Estratigrafia e análise paleoambiental de rochas Neoproterozóicas do Cinturão Araguaia, região de Redenção–PA, Brasil

Abstract

The Neoproterozoic age was marked by intense climatic changes that were accompanied by the reorganization of the continental masses at a global level. These climatic events are represented by the Sturtian and Marinoan Snowball Earth glaciations registered in the rocks of the Cryogenic age. The rearrangement of the continental masses was marked by the rupture of the Rodinia (870 - 750 Ma) and subsequent intracratonic collisions (600 Ma) that generated the amalgamation of Gondwana, which in turn originated several orogens such as the Araguaia Belt. This Neoproterozoic geotectonic unit located in the northern portion of the Tocantins Province, is composed of the sedimentary rocks of the Baixo Araguaia Supergroup, which subdivided into the Estrondo and Tocantins Groups. In the region of Redenção, the company WMC Resources Ltda drilled several cores, called SMD, to unveil the genesis of the São Martim prospect. Initially, the carbonate and siliciclastic rocks without evidence of metamorphism, in these cores (SMD 08 and SMD 15), were positioned in the Couto Magalhães Formation. However, the original definition of this unit involves rocks with low-grade metamorphism. This fact led to a lithostratigraphic review of the Couto Magalhães Formation that occurs on a subsurface in the Redenção region, in addition to the paleoenvironmental reconstitution proposed in the original work. Thus, this unit was redefined, in the study region, exclusively based on its lithological characteristics and named São Martim Formation (carbonate rocks) and Rio Arraias Formation (siliciclastic rocks). Due to the few testimonies that exist, it is not possible to define its lateral extension in subsurface. To achieve the proposed objectives, faciological, sedimentological and geochemical analyzes were performed, as well as interpretations of sedimentary microfacies and glacial microstructures. The most basal portion of the cores is characterized by the occurrence of banded iron formations (BIF) that are interpreted as the basin floor, these formations have age and compositional similarity correlated with the BIF’s of the Carajás Formation. In the overlying sequence, nineteen sedimentary facies were defined, which were grouped into four facies associations corresponding. In the São Martim Formation, carbonate rocks are interpreted as the record of a carbonate platform environment in an epicontinental sea (AF1) that is in discordant contact with the BIF’s. The Rio Arraias Formation is characterized by deposits of mud-sand-rich slope plain turbidites (AF2), glacial (AF3) and sand-rich slope turbidites (AF4). The ages of these rocks were inferred from the bibliographic survey. Meso-Archean ages (2,952.3 ± 7.3 Ma and 2,994.0 ± 5.8 Ma) of the BIF´s were determined based on U-Pb (SHRIMP) data of detritical zircons. There are no ages available for carbonates from the São Martim Formation. The ages obtained for the siliciclastic rocks of the Rio Arraias Formation are based on the Pb / Pb methodology in clastic pyrite (716 to 670 Ma), diagenetic pyrites (668–616 Ma) and vein pyrite (438 Ma - 394 Ma). As well as Sm / Nd ages of sedimentary provenance between 1660 Ma, 1710 Ma and 1720 Ma in diamictite clasts of the Rio Arraias Formation. Additionally, rhyolitic pyroclastic rocks (ages U-Pb) dated 634 ± 21 and 624 ± 11 Ma superimposed directly on the glacial deposits of the Rio Arraias Formation. According to these geochronological ages, a Cryogenian age for the carbonate-siliciclastic sedimentary sequence is inferred in this work. The first sedimentation event after the formation of BIF was the deposition of carbonate sediments that make up AF1, the contact between these two associations is abrupt and characterized by the presence of a thin layer of black shale. The AF1 is constituted in the basal portion by pseudonodular to pseudolaminar mudstone, which present structures generated by chemical compaction such as dissolution seams and stylolites, in addition to fine levels of volcanic tuff, in the upper portion are the carbonate and siltstone breccia facies with wavy lamination. The second deposition event was possibly marked by the subsidence of the basin generated by the post-collisional closure of the Amazon with São Francisco / Congo cratons, where the AF2 was deposited, which is composed of massive sandstones and with plane-parallel lamination, truncated by wave and deformed, rhythms with plane-parallel lamination and truncated by wave, massive mudstones and with plane-parallel lamination. The third sedimentation event occurred at the maximum glacial advance and subsequent lowering of sea level, with the deposition of materials corresponding to AF3. This association is made up of foliated and massive diamictites associated with the deposition of sediments from glaciers that advance in water bodies. The transition between AF2 and AF3, presents layers with deformations possibly produced by glaciotectonic. The fourth sedimentation event is marked by a rapid thaw accompanied by an increase in sea level which increased the potential for preservation of subglacial deposits and possibly controlled the deposition of AF4 materials. This association is composed of massive sandstones with planar-parallel, wavy, wave-truncated and deformed lamination, to a lesser extent rhythms with planar and wave-truncated lamination and thin layers of massive mudstones, in addition to two levels of siliciclastic breccias. In summary, a new Neoproterozoic lithostratigraphic definition for the Araguaia Belt is proposed that suggests for the first time that the platform, in certain periods, was subjected to glacial processes possibly within the context of Snowball Earth, precisely to the Marinoan glaciation.