TESTING THE PLEISTOCENE LAKE OTERO HYPOTHESIS: PRELIMINARY FINDINGS FROM A WHITE SANDS CORE

White Sands is famous for its gypsum sand dunes. However, the evolution of this terminal saline lake and eolian system is not well understood. A long-held hypothesis states that there was a deep, freshwater lake, Lake Otero, in the Pleistocene. The USGS/NPS White Sands MW-11 core, drilled to a depth of 192 feet in the center of the dune field, represents an opportunity to test the Lake Otero hypothesis. This core provides the only available resource to study subsurface sedimentology of White Sands. Here, we describe the detailed sedimentology of this core.

The core is dominated by gypsum. The approximate bottom half of the core contains bedded, bottom growth gypsum, displacive gypsum, gypsum sandstone, and siliciclastic sandstone and mudstone, and minor charcoal. Sedimentary structures include wavy, discontinuous lamina, ripple cross-bedding, mudcracks, and root features. In contrast, the middle of the core contains a mudstone with planar laminations with an overlying quartz and gypsum sandstone with low angle cross-bedding. The upper half of the core is dominated by gypsum sandstone and gypsum mudstone, containing both abraded bottom growth gypsum crystal grains and abraded displacive gypsum crystal grains with bimodal grain size distribution. These gypsum strata contain wavy lamina, cross-bedding, mudcracks, charcoal, and seeds.

Based on our preliminary observations, past environments at White Sands were dominated by shallow, saline lakes and groundwaters as well eolian processes. Lake Otero may have been a short lived freshwater lake, an atypical environment for White Sands.