200-MILLION YEARS OF KARST HISTORY, DACHSTEIN LIMESTONE, HUNGARY

Platform carbonates of the Upper Triassic Dachstein Limestone in Nasza ly Hill have been karstified extensively over the past 200 million yea rs. They provide an excellent example of polyphase karstic diagenesis that is probably typical of many subaerially exposed carbonate sequenc es. Seven karstic phases are recognized in the area, each of which inc lude polyphase karstic events. The first karst phase was associated wi th the Lofer cycles. Meteoric waters caused dissolution; enlarged frac tures and cavities were filled by marine and/or vadose silts and cemen t. The second karst phase was caused by local tectonic movements. Bedd ing-plane-controlled phreatic caves were formed, and filled by silts. The third karst phase lasted from the end of the Triassic to the Eocen e. This was a regional, multiphase karstic event related to younger co mposite unconformities. Bauxitic fill is the most characteristic produ ct of this phase. The karst terrain reached its mature or senile stage with very little porosity. Narrow veins and floating rafts of white c alcite marks karst phase 4, which resulted from hydrothermal activity associated with Palaeogene magmatism. The early Rupelian phase of Alpi ne uplift caused large-scale rejuvenation of the former karst terrain (karst phase 5). Subsequently Naszaly Hill was buried as an area of ju venile karst with significant porosity. A second period of hydrotherma l activity in the area (karst phase 6) was induced by Miocene volcanis m, which resulted in wide, pale green calcite veins. Finally karst pha se 7 was of tectonic origin. Following the most recent, Miocene uplift of the Naszaly Hill, the carbonates have again become the site of vad ose karst development.