Biostratigraphy and facies description of Middle Triassic rift-related volcano-sedimentary successions at the junction of the Southern Alps and the Dinarides (NW Croatia)

The Middle Triassic volcano-sedimentary successions deposited on the passive continental margin during a period of intense extensional tectonics related to the opening of the Neotethys Ocean were investigated in NW Croatia. A new palaeogeographic term, the Northwestern Croatian Triassic Rift Basin (NCTRB), is introduced for these successions. Pelagic sediments were deposited on top of older shallow-marine carbonates from the early Illyrian to possibly late Ladinian. Pelagic limestones containing Illyrian ammonites and redeposited benthic foraminifers of the same age indicate the existence of a contemporaneous shallow-marine carbonate environment that supplied material to the deeper parts of the basin. Stratigraphically stacked volcanic and volcanogenic rocks are intercalated with pelagic sedimentary rocks. Submarine basaltic rocks, geochemically characterized as trachy-basalts, are related to deep-rooted faults. Trachy-basaltic hyaloclastites, found intercalated within pelagic limestones, were formed by the quenching of magma that came into contact with cold sea water and subsequent resedimentation of the newly formed basaltic fragments. The majority of volcanogenic deposits belong to the Pietra Verde deposits found higher in the sections. The material for these deposits was produced by explosive volcanic eruptions and deposited by gravitational mechanisms, including pyroclastic density currents. Radiolarians from intercalated radiolarian cherts indicate late Illyrian to early Fassanian age for volcanic activity, as well as episodic eruptions and deposition of pyroclastic material. The uppermost part of the NTCRB successions is characterized by secondary volcaniclastic deposits generated by the rapid reworking of unconsolidated pyroclastic detritus and is deposited as medium- to fine-grained turbidites, marking the gradual filling of the basin. Based on regional correlations, late Ladinian is the most likely age for these deposits, indicating a significant stratigraphic gap in the NTCRB successions.