Pennsylvanian mixed siliciclastic-carbonate deposits of the Amazonas basin, North of Brazil: The record of an epicontinental sea in Western Gondwana

Abstract

Epicontinental seas dominated the central Amazonas Basin during the late Carboniferous configuring singular paleogeography of West Gondwana connected to the Panthalassa Ocean. The Pennsylvanian record is represented by 1,7 km-thick mixed siliciclastic-carbonate-evaporite deposits forming a complete transgressive-regressive megacycle in the Amazonas Basin. Outcrop and core-based stratigraphic and facies analysis carried out in 50 m-thick mixed siliciclastic-carbonate transgressive succession of the Amazonas Basin identified twenty-six facies and microfacies, representative of coastal to platform depositional systems and grouped in three facies associations: (1) coastal aeolian deposits, consisting of fine to medium-grained sandstone, lime mudstone and finely-crystalline dolostone that correspond to a complex association of aeolian dunes, sand sheets, interdunes, fluvial channels and lagoon deposits bioturbated by Palaeophycus, Lockeia, Thalassinoides, Roselia, and meniscate trace fossils; (2) a mixed tidal flat setting, constituted by fine to medium-grained sandstone, mudstone, shale, siltstone and limestone interpreted as supratidal, tidal channel, tidal delta and lagoon settings with evidence of subaerial exposure; and (3) carbonate epicontinental platform deposits, consisting of lime mudstone, wackestone, packstone and grainstone with allochemicals (ooids and peloids), terrigenous grains and abundant, diversified open shallow marine benthic and nektonic organisms,. A Bashkirian age is suggested based on the presence of the Neognathodus symmetricus, Streptognathodus sp., and Ellisonia sp. These mixed siliciclastic-carbonate deposits are correlated with several successions in the west and east Amazonia and sub-Andean basins in South America. These records indicate that the Itaituba deposits constitute a significant portion of a vast Pennsylvanian epicontinental sea connected to the west with the Panthalassa Ocean.