GEOCHEMICAL EVIDENCE FOR A COMPLEX ORIGIN OF THE KELSO DUNES, MOJAVE NATIONAL PRESERVE, CALIFORNIA

The Kelso dune field is intriguing because although it is of limited areal extent (~100 km²), it has a wide variety of dune forms and contains many active dunes (~40 km²), which is unusual in the Mojave Desert. Studies over the past half-century have concluded that the dunes are derived from a single source, Mojave River alluvium, under a dominant, westerly-to-northwesterly wind regime. Most of these studies have not presented data to support this conclusion, but an exception is Ramsey et al. (1999, GSA Bulletin 111, 646-662), who inferred from mineralogical data that other sources could be significant. We conducted mineralogical and geochemical studies of most of the 14 geomorphically defined dune groups of the Kelso dune field and potential sand sources, alluvial sediments derived from the surrounding mountain ranges. Results indicate that sands in the 9 western dune groups have Ca/Sr (from plagioclase) as well as K/Rb and K/Ba (from K-feldspar) compositions that are indistinguishable from Mojave River sands, supporting previous interpretations. In contrast, sands from the 5 northeastern dune groups have Ca/Sr, K/Rb, and K/Ba values that indicate significant inputs from alluvial fan deposits of the Providence Mountains. This requires either rare winds from the east or southeast or, more likely, eolian reworking of distal Providence Mountain fan sediments by winds from the west, at a rate greater than that from Mojave River inputs. The results indicate that even a small dune field can have a complex origin, with either seasonally varying winds or repeated distal fan-dune interaction.