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The Middle Badenian deep-marine sedimentation in the Central Paratethys: A case study of the Sava Depression in the North Croatian Basin

Continuous rock coring for the purpose of exploring unconventional hydrocarbon reservoirs in the wells of the NW area of the Sava Depression, Croatia, within the SW part of the Pannonian Basin System, has revealed Middle Miocene deep-marine sediments. These sediments indicate deposition in deeper parts of the basin during the Middle Badenian when the Sava depression, together with the remaining parts of the North Croatian Basin (NCB), formed part of the Central Paratethys (PAVELIĆ & KOVAČIĆ, 2018). Since similar sediments rarely appear on the surface in the investigated area, detailed research provided significant information regarding paleoenvironments of the Miocene series, relevant for the interpretation of depositional evolution of the NCB. Most of the sediments consist of marls, interbedded with sandstones. Detailed mineralogical, petrographic, biostratigraphic, sedimentological and geochemical analyses have been conducted. Marls are fossiliferous, predominantly homogeneous, partly silty and sandy, enriched with organic matter, and interbedded with thin layers of bioturbated sandstones. They are composed of micro- to cryptocrystalline carbonate minerals, primary calcite, and clay minerals, with the appearance of numerous microfossils and, less frequently, detrital siliciclastic and intrabasinal carbonate grains. The most abundant microfossils are planktonic foraminifera. Sandstones of different types appear as small interbeds. Mixed hybrid sandstones represented by calcarenaceous sandstones and calcarenites are the most abundant, while greywackes are also present. Layers of sand consist of fragments of calcareous red algae (Corallinaceae), benthic foraminifera (Amphistegina sp., Elphidium sp., Asterigerinata sp.), bryozoans, molluscs (bivalves) and echinoderms (urchins), appearing as bioclastic detritus, together with other siliciclastic grains (alkali feldspar, plagioclase, quartz, and rock fragments, mostly granitoids). Bioturbation in sediments is represented by traces of short-term infaunal activities of decapod crustaceans (Thalassinoides). Some of the most common foraminifera taxa are Praeorbulina glomerosa (BLOW), P. curva (BLOW), P. circularis (BLOW), Orbulina suturalis BRÖNNIMANN, Turborotalita quinqueloba (NATLAND), Globigerina concinna REUSS, Globigerina tarchanensis SUBBOTINA & CHUTZIEVA, Trilobatus trilobus (REUSS), Paragloborotalia mayeri (CUSHMAN & ELLISOR), Globorotalia bykovae (AISENSTAT) and Tenuitella sp. Determined planktonic foraminifera association implies Middle Badenian age, warm surface water, well-stratified water column and good connections with Mediterranean bioprovinces. Palynological analyses have revealed abundant and heterogeneous dinoflagellate cysts assemblage in which the most frequent and the most important species were Unipontidinium aquaeductum (PIASECKI 1980) WRENN 1988, Hystrichokolpoma rigaudiae DEFLANDRE & COOKSON 1955, Systematophora placacantha (DEFLANDRE & COOKSON 1955) DAVEY et al. 1969, Melitasphaeridium choanophorum (DEFLANDRE & COOKSON 1955) HARLAND & HILL 1979, Polysphaeridium zoharyi (ROSSIGNOL 1962) BUJAK et al. 1980 and Palaeocystodinium golzowense ALBERTI 1961. Index fossil Unipontidinium aquaeductum (PIASECKI 1980) WRENN 1988 defines D17-D18 (Uaq and Aan) dinoflagellate cyst biozones (POWELL, 1992) in the Tethyan succession and homonymous zone of the SW part of the Paratethys, specifying Middle Badenian age (15.0-13.4 Ma) (BAKRAČ, 2005). Marls contain organic matter, dispersed and rarely in laminae. Organic carbon content is in the range from 0.40 to 1.81% TOC. Kerogen is dominantly type II. Marls are good source rocks that reached early catagenetic stage of thermal transformation, i.e. the early phase of oil window. Sandstones contain small amounts of kerogen type III. All the facies characteristics of the cored sediments indicate deep-water marine sedimentation, most probably beneath the storm wave-base, on the outer shelf or slope, with occasional material inflow from proximal and shallower areas. Marls were continuously deposited from the suspension in a low water energy environment. Intrabasinal clasts and siliciclastic grains in sandstones and sandy marls, indicate periodic resedimentation from shallower zones caused by storms and turbidity currents. Granitoid orogen most probably spread alongside the shore. The shore was steep and particle transportation from the erosion site to the depositional area was relatively fast, with no major obstacles in shoreface and reef related parts of the basin. Connection with the open sea is indicated by the abundant appearance of planktonic foraminifera, as well as the presence of coccoliths. The deep-marine environment is further confirmed by significant ichnofossil features, as well as considerable pyritization and preserved organic matter, connected to more reductive conditions in the environment.

Central Paratethys ; Middle Badenian ; deep-marine sediments ; Sava depression ; hydrocarbon potential

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