New occurrences of glacial deposits in the Presidente Figueiredo region, northern margin of the Amazon Basin

Abstract

The Paleozoic sedimentary sequence outcropping in the municipality of Presidente Figueiredo, on the northern margin of the Amazon Basin, is bounded to the north by the Paleoproterozoic crystalline basement and to the south by the Cretaceous–Neogene sedimentary cover. Within this sequence, new records of glacial deposits occur associated with shoreface and foreshore sedimentary facies of the Nhamundá and Manacapuru formations (Trombetas Group). Glaciers primarily formed these deposits by sedimenting terminal moraines and depositing material in lakes. They consist mainly of coarse diamictites that contain clasts from igneous, sedimentary, and metamorphic rocks. Glaciers deposited these materials as they moved over marine sedimentary rocks and the underlying crystalline basement. This study highlights the unprecedented record of dropstones embedded in laminated pelite deposited in proglacial lakes from the release of clasts from the base of icebergs. The glacial deposits studied were associated with two events: one positioned in the Early Silurian (Llandovery–Wenlock) based on palynology, and the other corresponding to a newly identified glacial event in this study, placed in the Late Silurian–Early Devonian (Early Pridoli to Early Lochkovian), based on chitinozoans (Angochitina filosa, Angochitina strigosa, Angochitina sp. D, and Lagenochitina cf. L. navícula). In addition to the glaciotectonic deformation identified in some diamictite outcroppings in the Presidente Figueiredo region, the presence of faceted clasts and dropstones also stands out as strong evidence of Silurian–Devonian glacial activity in the Amazon Basin.