Mesozoic evolution of the eastern Adriatic continental margin preserved in the Internal Dinarides – a radiolarian perspective

The Dinarides are largely built of Mesozoic deposits derived from the Adriatic microplate and ophiolites originating from the Neotethys Ocean. While the External Dinarides are predominantly composed of shallow-marine carbonates deposited on the Adriatic Carbonate Platform (AdCP), the Internal Dinarides, together with the ophiolites, preserve remnants of the deeply submerged eastern Adriatic continental margin. Radiolarian research in the Dinarides has largely focused on pelagic sediments associated with ophiolites, whereas continental-margin successions remain poorly explored. This contrasts with the Southern Alps, where the preserved pelagic domains of the Adriatic margin became classical localities for Mesozoic radiolarian research.

The oldest radiolarian-bearing deposits attributed to the Adriatic margin in the Dinarides originated in Middle Triassic rift basins associated with the opening of the Neotethys. This pelagic episode is recorded in both the External and Internal Dinarides. Late Anisian and Early Ladinian radiolarian assemblages from these basins, comparable to those found atop the oldest Neotethyan oceanic crust, are generally diverse and well preserved, reflecting deposition in nutrient-rich, oxygenated environments. A low-diversity assemblage from a restricted intraplatform basin has also been reported from the External Dinarides. Pelagic conditions in the smaller of these basins ceased during the Ladinian, although seafloor spreading in the Neotethys continued throughout the Late Triassic.

Following Middle Triassic rifting, the eastern Adriatic margin was characterised by a horst-and-graben architecture comprising several basins and topographic highs aligned parallel to the Neotethyan mid-ocean ridge. This architecture is best preserved in the central and southern Dinarides. There, the Durmitor High separated the Bosnian Basin from the Lim Basin, which was in turn separated from the deep-water environment of the distal continental margin by the Drina–Ivanjica High. The Bosnian Basin, which occupied the proximal part on the margin relative to the AdCP, extended northwards, with correlative successions now exposed in the northern Dinarides and the Southern Alps. Here, however, there is no evidence of the large-scale horst-and-graben topography.

During the Late Triassic and Early Jurassic, regarded as the period of seafloor spreading in the Neotethys, carbonate sedimentation prevailed in the pelagic basins of the eastern Adriatic margin, and no radiolarian assemblages of this age have been reported. During the Middle Jurassic, sedimentation shifted from carbonate to siliceous, reflecting a regional change in ocean-surface fertility. At the same time, tectonic conditions within the Neotethys changed from extension to convergence. In the Lim Basin, a late Aalenian radiolarian assemblage has been reported from a shale-dominated interval, whereas Middle and Late Jurassic assemblages have been described from cherts in all pelagic domains of the eastern Adriatic margin.

In the latest Jurassic, with the rapid diversification of oligotrophic nannoplankton, pelagic limestones began to accumulate above radiolarites in the basins of the western Tethyan realm. This transition has also been recorded in the basins of the eastern Adriatic margin; however, radiolarian assemblages of this age have been reported only from the Southern Alps. Simultaneously, syn-orogenic deposits began to accumulate in the pelagic basins of the eastern Adriatic margin, documenting the onset of ophiolite obduction.