Sedimentology and stratigraphy of new outcrops of Silurian glaciomarine strata in the Presidente Figueiredo region, northwestern margin of the Amazonas Basin

New occurrences of Lower Silurian glaciomarine outcrops were identified in a belt about 80 kilometers long in the municipality of Presidente Figueiredo on the northern flank of the Amazonas Basin. This narrow belt of Paleozoic sedimentary rocks has a general E-W trend and includes Ordovician to Early Devonian and Middle Devonian to Early Mississippian sedimentary sequences. The belt is limited to the north by the Neoproterozoic Prosperança Formation and Paleoproterozoic igneous and metamorphic rocks of the Central Amazonia Province. To the south, the Paleozoic beds are bounded by Neogene sediments of the Alter do Chão Formation. Outcrops were studied along the BR-174 and AM-240 highways, secondary roads and river banks of the region, where nine distinct sedimentary facies allowed definition of shoreface, foreshore and glaciomarine environments. Shoreface deposits are the most abundant, represented by five distinct facies, and are associated with two foreshore facies. Included in marine strata are two levels of massive, coarse diamictite and laminated facies consisting predominantly of clasts of quartz-sandstone, quartz and mudstone immersed in a fine clay-silty-fine sandy matrix. The massive aspect of the diamictites and the presence of faceted and striated pebbles, associated with palynological data, allowed us to relate them to a Middle-Late Llandoverian glacial event, which was first recognized in the subsurface of the Amazonas Basin in 1971.