Integrated study and stratigraphic implications of Miocene volcaniclastic deposits on Mt. Medvednica (North Croatian Basin)

The North Croatian Basin (NCB) evolved during the Miocene as a part of the SW margin of the Pannonian Basin (Carpathian-Pannonian Region, CPR). A major transgressive-regressive sedimentary cycle in the NCB is represented by an initial deposition in alluvial and lacustrine environments, followed by Central Paratethyan marine environments, and a subsequent return to lacustrine and alluvial depositional environments (Pavelić & Kovačić, 2018). Indications for these variable depositional conditions that are linked to the marine flooding of the NCB are preserved in sediments on Mt. Medvednica (Pavelić and Kovačić, 2018 ; Avanić, 1997). Due to the intensive and long-lasting CPR magmatic activity, numerous referent volcaniclastic layers are preserved in stratigraphically and environmentally different sedimentary facies. We applied an integrated stratigraphic, compositional and geochronological approach on the three volcaniclastic horizons (ČUČ-1, ČUČ-6, PL) intercalated within the marine sediments on Mt. Medvednica to determine the flooding history and the timing of the initial Miocene Central Paratethys transgression of the western part of NCB, as well as to enable the correlation of the volcaniclastic layers. Volcaniclastic deposits ČUČ-1 and ČUČ-6 are fine to coarse tuffs intercalated with massive marls. The sequence of calcarenite, tuff, and calcisiltites recorded at site PL indicate a gradual deepening of the marine environment. The composition of volcaniclastic samples from all three sites is similar, with dominantly > 50% vitric juvenile particles (pumice and volcanic shards) with subordinate magmatic minerals (feldspar, biotite, quartz, and amphiboles). The samples are well preserved, with rarely visible volcanic glass alteration. Rhyolitic glass compositions (>77% wt. SiO2 ; ranging from rhyolites to high-K rhyolites) from all 3 volcaniclastic horizons suggest an origin from major silicic eruptive events of the CPR. Future trace element measurements of glass shards will provide additional constraints to distinguish primary from secondary volcaniclastics. New high-precision CA-ID-TIMS U-Pb zircon dates indicate that the ČUČ-1 volcaniclastic horizon (up to 15.44 Ma) is older than the PL horizon (14.937 ± 0.012 Ma), as suggested by Avanić (1997). Since ČUČ-1 zircon dates range from 15.44 and 15.9 Ma, it is not possible to interpret a high-precision age, however, the absence of zircons younger than ~15.44 Ma implies the existence of NCB marine deposits older than ~14.8 Ma (Marković, 2017 ; PL this study). This data is also indicating that the initial Middle Miocene Central Paratethys flooding of the western (Mt. Medvednica) and the eastern (Mt. Požeška gora, Brlek et al. 2020) parts of the NCB occurred earlier than the previously accepted ~15 Ma maximum flooding age. References: Avanić, R. (1997): Analiza facijesa srednjeg miocena jugoistočnog dijela Medvednice, Master Thesis, University of Zagreb. Brlek, M. et al. (2020): Miocene syn-rift evolution of the North Croatian Basin (Carpathian-Pannonian Region): new constraints from Mts. Kalnik and Požeška gora volcaniclastic record with regional implications. International Journal of Earth Sciences 109:2775–2800. Marković, F. (2017): Miocene tuffs of the North Croatian Basin, Dissertation, University of Zagreb. Pavelić, D., Kovačić, M. (2018): Sedimentology and stratigraphy of the Neogene rift-type North Croatian Basin (Pannonian Basin System, Croatia): a review. Mar Pet Geol 91:455–469.