Sedimentation of Late Pannonian clastic deposits in main and marginal basins B. Vrbanac, J. Velić & T. Malvić (Croatia) EGU General Assembly 2008, Vienna, April 13-18, 2008

INTRODUCTION The Sava depression and the Bjelovar subdepression are both located at the very southwest margin of the Pannonian basin. The main difference is in their scale: the Sava depression is independent regional geotectonic unit, while the smaller Bjelovar subdepression represents only the southern part of the Drava depression. Both basins were formed contemporaneously by Neogene extension. Previously observed differences were interpreted as a consequence of different source areas and variable palaeogeography. Here was proved that these area created and developed simultaneously, only with different depths.

Geographic positions of the Sava depression and the Bjelovar subdepression

Geotectonical position of the Sava depression and the Bjelovar subdepression SAVA depression BJELOVAR subdepression

Geological map of the study area (Upper Pannonian deposits are marked red) SAVA depression BJELOVAR subdepression

LITHOSTRATIGRAPHIC SECTIONS Lithostratigraphic sections are different for the Sava depression and for the Bjelovar subdepression. Upper Pannonian deposits are represented by alternation of marls, sandstones and siltites. There are two main sandstone members: -the Okoli ss. can be followed in both regions, -the IVA ss. are found only in the Sava depression. Pure sandstones are deposited only in the central parts of depressions. Marls completely substituted sandstone members in marginal areas (as a result of the basin plain sedimentation).

Correlation table of the chronostratigraphic and lithostratigraphic units and positions of EL-markers

MAPS AND FACIES Isopach map of the Rs5-Z’ interval (Ivanić-Grad formation of the Late Pannonian age) The maximum thickness of 800 m 600 m 500 m The maximum thickness of 800 m

Sandstones – fine-grained, rarely medium-grained (diameter up to 0.5 mm); porosity variable from 10-33%, porosity values decrease toward SE (in direction of palaeotransport). Petrographic composition is very homogeneous: quartz (>60%) and rock fragments - mostly carbonates, cherts, schists, gneisses, granites (18-35%). Source area is determined after accessory minerals indicating metamorphic rocks (from epi- and mesozones), limestones and cherts, all probably originating from the Alps. Siltites – the mineral composition is identical like in sandstones, only the mica content is increased. Marls – the matrix is clay and carbonate, cryptocrystalline. The content of CaCO 3 is ca. 60%. Lithological description

Facies found within core samples (1) F 4 – massive marls F 1 – thick-layered to massive sandstones Facies of massive marls (F 4 ) – homogeneous, massive, mostly unstratified marls with clearly visible bioturbations. Sometimes stratification can be recognized by relics of lamination, colour changes and thin siltite films. Marls were formed by continuous sedimentation of pelitic detritus. Facies description (1)

Facies found within core samples (1) F 4 – massive marls F 1 – thick-layered to massive sandstones Facies of thick-layered to massive sandstones (F 1 ) – this facies includes thick-layered to massive, homogeneous sandstones, rarely with thin beds or laminae of siltites and marls. Thickness of sandstone intervals varies between m; thicker beds are probably result of amalgamation. Gradation and cross-bedding are hardly visible. Convolution is more often recognizable. Lower bedding surfaces are characterized by erosional marks. Sandstones are deposited from denser turbidites. Sedimentation is result of directed, fast and massive deposition from suspension. Facies description (2)

Facies of thin sandstone layers (F 2 ) – represented by sandstone layers up to 0.5 m thick. Horizontal and cross bedded lamination is very easily observable in sandstones and siltites. Very often erosional marks can be observed on the upper bedding planes. Also, convolution and marls clasts can be found frequently. These deposits were also deposited from turbidites, but in areas distally from the main currents. F 3 – laminated sandstones, silts and marls F 2 – thin sandstones layers Facies description (3) Facies found within core samples (2)

Facies of laminated sandstones, silts and marls (F 3 ) – monotonous alternation of thin sandstone layers, which are in upper part gradually substituted by siltites and marls. Textures encompass horizontal and cross lamination, convolution and sandstone veins and dykes. These rocks were deposited from low-density turbidites, but probably some of these strata are results of redeposition caused by normal submarine currents along the sea bottom. F 3 – laminated sandstones, silts and marls F 2 – thin sandstones layers Facies description (4) Facies found within core samples (2)

Facies associations indicated by spontaneous potential and resistivity curves in the Sava depression FACIES ASSOCIATION Analysis can be followed through four (4) facies associations (F A, F B, F C, F D ). F A or Channel filling facies association – This facies association consists of thick-bedded sandstone facies (F 2 ) and thin-bedded sandstone facies (F 3 ). F B or Depositional lobe facies association – This facies association is composed of F 3 facies in lower part and facies F 2 in its upper part.

Facies associations indicated by spontaneous potential and resistivity curves in the Sava depression F C or Lateral or distal turbidite facies association – This facies association consists of a monotonous alternation of very thin, thin and medium thick sandstone beds passing into siltites and marls. They are represented by facies F 3 and F 2. F D or Massive marls facies association – this association is mostly represented by massive marl facies F 1 with rare intercalations of thin siltite or sandstone laminae or beds.

Facies associations indicated by spontaneous potential and resistivity curves in the Bjelovar subdepression

Vertical and lateral facies correlation section over SW part of the Žutica field in the Sava depression

Lateral facies correlation section over Letičani, Galovac-Pavljani and Velika Ciglena fields in the central part of the Bjelovar subdepression

Lateral facies correlation within the Žutica field in the Sava depression

Lateral facies correlation within the Šandrovac field in the Bjelovar subdepression; (modified after Bokor et al., 2000)

Schematic lateral facies correlation between the Sava and Bjelovar depressions

CONCLUSIONS (1) 1.Two mechanisms of transport and deposition of the Ivanić Grad formation were proposed: massive marls are product of “normal” basin hemipelagic deposition (F 1 facies), while periodic turbidity currents transported coarser sand-sized detritus, as well as finer grained detritus of siltite and mud size as a turbidite facies (F 2 -F 4 ). 2.It can be concluded that the depositional environment was calm and stable, owing sufficient water depth (more than 200 meters) that compensated for all water level changes caused by either tectonic movements or cyclic climate changes. 3.This low-energy environment was disturbed only by temporary turbidity currents which deposited most detritus in the depression.

CONCLUSIONS (2) 4.Deposition of the Iva sandstones (lower part of Upper Pannonian) occur only in the Sava depression. Contemporaneous deposits of the Bjelovar subdepression are represented only by basin marls. 5.Younger Okoli sandstones are found in both depressions, as a result of the deeping and opening of the Bjelovar subdepression to source areas and transport by turbidity currents.

REFERENCES 1.Bokor, N., Hernitz, Z., Sečen, J. and Steiner, I. (2000): Bypassed oil: Šandrovac oilfield (Northern Croatia). 62 nd EAGE Conference & Techbical Exhibition, Glasgow, Extended Abstracts Proceedings. 2.Malvić, T. (2003): Vjerojatnost pronalaska novih zaliha ugljikovodika u bjelovarskoj uleknini (Oil-Geological Relations and Probability of Discovering New Hydrocarbon Reserves in the Bjelovar Sag). Unpublished PhD Thesis, Faculty of Mining, Petroleum and Geology, University of Zagreb, 123 p. 3.Velić, J. (1980): Geološka građa zapadnog dijela Savske depresije (Geological framework of the western part of the Sava depression). Unpublished PhD Thesis, Faculty of Mining, Petroleum and Geology, University of Zagreb, 137 p. 4.Vrbanac, B. (1996): Paleostrukturne i sedimentološke analize gornjopanonskih naslaga formacije Ivanić grad u savskoj depresiji (Palaeostructural and sedimentological analyses of Late Pannonian sediments of Ivanić Grad formation in the Sava depression). Unpublished PhD Thesis, Faculty of Natural Sciences, Geological department, University of Zagreb, 303 p., Zagreb. 5.Vrbanac, B. (2002): Chronohorizons Based on Resistivity Curve Variations – Upper Miocene Sediments of the Ivanić Grad Formation in the Sava Depression (NW Croatia). Geologia Croatica, 55, 1, 11-24, Zagreb. 6.Vrbanac (2002): Facies and Facies Architecture of the Ivanić Grad Formation (Upper Pannonian) – Sava Depression, NW Croatia. Geologia Croatica, 55, 1, 57-78, Zagreb.