Attila Vörös

MIDDLE TRIASSIC PLATFORM AND BASIN EVOLUTION OF THE SOUTHERN BAKONY MOUNTAINS (TRANSDANUBIAN RANGE, HUNGARY)

Middle Triassic history of the Southern Bakony Mts. is outlined on the base of horizontal and vertical facies changes of the formations. During the Pelsonian (Balatonicus Chron) the evolution of the basins and platforms was determined basically by synsedimentary tectonics. The Felsoors basin of the Balaton Highland opened due to the block-faulting of the Bithynian carbonate ramp (Megyehegy Dolomite). Above the drowning blocks „halfgraben” basins were formed (Felsoors Formation), while isolated platforms developed on the uplifted ones in the middle part of the Balaton Highland and on the Veszprem plateau (Tagyon Formation). Due to the relative sea-level fall in the early Illyrian, the platforms became subaerially exposed and karstified. As a consequence of the late Illyrian tectonic subsidence (manifested by neptunian dykes) the central platform of the Balaton Highland has been drowned (Camunum Subchron). On the contrary, the Anisian platform of the Veszprem plateau was totally flooded only during the latest Illyrian (Reitzi Subchron) due to eustatic sea-level rise. It was followed by a short highstand period (Secedensis Chron), characterised by the first progradation of the Budaors platform on the Veszprem plateau and highstand shedding in the basins and on the submarine high (Vaszoly Limestone) in the centre of the Balaton Highland basin. Due to the following rapid sea-level rise, carbonate sedimentation continued in eupelagic basin from the Fassanian (Buchenstein Formation). At the beginning of the late Longobardian highstand period (Regoledanus Chron) the Budaors platform intensively prograded from the Veszprem plateau to the southwest, causing highstand shedding in the Balaton Highland basin (Fured Limestone).

Influence of upwelling on the sedimentation and biota of the segmented margin of the western Neotethys: a case study from the Middle Triassic of the Balaton Highland (Hungary)

Abstract In the Middle Anisian, extensional tectonic movements led to the development of a small isolated carbonate platform in the middle part of the Balaton Highland, Transdanubian Range, Hungary. In the Late Illyrian, a condensed pelagic carbonate succession with phosphorite horizons was formed on the top of the already drowned platform. These strata contain an extraordinarily diverse ammonite fauna. This unit is overlain by radiolarian-rich carbonates, locally with radiolarite interbeds. We suggest that the drowning process and the post-drowning sediment deposition were controlled partly by regional factors, i.e., the onset of opening of the Neotethys Ocean, and partly by local factors such as the bottom topography and related current activity, which may also be connected with the opening of the ocean. The predominance of the radiolarian-rich sediments suggests eutrophic surface water, which may be explained by a monsoon-driven upwelling model. The segmented sea-floor topography together with the high-fertility surface water conditions may have provided favorable habitats for the ammonites, which may have adapted to various ecological conditions, leading to extreme diversification of this group. Since similar Middle to Late Anisian evolution was reported from many other units of the western Neotethys margin, regional factors such as the establishment of an extensional tectonic regime and related marginal basin formation, monsoon-driven upwelling, and related high surface water productivity seem to be of critical importance in controlling the depositional conditions.