The origin and evolution of the Gran Desierto Sand Sea, Sonora, Mexico
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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 km² and is the largest active dunefield in North America. The Gran Desierto is well-suited for investigation into the timing and nature of aeolian activity and regional climate change as it contains multiple dune geomorphic elements that compose the complex pattern observed today. Individual generations of dune geomorphic features are identified and "backstripped" through the utilization of several techniques. Methods include remote sensing and statistical analysis of physical parameters of the dune geomorphic forms, as well as absolute age determination of individual generations through optically stimulated luminescence (OSL) age dating techniques. Individual generations of dune geomorphic features are digitized from a Landsat 7 multispectral composite. Crestline lengths, spacings and orientations are statistically analyzed in order to identify distinct trends among individual groups of dunes. These groups are then compared with dune geomorphic features visible on a satellite image overlain on a vertically exaggerated digital elevation model. OSL ages are utilized to determine individual generations within each dune group. These generations are then correlated with global climatic events, and associated variability in predominant wind regimes. Aeolian units sit on top of alluvium that is sourced from distal alluvial fans of Tertiary granitic outcrops, and from fluvial sediments deposited as the ancient Colorado River avulsed westward across the region throughout the Pleistocene. Alluvium is dated from ~87 ka to ~26 ka. Six generations of dune geomorphic features ranging in age from 26,000 years to modern are identified. Based on gross bedform normal transport, wind regimes are reconstructed since LGM. From ~26 ka to ~12 ka, winds shifted from N and NNW to NW and S. From ~12 ka to ~ 10 ka, winds shifted to SE dominant with a subordinate NW component. From ~10 ka to ~6 ka, NW winds became dominant over the south winds and more northerly. Modern conditions were reached by (at latest) ~6 ka, with the exception of enhanced ENSO cycles ~2.4-2.8 ka.