Climate-controlled early dolomite, Late Triassic cyclic platform carbonates, Hungary

The 2 km thick Late Triassic Hungarian carbonate platform has a completely dolomitized lower part and a limestone-dominated upper part. The platform s uccession is made up of meter scale, possibly precessional (similar to 20 k y) carbonate cycles. In the lower platform (Main Dolomite), cycles are comp letely dolomitized and tapped by caliche laminites and pisolites, In the up per platform (Dachstein Limestone), dolomite is confined to laminite caps o f 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 sli ghtly coarser grained, are low in Fe2+ and Mn2+, and have the heaviest delt a(18)O signature. This indicates they formed from evaporative, oxidizing br ines sourced from supratidal flats. Intertidal-supra tidal dolomites are fi ne grained, commonly Fe2+ and/or Mn2+ rich, and slightly enriched in O-18 c ompared to the marine calcite cement, They formed from marine water that wa s weakly to moderately reducing. Dachstein paleosols have light delta(18)O and delta(13)C signatures reflecting meteoric soil waters. Repeated emergen ce stabilized the dolomites to low Sr2+ and Na+ dolomite similar to Cenozoi c platform dolomites, In contrast to these early cyclic dolomites, coarse-g rained platform-margin dolomites with very low Mn2+ and Fe2+ and light delt a(18)O 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 s ea-level changes, but the overall vertical distribution of ear ly dolomite on the platform does not reflect long term eustasy, Rather, the intense dol omitization of the lower platform reflects a semiarid, hot subtropical, sea sonal setting and megamonsoonal climate. Global cooling and increased humid ity toward the latest Triassic and Early Jurassic inhibited pervasive early dolomitization, leaving the upper platform largely as limestone.