Tectonic origin of the unique Holocene travertine from the Holy Cross Mts.: microbially and abiologically mediated calcium carbonate, and manganese oxide precipitation

Abstract

Recent tectonic activity of the main dislocation within the Paleozoic core of the Holy Cross Mts. led to formation of a large travertine dome of Holocene age. The main body of the travertine is built up of extremely fast crystallized calcite from highly supersaturated solutions derived from hydrothermal waters circulating through the tectonic dislocation. Many calcite crystals display the remains of calcified bacilliform bacteria rods suggesting an essential part, of the calcite crystallization process was on a bacterial precursor. Successively, after the micrite calcite travertine frame had been formed, almost pure monoclinic manganese oxide (a-MnO₂) precipitated filling part of the remaining porosity. The unique characteristics of manganese oxide crystallization also suggest a very fast process of manganese oxidation due to increase in Eh and the activity of abundant fungal species which might be associated with a specific symbiotic bacterium. Specfic arrangement of the a-MnO₂ crystals into the rosettes seems to be achieved by fast evaporation of upwards migrating solutions from the tectonic zone. The last in this succession is calcite cement filling the pores, mainly in the upper part of the travertine frame. Generally, this was abiologically mediated process of precipitation of calcium carbonate due to rapid degassing of carbon dioxide and occasionally, in the top of the travertine dome, due to direct precipitation from the ground waters.