Sedimentary features and palynofacies of the Triassic/Jurassic boundary interval (Csövár section, N Hungary)

The studied Csövár 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. A predominantly limestone succession is exposed in two outcrops: the Pokol-völgy quarry and the southern slope of the Vár-hegy. Based on ammonites and the last appearance of conodonts, the Triassic/Jurassic boundary can be drawn within the Csövár Formation. Facies analysis of the Rhaetian-Hettangian deposits reveals a long-term change in sea level, superimposed by short-term fluctuations. 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 buildups 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 characterized 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 of the sedimentary series exposed in the Pokol-völgy quarry is dominated by terrestrial components, reflecting a high supply from the hinterland. Numerous needle-shaped opaque particles, as well as a high amount of large translucent plant fragments within the phytoclast group, may point to the transport mechanism of sedimentary organic matter, strongly related to the occurrence and frequency of turbidites along the slope. The studied samples of the Pokol-völgy quarry yield a typical Upper Rhaetian palynomorph assemblage, characterized 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 carbonates exposed in the upper part of the Vár-hegy section, dated as 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.