Milankovitch-driven cyclicity and climate controlled dolomitization of a Late Triassic carbonate platform, Hungary
MetadataShow full item record
The Late Triassic platform carbonates of the Transdanubian Range, Hungary were part of a passive margin platform at the southwestern end of the Triassic Tethys now occurs in a single fault-bounded terrain The Hungarian platform is made up of meter-scale, precessional (-20 k.y.) carbonate cycles. It contains a lower unit, the Main Dolomite Formation (600-1500m thick), which is totally dolomitized. It is overlain by the Transitional Unit (150-400m thick). The overlying Dachstein Limestone is up to Soom thick. The platform is a cyclic succession of subtidal carbonate, laminated tidal flat limestone or dolomitic caps, and reddish or greenish paleosols or reworked paleosols. The Triassic was a time of global greenhouse conditions and Milankovitch climate forcing has been well documented from lakes and off-shelf facies. The Triassic Hungarian carbonate platform records an imperfect Milankovitch eustatic signal. They lack the bundling of 5 precessional cycles into 100 k.y. eccentricity cycles or 20 cycles/400 k.y. bundle. This is interpreted to be due to many missed beats evidenced by caliches and paleosols, and thick amalgamated subtidal carbonates. These result from precessional sea-level fluctuations either not flooding the platform, or flooding it too deeply to allow shallowing up to sea-level in one precessional beat. Spectral analysis of the Hungarian carbonates was used to compare the amplitude spectra of different time series including lithology, gamma ray, self potential and neutron density. The spectra based on lithology were compared to synthetic spectra generated by computer from platforms subjected differing Milankovitch signals. Most dolomitization of the Hungarian carbonates occurred early in tidal flat settinfs during each high frequency cycle. Intertidal-supratidal dolomites are fine grained, Fe + and Mn2+ rich and slightly enriched in 01S0 compared marine calcite cement, and formed from weakly to moderately reducing marine waters. Subtidal dolomites are slightly coarser grained, low in Fe2+ and Mn2+ and have heaviest d ISO signatures, indicating more evaporative oxidizing brines beneath flats. Repeated emergence stabilized the dolomites to low Sr2+ and Na+ ~pes similar to Cenozoic dolomites. Later, coarse-grained dolomites with very low Mn + Fe2+ and light 0 ISO signatures were formed along the platform margin by thermally driven, warm oxidizing marine water associated with Jurassic rifting of the Pennini Ocean (Neo-Tethys), The overall vertical distribution of early dolomite on the platform does not reflect long term ecstasy. Instead the regional stratigraphic trends in climatically sensitive sediments, as well as stable isotopes, suggest that intense dolomitization of the lower platform reflects a semi-arid, hot subtropical setting and megamonsoonal climate. Global cooling and increased humidity toward the latest Triassic and Early Jurassic, inhibited pervasive early dolomitization, leaving the upper platform little dolomitized.
- Doctoral Dissertations