New Mexico Geology

The surficial geology of the central Jornada del Muerto is composed of 12 stratigraphic units identified from deep backhoe trenches. Their age is provided by 54 optically stimulated luminescence (OSL) dates, from 173.1 to 0.10 ka. Upper piedmont-slope alluvium from both the Caballo and San Andres Mountains is >65.6 ka and 49.5 to 23.4 ka; lower piedmont alluvium is >66.2 to >48.5 ka and may be middle Pleistocene. Piedmont alluvium was deposited during the wet climate of the late (and perhaps middle) Pleistocene. Jornada Draw alluvium (50–45 ka) accumulated within the same period as the upper piedmont-slope alluvium. Piedmont alluvium is commonly mantled by cover sediment (14.9–5.0 ka) and the eolian Jornada sand sheet (10.3–1.38 ka). Radioisotope (K₂O, U, Th) signatures and particle size indicate that the eolian sand sheet is derived from local Caballo Mountains upper piedmont alluvium. The sand sheet formed during the comparatively dry climate of the Holocene. The rate of eolian sand deposition increased during the arid middle Holocene, likely due to a greater supply of sand. All units have high percentages of silt that peaked 23 ka, similar to the last glacial maximum loess in the Central Plains and Midwest. A cumulic paleosol A horizon formed on the sand sheet within the period A.D. 1400 to 1550 during the wetter and cooler conditions of the Little Ice Age, correlating with a similar soil A horizon on other sand sheets in the region. By A.D. 1860, renewed availability of sand and eolian activity across the Jornada valley produced a thin cover sand and, where mesquite shrubs were present, coppice dunes. The historic eolian activity may be related to livestock grazing pressure.