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THE DEVELOPMENT OF THE NEOGENE NORTH CROATIAN BASIN: FROM THE LAND TO THE SEA AND BACK

The Neogene North Croatian Basin (NCB) is situated in the southwestern part of the Pannonian Basin System (PBS), and except for its northwesternmost segment, geographically covers the entire area of Pannonian Croatia. It is a rift type basin whose formation was controlled by tectonics, changes in climate, volcanic activity and eustatic fluctuations. The accumulated sedimentary succession thickness in places reaches up to 7 km (Saftić et al., 2003). These sediments were initially deposited in alluvial and lacustrine environments, followed by marine environments, and then a return to lacustrine and alluvial, thus displaying a major transgressive- regressive sedimentary cycle (Pavelić & Kovačić, 2018). The time span of the basin formation and duration of the individual phases in its development were, until recently, chiefly defined based on mainly fossil communities and superposition. However, temporary endemism caused stratigraphic problems within the NCB and correlation with neighboring areas. The problems were not resolved until using multidisciplinary investigations which were carried out during the last ten years. Combined radiometric dating of tuff layers and integrated bio- magnetostratigraphy improved the time span for the formation and development of the NCB. Deposition within the NCB commenced in the Early Miocene within terrestrial sedimentary environments which were under strong influence of cyclic shifts between arid and humid climate conditions. Alluvial sediments were deposited over an unconformity above basement rocks of the NCB, and are in places interlayered with pyroclastics and loess deposits, while sediments deposited in a salina type lake have also been documented (Kovačić & Pavelić, 2017). Radiometric dating of tuff interlayers from the lower part of the alluvial sequence (Mandic et al., 2012), as well as tuffs from the overlying lacustrine deposits (Marković, 2017), has confirmed the previously supposed Ottnangian age of the oldest Miocene sediments in the NCB. Also, it has been newly determined that the deposition of alluvial and salina lake sediments continued into the Karpatian. Terrestrial depositional environments persisted in the NCB into the early Badenian within lacustrine environments where mainly clastic pelitic sediments with tuff layers were deposited. Their fossil assemblage indicates deposition within interchanging freshwater and brackish lacustrine conditions without any connection with the sea (Mandic et al., 2018). Radiometric dating on tuff layers indicates that lacustrine environments persisted in the NCB area up to approximately 15 Ma (Marković, 2017). As a consequence, the beginning of the marine transgression, previously assigned to the Ottnangian/Karpatian boundary based on superposition, has been shifted to the beginning of the middle Badenian. The marine phase of the NCB covers the middle to late Badenian and Sarmatian time period, during which the area represented the southwestern marginal segment of the Central Paratethys (CP) (Pavelić & Kovačić, 2018). During the middle and late Badenian diverse pelitic, carbonate and carbonate-clastic sediments with pyroclastic interlayers were deposited continuously onto lacustrine deposits or above an unconformity, covering various basement rocks. Radiometric dating of tuffs (Marković, 2017) confirmed their stratigraphic age which had previously been defined based on the rich fossil community present in the marine sediments. Following the late Badenian transgression, which marks the peak of the transgressive cycle in the NCB, a general regressive sedimentary succession begins. Within it, different Sarmatian clastic and carbonate sediments were deposited over the upper Badenian carbonate sediments in conditions of reduced volcanic activity and weakening connections between the CP and surrounding marine realms. Gradual isolation of the CP and a reduction in water salinity led to an extinction of stenohaline marine organisms at the Badenian/Sarmatian boundary, and allowed the development of a new community adapted to the life in a marine environment of reduced salinity. The continuing trend of CP's isolation led to a complete isolation of the PBS from surrounding marine realms, the termination of marine sedimentation and the formation of the brackish Lake Pannon. In the newly formed lake, a stratigraphically ambiguous endemic community of organisms developed. This circumstance, together with a lack of pyroclastics, has caused problems in the subdivision of the thick succession of Upper Miocene lacustrine deposits in the NCB area, and their correlation with neighboring regions. It has been shown that the fossil communities of mollusks based on which they are divided into Croatica, Banatica, Abichi and Rhomboidea layers, depended on depositional facies and are diachronous, as well as that the Pontian stage is incorrectly applied in the PBS area (Piller et al., 2007). The deposition of the Upper Miocene lacustrine sediments in the NCB is characterized by a transgressive-regressive cycle. In the older, transgressive part of the cycle, limestones predominate. These are overlain by marly sediments with rare interlayers of sand and gravel while the younger, regressive part of the cycle, is characterized by deposition of sandy-silty clastic detritus supplied into the lake by deltaic systems (Pavelić & Kovačić, 2018). These systems prograded into the area of the NCB from the north and the northwest that generated gradual shallowing of the lake and final infilling (Kovačić et al., 2004). Notably, before the end of the Miocene the north and northwest part of the NCB were transformed into an alluvial plain. In the southeastern part of the basin the Lake Pannon persisted into the early Pliocene, when it was replaced by the freshwater Lake Slavonia. Within it, from the middle Pliocene to the early Pleistocene, a variety of clastic sediments were deposited, previously known as Viviparus beds, which have been found to represent a new independent phase of the development of the basin, and have accordingly been defined as the newly proposed regional Cernikian stage (Mandic et al., 2015). Since the early Pleistocene, the entire area of the NCB is once again represented by terrestrial sedimentary environments.

North Croatian Basin, Neogene, transgressive-regressive cycle

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