Paleoenvironmental settings and assemblage changes of foraminifera and palynomorphs across the eocene-oligocene boundary of Southern Tanzania

Abstract

A quantitative micropaleontological analysis was performed on outcrop and core samples across a shallow borehole drilled in the southern coastal basin of Tanzania with the aim of characterizing foraminifera and palynomorphs assemblage changes aiming at reconstructing paleoenvironmental settings across the Eocene-Oligocene transition (EOT). The data reveal high diversity and abundance of calcareous benthic foraminifera assemblages in the Late Eocene succession and a decline of their abundance and diversity across the EOT to Early Oligocene. Planktonic foraminifera assemblages were low in abundance and diversity in the Late Eocene succession and decreased through the EOT when most planktonic foraminifera species from Hantkeniide family and Turborotaloiide groups went extinct. Additionally, marine palynomorphs/dinoflagellate dominated the oldest sedimentary succession (Late Eocene). Their abundance and diversity declined towards the EOT to the Early Oligocene while terrestrial palynomorphs (spores and pollens) dominated the youngest succession. The palynomorphs assemblage changes responded rapidly to environmental variations across the Eocene-Oligocene boundary which was associated with a global cooling event. Both foraminifera (i.e. calcareous benthic foraminifera) and palynomorphs assemblages as well as planktonic/benthic ratios indicate that the EOT paleoenvironment settings were compatible with shallow marine of inner to outer shelf environments.

Keywords: Eocene-Oligocene, foraminifera, palynomorphs,  paleoenvironment, Tanzania