Mesoproterozoic, the absence of rooted-vegetation capable of acting as a sand stabilising agent allowed the widespread generation of aeolian systems in humid as well as arid environments. In humid environmental settings, water played a significant role in the accumulation and preservation of aeolian deposits, preventing their reworking by the wind or other exogenous agents.