Bartonian-Priabonian Larger Benthic Foraminiferal Events In The Western Tethys

Abstract

The composition of Western Tethyan larger benthic foraminifera (LBF) significantly changed as a consequence of the general climate cooling trend in the late Bartonian and Priabonian. Major events are characterized by the disappearance of giant forms of both Nummulites and Assilina and also of the genus Alveolina and some orthophragminid lineages. Simultaneously, radiate and especially reticulate Nummulites, the N. fabianii lineage, and nummulitids with secondary chamberlets such as Heterostegina and Spiroclypeus emerged. Based both on first/last occurrence (FO/LO) data and the evolution of these forms and integrating geological information such as field observations, other LBF and planktonic data, a high-resolution stratigraphy of the Bartonian and Priabonian could be established in the frame of the Tethyan shallow benthic zonation (with SBZ zones and newly erected subzones for SBZ 18 and 19 based on the exceptionally rapid evolution of Heterostegina). We distinguish eight Western Tethyan LBF events in this time-span, mainly based on FO/LO data of selected lineages. These are: (1) FO of the Operculina gomezi-group (SBZ 16/17), (2) FO of genus Heterostegina (SBZ 17/18a), (3) FO of the H. reticulata-lineage (SBZ 18a/b), (4) LO of giant Nummulites (with supposed eastward migration during SBZ 18b-c), (5) FO of genus Spiroclypeus (SBZ 18c/19a), (6) LO of some survivor Middle Eocene orthophragminid lineages (around SBZ 19a/b), (7) FO of Heterostegina with granules (SBZ 19b/20) and (8) LO of the majority of survivor Eocene LBF (SBZ 20/21). The most dramatic change happened between event 4, followed by the successive expansion of reticulate Nummulites, and event 5. Due to its easy recognition by field methods, the Middle/Late Eocene boundary is traditionally drawn at the base of SBZ 19 (Serra-Kiel et al., 1998), corresponding to event 5 and marked also by the intraphyletic appearance of both Nummulites fabianii and Heterostegina reticulata mossanensis. This change falls, however, within P 15 (planktic foraminifera) and NP 19-20 (nannoplankton) zones, and is therefore considerably younger than the Bartonian/Priabonian boundary placed at the boundary of NP 17/18 zones by planktonic experts.