Neogene Eastern Amazon carbonate platform and the palaeoenvironmental interpretation

Abstract

The Early Miocene Pirabas Formation represents extensive carbonate and siliciclastic belts deposited in marine coastal environments along the Eastern Amazon coast. This Formation was studied in its palynological, palaeocarcinological and sedimentary facies, in order to develop a model of its depositional processes, faunal and floral assemblages and the sedimentary environments, as observed in the B17 mine (Capanema, Pará). The sedimentary records consist mostly of light gray to greenish gray mudstones with tidal bedding, calciferous sandstones, stratified biocalcirudites and levels with conglomerates deposited in a carbonate/siliciclastic transitional system with lagoon, tidal flat, flood-tide delta and foreshore environments. Eighteen palynomorph species were identified in the tidal flat deposits including 1 alga, 3 fungi, 5 pteridophytes, and 9 angiosperms. Among the latter, mangrove pollen and spores of Zonocostites ramonae, Zonocostites minor, and Deltoidospora adriennis are reported. Nine species of crustacean decapods were recognized in the foreshore deposits, comprising 1 Callianassoidae, 2 Calappidae, and 6 Portunidae, represented by Euphylax, †Necronectes, Portunus, and Scylla, the ecology of which is associated with beaches of marginal lagoons. The main factor of the formation of these carbonate environments is assumed to have been related to the lack of a large-scale drainage system such as the Amazon River basin in the transition Oligocene–Miocene, which may also have influenced carbonate production and resulted in a strong decrease of palaeodischarges from incipient river systems along the Amazon coast.