QUATERNARY STRATIGRAPHY AND OSL DATING OF LUNETTES IN THE NORTHERN SAN LUIS VALLEY, COLORADO

The Great Sand Dunes National Park and Preserve (GRSA), located near Mosca, Colorado, contain the highest dune field deposits in the United States and include the tallest dunes in North America (several are taller than 200 m). The surficial deposits within the GRSA generally consist of three distinct areas; the dune fields, sand sheets, and the sabkha. Within the sabkha area, complex aeolian and fluvial deposits with associated playa environments are present. These complex deposits include several distinctive lunette features which are commonly related to the presence of playas. Lunettes are aeolian features that are characterized by being broad, low, even-crested crescentic ridges that are rarely more than six to nine meters tall. These ridges border the leeward side of playas in the San Luis Valley and are produced and formed by dust-laden winds. The purpose of our study is to understand the evolution of the lunettes, their depositional phases, and their connection to the surrounding aeolian and fluvial systems. In addition to coring the lunettes on a systematic basis, another part of this project was to apply OSL dating to selected sand samples at various stratigraphic intervals within the lunettes in order to evaluate the age and preservation characteristics of the lunette deposits. Previous studies using OSL on the dunes and sheets that are part of, and surround the GRSA, (Madole et al, 2008; Forman et al, 2007; Madole and Mahan, 2007; Marin et al, 2005) provide a wealth of background data with which to compare or contrast trends and climatic connections. Preliminary results from the OSL dating have determined that the current lunette deposits were deposited and have been relatively stable for at least a minimum of 8,000 years. The OSL results, in combination with radiocarbon dating, grain size analysis, thin-section, and XRD, also suggest that some of the lunettes may have been reworked by fluvial processes resulting in the unique topography observed today.