LOCAL SOURCES OF SEDIMENT ON DUNES AT WHITE SANDS NATIONAL MONUMENT, INTEGRATION OF SEDIMENTOLOGY AND END MEMBER MIXING ANALYSIS

Most studies of dune seas have described the large-scale trends within dune seas. However, the gypsum dunes at White sands National Monument provide data about local sources of sand and the cycling of sand within and between dunes. This study illustrates how small scale variation in grain populations resulting from mixing of sands from local sources can make inference of larger trends ambiguous, both on earth and Mars. Recrystallization of gypsum and the generation of fresh grains in significant quantities within both dunes and interdunes at White Sands is the primary control on observed variation in grain size within the dune field. Field observations indicate that evaporate interdunes, vegetated interdunes and erosional interdunes at White Sands each generate their own distribution of grains, which mix with the more rounded grains that represent transport farther than a few 10’s of meters. End-member mixing analysis and more traditional statistics support this interpretation.

End-member mixing analysis (EMMA) is a technique only recently applied to granulometric studies. It is a form of Q-mode factor analysis where each sample is assumed to represent a mixture of one or more end member populations. EMMA at White sands derives four end members, which explain 95% of the variation in grain size between samples. Each end member corresponds to grain populations with different microscopic characteristics. End Member 1 is the finer-grained sands that are equant and more rounded. End member 4 is associated with granule ripples, derived from sand recrystallized in the migrating dune. End Member 3 contains fine-sand sized needles and corresponds to samples collected near plants in adjacent to vegetated interdunes. End Member 2 is associated with erosional interdunes, where sand recrystallized at the base of the migrating dunes, creates cemented interdune surfaces.

Each dune exhibits a statistically distinct mean grain size resulting from the relative percentages of these different end members. No trends across dunes, or along the transects are evident as these are swamped by the local variability resulting from the different dune sources. The results show that little sand is passed across the interdune surfaces, and is largely only transported from dune to dune where they are linked, forming coalesced dunes.