2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 4
Presentation Time: 8:00 AM-12:00 PM


LEPPER, Kenneth, Luminescence Geochronology Lab, Los Alamos National Lab, EES-10 MS J495, Los Alamos, NM 87545, MAHAN, Shannon A., U.S. Geol Survey, P.O. Box 25046, MS 974, Denver Federal Center, Denver, CO 80225 and VINCENT, Kirk, U.S. Geol. Survey, 3215 Marine St, E127, Boulder, CO 80303, lepper@lanl.gov

The Chaco Culture National Historical Park in northwestern New Mexico preserves a rich history of pre-Columbian Americans and of the many famous scientists who have worked there over the past century. Chaco Wash, the principle stream transversing the park, has experienced several episodes of incision and filling during the Holocene. Presently, park custodians are concerned that these ongoing processes, particularly arroyo wall retreat may threaten several of the “Great House” ruins found within Chaco Canyon.

The primary objective of this investigation is to develop a geochronology of late Holocene stream dynamics for the contemporary Chaco arroyo and the inner channel of Chaco Wash. To achieve this goal, several optically-stimulated luminescence (OSL) dating techniques have been applied to sediment samples from the wash. The sediments collected represent modern (April 2000) and late Chacoan aged (950-1100 yrs. BP) deposits as well as a set of samples without age control, from an inset terrace exposed in the modern arroyo wall, believed to be intermediate in age. The OSL techniques employed included multiple aliquot additive dose measurements using infrared-OSL on polymineral fine-grains (IRSL MAAD), blue-OSL on single aliquots of quartz sand (OSL SAR), and single-grain green laser luminescence (SGLL) on quartz sand. These last two methods, OSL SAR and SGLL, allow hundreds of age estimates to be made from a single sample, thereby facilitating the use of advanced analytical techniques.

OSL ages for the modern sediments were largely over-estimates. However, stream flow data indicates that the April 2000 runoff event peaked at 8:30AM. Given the origin of the event, high in the watershed, it can be inferred that most of the transport period was at night. Also, streams in the region carry exceedingly high sediment loads (5-40% by volume). Successful OSL dating is unlikely for such short duration, high turbidity, nighttime events.

OSL ages for the late Chacoan aged samples were consistent with radiocarbon and other age controls within error limits, independent of the OSL technique used, suggesting less extreme pre-modern transport scenarios. Results from OSL analysis and dating of the inset terrace deposits highlight the potential of advanced OSL tools, such as SAR/SGLL and dose distribution analysis.