Integrated stratigraphy of the Priabonian (upper Eocene) Urtsadzor section, Armenia

The Global Stratotype Section and Point for the Priabonian has yet to be formally defined and is currently under discussion. The transition from the upper Bartonian to lower Priabonian, as traditionally understood, is associated with a series of extinctions and originations in several microfossil groups, although the events are not necessarily synchronous. The planktonic foraminifer genus Morozovelloides and large species of Acarinina suffered a rapid global extinction, as do many radiolarian. Calcareous nannofossils show several assemblage changes including the last occurrence of Sphenolithus obtusus and first occurrences of Cribrocentrum isabellae, Isthmolithus recurves and Chiasmolithus oamaruensis. In shallow water environments, larger foraminifera also show an extinction among large species of Nummulites, as well as the first occurrences of the stratigraphically important genus Spiroclypeus. The correlation between shallow and deep water records remains uncertain, as do the mechanisms driving these extinction events. Here we present the first results of a new integrated stratigraphical study (nannofossils, planktonic foraminifera, larger benthic foraminifera, and low-resolution magnetostratigraphy) of the Urtsadzor section in south-western Armenia which appears to be continuous through this interval. The Urtsadzor section consists of calcareous siltstones rich in micro- and nannofossils, with interbedded limestones containing abundant larger benthic foraminifera. Our new data enable us to correlate larger foraminiferal events with global plankton biostratigraphy, in a section outside of southwest Europe where most previous correlations have been based. At Urtsadzor, the large Nummulites species N. millecaput decreases in abundance toward the top of the section. The first occurrence of Spiroclypeus also occurs in the uppermost limestone bed. These larger foraminiferal events occur well above the planktonic foraminiferal extinction level and nannofossil assemblage changes indicating the events are not synchronous across groups, with implications for biostratigraphy and recognition of the basal Priabonian in different areas.