SEDIMENTARY FEATURES AND PALYNOFACIES OF THE TRIASSIC/JURASSIC BOUNDARY INTERVAL

The studied Csővár section is situated NE of Budapest (Hungary) and exposes a continuous, marine Triassic–Jurassic boundary section of slope to basinal facies. Late Triassic palaeogeographic reconstructions indicate that this area was located close to the offshore margin of the Dachstein carbonate platform system, which was segmented by intraplatform basins.

Rhaetian–Hettangian deposits reveal a long-term change in sea level: After a period of highstand platform progradation in the Late Norian, a significant sea-level fall occurred in the Early Rhaetian, exposing large parts of the platform. A renewed transgression led to the formation of smaller build-ups fringing the higher parts of the previous foreslope that served as habitat of crinoids, representing the main source of carbonate turbidites. The higher part of the Rhaetian is characterised by proximal turbidites with intercalated lithoclastic debris flows. Distal turbidites and radiolarian basin facies become prevalent upsection, dominating in the earliest Hettangian. The next significant facies change in the Early Hettangian is marked by the appearance of redeposited oncoid–grapestone beds, indicating the end of the Rhaetian to earliest Hettangian sequence.

Palynofacies is dominated by terrestrial components, reflecting a high supply from the hinterland. The studied samples yield a typical Upper Rhaetian palynomorph assemblage, characterised by a high amount of Circumpolles (Classopollis), Rhaetipollis germanicus, Ovalipollis pseudoalatus and numerous trilete spores. The marine fraction is marked by foraminiferal test linings and prasinophytes of the genus Pterospermella; acritarchs are very rare and dinoflagellate cysts are absent. This microplankton assemblage is characteristic of a permanently stratified basin. Palynofacies of the Lower Hettangian is dominated by degraded organic matter, small equidimensional phytoclasts and foraminiferal test linings, pointing to a distal basinal setting.

The integrated analysis of sedimentary and organic facies enables one to reconstruct the depositional environment and to detect major sedimentary processes. The dominance of turbidites, together with the preservation and composition of sedimentary organic matter, supports the complex basin topography.