Análisis bioestratigráfico mediante nanofósiles calcáreos para el Oligoceno-Mioceno medio del norte de Colombiasección de superficie del Arroyo Alférez, Colombia

Abstract

[Biostratigraphical analysis from calcareous nannofossils of the Oligocene�Middle Miocene of North Colombia. Outcrop of Arroyo Alférez, Colombia] A calcareous nannofossil-based biostratigraphy is presented for northern Colombia. The samples were taken from the Arroyo Alférez on-shore section belonging to El Carmen Formation. Mostly it consists of a massive succesion of dark gray silty mudstones with occasional glauconitic sandstone intercalations. Very few stratrigraphic and biostratrigraphic studies have been done in the land areas of NE South America based on marine fossil faunas and there are not any modern paleoceanographic and biostratigraphy calcareous nannofossil reconstruction. In this research, the classical biostratrigraphic schemes proposed by Martini (1971) and Bukry (1973, 1975) for the subdivision of Oligocene and Miocene have been chosen, because those were established in detailed sequences of the Caribbean Sea. In addition, other events proposed by several authors have been considered. The base of the Arroyo Alférez sequence studied is localized in the lower Oligocene NP23 (CP18) to Middle Miocene NN5 (CN4), showing a continuous record and an excellent preservation of coccolithophore assemblages. The most representative taxa identified in the sequence are Cyclicargolithus floridanus, Cyclicargolithus abisectus, Reticulofenestra minuta, Reticulofenestra minutula and Reticulofenestra pseudoumbilicus. We also note that the Helicosphaera spp. was abundant in some intervals, while discoasterids increase toward the top of the section indicating more temperate superficial waters. It also shows a potential paleoceanographic key for the future correlation with the off-shore record.