Sedimentary facies and palynoflora of the Andirá and Arari formations in the Autazes region, Amazonas Basin

Abstract

This paper presents sedimentological, stratigraphic, and palynological data from borehole PBAT-15-43, in which a 230 m long continuous core was sampled in the central portion of the Amazonas Basin, where it was possible to define the contact relationship between Paleozoic deposits of the Andirá and Arari Formations (Tapajós Group). The drilling hole is located in the southern portion of the municipality of Autazes (Amazonas State) and was made available by the Potássio do Brasil Company. Six sedimentary facies were identified in the borehole and grouped into two associations, informally referred to as I and II, representing the Andirá (upper part) and Arari (lower part) formations, respectively. Association I was interpreted as a fluvial environment with channel deposits, flood barriers, and crevasses splay. Association II, on the other hand, represents an alluvial fan environment with predominantly gravitational flow deposits. The facies of association II proved to be sterile for palynomorphs, whereas facies I contain a rich palynoflora, including especially the species Lueckisporites virkkiae, Corisaccites alutas, Hamiapollenites andiraensis, H. karooensis, Vittatina costabilis, V. saccata, V. subsaccata, and Tornopollenites toreutos; secondarily by species of the genera Punctatisporites, Verrucosisporites, Limitisporites, Cycadopites and Stratopodocarpites, which are considered as belonging to the Permian age. The fact that no marine palynomorphs were detected supports the present interpretations regarding the age and the depositional environments previously predicted for these formations as predominantly fluvial deposits.