Abstract The Corumbá Group is a Neoproterozoic succession of terrigenous and carbonate sedimentary rocks located at the southern Paraguay Belt, central Brazil. The upper units of the Corumbá Group include the Ediacaran carbonate Bocaina and Tamengo formations, whose limit is characterized by polymictic breccias recognized in several sites from Corumbá to Serra da Bodoquena, Mato Grosso do Sul. Despite the widespread occurrence, the breccias are poorly described and their origin is uncertain. The aim of this study is to present the differences between sedimentary and tectonic breccias of the Corumbá Group and propose a genesis model for each. The sedimentary breccias comprise mainly matrix-supported chaotic facies that formed by submarine mass flows on slope aprons. Sea level fall and/or increased faulting rates exposed the underlying units and triggered the gravity fluxes by creating a steep slope. The base of the sedimentary breccia represents a major unconformity within the carbonate sedimentation of the Corumbá Group, with potential correlation to other Ediacaran units. The subsequent development of the Paraguay fold-thrust belt caused the formation of tectonic breccias in reverse fault zones. Cataclasis and mylonitization deformed the dolomitic host rock by fracturing and produced a fine foliated matrix.

RESUMO: Dados paleomagnéticos obtidos para o Cráton Amazônico nos últimos anos têm contribuído significativamente para elucidar a participação desta unidade cratônica na paleogeografia dos supercontinentes pré-cambrianos. Dados paleomagnéticos do Paleo-Mesoproterozoico favoreceram a inserção do Cráton Amazônico no supercontinente Columbia há 1780 Ma, em um cenário que se assemelhava à configuração "South AMerica and BAltica" (SAMBA). Estes mesmos dados também sugerem a ocorrência de movimentos transcorrentes dextrais entre os Escudos das Guianas e do Brasil-Central após 1400 Ma, ou que movimentos de rotação do bloco Amazônia-Oeste África ocorreram dentro do Columbia entre 1780 e 1400 Ma. Os dados paleomagnéticos atualmente disponíveis do final do Mesoproterozoico são compatíveis com dois cenários diferentes para a Amazônia no supercontinente Rodínia. O primeiro cenário envolve uma colisão oblíqua da Amazônia com a Laurentia, começando no Texas há 1200 Ma, seguida por movimentos transcorrentes até o final da colisão da Amazônia com a Báltica há 1000 Ma. No segundo cenário, a ruptura da Amazônia e da Báltica do Columbia ocorre após 1260 Ma e é seguida por uma rotação horária e pela colisão desses blocos com a Laurentia ao longo do cinturão Grenville há 1000 Ma. Finalmente, a época em que a Amazônia colidiu com a parte central do Gondwana tem sido objeto de muita disputa. Todavia, os poucos polos paleomagnéticos do final do Neoproterozoico/Cambriano para o Cráton Amazônico, para a Laurentia e outros blocos do Gondwana Ocidental sugerem que o Oceano Clymene que separou estes blocos ocorreu entre o final dos períodos Ediacarano e Cambriano, após a separação do Cráton Amazônico da Laurentia há 570 Ma.

ABSTRACT: In the last decade, the participation of the Amazonian Craton on Precambrian supercontinents has been clarified thanks to a wealth of new paleomagnetic data. Paleo to Mesoproterozoic paleomagnetic data favored that the Amazonian Craton joined the Columbia supercontinent at 1780 Ma ago, in a scenario that resembled the South AMerica and BAltica (SAMBA) configuration. Then, the mismatch of paleomagnetic poles within the Craton implied that either dextral transcurrent movements occurred between Guiana and Brazil-Central Shield after 1400 Ma or internal rotation movements of the Amazonia-West African block took place between 1780 and 1400 Ma. The presently available late-Mesoproterozoic paleomagnetic data are compatible with two different scenarios for the Amazonian Craton in the Rodinia supercontinent. The first one involves an oblique collision of the Amazonian Craton with Laurentia at 1200 Ma ago, starting at the present-day Texas location, followed by transcurrent movements, until the final collision of the Amazonian Craton with Baltica at ca. 1000 Ma. The second one requires drifting of the Amazonian Craton and Baltica away from the other components of Columbia after 1260 Ma, followed by clockwise rotation and collision of these blocks with Laurentia along Grenvillian Belt at 1000 Ma. Finally, although the time Amazonian Craton collided with the Central African block is yet very disputed, the few late Neoproterozoic/Cambrian paleomagnetic poles available for the Amazonian Craton, Laurentia and other West Gondwana blocks suggest that the Clymene Ocean separating these blocks has only closed at late Ediacaran to Cambrian times, after the Amazonian Craton rifted apart from Laurentia at ca. 570 Ma.