Proveniência sedimentar dos depósitos cretáceos da Formação Alter do Chão, Bacia do Amazonas, região de Ponta do Curuá, Prainha-PA

Abstract

Deposits of the Alter do Chão Formation are found along the right bank of the Amazon River and are well registered in the locality of Ponta do Curuá, municipality of Prainha, the western region of the State of Pará. The outcrops are characterized by thick layers of fine to coarse sandstones interbedded with siltstones and claystones, and subordinate subordinate conglomerates. The facies analysis carried out in these outcrops allowed the individualization of 8 sedimentary facies, grouped in three facies associations (AF), configuring a high sinuosity and mixed load fluvial meandering depositional system. AF1 comprises facies with a thicker granulometry of the studied succession and was interpreted as the filling of the fluvial channel, characterized by massive to stratified conglomerates, massive sandstones, sandstones with trough and tabular cross-bedding, sandstones with convoluted laminations and climbing crosslaminations. AF2 was interpreted as abandoned channel filling deposits and corresponds to thick layers of massive to laminated mudstones that are sporadically associated with thin layers and very thin sandstone lenses forming wavy-linsen bedding. Well-preserved leaves and trunks are present in this association. AF3 corresponds to the flood deposits, composed of massive to laminate mudstones, massive sandstones, and sandstones with trough, tabular and sigmoidal cross-stratifications. The study of heavy minerals in the sandstones of this formation showed an assembly with a predominance of ultra-stable minerals such as zircon, tourmaline, rutile and anatase, and less frequent minerals such as kyanite, staurolite, sillimanite, andalusite, and garnet. Metasedimentary sources are suggested due to the presence of aluminum-rich metamorphic minerals. Whereas, rutile and tourmaline are present in both igneous and metamorphic rocks, being more common in the latter. Well-rounded minerals suggest an origin from sedimentary deposits, in the same way that subhedral and euhedral minerals suggest first cycle sediments. The high stability of this assembly is attested by the high compositional maturity exhibited by these sandstones, with values of the ZTR index ranging from 69% to 99%. These high values indicate that chemically unstable minerals have been eliminated over geological time, probably due to the action of interstitial fluids that circulate in the porous during weathering and diagenetic processes. However, the analyzed mineral assembly is more compatible with conditions of exposure to intense chemical weathering. The effects of intrastate dissolution in the area is directly proportional to the increase in the ZTR index, being attested by the presence of corrosion textures on the surface of the minerals that were analyzed from SEM images. Thus, the mineralogical assembly would reflect the effects of the intense chemical weathering, under a humid tropical climate, imposed on the deposits of the Alter do Chão Formation that would be associated with the lateritization event responsible for the genesis of bauxite deposits during the Paleogene. Recently, the luminescence (LC) characteristics of quartz grains have been used as provenance indicators, mainly in rocks affected by weathering or rich in quartz. In this work, luminescence signals were acquired by polychromatic cathodoluminescence to establish the relationship between luminescent signals and the genesis of this mineral. The studied grains present red, blue, violet and brown colors, with different intensities: the reddish-brown tones, are generally attributed to metamorphic rocks; dark blue luminescence is a characteristic observed in quartz of plutonic origin; grains with intense LC such as blue, red and violet of high bright, are volcanic quartz. Geochronological analyzes carried out on detrital zircons using the U-Pb method allowed to determine that the main sources for the Alter do Chão Formation are Paleoproterozoic ages, subdivided into two groups: (1) 1771 to 1906 Ma; and (2) 1957 to 2037 Ma, in addition to a small Archean contribution (Neoand Meso-Archean) aged between 2529 and 2977 Ma. The data from the first group, 1771 and 1906 Ma, were correlated with the age of provenance of metasedimentary rocks from the Araguaia Belt, which occurs on the eastern edge of the Amazon Craton. Ages around 1957 and 2037 Ma are consistent with rocks from the Maroni-Itacaiúnas Province, located on the eastern edge of the basin. It is possible to mention rocks associated with late to post-collisional orogenic magmatism such as the Igarapé Careta Intrusive Suites (2065 ± 33 Ma), Parintins (2030 ± 3 Ma) and paleoproterozoic indistinguishable granitoids occurring in the Amapá Block and Carecuru Domain, in addition to rocks related to post-orogenic magmatism such as the Granodiorito Sant'Ana (1986 ± 5 Ma) of the Bacajá Domain. Archean contributions can be associated with rocks from the Central Amazon Province or even to Archean nuclei distributed in the region.