engleski

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

During the Miocene, Medvednica Mt. in Northern Croatia was an island situated at SW margin of the Central Paratethys Sea. Middle Miocene sedimentary rocks (belonging to the Upper Badenian Ammonia beccarii Zone and Bulimina– Bolivina Zone), from the SW slopes of Medvednica Mt. clearly reflect a transgressive- regressive cycle with emersion during the late Middle Miocene (Badenian/Sarmatian boundary in local Paratethys stratigraphy ; equivalent of the Middle Serravalian Mediterranean substage). Shallow marine sediments of Borovnjak section were transgressively deposited over the Mesozoic carbonate basement. Marginal marine, extremely stressed, highly oxygenated environment of normal salinity is represented with pioneer Elphidium–Asterigerinata–Ammonia benthic foraminiferal community, with low diversity and strong domination. Highly diverse inner/middle shelf Elphidium–Asterigerinata community at Gornje Vrapče section is typical for well-oxygenated environment and rich and diverse food supply indicative for the advanced transgression. Shallowing upward sequences with increase of siliciclastic input in depositional basin are present in the middle and upper part of studied sections. In marginal shoal area (Borovnjak locality) fluctuations in salinity finally lead to the brackish conditions and Ammonia–Elphidium community appears. Deeper and more sheltered environment of Gornje Vrapče reflects regressive trends in cyclic lamination. Laminae differ in colour, calcium content and benthic foraminiferal communities (Heterolepa–Bolivina community / Bolivina– Cassidulina community / Elphidium– Asterigerinata community). Dominant controlling factors for foraminiferal communities in this part of the section were fluctuations in bottom oxygen content and quantity and quality of food supply. Similar ciclycity and distribution of foraminiferal communities is known from modern seasonally controlled shelf environments. The uppermost part of both sections is represented with massive biocalcrudite or coralgal biolitite, and, finally, the emersion between the Upper Badenian and Lower Sarmatian deposits.

Middle Miocene ; Late Badenian ; Central Paratethys ; Benthic Foraminifera ; Palaeoecology

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