ORIGIN AND EVOLUTION OF THE GRAN DESIERTO SAND SEA

The Gran Desierto Sand Sea of Sonora, Mexico, is located on the northeastern shore of the Gulf of California, east of the Colorado River Delta. The modern sand sea covers an area of 5700 sq km and is the largest active dunefield in North America. The Gran Desierto is well-suited for investigation of the timing and nature of aeolian activity and regional climate change because it contains multiple dune geomorphic elements that make up the complex pattern observed today.

Individual generations of dune geomorphic features were identified and ‘backstripped’ through the utilization of several techniques including analysis of Landsat images, statistical analysis of dune morphometry, and absolute age determination through optically stimulated luminescence (OSL) dating techniques. These dune generations were then correlated with global and regional climatic events, and associated variability in predominant wind regimes.Wind regimes since the Last Glacial Maximum were reconstructed based on the concept of gross bedform normal transport.

Aeolian units overlie alluvium that is sourced from distal alluvial fans of Tertiary granitic outcrops, and from fluvial sediments deposited by the ancient Colorado River, which avulsed westward across the region in the late Pleistocene. The alluvium is dated to ~87 ka to ~26 ka. The oldest dune generation consists of N-S linear dunes located in the northern half of the sand sea, bracketed in age from ~26 ka to ~12 ka, and formed by north and southwest winds during the period that corresponds to the LGM. The next oldest generation of dunes is ~12 ka old degraded crescentic dunes in the north central part of the sand sea which were formed by northwest and southeast winds in the period corresponding to the Younger Dryas and Early Holocene. A southern group of large crescentic and reversing dunes ~ 7 ka old and a northern group of modern crescentic dunes were formed by north-northwest and south-southeast winds during the middle to late Holocene. A large group of modern star dunes formed under a similar wind regime,although actively reorienting secondary arms possibly correspond to increased ENSO intensity during the Late Holocene.