Ostracofauna da Formação Solimões (Atalaia do Norte, Amazonas, Brasil): taxonomia, implicações paleoambientais e bioestratigráficas.

Abstract

The study of the ostracods from the boreholes 1AS-7D-AM, 1AS-8-AM and 1AS-31- AM, drilled at the locality of Atalaia do Norte, in Amazonas State, Brazil, allowed to recognized 9 genera and 30 species; the eurialyne genera Cyprideis is the most abundant and diverse, with 19 known species and two new species described herein: C. atalaiensis sp. nov. and C. diction sp. nov. Other marine and/or transitional (Paracypris, Perissocytheridea, Rhadinocytherura, Pellucistoma and Skopaeocythere) and non-marine (Cypria, Cytheridella and Penthesilenula) genera have been found, as well as other microfossils (foraminifera, fishs, mollusks and palinomorphs) which have been used for the paleoenvironmental interpretations and biostratigraphy of the Solimões Formation. The integrated stratigraphic analisys of ostracods and palinomorphs in the boreholes 1AS-8-AM and 1AS-7D-AM allowed to date the sequence from Early Miocene to Late Miocene. Through the index fossils, were identified five palinological zones already proposed from the Neogene of western Amazon: Verrutricolporites, Early Miocene; Psiladiporites-Crototricolpites, Late Early Miocene to Early Middle Miocene; Crassoretitriletes, Middle Miocene; Grimsdalea, Late Middle Miocene to Early Late Miocene; and Astaraceae, Late Miocene. The distribution of the Cyprideis species allowed to recognize five zones corresponding to the palynological zones, four of which previously established, although its age limits have been changed in this study: C. aulakos, renamed as C. sulcosigmoidalis, from late Early Miocene to early Middle Miocene, C. caraione, Middle Miocene to early Late Miocene, C. minipunctata, late Late Miocene and C. cyrtoma Late Miocene. Furthermore, is proposed here a new ostracod zone: C. paralela, from Late Miocene. The biostratigraphic analysis, based on palinology and ostracods, as well as other microfossils reveals a sequence which starts at the Early Miocene, with influence of coastal environments attested by the presence of palinomorphs and foraminifera typically from mangrove. At the Middle Miocene, the paleoenvironments are characterized as fluvio-lacustrine with marine influence. Finally, in the Late Miocene although still marked by marine influence, predominate fluvio-lacustrine environment upward to the section.