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The Triassic/Jurassic boundary in three contrasting facies in Hungary

Pálfy J., Barbacka M., Császár G., Demeny A., Görög Á., Haas J., Götz A., Oravecz-Scheffer A., Ozsvárt P., Piros O., Ruckwied K., Szeitz P., Zajzon N.

Volumina Jurassica

2006

Vol. 4, no. 1

292-293

Using an integrated stratigraphic approach, the Triassic/Jurassic (T/J) boundary has been studied in two different tectonostratigraphic units and three contrasting facies types in Hungary. In the northeasternmost part of the Transdanubian Range unit (part of the Alcapa terrane), the Csövár section has been intensively studied recently. Here a marine carbonate succession represents slope to basinal facies, deposited in an intraplatform basin near the margin of a Late Triassic Dachstein-type platform system. The T/J boundary is defined on the basis of ammonoid, radiolarian, conodont, and foraminiferan faunas. Paleontological data not only contribute to a biostratigraphic subdivision, but are also interpreted in the context of end-Triassic extinction and earliest Jurassic recovery of different fossil groups. A significant negative carbon isotope excursion is recorded in both carbonate and organic matter in the boundary interval. In other parts of the Transdanubian Range, a Late Triassic carbonate platform is preserved as the thick Dachstein Limestone Formation. In the Gerecse Mts. and the Tata horst, a T/J unconformity truncates the topmost Rhaetian part of the Dachstein Fm. and separates it from the overlying Hettangian strata, deposited in a deepening marine environment. Earliest Hettangian deposits and fossils are absent. A potentially more complete succession occurs farther to the west, in the Bakony Mts. Here the shallow marine carbonate deposition continues into the Hettangian, represented by the Kardosrét Limestone Fm. that overlies the Norian-Rhaetian Dachstein Fm. The lithostratigraphic boundary corresponds to the T/J boundary that marks a significant break in platform development. A disconformity and small hiatus is assumed but its duration has not yet been satisfactorily determined. A surface section on Körishegy and core material from boreholes Zt-62 (near Zirc) and Süt-28 (near Sümeg) have been investigated. The T/J boundary is marked by a sharp disappearance of Triasina hantkeni and changes in the accompanying foraminiferan and dasycladacean algal assemblage. A preliminary stable isotope study of the cores has failed to identify a negative δ ¹ ³C excursion, suggesting a hiatus at the boundary. The Mecsek Mts. in southern Hungary is part of the Tisza unit (or Tisza terrane). Here the T/J boundary falls within the coal-bearing, terrestrial to marginal marine, locally more than 1000 m thick Mecsek Formation but its precise placement has been proved difficult. Palynology offers the best potential for biostratigraphic subdivision. New palynological and paleobotanical studies are underway, aimed at a more precise palynostratigraphy and a reconstruction of climate and vegetation history in the boundary interval. In summary, the T/J boundary sections in Hungary occur in terrestrial, shallow marine (carbonate platform), and deeper marine (slope to basinal) facies. Their study provide new data towards a better understanding of the biotic and environmental changes at this critical interval of Earth history.

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

Ruckwied K., Götz A., Haas J., Pálfy J.

Volumina Jurassica

2006

Vol. 4, no. 1

296-296

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.