Litofácies e minerais pesados da Formação Alter do Chão (cretáceo), região de Óbidos-PA, parte central da bacia do Amazonas

Abstract

Cretaceous deposits exposed in riverbanks of the Amazon River, near the town of Óbidos (state of Pará), record a succession of sandstones, conglomerates and mudstones. Seven lithofacies types including massive conglomerate (Cm), trough cross-bedded sandstone (St), planar cross-bedded sandstone (Sp), cross-laminated sandstone (Sl), massive mudstone (Mm), bioturbated mudstone (Mb) and deformed mudstone (Md) have been recognized that are organized in fining-upward cycles each 1 to 6 m thick. These lithofacies have been arranged in two associations: 1-Channel fill, composed of gravel bars, sandy bedforms and lateral-accretion deposits and, 2- Overbank deposits composed of levee, crevasse splay and floodplain sediments. Cross-bed measurements in the facies St and Sp indicate a general flow system to SW. The facies analysis evidenced that the Alter do Chão formation was deposited in a fluvial meandering system, with sinuosity and mixed-load variation. Petrographic studies classified the sandstones as quartz-sandstones with kaolinitic matrix, the latter being generated by weathering. The quartz-sandstone generally consists of moderately sorted, fine to coarse, angular to subangular, and secondly, subrounded grains. The heavy mineral assemblage has high compositional maturity, with average ZTR index of 85%, represented by zircon, tourmaline, kyanite, rutile, staurolite, monazite and occasionally andalusite and spinel. Conchoidal fractures and impact marks are common to zircon and tourmaline while dissolution features occur in kyanite and staurolite. The quartz grains as well as the heavy mineral originated from igneous and metamorphic rocks. Additionally, embayments and vacuoles in quartz grains suggest volcanic origin for, at least one part of this mineral. The informations from heavy minerals, combined with paleocurrent data and U-Pb ages (Mappes, 2009), suggest as source areas for the Alter do Chão formation near Óbidos, regions located north/northeast of the study area. The Maroni-Itacaiúnas Province characterized by metavolcanic and metasedimentary rocks, as well as gneisses and granites may be considered as the main potential source for the sandstones of the Alter do Chão formation.