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Paleoenvironmental interpretation of the Eocene mud-mounds based on larger benthic foraminifera (Promina Beds, Croatia)

Larger benthic foraminifera (LBF) from two spectacularly exposed carbonate mounds that crop out in the SE part of the Promina piggy-back basin (Outer Dinarides, Croatia) were used to define the origin and the mechanisms that produced them. The Promina Beds are ca 2000 m thick and are a regressive molasse succession of neritic to terrestrial calciclastic deposits of mid Eocene to Oligocene age accumulated in the Dinaric foreland zone. Limestones with LBF and corals occur in the upper part of the clay/marls neritic succession. In order to estimate the relative bathymetry of deposition and to build a depositional model, textural analysis, nummulitid (Nummulites s.str and Asterigerina/Amphistegina sp.) and orthophragminid test size and shape, and the types of light-dependent skeletal components were studied. Five types of facies were recognized: Facies #1, packstones in which LBF (nummulitids) make up 20% of the rock volume while small bioclasts (fragments and whole tests) comprise 10 % are dispersed in micrite matrix ; Facies #2, a red-algae-rich coral rudstone with a pack-wackestone matrix ; Facies #3, well sorted, fine-grained or poorly sorted coarse-grained skeletal packstones with numulitids, bryozoans, coralline algae, and very rare, smaller benthic foraminifera ; Facies #4, coral boundstone with abundant red algae in a mud-dominated matrix ; and Facies #5, nummulitid packstones, with Nummulites sp., Operculina sp. Discocyclina radians and Asterocyclina stellata, coralline algae, very rare smaller benthic foraminifera. Facies associations, textures, and photodependent components (T/D ratio, test morphology) indicate these cycles with corals and coralline algae to have grown in the mesophotic (facies # 2, #3 and #4) and euphotic (facies #1 and #5) zones and in low energy conditions below fair-weather wave base, where they were only occasionally influenced by exceptional storms. Facies #1 is assigned to a maximum water depth of 10 m and the deposits are winnowed, probably from the action of bottom currents. The prevalent deposits are unsorted Discocyclina–Asterocyclina micrites (facies #5), which were deposited at a maximum water depth of 50–60 m suggested by this large, hyaline foraminiferal assemblage.

Larger benthic foraminifer a; Promina basin; Eocene

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