engleski

Marine Miocene succession in NW Sava Depression (SW Central Paratethys): biostratigraphy and paleoenvironments

The Miocene marine sedimentary succession of the Sava Depression belongs to the SW margin of the Central Paratethys, i.e. to the North Croatian Basin (Pavelić and Kovačić, 2018). This study presents the results of the detailed biostratigraphic, petrographic and sedimentological analyses from a deep exploration well in the SW part of the depression. The analyses were based on the rock material from drill-cuttings and cored intervals in order to determine the timing and paleoenvironments of the marine sediments in the Miocene. The Upper Karpatian marine sediments have been determined in over 300 m interval. These sediments consist of marls, sandstones (subarkose, arkose and rarely sublitharenite, quartzarenite and lithic greywacke) and siltstones. In the lower part of the interval moderately preserved agglutinated benthic foraminifera such as Cyclammina and Haplophragmoides, and calcareous benthic foraminiferal genera Nonion, Anomalinoides, Uvigerina and Cibicidoides have been identified. Upper parts of the Karpatian sediments contain well preserved and moderately diverse benthic foraminiferal assemblage: Heterolepa dutemplei, Sphaeroidina bulloides, Chilostomella ovoidea and Bolivina sp. and Bulimina sp. The planktonic foraminiferal association consists of rare small-sized, cold-preferring species Tenuitella angustiumbilicata, Globigerinita uvula, Turborotalita quinqueloba, and a few warm-water but opportunistic species Cassigerinella chipolensis, Globigerinella obesa and Trilobatus trilobus. Species Trilobatus bisphaericus occurs specifically in the upper Karpatian interval and continues to lowermost Badenian. Microfossil content and lithofacies characteristics of the sediments indicate middle to inner shelf environment with occasionally open sea influence. Lower to Middle Badenian sediments (about 200 m) consist of alterations of silty to sandy marls and calcite-rich marls and intercalations of the fine to medium-grade, rarely coarse- grained sandstones (calcarenite and biocalcarenite). Thesesediments were deposited in a deep marine environment (submarine outer fan). The base of the Lower Badenian is marked by the first occurrence of the Trilobatus sicanus. Throughout this interval occur: Praeorbulina glomerosa, Praeorbulina circularis, Praeorbulina curva, Orbulina suturalis together with other planktonic taxa such as Globigerina concinna, Globigerina diplostoma, Globigerina bulloides, Globigerina tarchanensis, Globorotalia bykovae, Globorotaloides spp., and Tenuitella spp. indicating warm surface water, well-stratified water column and excellent connections with Mediterranean bioprovinces. Calcareous benthic foraminifera are represented by infaunal and epifaunal taxa which inhabit various middle to outer shelf environments. Biocalcarenite contains shell remains of shallow water organisms (echinoderms, bryozoans, corallinaceans, and molluscs). The deepest palynologically explored sample contains moderately rich dinoflagellate assemblage (Spiniferites sp., Achomosphaera sp., Hystrichokolpoma sp., Systematophora sp., Systematophora placacantha, Hystrichostrogylon membraniphorum, Polysphaeridium zoharyii) which indicates Badenian age and relatively proximal marine environment with significant terrestrial input. Further palynological samples contain abundant dinocyst association (Spiniferites ramosus, Spiniferites pseudofurcatus, Achomosphaera sp., Hystrichokolpoma sp., Hystrichostrogylon membraniphorum, Unipontidinium aquaeductum, Melitasphaeridium choanophorum, Batiacasphaera sphaerica, Areoligera sp., Operculodinium centrocarpum, Lingulodinium sp., Palaeocystodinium golzowense, Cribroperidinium sp., Nematosphaeropsis lemniscata, Systematophora placacantha, Polysphaeridium zoharyii, Leptodinium sp. and Impagidinium sp.), which indicates deepening of the whole realm and sedimentation in a more distal marine environment with occasionally derived terrigenous material. At the Middle to Late Badenian boundary the shallowing of the whole area occurred. In Late Badenian deep-water sediments were replaced by the reef and peri-reef deposits (biocalcarenite, calcarenite and fossiliferous marls) rich in carbonate shell fragments of the shallow water organisms (interval of about 50 m). Benthic foraminiferal assemblage is dominated by warm water preferring taxa: Amphistegina, Elphidium, Cibicides and Ammonia. Numerous fragments of molluscan shells, echinoderms, bryozoans, and fragments of corallinacean thalii are also present. Sarmatian sediments (about 40 m) are composed of calcite-rich marls intercalated by thin beds of sandstones (biocalcarenites and calcarenites). These sediments contain rich, moderately diversified and well-preserved assemblages of calcareous benthic foraminifera, in particular warm and shallow water preferring genera such as Elphidium, Anomalinoides, Ammonia, Nonion and Cibicidoides. The benthic foraminiferal assemblage and presence of the Sarmatian index species Anomalinoides dividens indicate sedimentation in inner shelf environment with normal to slightly reduced salinity (Filipescu, 2004). Dinocyst association is relatively abundant and the most frequent species are Polysphaeridium zoharyii, Lingulodinium machaerophorum, Spiniferites bentori budejenoensis, Spiniferites ramosus, Spiniferites sp., Operculodinium centrocarpum, Hystrichokolpoma sp. and Selenopemphix sp. The whole association, and especially taxa Polysphaeridium zoharyi and Lingulodinium machaerophorum, imply Sarmatian age (Bakrač 2005 ; Bakrač et al. 2012) and sedimentation in a lagoonal environment. The presence of the green algae Botryococcus braunii implies slightly reduced salinity and minor terrigenous input.

Biostratigraphy ; Sedimentology ; Paleoenvironments ; Miocene ; Sava Depression ; Central Paratethys

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