Abstract: Regional Climate Controlled Dolomitization of a Late Triassic Carbonate Platform, Hungary

Anna Balog, J. F. Read, J. Haas

The large, Late Triassic carbonate platform in the Transdanubian Range, Hungary, which was originally part of the Alpine Triassic platform, contains a well developed precessional cycle record. The 2 km thick Late Triassic carbonate platform developed above an incipiently drowned and fragmented Middle Triassic platform. The Late Triassic platform consists of a pervasively dolomitized lower unit (Maindolomite Formation) and a less dolomitized upper unit (Transitional Unit and Dachstein Limestone). The platform contains abundant carbonate cycles (2 to 5 m thick) whose facies include reworked regolith, transgressive laminite, subtidal muddy carbonate, regressive laminite, and capping paleosol/disconformity; cycles are stacked into 400 k.y. and 1 to 2 m.y. sequences. Dolomitization was con rolled by Milankovitch sea level fluctuations and climate. A semi-arid climate during deposition of the Maindolomite Formation is suggested by common caliches, scarcity of paleosols, and abundant pervasive dolomitization. The dolomites are isotopically heavy (^dgr¹⁸O+1 to +3.5 per mil PDB; ^dgr¹³C+1.5 to +3.3 per mil PDB) reflecting evaporative conditions. Dolomites most enriched in ¹⁸O replace subtidal facies and formed from evolved brines; supratidal and intertidal dolomites are less enriched but still isotopically heavy relative to marine calcites. Dolomite in the Dachstein Limestone is restricted to laminite caps and is isotopically depleted (^dgr¹⁸O -1 to +1.5; ^dgr¹³C +1.5 and +2.4) and similar to or slightly heavier than the m rine calcites. These dolomites formed from less evaporative waters under a more humid climate, indicated by common algal tufas, abundant early leaching of aragonite, common paleosol caps, and little dolomitization of subtidal limestone. Thus, the change from the intensely dolomitized lower platform to the less dolomitized upper platform was coincident with decreasing aridity accompanying Late Triassic global cooling and may have important implications for other Mesozoic-Cenozoic platforms.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994