Geochronology of the Bolson sand sheet, New Mexico and Texas, and its archaeological significance

The Bolson sand sheet occurs in the Tularosa Valley, New Mexico, and the Hueco Bolson, Texas and consists of two principal eolian sand units. Optically stimulated luminescence (OSL) dating provides a new chronology of the sand sheet that relates as well to the formation, preservation, and visibility of the local archaeological record. The lower sand (unit Q2) (44.8 +/- 2.9 ka) and the upper sand (unit Q3) (22.2 +/- 1.6-5.2 +/- 0.3 ka) have a combined thickness of less than 2 m. The Q2 sand is characterized by a red Bt paleosol, and the Q3 sand has a weak calcic paleosol with stage I carbonate morphology. Elevated amounts of airborne silt were incorporated in the Q3 sand during the period 24-14.5 ka, representing higher amounts of dust in the atmosphere during glacial and late-glacial time. Multiple OSL ages from the Q3 sand indicate a slow net sedimentation rate of 0.06-0.09 mm/yr, similar to other OSL-dated sand sheets in the region; sand deposits in dune fields have higher accumulation rates. The McGregor A horizon soil, radiocarbon-dated to younger than 0.5 ka, occurs at the top of the sand sheet and likely formed with desert grassland vegetation. Thousands of recent mesquite coppice dunes (unit Q4d) mantle the sand sheet, and two are dated to the twentieth century by OSL and Cs-137. Archaeological sites that postdate 3000 B. C. are concentrated, sometimes together, on the surface of the sand sheet, while sites that predate 3000 B. C., although rare, may be buried within the Q3 eolian sand. The Q2 sand is too old to contain archaeological sites, although site features may intrude into the sand. Previous chronologies of the sand sheet are based on radiocarbon dates of charcoal from archaeological sites, on radiocarbon dates of soil carbonate, and on soil-geomorphology correlations with Rio Grande Valley alluvium. The optical chronology does not support these various correlations. We recommend that the alluvial names Isaacks' Ranch, Fillmore, and Organ no longer be applied to the Bolson sand sheet.