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40Ar/39Ar dating of Tuffs from the North Croatian Basin

North Croatian Basin (NCB) is located in the Central and Eastern Croatia and represents a south-western part of the Pannonian Basin System. The formation of the NCB commenced 18 million years (Ma) ago and is still lasting today. In this period, except the period of Middle Miocene when it was a part of the Paratethys sea, NCB represented an isolated basin characterized by alluvial, fresh-water and brackish lake deposition (RÖGL, 1996 ; PAVELIĆ, 2001 ; PAVELIĆ & KOVAČIĆ, 2018). These periods were characterized by the evolution of endemic species which make the correlation of the beds from the NCB with the beds from the surrounding basins difficult (HARZHAUSER & PILLER, 2007). The most intensive period of volcanic activity was during the Karpatian and the Early and Middle Badenian. Karpatian and Early to Middle Badenian ages are marked by the numerous layers of tuff in the investigated area (PAMIĆ, 1997). Although layers of tuff represent a potential stratigraphic marker, especially in non-marine sediments they haven’t been used for this purpose in the NCB. The aim of this research is the reconstruction of the time frame of the evolution of NCB based on the 40Ar*/39Ar dating of the tuffs from different localities and different stratigraphic levels. Facies analysis and the determined fossil assemblage found in sediments of the investigated area point to freshwater, brackish and marine depositional environment. The palynomorph assemblages imply that the deposition took place in a moderate to subtropical climate. According to the results of the 40Ar*/39Ar dating on sanidine, tuffs have been dated in the range from 17 Ma to 14.4 Ma (Table 1). The fossil assemblage of foraminifera, ostracods, nannoplankton and palynomorphs shows that the tuffs older than 15.1 Ma were deposited in freshwater or brackish lake environments, while those dated as 14.8 to 14.4 Ma old were deposited in marine environments. Formation of the NCB started in Ottnangian with the deposition of coarse grained sediments, dominantly conglomerates intercalated with layers of tuffs in the Kalnik area which have been dated at 18 Ma (MANDIC et al., 2012). Alluvial environments have been gradually replaced with freshwater lake environments. This type of environments lasted up to 15 Ma ago. Layers of tuff found within freshwater lake sediments at the sites of Sjeničak and Paripovac in Banovina area, whose age is determined at approximately 16 Ma (MANDIC et al., 2012) and the tuff determined in the same area at Jovac locality with a determined age of 15.1 Ma confirm that claim. The existence of the freshwater lake environment, with possible marine influences, was also revealed at the locality of Laz on Medvednica Mt. where the age of the tuff was determined at 15.4 Ma. The layers of tuff found within the marine sediments have an age of 14.8 Ma, at the locality of Čučerje-1 on Medvednica Mt. and 14.4 Ma on the locality of Nježić in Slavonija. The mentioned results indicate that the marine transgression in the area of the North Croatian Basin took place around 15 Ma ago, while according to the previous studies (PAVELIĆ, 2001) it was supposed to have started at the beginning of the Karpatian.

40Ar/39Ar dating, Miocene, North Croatian Basin, tuffs

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