Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 9
Presentation Time: 11:00 AM


TAFOYA, April Jean1, CROSSEY, Laura J.1, KARLSTROM, Karl E.1, ASMEROM, Yemane1, POLYAK, Victor J.1 and COX, Chris2, (1)Earth and Planetary Science, University of New Mexico, Albuquerque, NM 87131, (2)Albuquerque, 87131,

High precision uranium-series geochronology at Soda Dam, New Mexico provides a record of paleohydrology and incision history for the upper reaches of the Jemez River over the last 700 ka. Active travertine-depositing hot springs occur along the intersection of the Soda Dam fault, and the Jemez River; new dates on active and extinct deposits provide improved geochronologic and geologic context with respect to the timing of travertine accumulation. The largest volume-accumulation, Deposit A, yields ages ranging from >700 ka to 259.6 ± 14.5 ka. Inset into Deposit A, Deposit A1, contains travertine-coated Jemez River gravels with a strath 30 m above the modern river; these yield an age of 200.6 ± 2.1 ka, giving a bedrock incision rate of 150 m/Ma over the last 200 ka. A cross cutting vein, at the same elevation as the 200 ka sample, yields an age of 110.9 ± 1.5 ka, suggesting substantial artesian head during the 5e substage of the Eemian interglacial. The oldest ancestral Jemez River gravels are preserved beneath Deposit A, at a maximum elevation of 132 m above the modern river; efforts to date pumice clasts are ongoing, but model ages of ~700 ka provide an estimate of 188 m/Ma over the last 700 ka. This agrees with longer term bedrock incision rates of 200 m/Ma, from the base of the 1.1 Ma Bandelier tuff. Deposit B, at lower elevations, developed on a banded central fissure ridge; combined ages from the fissure and mound accumulation indicate the system was active from 138-78 ka, a 60,000 yr interval spanning the transitional period during termination II. Deposit C, at 23 and 16.5 m above the modern river, respectively, yielded ages of 103.2 ± 0.5 ka and 101.7 ± 0.5 ka, giving a river incision rate of 160 m/Ma over the last 100 ka. Our results suggest semi-steady bedrock river incision since ~1 Ma with episodic travertine deposition along the Soda Dam fault system at 400-700 ka, 260-360 ka, 134-96 ka, and < 5 ka; reflecting the changes in climate, regional volcanic processes and the existence of the Valles Caldera paleolakes. Stable isotope values of the dated travertines range from δ180 = -19 to -6.5 per mil (PDB), reflecting variations in local spring chemistry and temperature. Compared to global travertine data, Soda Dam travertines have relatively positive C isotope values of +1.4 to +11.7, suggesting degassing of CO2 in the hydrothermal system.