QUATERNARY HISTORY AND DEVELOPMENT OF THE SAN LUIS LAKE LUNETTE, GREAT SAND DUNES NATIONAL PARK AND PRESERVE, COLORADO

The large and complex lunette that is adjacent to San Luis Lake was recently studied in order to understand how the system evolved and continues to respond to climatic influences. This lunette exhibits geometry unlike any of the other lunettes located within the surrounding area. In contrast to the adjacent lunettes, the San Luis Lake lunette features multiple surficial ridges, with a complex internal topographic expression between the ridges. All other lunettes have a single ridge and lack any zone of internal topographic alterations. Sediment samples were collected across the lunette at San Luis Lake, analyzed to determine the preserved depositional history of the system, and the subsequent data was evaluated to determine the dominant influence on this distinct evolutionary history. Additionally, the samples were dated by luminescence and radiocarbon (when applicable) in order to establish the timing and duration of events.

Results show that the system initially formed prior to 8,500 years ago, but the initial lunette front has remained relatively stable over time. This age on the in-situ sediments (under a meter from the surface) of the lunette implies that the core of the lunette is far older. This leads to the conclusion that the lunette crests, in this area, are behaving more like sediment bypass surfaces, with younger sediment not being deposited or preserved for any length of time. The complex topographic expression within the remainder of the system is a response of the San Luis Lake system to alternating wet/dry climatic cycles that have affected the area in the past 8,000 years. One of the multiple ridge layers within the lunette is believed to be related to a climatically forced dropping of the perennial lake level, dated between 5,000-4,000 years ago, correlating with a known dry cycle. These ridges, which likely initially formed a complete lunette, were then subsequently modified by an increased fluvial input into the lake, during a wet cycle, which has since ceased. OSL dating records this fluvial cycle between 2,000 and 3,000 years ago, and this corresponds to a previously recorded wet cycle of 2,000 years ago for the region.

The dating and correlation of events manifest the unique history for the San Luis Lake lunette; at present it is the only system in the area to preserve climatically influenced shifts in such detail.