Dulai Alfréd: A Bakony természettudományi kutatásának eredményei 26. - A Dunántúli-középhegység hettangi és kora-szinemuri (kora-jura) brachiopoda faunája I. Diverzitás, rétegtani elterjedés, paleoökológia, paleobiogeográfia, faunafejlődés (Zirc, 2002)

Summary

1982b, 1997) on the stratigraphie distribution of the Jurassic brachiopods of the Transdanubian Central Range. Twelve species are recorded for the first time from Hungary and the stratigraphie ranges of another 44 species are amended for the study area. These latter taxa were already known from the Late Sinemurian and/or Pliensbachian deposits of the Bakony and/or Gerecse Mts. Now, the distributions can be extended downwards to the Hettangian and/or Early Sinemurian. The global stratigraphie ranges of 14 species are updated: these species appeared earlier in the Transdanubian Central Range than elsewhere. Detailed taphonomical analysis of the brachiopod faunas has revealed the ancient depositional environments at the different localities. A differentiated paleoenvironment has been suggested for the Hettangian in the Bakony Mountains. The outlined paleogeography is similar to the pattern of horsts and intervening basins postulated for the Pliensbachian, indicating that the tectonic collapse of the platform started during the Hettangian. A basinal, partly resedimented sequence can be found at Lókút Hill between the Hajag and Amos horsts. From these submarine horsts a considerable amount of material was resedimented into the basin along the slopes. Taphonomical analysis of the brachiopod fauna suggested strong resedimentation at the basal layers of the section, shown by the great diversity and the large number of articulated brachiopod valves. By developing newer stepped foults, the source area gets more and more distant from the deposition area. The quantity of the redeposited sediment, as well as the number and size of brachiopods decreased but the disintegration increased because of the longer transportation. The resedimentation is stronger again at the upper part of the examined section, referring to the rejuvenation of the tectonic activity. A new source area maybe also appeared at the surrounding of the Lókút Basin. Som Hill locality is situated at the edge of the Hajag-Papod horst, therefore a large part of the sequence is of redeposited sediments from the higher areas. The crinoidal limestone interbeds are frequent and sometimes show cyclicity. The ratio of disarticulated valves is very high along the whole section. The percentage of the genus Rhynchonellina is very high at Som Hill (72%), although this taxon is missing or very scarce at other localities. Rhynchonellina has been identified as a cold-seep related taxon. This locality is situated at the edge of the horst, where nutrient-rich cold-seeps could have occurred along the tectonic faults. Tölgyhát Quarry, Póckő and Kisgerecse is situated in the Pisznice basin of the Gerecse Mountains. The brachiopods are generally articulated and infilled by micritic lime mud at these localities and the specimens are not sorted by size. These taphonomical features suggest slow sedimentation, without any resedimentation or transportation. The relatively diverse brachiopod fauna of these localities represent the composition of an Early Jurassic moderate to deep-water community. The resedimentation is more important in the Vöröshíd Quarry, at the margin of the Piszice basin. The cyclicity of the taphonomical features (disarticulated valves, sparitic infilling of brachiopods) can be correlated with the cyclicity of resedimentation events. The relatively low ratio of isolated brachiopod valves confirms LANTOS' opinion (1997), that the slopes between the horst and the basin were less steep in the Gerecse than in the Bakony Mountains. The taxonomic compositions of the brachiopod faunas reveals that the evolutionary niche replacement within the Brachiopoda, thought to occur at the Triassic/Jurassic boundary (SANDY, 1995a) took place only after the Hettangian in the Mediterranean region. At the Gerecse localities, the distribution of spiriferinids contradicts to what was found by SANDY (1995a): the proportion of spiriferinids increases from the basin towards the sub-