| 英文摘要: | This work studies a fossil system of perched and fluvial travertines passing distally to fluvial tufas within a volcanic ravine. Sedimentology, petrology and geochemistry of fossil aragonitic-calcitic travertines and downstream calcitic tufas from the Azuaje volcanic ravine were studied. These spring-related carbonates seem to be formed after the Mid-Holocene climate change, the transition from a monsoon-dominated humid climate to an arid-semiarid climate controlled by trade winds.
The main travertine facies include rafts, dendrites/shrubs, ooids, oncoids and stromatolites among others, whereas tufas are characterised by phytoclasts, oncoids, coated stems, intraclasts and stromatolites.
Fades observed can be (i) microbial-influenced when the microbial growth rate is greater than the precipitation rate and flow energy is not above the threshold value tolerated by microbes, or (ii) inorganic-dominated if the precipitation rate exceeds that of the microbial growth rate and/or flow energy is above the threshold tolerated by microbes.
Travertine facies vary from mostly inorganic to microbially-dominated, whereas tufa facies are mostly microbially-influenced. Observed changes of fades in both travertines and tufas were interpreted as due to changes in environmental conditions from (a) less to more evaporative, (b) less saturated to oversaturated. and (c) high to low energy. Changes in textures, mineralogy, geochemistry and stable isotope composition downstream from travertine to tufa suggest a decrease in the CaCO3 precipitation rate and an increase in microbial influence from travertines (proximal part of the system) to (distal) tufas.
Our study case illustrates the wide variety of facies and processes operating in spring-related travertine and tufa deposits. The details of arrangement, mineralogy, facies and geochemistry of the deposits were mostly controlled by climate and hydrogeology, although the volcanic setting, provided suitable conditions for spring-carbonate deposition. (C) 2018 Elsevier B.V. All rights reserved.
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