A LACUSTRINE FACIES MODEL FOR WHITE SANDS NATIONAL MONUMENT, NEW MEXICO

It has long been suggested that White Sands, the Earth’s largest gypsum dune field and an analog for Mars, is sourced from adjacent ephemeral Lake Lucero. However, lacustrine processes resulting in the formation of gypsum remain poorly understood. A study of this system was undertaken to determine if changes in this sediment source could impact the dunes. Here, we present new evidence from field sedimentology and remote sensing that a much larger area, including Alkali Flats, irregularly but actively produces bottom-growth gypsum that is eroded by wind to form the dunes. Facies were mapped in the field, described and samples were collected for microscopy and geochemistry. Pits were dug in each faces and documented. Landsat 7 images were used for a qualitative assessment of flooding, evaporation, and desiccation since 1999.

Four facies assemblages form north-south trending bands. The alluvial facies assemblage consists of large composite alluvial fans extending from the mountains. The ephemeral saline lake and mudflat facies assemblage form a 4 - 11 km strip along 40 km of the western edge of the basin. The eolian facies assemblage is comprised of sheets of coarse to fine gypsum and dolomite sands, dunes, interdunes, and stabilized dunes and stretches for over 16 km east along the entire length of ephemeral saline lakes and mudflats. The eolian system has a high water table and high sediment moisture disconnected from the ephemeral lake system. An ephemeral stream floodplain of well-developed soils, fossil dunes stabilized by efflorescent crusts, lava flows, and ephemeral fluvial drainages comprise the final facies assemblage covering the reaming valley floor.

Lake Lucero and Alkali Flats are complex facies consisting of dry saline mudflats, wet saline mudflats, spring deposits, and ephemeral lakes. From 1999 to 2015 many small (>1 sq km) flooding events and one major flooding event (Nov 06 to Mar 07) occurred. Although there is evidence that bottom-growth gypsum precipitated over the entire flood area in 2006, the sediment has since been reworked by wind. White Sands is a groundwater dominated dry lacustrine system where only rare and short lived major flooding is still capable of producing bottom growth gypsum. The ephemeral lakes and mudflats must be studied in further detail before the dunes can be understood or managed.