The Villaviciosa tufa: a scale model for an active cool water tufa system, Guadalajara (Spain)

摘要

The Villaviciosa tufa building, on the banks of the Tajuna River, central Spain, is a recent perched springline tufa with four architectural elements: cascades, pools, channels, and barrages. The facies of the tufa building are LMC (low magnesium calcite) framestones of bryophytes, hanging stems, charophytes, phytoclastic rudstones, and rudstones of oncoids. Isotopic analyses give delta C-13 values ranging from - 8.05 to - 10.3% VPDB, and delta O-18 values from - 6.9 to - 7.45% VPDB. These values are in the range of most common tufa systems and such a short range of values is consistent with the Villaviciosa tufa building's reduced dimensions. The mechanisms involved in precipitation vary within the building's different environments, as reflected in the isotopic composition of the samples. Non-biogenic degassing values in cascade deposits (framestones of bryophytes) are the heaviest, whereas in pools, dominated by biogenic degassing, values for framestones of charophytes and phytoclasts are the lightest. Microbial bio-mediation is also strongly influenced by the development of oncoid, stromatolite, and bryophyte framestones. Temperature variation throughout the system of approximately 3 degrees C is not clearly reflected in the isotope signals. In short, in spite of the reduced dimensions of this tufa deposit, its notable environmental, facies, and geochemical variations may aid not only in the interpretation of detail of other tufa systems but also in the discussion of detailed facies analyses in paleoclimatic interpretations of this type of deposits.