The Amazonas Basin is a geological structure developed as intracratonic syneclisis with an area estimated at 500,000 km², located in northern Brazil covering part of the States of Amazonas and Pará. The Monte Alegre Formation (Bashkiriano) is the basis of the neocarboniferous-neopermian regressive sequence of the Basin, represented by the Tapajós Group. From the description of boreholes sampled in the municipality of Uruará/PA, 4 (four) sedimentary facies were identified: 1) sandstone with plane-parallel lamination (Ap); 2) massive sandstone (Am); 3) sandstone/siltstone with plane-parallel lamination (ASp); and 4) sandstone with tabular cross-stratification (At). These were grouped into two facies associations: sand sheet and interdunes (AF1) and dune fields (AF2), representatives of a humid/coastal desert system. From the petrography of 18 polished thin sections, the rocks were classified as quartzoarenites, with very fine to medium sand granulometry, compositional and texturally mature. Supported by X-ray diffraction (XRD), scanning electron microscopy (SEM) and catodoluminescence (CL) of the samples, the diagenetic sequence was proposed to the sandstones of the Formation, which includes carbonic cementation, silica overgrowth and infiltration of clays during eodiagenesis, mechanical and chemical compaction, alteration of feldspars and mineral authigenesis during mesodiagenesis, as well as late percolation of oxidizing fluids and precipitation of hematite. The porosity of the set was partially obliterated by cementation events, but secondary porosity had been generated by mineral dissolution, culminating in intervals between 2 and 13% of total porosity for rocks. The application of catholuminescence on quartz grains identified emissions ranging between violet, dark blue and light blue, indicative of plutonic granite sources for sediments.
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