Facies changes in the Triassic-Jurassic boundary interval in an intraplatform basin succession at Csoevar (Transdanubian Range, Hungary)

摘要

A continuous, pelagic marine Triassic-Jurassic boundary section is exposed in newly excavated trenches in the surroundings of Csoevar, NE of Budapest, Hungary. In the late Triassic, this area was located close to the offshore margin of the Dachstein carbonate platform system that was segmented by intraplatform basins. Based on detailed facies analysis of the Rhaetian-Hettangian platform foreslope-basin succession, a long-term (second order) and superimposed shorter-term (third and fourth order) changes of the relative sea-level could be revealed. After a period of highstand platform progradation in the late Norian, large amounts of larger plant fragments and sporomorphs of continental plants mark a significant sea-level drop in the early Rhaetian, presumably 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. Crinoid meadows may have occupied the slopes, a potential source area of the bioclastic carbonate turbidites. Rising relative sea-level that followed the marked early Rhaetian lowstand is also reflected in the general facies trend from lithoclastic debris-flows and proximal to the very distal turbidites and radiolarian basin facies up to the earliest Hettangian. Meter-scale high frequency (probably fourth order) deepening upward cycles, probably indicating sea-level oscillation, could also be recognized within this interval. At the base of the last Rhaetian cycle, about 10 m below the assumed Tr-J boundary, the amount of bioclasts, including conodonts, drastically decreases (biotic decline). In the radiolarian basin facies characterizing the topmost part of this cycle, a significant negative shift in both the δ~(13)C_(carb) and δ~(13)C_(org) and a marked decrease in the organic material (TOC) was recognized that can be interpreted to reflect reduced productivity in connection with the Triassic―Jurassic "boundary event". It is overlain by a lithoclastic horizon, then by laminites where the last conodonts were found. The next significant facies change in the early Hettangian is marked by appearance of redeposited oncoid-grapestone beds suggesting survival of shallow marine carbonate factories in the neighborhood of the basin and the end of the Rhatian to earliest Hettangian sequence (third order). The recurrence of basin facies above this interval indicates a new short-term (third order) transgression and continuation of the long-term (second order) deepening trend in the early Hettangian.