Sedimentary aspects of the onset of Middle Triassic continental rifting in the western end of Neotethys; inferences from the Silica and Torna Nappes, NE Hungary: a review

Abstract

The initiation of continental rifting from the latest Early Triassic was reconstructed by correlation of sedimentary formations deposited in the western end of Neotethys (in the Dinaric–Alpine oceanic branch). The shallow-marine and basinal strata of the Silica Nappes and the Bódvarákó Series from the Torna Nappe (located in the southern part of the Inner Western Carpathians) were studied and compared to sedimentary successions described from the Alps, the Carpathians and the Dinarides. The depositional zonation, developed on the shelf during the Late Permian‒Early Triassic transgression, was dissected and rearranged from the latest Early Triassic. The facies pattern and the differential sedimentary evolution of the shelf domains suggest that the accelerated subsidence began in the latest Early Triassic, and was connected to the onset of continental rifting. Three stages are reconstructed in the studied time-frame. (1) Dark grey carbonates, very poor in fossils, were deposited in restricted and hypersaline intraplatform basins in many shelf domains. In the external domains, shallow-marine carbonates, depositional gaps and terrestrial deposits are typical (formations in the Southern Alps, the External Dinarides and the Serbian–Macedonian Massif). From the latest Early Triassic, this latter shelf segment formed a threshold that restricted water circulation from the intraplatform basins. (2) Shallowing-up carbonate successions mark the next stage that implies a period of tectonic quiescence on the shelf from the late Early Anisian to late Middle Anisian. A peculiar change in biota occurring in previously restricted domains was coeval in shallow-marine and deep-marine settings. The biotic change is revealed by observations that dark grey carbonates, which are very poor in fossils, are overlain by carbonate successions rich in fossils typical for normal-marine water. The biota and environmental changes indicate the opening of a passage which allowed the circulation of well-oxygenated and normal-salinity marine water towards the previously restricted depositional areas. The geodynamic setting switching from continental rifting to spreading in the southern sector of the Dinaric–Alpine oceanic branch (Hellenides and Albanides), triggered the opening of the gateway between the future continental margins, i.e., between the External Dinaridic domain (Adria) and Serbian–Macedonian Massif (Eurasia). (3) Following the biotic event in the northern sector of the shelf, subsidence accelerated and additional intraplatform basins opened from the latest Pelsonian.