BALOG, A., J. F. READ, and R. J. BODNAR, Department of Geological Sciences, Virginia Tech, Blackburg, VA, USA; and J. HAAS, Eotvos Lorand University, Budapest, Hungary

ABSTRACT: Dolomitization of a Late Triassic Carbonate Platform, Hungary

The Late Triassic carbonate platform in Hungary contains a superb diagenetic record of sea level fluctuations and tectonics.The platform is up to 2 km thick and consists of the Main Dolomite and the Dachstein Limestone. Platform cycles contain from base to top, basal reworked soil, local transgressive laminite, subtidal carbonate, regressive laminite, and capping soil and associated disconformity. Main dolomite cycles are completely dolomitized, whereas the Dachstein Formation has many subtidal units that are still limestone. Evidence that the platform cycles formed by fluctuations in sea level includes abundant soils, disconformities, possible rootmolds, and large scale leaching of aragonite shells beneath cycle disconformities, and the short average periods (20-40 k.y.). Much of the early diagenesis was controlled by these high frequency sea level fluctuations. A semi arid tropical climate during the Main Dolomite deposition is suggested by lack of evaporites, but this changed to a more humid climate during Dachstein deposition (abundant calcified filament molds in algal tufas and abundant early leaching of aragonite).

Fine-grained, nonluminescent dolomite replaces laminites in Dachstein cycles, as well as in the Main Dolomite subtidal carbonates. The dolomites are isotopically heavy and are near-surface evaporative type. During emergence, the tops of cycles underwent soil formation, fracturing and brecciation, and aragonite shells were leached and infiltrated with internal sediment. Abundant high-Mg calcite bladed cements and less commonly botryodale aragonite cements were deposited in pores and fractures in

cycle caps. Marine cement and deeper water sediment fill neptunian dykes at the top of the platform, reflecting post-Triassic breakup. Later dolomites of obviously burial origin are common in the poorly cyclic platform margin and are fabric destructive, coarse-grainded, have light O isotopic composition. Fluid inclusions suggest hydrothermal, marine waters as the dolomitizing fluid.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.