International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Shallow-water benthic foraminiferal assemblages and their response to the palaeoenvironmental changes — example from the Middle Miocene of Medvednica Mt. (Croatia, Central Paratethys)

Pages: 329 - 345

Authors: ÐURÐICA PEZELJ, JASENKA SREMAC, VLADIMIR BERMANEC

Abstract: During the Middle Miocene, the northern Croatian Medvednica Mt. was an island within the Pannonian Basin System, situated on the SW margin of the Central Paratethys Sea. Miocene sedimentary rocks (the Late Bade- nian Bulimina–Bolivina Zone and Ammonia beccarii ecozone), from the SW slopes of Medvednica Mt. clearly re ect a transgressive-regressive cycle with emersion during the Badenian/Sarmatian boundary. After the initial phase of transgression, the pioneer Elphidium–Asterigerinata–Ammonia benthic foraminiferal assemblage is present in bioclas- tic limestones, such as those at the Borovnjak locality. This marginal marine assemblage from a highly energetic, normally saline environment is characterized by poor preservation of foraminiferal tests, low diversity and strong domination. Advanced transgression is followed by establishment of the Elphidium–Asterigerinata assemblage, which is found in biocalcsiltites from the laterally deeper and more sheltered environment at Gornje Vrap e. This diverse assemblage is typical for inner/middle shelf environment with suf cient oxygen content. A general shallowing upward trend can be recognized at both localities, followed by visible interchange of different sedimentological and biotic features. Successive and oscillatory regression in the marginal marine environment was followed by salinity uctua- tions and nal brackish conditions with AmmoniaElphidium assemblage. The laterally deeper environment reacted to regressive trends on ner scale with almost regular changes of benthic foraminiferal assemblages in the laminae (Heterolepa–Bolivina assemblage/Bolivina–Cassidulina assemblage/Elphidium–Asterigerinata assemblage). It might re ect sea-level oscillations with periodically increased siliciclastic and nutrient input from land or in uence of sea- sonality on benthic assemblages, which occurred in the advanced phase of the regression near the Badenian/Sarmatian boundary.

Keywords: Middle Miocene, Late Badenian, Central Paratethys, palaeoecology, benthic foraminifera

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