Mid-Miocene (Badenian) transgression on Mesozoic basement rocks in the Mt. Medvednica area of northern Croatia

Abstract

Rocky shorelines in the geological record, which represent major transgressive surfaces and provide us with crucial information about paleoshorelines and ancient sea levels, often involve a basal conglomerate associated with an unconformity. The unconformity between the Mesozoic basement and the overlying Middle Miocene (Badenian) deposits (which belong paleo-geographically to the southwestern margins of the Central Paratethys, and geotectonically to the Pannonian Basin System) in the Gornje Oresje section (northeast Mt. Medvednica, northern Croatia) is marked by basal Badenian conglomerates. The Upper Cretaceous limestone lithoclasts occurring in basal conglomerates show abundant truncated Gastrochaenolites and Entobia borings (represented by an in situ rocky substrate community of bivalves and sponges, respectively), with Gastrochaenolites being the dominant ichnogenus. Gastrochaenolites-Entobia ichnofossil assemblage related to the Entobia subichnofacies and in turn assignable to the Trypanites Ichnofacies, is very typical of Neogene rocky shores. This association characterizes littoral rockground environments, indicating wave-cut platforms and marine flooding surfaces (transgressive surfaces) with a low or null rate of sedimentation. Erosion of a pre-existing Mesozoic basement rocky shore during a marine transgressive phase in these high-energy littoral conditions, which formed basal conglomerates analyzed here, is also evidenced by truncation and the occurrence of Gastrochaenolites borings on all sides of limestone clasts. Calcareous red algae encrusting bored lithoclasts and often-forming rhodoliths also imply that the basal conglomerates occur at the start of a deepening-upwards succession of a transgressive systems tract deposits. The biostratigraphic correlation of the transgressive deepening-upward sequence from the Gornje Oresje section (ascribed to a single Badenian transgressive event that affected the Mesozoic basement) is based on the presence of the orbulinas in the uppermost outer shelf marl interval. The co-occurrence of Praeorbulina glomerosa circularis and Orbulina suturalis points to the short interval between FO (first occurrence) of Orbulina suturalis and LO (last occurrence) of Praeorbulina glomerosa circularis. These samples belong to the plankton zone M6 (Orbulina suturalis Zone) or the base of NN5 zone, marking the third-order sequence TB 2.4, which represents the second, main Badenian transgression in the Central Paratethys.