Diversity and biolithostratigraphy of foraminifera from the Upper Eocene–Lower Oligocene Malishka section, Armenia

The Eocene-Oligocene transition marks significant global climatic changes, coinciding with the closure of the southern Neotethys. The Priabonian-Rupelian transition in the Malishka section of the Vayots-Dzor area is represented by alternation of shallow-water limestones-sandstones with hemipelagic claystones and siltstones. These facies provide essential evidence for revising planktonic and benthic foraminiferal biozonations and analyzing the sedimentological characteristics of the section. In Malishka section turbidity currents are inferred from flute cast structures, reverse-graded bedding, and pebble-bearing limestone in the Late Eocene, which was followed by greywacke sedimentation in the Early Oligocene. Larger benthic foraminifers (LBF), fragments of solitary and colonial corals, red algae, echinoderms, bryozoans, bivalves and rare carbonate sponges, together with planktonic foraminifera (PF) in limestones suggest transport from the euphotic zone. The LBF exhibit significant abundance and diversity in the upper Eocene sediments, with about 20 species and subspecies of orthophragminids (Discocyclina, Orbitoclypeus, Asterocyclina) and nummulitids (Nummulites, Operculina, Heterostegina, Spiroclypeus). The Oligocene LBF are represented by only five species of Nummulites. The last occurrences of orthophragminids were observed before the Eocene/Oligocene boundary, as defined by PF zonation. About 40 species of PF are identified in the claystones and siltstones of uppermost Eocene and lower Oligocene. The 46-meter-thick Malishka section, spanning the Eocene/Oligocene boundary, corresponds to the PF zones E16, O1, O2 and O3 as well as the LBF zones SBZ20 and SBZ21. The study of PF and LBF from the Malishka section facilitated the correlation of bioevents between these two groups.