GEOL 340 Sedimentology and Stratigraphy
Lecture 38
Case Study: Mammoth Hot Springs, Yellowstone National park
Fouke, B.W., 2001, Depositional facies and aqueous-solid geochemistry of travertine-depositing hot springs (Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, USA)-REPLY. Journal of Sedimentary Research, Vol. 71, 497-500.
Fouke, B.W., Farmer, J.D., Des Marais, D.J., Pratt, L., Sturchio, N.C., Burns, P.C., and Discipulo, M.K., 2000, Depositional facies and aqueous-solid geochemistry of travertine-depositing hot springs (Angel Terrace, Mammoth Hot Springs, Yellowstone National Park, USA): Journal of Sedimentary Research, Vol. 70, p. 565-585.
Abstract from Fouke et al. (2001)
Petrographic and geochemical analyses of travertine-depositing
hot springs at Angel Terrace, Mammoth Hot Springs, Yellowstone
National Park, have been used to define five depositional facies
along the spring drainage system. Spring waters are expelled in
the vent facies at 71 to 73oC and precipitate mounded travertine
composed of aragonite needle botryoids. The apron and channel
facies (43 - 72oC) is floored by hollow tubes composed of aragonite
needle botryoids that encrust sulfide-oxidizing Aquificales
bacteria. The travertine of the pond facies (30-62oC) varies in
composition from aragonite needle shrubs formed at higher temperatures
to ridged networks of calcite and aragonite at lower temperatures.
Calcite "ice sheets", calcified bubbles, and aggregates
of aragonite needles ("fuzzy dumbbells") precipitate
at the air-water interface and settle to pond floors. The proximal-slope
facies (28-54oC), which form the margins of terracette pools,
is composed of arcuate aragonite needle shrubs that create small
microterracettes on the steep slope face. Finally, the distal-slope
facies (28-30oC) is composed of calcite spherules and calcite
"feather" crystals.
Despite the presence of abundant microbial mat communities and
their observed role in providing substrates for mineralization,
the compositions of spring-water and travertine predominantly
reflect abiotic physical and chemical processes. Vigorous CO2
degassing causes a +2 unit increase in spring water pH, as well
as Rayleigh-type covariations between the concentration of dissolved
inorganic carbon and corresponding d13C. Travertine d13C and d18O
are nearly equivalent to aragonite and calcite equilibrium values
calculated from spring water in the higher-temperature (~ 50-73oC)
depositional facies. Conversely, travertine precipitating in the
lower-temperature (< ~ 50oC) depositional facies exhibits d13C
and d18O values that are as much as 4 less than predicted equilibrium
values. This isotopic shift may record microbial respiration as
well as downstream transport of travertine crystals. Despite the
production of H2S and the abundance of sulfide-oxidizing microbes,
preliminary d34S data do not uniquely define the microbial metabolic
pathways present in the spring system. This suggests that the
high extent of CO2 degassing and large open-system solute reservoir
in these thermal systems overwhelm biological controls on travertine
crystal chemistry.