Abstract
Abstract The aim of this work is to propose a new stratigraphic scheme of the Barriga Negra Formation, in order to solve the controversies in one of the most discussed units of Uruguay. Here, we introduce a new map (unmodified since 1984) and amend the stratigraphic profile for the Barriga Negra Formation, establishing the correct sequence, order and thickness, dividing it in four members (Volcaniclastic, Arkosic Conglomerate, Calcareous Conglomerate and Petromictic Conglomerate Member). We describe by first time the presence of basal volcanic and volcanoclastic rocks that implies a tectonic active environment. Volcaniclastic rock dating yield an U-Pb concordia age of 633 ± 3.4 Ma. Moreover, we show that this unit has no stratigraphic continuity with other units that previously were used to constrain its stratigraphic position, age and paleoenvironment. We confirm unequivocally that the unit is as a continental conglomeratic sequence deposited in arid and active tectonic conditions, and not transitional passive margin deposits as some works claim. Finally, we revisit the metamorphism, deformation, stratigraphy, structural relationships of this unit and identify new basement units. As consequence of this new evidence, we reexamine its correlation with other regional units. This work is also a key to understand the chaotic stratigraphic proposals of Uruguay..
Highlights
Several low-grade metamorphic volcano-sedimentary and sedimentary units have been described overlying the igneous and metamorphic basement of Uruguay and Rio Grande do Sul (Midot 1984, Fragoso-Cesar et al 1987, Basei et al 2000)
The Barriga Negra Formation is composed of well-preserved immature conglomerates and sands rich in feldspar and lithic fragments
Size, composition, and detrital zircon ages indicate a clear provenience of the surrounding basement (Gaucher 2000, Fambrini et al 2005a, Núñez Demarco 2014)
Summary
Several low-grade metamorphic volcano-sedimentary and sedimentary (conglomerate) units have been described overlying the igneous and metamorphic basement of Uruguay and Rio Grande do Sul (Midot 1984, Fragoso-Cesar et al 1987, Basei et al 2000). The Barriga Negra Formation is one of these controversial units; it is a thick succession of reddish conglomerates, arkoses and pelites, which is assumed to be Neoproterozoic in age (e.g., Midot 1984, Fambrini et al 2005a) It has been described as it rests unconformably over its basement (Walther 1919, Midot 1984, Fambrini et al 2005a, Masquelin et al 2017), as concordant (Preciozzi & Fay 1988, Gaucher 2000) or even interbedded with it (Caorsi & Goñi 1958). Others authors (Gaucher 2000, Gaucher et al 2003, Bossi & Gaucher 2004, 2014, Blanco et al 2009) do not accept the existence of the Dom Feliciano Belt in the area, and interpret the opposite scenario; they consider Barriga Negra Formation as the middle or bottom of a marine-coastal sequence, that overlies
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