Peleoecological and sedimentological characteristics of middle Miocene sediments from the Hrvatska Kostajnica area (Croatia)

The Miocene deposits of the Hrvatska Kostajnica area belong to the south-western marginal belt of the Pannonian Basin System. Sedimentological, paleontological and mineralogical investigations were performed on Kubarnovo brdo – Stari put landslide. Soil breakdown and sliding formed 30 m outcrop of the Badenian–Sarmatian marly deposits. The landslide was initiated by sudden snow melt with simultaneous precipitation which was accompanied by extremely high water level of the Una River. As a consequence of climatic conditions, the marly sediments rich in swelling mineral smectite were completely saturated with water. The slope collapse was also influenced with other unfavorable conditions: slope inclination toward Una river, geological conditions and anthropogenic activity. Badenian and Sarmatian sediments of this area were deposited in marine offshore with local input of terrigenous material represented with marls and silty marls with sand intercalations. The calcite content in the geological column varies from 33 to 88 %. Marls predominantly consist of calcite and clay minerals, while quartz is less abundant. Among clay minerals, in the less than 2 μm insoluble residue fraction, smectite is the most abundant while illite and kaolinite and in some samples chlorite were also determined. The heavy minerals association of the silty marls consists of most resistant minerals: garnet, tourmaline, zircon and rutile together with clinoptiolite indicating local origin of the material. According to palynofacies deposition occurred in a marginal environment under the periodically high terrestrial influence. The land was partially covered by swamp vegetation (Taxodiaceae), riparian forest (mostly Carya) and hillside–piedmont forest (mostly Pinus). Palynomorph assemblage is dominated by dinocysts such as Lingulodinium machaerophorum, Spiniferites div. sp., Cleistosphaeridium placacanthum, Melitasphaeridium choanophorum that point to the Polysphaeridium zoharyi– Lingulodinium machaerophorum Zone (Pzo-Lma) of the latest Badenian–Sarmatian age (Bakrač et al. 2012). Occurrence of small, microperforate planktonic species of the genera Tenuitella and Tenuitellinata are well documented in the Transylvanian Basin. According to Filipescu and Silye (2008), these small planktonic foraminifera occur in the uppermost Badenian– lowermost Sarmatian, just above the Velapertina Zone and slightly before the Anomalinoides dividens Zone. Highly oxic, shallow to middle water depth is the ideal environment for this foraminiferal group (Majewski 2003). The para-acme of Sphenolithus heteromorphus and the (FAD) of Helicosphaera HOL dalmaticus type (Syracolithus dalmaticus), noticed at the base of the section, marks Badenian NN6a subzone, while FAD of Discoaster kugleri, in its upper part, marks the early Sarmatian NN7 Zone of calcareous nannofossils for the Paratethys (Mărunţeanu 1999). Based on higher species diversity characteristic for the world’s ocean of lower latitudes (O’Brien et al. 2016) and dominations of warm water species (larger Coccolithus pelagicus, Umbilicosphaera jafari, small reticulofenestrids) indicate the MMCT. References Bakrač K., Koch G. and Sremac J. 2012. Middle and Late Miocene palynological biozonation of the south-western part of Central Paratethys (Croatia). Geologia Croatica, 65/2: 207–222. Filipescu S. and Silye L. 2008. New paratethyan biozones of planktonic foraminifera described from the Middle Miocene of the Transylvanian Basin (Romania). Geologica Carpathica, 59: 537– 544. Majewski W. 2003. Water-depth distribution of Miocene planktonic foraminifera from ODP site 744, Southern indian Ocean. Journal of Foraminiferal Research, 33: 144–154. Mărunţeanu M. 1999. Litho- and Biostratigraphy (calcareous nannoplankton) of the Miocene deposits from the outer Moldavides, Geologica Carpathica, 50: 313–324. O’Brien C.J., Vogt M. and Gruber N. 2016. Global coccolithophore diversity: Drivers and future change, Progress in Oceanography, 140: 27–42.