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Preservation of Nummulites tests in shallow-water limestones regarding the Middle Eocene Climate Optimum (Dinaric foreland basin, Croatia)

The Eocene sediments along the eastern Adriatic coast, deposited in the Dinaric foreland basin, are characterized by an abundance of larger benthic foraminifera (LBF). Representatives of the Nummulites are abundant in limestones deposited in oligotrophic, euphotic to mesophotic carbonate ramp settings and in deep basin sediments (being redeposited in mass - transport deposits). Nummulite-rich Lutetian to Bartonian sediments occur in the northwestern part of the basin and Bartonian to Priabonian in the SE part. For this study, nummulite tests from 60 thin sections of shallow-water limestones collected in the northern and southern parts of the foreland basin were examined in detail. The foraminiferal tests from the different areas show signs of mechanical, biological, or chemical degradation, which varies greatly depending on the area. The study of Lutetian carbonates from Istria and Northern Adriatic region has shown that 2 to 5% of all Nummulites tests show boring marks. The A– and B– generation representatives were equally affected by bioerosion. The traces are small and randomly distributed holes near to the outer walls. The Nummulites tests of Bartonian – Priabonian (Hvar Is., Pelješac) are more frequently bioeroded, about 10% of all specimens show boring marks, and preferably B-generation specimens are destroyed. Destruction varies from multiple holes of the same size distributed in a line along the test diameter to complete destruction of the internal test structure. There is a kind of size selection of the bioeroders, because the boring marks in the B- generation tests are larger, than those in the A- generation tests. The more intense bioturbation suggests greater biological competition in the middle to outer ramp settings around the Middle Eocene Climate Optimum in this part of the Neotethys. There are several possible causes for this: (i) a slightly higher seawater temperature around the MECO led to a higher occurrence of bioeroders (even the appearance of some new forms, such as the recorded crabs from the eastern Neotethys, Schweitzer et al., 2007), or (ii) a difference in sedimentation rate (burial history) between the two areas, or (iii) the Lutetian LBF assemblages were more resistant to biological destruction?

Dinaric foreland basin, Bartonian, Priabonian, Nummulites, bioerosion

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