Transição siluro-devoniana na borda Sul da bacia do Amazonas, entre Uruará- Rurópolis, Oeste do Estado do Pará, Norte do Brasil.

Abstract

The Silurian-Devonian transition is characterized by drastic geographical variations in the paleocontinents geographical setting. On the Amazonas Basin South margin, whose register is observed in the contact between Pitinga and Maecuru Formations. These deposits were studied among Rurópolis and Placas regions, in the west of the State of Pará, aiming recognition and association of their sedimentary facies for paleoenvironmental reconstitution and its relation with collisional events of the supercontinent Gondwana. The deposits provenance is also desired. In this study, eight sedimentary facies were identified and grouped into two facies association: a) Shallow platform with tidal and wave influences (AF-1) corresponding to Pitinga Formation and; b) Proximal braided lowland (AF-2) corresponding to Maecuru Formation. The contact between Pitinga and Maecuru Formations is interpreten as regional erosive discordance observed in more than 300 km along all the Amazonas Basin south margin, which is tied up to accretionary stages from the supercontinent Gondwana during the Precordillera and/or glacio-eustatic effects caused by the end of Silurian glaciation. Sandstones from Pitinga Formation were classified as fine- to medium-grained, moderately well-sorted sub-arkoses and quartz-arenites. While the sandstones of Maecuru Formation are medium- to very coarse-grained, moderate- to poorly-sorted arkoses and quartz-arenites. The petrographic analysis of these deposits suggests quartz enrichment due to diagenetic processes that were also effective on the elimination of less stable minerals as indicated by the presence, in great amount, of secondare pores. The quartz-grains imaging by cathodoluminescence (CL) indicated the predominance of quartz-grains from igneous and metamorphic rocks. This technique results together with paleocurrent measurements provided substantial data for deducing that the Maecuru Formation are lithotypes from the Bacajá and Iriri-Xingu Domain. The rock bodies alignment from the source areas corroborate these interpretations. Even with reliable and convincing CL data, the lack of paleocurrent measurements made it unfeasible to accurately interpret the Pitinga Formation provenance.