Abstract
ABSTRACT The 2 km thick Late Triassic Hungarian carbonate platform has a completely dolomitized lower part and a limestone-dominated upper part. The platform succession is made up of meter-scale, possibly precessional ( 20 ky) carbonate cycles. In the lower platform (Main Dolomite), cycles are completely dolomitized and capped by caliche laminites and pisolites. In the upper platform (Dachstein Limestone), dolomite is confined to laminite caps of cycles that are bounded by clayey paleosols. Most of the dolomitization in the platform interior occurred early in tidal-flat settings during each high-frequency cycle. Subtidal dolomites are slightly coarser grained, are low in Fe2+ and Mn2+, and have the heaviest 18O signature. This indicates they formed from evaporative, oxidizing brines sourced from supratidal flats. Intertidal-supratidal dolomites are fine grained, commonly Fe2+ and/or Mn2+ rich, and slightly enriched in 18O compared to the marine calcite cement. They formed from marine water that was weakly to moderately reducing. Dachstein paleosols have light 18O and 13C signatures reflecting meteoric soil waters. Repeated emergence stabilized the dolomites to low Sr2+ and Na+ dolomite similar to Cenozoic platform dolomites. In contrast to these early cyclic dolomites, coarse-grained platform-margin dolomites with very low Mn2+ and Fe2+ and light 18O signatures formed as thermally driven, warm, oxidizing marine water associated with Jurassic rifting of the Neo-Tethys Pennini Ocean circulated through the margin. Early dolomitization of cycles may have been controlled by high-frequency sea-level changes, but the overall vertical distribution of early dolomite on the platform does not reflect long-term eustasy. Rather, the intense dolomitization of the lower platform reflects a semiarid, hot subtropical, seasonal setting and megamonsoonal climate. Global cooling and increased humidity toward the latest Triassic and Early Jurassic inhibited pervasive early dolomitization, leaving the upper platform largely as limestone.
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