112 NEW MEXICO GEOLOGY November 2007, Volume 29, Number 4 Abstract Quaternary lacustrine deposits exist within several valleys in the Valles caldera in north- central New Mexico. These deposits contain potentially valuable paleoclimatic records. We report OSL ages for a section of uncon- solidated Quaternary sediments exposed in the southwest part of Valle Toledo within Valles caldera. The sequence represents the transition over time from lacustrine to fluvial deposition at the site. We present a strati- graphically coherent depositional chronology for the Valle Toledo section consistent with the available radiocarbon constraint based on analysis of bootstrapped dose distributions derived from data collected by IRSL MAAD procedures. The ages suggest the existence of a late Pleistocene lake in Valle Toledo from at least 48.5 ka to ~44 ka, which is consider- ably younger than age interpretations based on correlation to a >500 ka rhyolite dam across San Antonio Creek. This study helps to emphasize the need for additional research to decipher the geologic history of the intra-cal- dera lakes as well as to correlate the records of climate and environmental change among the lacustrine deposits within Valles caldera. Introduction The Valles caldera, in the Jemez Mountains of northern New Mexico (Fig. 1), was formed at ca. 1.25 Ma following eruption of volumi- nous ignimbrites of the Tshirege Member of the Bandelier Tuff (Smith and Bailey 1968; age from Phillips 2004). After breach of the caldera rim associated with resurgence at ≥ 1.2 Ma, multiple lakes formed at differ- ent times when post-resurgence volcanic eruptions dammed drainages within the caldera (Reneau et al. 2007). The lacustrine deposits in the caldera contain potentially valuable paleoclimatic records (e.g., Sears and Clisby 1952; Fawcett et al. 2006), and accurate geochronologic data are required to best interpret these records and under- stand the history of the lakes. The most extensive outcrops of lacustrine sediment in the Valles caldera are found in the northern moat along San Antonio Creek and its tributaries, as first mapped by Smith et al. (1970; see also Gardner et al. 2006 and Goff et al. 2006). Topographic relations and the distribution of lacus- trine sediment are consistent with most of these deposits being associated with a lake impounded behind ca. 557 ka rhyolite flows from San Antonio Mountain (Reneau et al. 2007; age from Spell and Harrison 1993). The easternmost outcrops along San Antonio Creek are found in Valle Toledo, OSL dating of a lacustrine to fluvial transitional sediment sequence in Valle Toledo, Valles caldera, New Mexico Kenneth Lepper, Department of Geosciences, North Dakota State University, Fargo, North Dakota 58105; Steven L. Reneau, Environmental Geology and Spatial Analysis Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545; Jennifer Thorstad, Boone Pickens School of Geology, Oklahoma State University, Stillwater, Oklahoma 74078; Anne Denton, Computer Sciences Department, North Dakota State University, Fargo, North Dakota 58105 FIGURE 1—Digital elevation model (DEM) map of the Valles caldera showing estimated maximum lake extent (blue) in Valle Grande and along San Antonio Creek, and other locations mentioned in text. SAC = San Antonio Creek; VG = Valle Grande; VSA = Valle San Antonio; VT = Valle Toledo; star indicates OSL sample location. Modified from Reneau et al. 2007; reprinted by permission of the New Mexico Geological Society.