A Formação Barriga Negra (Ediacarano do Uruguai): caracterização estratigráfica e correlação com unidades do estado do Rio Grande do Sul, Brasil

Abstract

The main regional unconformities found in Uruguay separate the following geological elements from each other: (i) pre-Ediacaran metamorphic basement, (ii) Ediacaran tafrogenic sedimentary units and (iii) Phanerozoic sedimentary uccessions. Although there is a general agreement about the geological meaning of the last unconformity, there is debate about the nature of the boundary between the basement and the tafrogenic cover. The present paper reviews this boundary and discusses the Barriga Negra Formation and its relationships with the metamorphic (Lavalleja Group) and plutonic (Polanco Granite) basement. The Barriga Negra formation constitutes a thick succession of conglomerates, pebbly sandstones, sandstones and pelites. At its base, above a lithologic unconformity with the basement, the formation is represented by conglomeratic facies that vary from angular cobble and boulder breccias to pebbly sandstones with lenses of pebbly conglomerates, interpreted as alluvial fan and braided river deposits. These basal lithofacies show a diagnostic feature of their stratigraphic position and depositional stile: the clasts of the conglomerates always reveal a source-area that matches the units that are now exposed in the vicinity of the deposits. The overlying sandy and muddy facies association represent a transgressive event registered in storm-affected shoreface and offshore deposits. These deposits are overlain by thickening-upward alluvial fan and fandelta successions. In southern Brazil, the correlative stratigraphic succession is the Santa Barbara Group (Camaqua Supergroup), that is also posterior to granites relatable to the Polanco Granite.