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Middle Miocene warm-temperate carbonates of Central Paratethys: An example from the Mt. Zrinska Gora, Croatia

Investigated Zrin section in the Mt. Zrinska Gora (Croatia) represents Middle Miocene carbonate deposits from the SW part of the Central Paratethys. Lithological, sedimentological and palaeontological record enabled reconstruction of depositional settings and biostratigraphic position of the studied section, allowing comparison with similar deposits from the Central Paratethys area. Sampling was done bed-by-bed or at each observed lithofacies change within single bed in order to determine facies types, textures, microfossil assemblages, and possible terrigenous influence. The main lithological feature is an alternation of two lithofacies: fully lithified grainstones–rudstones and packstones, and semi-lithified rudstones–floatstones with a carbonate sandy matrix. Therefore, samples of fully lithified limestones were prepared as thin-sections and samples of semi-lithified limestones were sieved and studied in fractions from 63 to 1000 microns. The studied section contains rich fossil community mainly composed of non-geniculate coralline red algae, bryozoans, benthic foraminifera and echinoderms, and of ostracods, molluscs, planktonic foraminifera and calcareous nannoplankton to a lesser extent. According to the presence of foraminifera Orbulina suturalis and O. universa, zonal markers for the N9a and N9b planktonic Subzones, and calcareous nannoplankton Sphenolithus heteromorphus, zonal marker for Zones NN4 and NN5, studied deposits are determined as Middle Badenian. Based on lithological variations and changes in biogenic components, four facies associations are distinguished. Their distribution points to skeletal production and sedimentation on a middle to proximal outer carbonate ramp. Depositional environments on the ramp were presumably periodically influenced by minor high-frequency sea-level changes and/or changes of hydrodynamic conditions, which are suggested as the driving mechanisms causing the alternation of these two lithofacies. High carbonate content (89–100%) found in all of the studied samples indicates that the depositional environment was isolated from the input of terrigenous material, which brought to the high carbonate skeletal production and accumulation. Non skeletal grains, such as ooids or peloids are not observed. Patterns in relative abundance of bryozoan colony growth types (vinculariiform, cellariiform, adeoniform, membraniporiform, celleporiform, and reteporiform), size and abundance of rhodoliths and coralline branches, and benthic foraminifera are interpreted by comparison with data from modern and fossil environments. Based on these data it can be hypothesized that relative sea-level fluctuated in the water depth range from 30 to 80 m, while in the uppermost part of the section, enriched in planktonic foraminifera and calcareous nannoplankton water depth was probably greater. Causes of these relative sea-level fluctuations and a general deepening trend are probably related to the subsidence of the depositional basin. This is supported by alternation of typical rhodalgal and bryorhodalgal skeletal assemblages and abundance of planktonic foraminifera species in vertical facies pattern. Inherent skeletal assemblages are typical for warm-temperate carbonates which accumulated in SW part of the Central Paratethys Sea.

Warm-temperate carbonates; Middle Miocene; Central Paratethys; Croatia

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