The Gran Desierto de Altar is one of the major sub-ecoregions of the Sonoran Desert, located in the State of Sonora, in northwest Mexico. It includes the only active erg dune region in North America. The desert extends across much of the northern border of the Gulf of California, spanning more than east to west and over north to south. It constitutes the largest continuous wilderness area within the Sonoran Desert.
The eastern portion of the area contains the volcanic Pinacate Peaks region; together with the western portion, the area forms the El Pinacate y Gran Desierto de Altar Biosphere Reserve and a UNESCO World Heritage Site.
Geography
The Gran Desierto covers approximately , most of it in the Mexican state of Sonora. The northernmost edges reach across the international border into Organ Pipe Cactus National Monument and Cabeza Prieta National Wildlife Refuge in southwestern Arizona, United States. The region is dominated by sand sheets and dunes ranging in thickness from less than to greater than . The total volume of sand in the Gran Desierto is about . Most of that volume was delivered by the Colorado River during the Pleistocene, which flowed through the present-day Gran Desierto area approximately 120,000 years before present. This Pleistocene delta migrated westward concomitant with strike-slip faulting and rifting associated with the opening of the Salton Trough and the Gulf of California.
The eastern margin of the Gran Desierto abuts the Cenozoic volcanic complex of the Sierra Pinacate, a composite volcanic field covering more than with a summit elevation of . Aeolian sands have climbed onto many of the western slopes of the Sierra Pinacate, defining the eastern limit of the dune field. To the north, the sands thin out against the distal margins of alluvial fans from the Tinajas Altas and Tule Mountains along the Arizona–Sonora border. The southern border of the sand sea is the northern shore of the Gulf of California.
The southernmost extension of the San Andreas Fault cuts across the area and lies beneath several prominent granite inselbergs, most notably the Sierra del Rosario mountains, which are surrounded by the erg on all sides. The Sierra Enterrada is a smaller inselberg almost completely buried by the sand near the boundary of the Gran Desierto and the Pinacate volcanic complex.
Sand dune distribution
The Gran Desierto is best known for its magnificent star dunes, many in excess of high. More than two-thirds of the Gran Desierto is covered by sand sheets and sand streaks. The remaining area is split equally between a western population of star dunes and an eastern set of transverse or crescentic dunes. Some of the larger crescentic dunes in the northeastern sand sea exhibit reversing crests, a transitional morphological feature associated with star dunes.
Flora
Vegetation assemblages of the Gran Desierto are typical of the lower Sonoran Desert with a marked difference in vegetation type and density with location. Large areas of the southern and eastern sand sea, especially near the margins, have a moderately dense (up to 20%) cover of perennial low shrubs and herbs such as bursage (Ambrosia dumosa) and longleaf jointfir (Ephedra trifurca) with creosote bush (Larrea tridentata) in areas of thin sand cover. Palo verde/acacia/ocotillo communities occur on alluvial slopes on the northern side of the sand sea, particularly in arroyos and washes. The region's estimated total vegetation cover is 15% in the star dunes and about 10% in the low transverse or crescentic dunes areas. These percentages are substantially greater than in most active dune fields, where vegetation covers of 15% are more typical.
Several teams have examined the middens built by pack rats as a proxy for ancient vegetation regimes. All have concluded that the Gran Desierto has been an ecological refuge for desert plants since at least the late Pleistocene. The Gran Desierto has served as a refuge for most dominant Mojave Desert plant species during cooler pluvial epochs as well. Carbon-14 dating for a midden from the Tinajas Altas Mountains shows assemblages of juniper and Joshua trees coexisting with contemporary Gran Desierto flora and fauna more than 43,000 years before present. Although midden studies do not provide information beyond the late Pleistocene, they do indicate that, in gross form, the climate of the Gran Desierto as recorded by plant communities has been desert-like since at least the peak of the Wisconsinan glaciation.