Strata forming a 113 m long core through Mammoth Hot Springs record the Holocene evolution of this travertine accumulation from deposition as part of a lacustrine to a terraced mound environment. The deposit is readily divided into four intervals: 113–67 m, carbonate-cemented volcaniclastic with intercalated layers of travertine; 67–60 m, moderately pure travertine with some volcaniclastics; 60–50 m, carbonate-cemented volcaniclastic-rich interval; and 50–0 m, essentially pure travertine. Lithologic composition, sedimentary structures, and the rare ostracode fossils indicate that the lower 67 m predominantly accumulated in a lacustrine environment, whereas the upper 40 m are terraced mound deposits. All of the travertine is calcite, some after aragonite. Layers of shrubs, oncoids, and peloids, all bacterial in origin, form the dominant allochems within the travertine. Stable isotopic carbon and oxygen values (n = 128) are strongly positively correlated and decrease up-core ∼4‰ and 8‰, respectively, reflecting a change in depositional environment from lacustrine to terraced mound upsection. Other stable isotopic trends indicate a pronounced difference between travertine allochems and immediately adjacent spar, e.g., spar averages 0.9‰ and 0.6‰, respectively, lower than immediately adjacent shrubs (n = 7 pairs). This difference is interpreted to reflect degassing and evaporation in the surface waters prior to precipitation of the allochems. The trends in stable isotopic values provide valuable corroborative data with regard to the depositional environment and diagenesis of the travertine.

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