Fine-Scaled Basin Evolution during the Middle Triassic Based on Conodont Stratigraphy and Ostracod Paleoecology in the Western Tethys

The Transdanubian Range (TR) unit forms the uppermost part of the Austroalpine nappe system, which is part of the Alpine–Carpathian–Pannonian (ALCAPA) megaunit. During the Mesozoic, it was situated on the Western Tethys shelf, between the Northern Calcareous Alps and the Southern Alps. During the middle Anisian, extensional tectonics driven by the westward propagation of Neotethyan rifting disrupted the uniform early Anisian carbonate ramp. This tectonic activity resulted in a complex horst-and-graben topography. During the late Anisian, the region experienced a relative sea-level rise and volcanic activity, leading to the gradual expansion and deepening of basins. The basins reached their maximum depths during the Ladinian. In TR numerous outcrops and cores document Middle Triassic marine successions that provide a unique opportunity for investigating the environmental and evolutionary dynamics of marine habitats. The continuous stratigraphical record dated by conodonts and ammonites enables the temporally and spatially comprehensive investigation of the marine ostracod assemblages. Research focusing on detection the diversity changes of the marine ostracod faunas in the Western Neotethys and to reveal links between biotic changes, water depth, nutrient supply and oxygenation of the water column during the Middle Triassic. The onset of open-marine sedimentation on elevated highs (Kádárta, Barnag and Tagyon highs) of the Balaton Highland can be dated late Pelsonian based on conodonts (Paragondolella bulgarica, P. bifurcata, P. hanbulogi, Nicoraella germanica, Ni. kockeli). The Illyrian beds of the studied sections were dated by diverse neogondolellids and contained very similar ostracod faunas (dominance of smooth bairdiids, bairdiacypridids and healdoids) on the separate submarine highs. The ostracod assemblages indicate deep neritic open shelf environment with water depth below 30-50 m (below the storm wave base). Changes in the composition of the ostracod community of the former Anisian/Ladinian GSSP candidate section near Felsőörs clearly reflect environmental changes associated with the opening of the basins, the increase of water depth and the more open marine conditions during the late Anisian. The Illyrian assemblages of Felsőörs are very similar to the faunas that can be found above the submarine highs. This is in contrast with the low diversity Pelsonian ostracod fauna, dominated by Spinocypris and Bektasia, which indicates a relatively shallow marine environment with restricted circulation at the sea floor, causing slight oxygen deficiency. During the Longobardian (Budurovignathus mungoensis and Bu. diebeli conodont Zones), uniform ostracod fauna with the thin-shelled ‘palaeopsychrosphaeric elements’ possessing long spines such as bairdiid Acratia, beecherellid Acanthoscapha, tricorninid Nagyella and macrocypridid Praemacrocypris appeared in the area of Balaton Highland suggesting the climax deepening of the basins with water depth below 200 m (upper bathyal zone). The ostracod record effectively mirrors the large-scale tectonic and eustatic changes that reshaped the Western Neotethys margin. The research was supported by the National Research, Development and Innovation Office (NKFIH STARTING 149443 grant).