| Popis: |
Speleothems and vadose crystal silt are effective indications for karstification processes in the fossil record. Upper Jurassic limestones in Southern Germany that have undergone vadose diagenesis contain on crystal margins and tips of coarse bladed calcites numerous fibrous calcite crystals, formed by abnormal growth conditions, and internal sediment within fractures and vugs. Fibrous calcite crystals grew as crusts, in fence- and mesh-like arrangements. Fibrous crystals, which have a length:width ratio of greater than 1:10, are made up of stacked subcrystals composed of an alternation of hexagonal prisms and rhombohedra. They exhibit a central to somewhat eccentric capillary. Electron probe microanalysis shows low-Mg calcite mineralogy with negligible amounts of Fe, Mn, and Sr as well as disseminated clay and metal hydroxide impurities. Stable-isotope data show relatively 13C-enriched and 18O-depleted values (13C ~ +2o/oo PDB, 18O ~ -6o/oo PDB), suggesting a meteoric environment and CO2 degassing as the main process of formation. Fibrous calcite crystals form from capillary fluids that are highly supersaturated with respect to calcium carbonate, contaminated with alien mineral impurities. The abnormal growth pattern is suggested to be substrate-controlled and attributed to mineral impurities that produce numerous crystallization nuclei. Fibrous calcite crystals are comparable to helictites of the filiform type that are reported only from Quaternary caves. Nevertheless, the diagenetic sequence and oxygen isotope data suggest a Late Cretaceous to early Tertiary age for their formation. The internal sediment consists exclusively of silt-size fragments of fibrous crystals and therefore is comparable to vadose crystal silt. Crystal silt is generated by the erosion of fibrous crystals both by vadose seepage and air currents. This study is the first observation of ancient helictites and related vadose crystal silt, documenting the close relationship between pore-ceiling vadose cements and the generation of crystal silt. |
