|dc.description.abstract||High precision uranium-series (U-series) geochronology provides data on timing of travertine deposition at Soda Dam, and a paleohydrologic record for the Valles Caldera hydrothermal system in Northern New Mexico for at least the last 500 ka. Travertine-depositing springs occur in San Diego Canyon, the only surface drainage outlet for the Valles Caldera, along the intersection of the Jemez fault zone (JFZ) and the Jemez River. This travertine-depositing hot spring system is part of the Valles Caldera geothermal system, which has been active throughout the Quaternary and explored for the past three decades for geothermal energy potential. New dates, compiled with previous dating, provide improved geochronologic and geologic context with respect to incision rates of the Jemez River and the episodic nature of travertine deposition.
The largest volume deposit (deposit A), high in the landscape on the west side of the river, yields stratigraphic ages ranging from >500 ka (outside of U-series range) to 287 ka, and caps paleo Jemez River gravels at 132 m above the modern river. Deposit A is intruded by spar calcite dikes of 486, 455, and 339 ka that are interpreted as episodes of elevated hydrologic head in the hydrologic system generated by caldera paleolakes that were present in several unresolved intervals throughout the recent Quaternary, beginning immediately after the formation of the caldera 1.25 Ma. Intrusions also occur at 207, 211, 111 and 96 ka, during interglacial Marine Isotope Stage’s (MIS) 9, 7 and 5. Deposit Ai is inset into deposit A and yields stratigraphic ages of 183 to 132 ka. Ancestral Jemez River gravels cemented by calcite rinds provide a terrace 30 m above the modern river and yield an age of 201 ka, producing a robust bedrock incision rate of 160 m/Ma for the last 200 ka. A cross-cutting calcite sill in deposit Ai yields ages of 111 and 210 ka, again suggesting locally elevated head during MIS 5 and MIS 7. Deposit B is a fissure/mound deposit on the east side of the river that developed along a fissure ridge straddling major Quaternary faults in the JFZ, much like the modern Soda Dam. The mound accumulation is 138 ka near the base and 78 ka at the top. The central fissure has a 1 m thick calcite vein system that displays mm scale banding; calcite vein growth took place approximately from 210 ka to 93 ka. These combined data indicate that the deposit B fissure/mound system was active for a 130 ka interval that spans glacial MIS 6 and the transition into interglacial MIS 5. Deposit C, a minor stratigraphic continuation of deposit B, is 103 ka at the base, capping river gravels 16.5 m above the modern river, and 101 ka at the top, indicating extensive platform development at about 100 ka. Deposit C yields a bedrock incision rate of 150 m/Ma since 101 ka, indicating semi-steady bedrock incision rates over the last 200 ka. Deposit D, the Soda Dam itself, began mound accumulation at 10 ka and is still actively depositing travertine.
Stable isotope values were obtained on 27 dated, and an additional 52 undated travertine samples. Oxygen isotope values (δ180) have an exceptionally large range from δ180 = -19 to -4‰ (PDB), reflecting variations in local spring chemistry, source water composition, temperature, as well as fractionation effects during deposition. Carbon isotope values (δ13C) range from +1.4 to +11.7‰ (PDB); these relatively positive values suggest kinetic enrichment during degassing of CO2 and likely the influence of a magmatic/mantle endmember. Geochronology, combined with volume calculations, indicate episodic travertine deposition along the Soda Dam fault system, with major accumulations broadly at >500 ka, 200-92 ka, and <10 ka. Over all, the Soda Dam travertine system provides an episodic record of travertine accumulation, river incision history and insights into paleohydrolgic conditions through stable isotope analysis for part of the Jemez hydrothermal system.||en_US