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Structural Dynamics at the Southern Part of the Drava Depression: Sedimentation and Structures in the Northern Part of Bjelovar Sag, Croatia

Investigated area covering approximately 85 km2 is located in the northern part of the Bjelovar sag, from the southwestern slopes of the Bilogora Mt. and Narta village on the northeast and southwest, respectively. It is a part of much larger Drava depression created since late Oligocene time and comprises wide area of Galovac-Pavljani oil field. Geological relations are reconstructed based on data obtained from 16 deep wells and 9 reflective seismic lines. Drava depression is formed along dextral strike-slip fault system. Start of opening is approximately constrained as being related to the late Oligocene - early Miocene extensional event(s) (HERNITZ, 1983). Later on, most probably during the Middle Miocene time, NE-SW oriented sinistral strike-slip fault system had been generated striking diagonally to the main NW-SE depression strike. Bjelovar sag, which shows a rhomboidal shape, had been formed along this later sinistral strike-slip fault system most probably as a pull-apart basin (PRELOGOVIĆ et al., 1995). The sag was, however, connected with the Drava depression to the east-northeast, representing one of its western branches. More to the southwest it was separated from the Sava depression by the massif of Moslavačka gora Mt. Most of the sag sin-extensional, siliciclastic fill derived from elevated parts of the basement rimmed by reef build-ups that were the major source for carbonate clastic material. Age of the sag fill was determined as Middle Miocene (mostly Badenian, Sarmatian, Early Pannonian), locally with some Ottnangian and Karpatian strata, based on bio- and lithostratigraphical correlation. Coarse-grained clastic material was transported by relatively short and strong currents and deposited within alluvial and delta fan environments (MALVIĆ, 1998), sporadically in a form of thick breccia bodies, proved to be a good hydrocarbon collectors on some fields in the Drava depression (HERNITZ, 1983 ; TIŠLJAR, 1993). Middle Miocene is unconformably overlain by post-extensional clastic sediments of Late Pannonian, Pontian and Pliocene age. Clastic material was derived form distant sources deposited in submarine deltas (ROYDEN, 1988), and partly in fresh water environments of various depths (VRBANAC, 1996) by turbidite current mechanism. Due to the fact that the sag was located away form the main clastic material transport direction the sag fill thickness does not exceed 2500 m and is relatively small when compared with 7000 m thick correlative deposits from the Drava depression. Due to Pliocene-Quaternary tectonics the sag was inverted with older structural pattern only partly preserved as shown on the base Late Pannonian structure contour map (FIGURE 1) with the anticline Pavljani in the central part bounded from the northeast and southwest by reverse, generally south-dipping faults. To the west and east the anticline is bounded by normal faults. More to the east the principal transversal NNE-SSW-striking normal fault of the Bjelovar sag is located, accompanied by some smaller normal faults. That principal fault, on the southern part, is associated by another oppositely dipping normal fault that both created a structure that looks like a smaller negative (inverse) flower structure. Observed structures, tectonics and accompanying sediments can be compared with similar features found in other parts of the Pannonian basin system (ROYDEN & DÖVÉNYI, 1988) and could be explained by repeated change in principal stress filed directions or local deviation of strike of major basin lineaments, e.g. the Drava fault zone, both leading to local formation of tranpressional and transtensional zones. Acknowledgement: Authors wish to express thanks to INA Inc. for its permission to publish data on the realised research. References: 1. HERNITZ, Z. (1993): Dubinski strukturno-tektonski odnosi u području istočne Slavonije (Subsurface Structure-Tectonic Relations in the East Slavonia Region).- Dissertation, Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, p. 221, Zagreb. 2. MALVIĆ, T. (1998): Strukturni i tektonski odnosi te značajke ugljikovodika širega područja naftnoga polja Galovac-Pavljani (Structural and Tectonics Relations and Characteristics of Hydrocarbons in the Wider Area of Oil-Field Galovac-Pavljani).- Master thesis, Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, p. 111, Zagreb. 3. PRELOGOVIĆ, E., JAMIČIĆ, D., ALJINOVIĆ, B., SAFTIĆ, B. & DRAGAŠ, M. (1995): Dinamika nastanka struktura južnog dijela Panonskog bazena (Dynamics of Structural Forming in the South Part of the Pannonian Basin). Proceedings 1st Croatian Geological Congress- Opatija 18-21.10.1995., 2, 481-486, Instit. za geol. istraž., Zagreb. 4. ROYDEN, L.H. (1988): Late Cenozoic Tectonics of the Pannonian Basin System.- AAPG Memoir 45 (Chap. 3) - The Pannonian Basin (eds. Royden, L.H. & Horváth, F.), 27-48, Tulsa. 5. ROYDEN, L.H. & DÖVÉNYI, P. (1988): Variations in Extensional Styles at Depth Across the Pannonian Basin System. AAPG Memoir 45 (Chap. 17) - The Pannonian Basin (eds. Royden, L.H. & Horváth, F.), 235-255, Tulsa. 6. TIŠLJAR, J. (1993): Sedimentary Bodies and Depositional Models for the Miocene Oil-Producing Areas of Ladislavci, Beničanci and Obod.- Nafta, 10, 531-542, Zagreb. 7. VRBANAC, B. (1996): Paleostrukturne i sedimentološke analize gornjopanonskih naslaga formacije Ivanić-Grad u savskoj depresiji (Palaeostructural and Sedimentological Analysis of the Upper Pannonian Sediments of the Ivanić-Grad Formation in the Sava Depression).- Dissertation, Faculty of Natural Sciences, Geological Department, University of Zagreb, p. 303, Zagreb.

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