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<p>The Eocene-Oligocene transition marks significant global climatic changes, coinciding with the closure of the southern Neotethys. In the Malishka section (Vayots Dzor, Armenia), this transition is represented by an alternation of shallow-water limestones and sandstones with hemipelagic claystones and siltstones. These facies provide essential evidence for revising planktonic and benthic foraminiferal biozonations and analyzing the sedimentological characteristics. Gravity-induced turbidity currents in the late Eocene are inferred from flute casts, graded bedding structures, and pebble-bearing limestones, followed by greywacke sedimentation in the early Oligocene. Limestones containing larger benthic foraminifers (LBF), fragments of solitary and colonial corals, red algae, echinoderms, bryozoans, bivalves and rare carbonate sponges indicate transport from different levels of photic zones. The LBFs show high abundance and diversity in upper Eocene beds, with about 20 species and subspecies of orthophragminids (<i>Discocyclina</i>, <i>Orbitoclypeus</i>, <i>Asterocyclina</i>) and nummulitids (<i>Nummulites</i>, <i>Operculina</i>, <i>Heterostegina, Spiroclypeus</i>). In the Oligocene, only five <i>Nummulites</i> species are present. Orthophragminids disappear below the Eocene–Oligocene boundary, as defined by planktonic foraminifera (PF) zonation. About 40 PF species are identified in uppermost Eocene and lower Oligocene claystones and siltstones. The 46 m thick Malishka section spans PF zones E16, O1, O2 and O3 as well as the LBF zones SBZ20 and SBZ21. Integrated PF and LBF analyses enabled the correlation of key bioevents. However, due to the rarity of LBF-bearing limestone interbeds, the extinction of orthophragminids can only be approximately correlated with the Eocene–Oligocene boundary, as determined by PF.</p>

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Diversity and biolithostratigraphy of foraminifera from the upper Eocene–lower Oligocene Malishka section, Armenia

  • 期刊论文

  • 发表日期：

    2026年1月29日

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  • Taron Grigoryan,
  • Lilit Sahakyan,
  • Elena Zakrevskaya,
  • Pavel Fokin,
  • Ara Avagyan

摘要

The Eocene-Oligocene transition marks significant global climatic changes, coinciding with the closure of the southern Neotethys. In the Malishka section (Vayots Dzor, Armenia), this transition is represented by an alternation of shallow-water limestones and sandstones with hemipelagic claystones and siltstones. These facies provide essential evidence for revising planktonic and benthic foraminiferal biozonations and analyzing the sedimentological characteristics. Gravity-induced turbidity currents in the late Eocene are inferred from flute casts, graded bedding structures, and pebble-bearing limestones, followed by greywacke sedimentation in the early Oligocene. Limestones containing larger benthic foraminifers (LBF), fragments of solitary and colonial corals, red algae, echinoderms, bryozoans, bivalves and rare carbonate sponges indicate transport from different levels of photic zones. The LBFs show high abundance and diversity in upper Eocene beds, with about 20 species and subspecies of orthophragminids (Discocyclina, Orbitoclypeus, Asterocyclina) and nummulitids (Nummulites, Operculina, Heterostegina, Spiroclypeus). In the Oligocene, only five Nummulites species are present. Orthophragminids disappear below the Eocene–Oligocene boundary, as defined by planktonic foraminifera (PF) zonation. About 40 PF species are identified in uppermost Eocene and lower Oligocene claystones and siltstones. The 46 m thick Malishka section spans PF zones E16, O1, O2 and O3 as well as the LBF zones SBZ20 and SBZ21. Integrated PF and LBF analyses enabled the correlation of key bioevents. However, due to the rarity of LBF-bearing limestone interbeds, the extinction of orthophragminids can only be approximately correlated with the Eocene–Oligocene boundary, as determined by PF.