WHITE SAND AND WHITE LOESS IN SOUTHERN NEW MEXICO

The White Sands dune field in the Tularosa Basin of southern New Mexico is the largest gypsum dune field in the world (~715 km²). A sheet of gypsum silt or loess lies immediately northeast of the gypsum sand covering ~390 km². The loess is up to 2m thick to the southwest and thins to a feather edge to the northeast. It rests unconformably on a well-expressed Aridisol (Btky horizonation) formed in either Pleistocene fan alluvium, siliclastic eolian deposits, or locally on early Holocene(?) eolian sand or gypcrete. OSL dates of ~2.5k years from near the top of the loess, ~5.3k years from the middle, and ~5.7k years just above the base document middle to late Holocene deposition, with perhaps more rapid middle Holocene accretion. This dating is supported by four radiocarbon dates averaging ~8.3k cal years B.P. on archaeological charcoal beneath the loess. The dating of the loess overlaps with archaeological carbon that places a phase of gypsum dune migration from ~4.5k to ~0.6k cal years B.P. The loess is believed to represent downwind deposition of silt dust derived from dune migration. Recognition of the loess is significant stratigraphically because it marks the terminal Pleistocene/early Holocene landscape and buries a significant Paleoindian and early Archaic archaeological record.