Origin and Evolution of Lunettes on the High Plains of Texas and New Mexico

Lunettes—isolated dunes on the lee side of playa basins—are common landforms on the Southern High Plains of northwest Texas and eastern New Mexico. The dunes contain calcareous (15–40% CaCO3) sandy loam or loamy sand, with minor amounts of sepiolite, deposited 25,000–800014C yr B.P. and derived by deflation of lacustrine carbonate in the basins. The dunes also contain low carbonate (0–15% CaCO3) sand or loamy sand that was deposited 25,000–15,000 yr B.P. and 8000–5000 yr B.P. and was derived by deflation that created the basins or deflated from sand deposited in the basins. Buried soils are common in the lunettes: A-Bk profiles characterize soils formed in the calcareous sandy loam; the sandy low-carbonate sediments contain A-Bt profiles in the oldest sand of some dunes, and A-Bw, A-Bt, or A-Btk in the early and middle Holocene sand. The dune stratigraphy, combined with carbon isotope data (derived from dated A-horizons in lunettes), suggests the following scenario for the Southern High Plains. The lunettes began forming as low-carbonate sand dunes in the late Pleistocene as playa basins were formed or deepened by wind erosion. The erosion repeatedly alternated with stability. The environment probably was cool and dry, but one or more cool and wet intervals 25,000–15,000 yr B.P. resulted in a rise in the water table and deposition of lacustrine carbonate in the deepest basins. There may have been short departures toward warmer (and probably toward drier) conditions throughout this time. Episodically dry conditions 15,000–8,000 yr B.P. resulted in deflation of the carbonate and further dune construction by repeated accretion of calcareous sandy loam or loamy sand. The low carbonate sand was deposited during widespread drought and deflation 8000–5000 yr B.P. The dunes have been largely stable in the late Holocene.