Gran Desierto de Altar - Origin

Origin

The geological history of the Gran Desierto is intimately linked to the opening of the Gulf of California and the capture of the ancestral Colorado River; source areas which were adjacent to the Gran Desierto have shifted in position, basement topography has been altered continuously, bedforms have been created, modified or completely destroyed and then reworked.

The Gran Desierto sand sheets and dunes are located atop deltaic deposits of the Pleistocene Colorado River. Lucchitta showed that the lower Colorado River was captured by the Gulf of California 1.2 million years before present. This event places an upper bound on the age of the Gran Desierto with the Colorado's major clastic sediment sources. Conglomeritic sands and silts beneath the Mesa Arenosa were examined by Colletta and Ortlieb and dated at between 700,000 and 120,000 years before present. Vertebrate fossils found by Merriam within the deltaic deposits include Equus, Gomphotherium and Bison and were assigned to Irvingtonian age (0.5 to 1.8 million years before present); dates consistent with the aforementioned capture of the lower Colorado River. Shaw and McDonald documented evidence of a giant anteater Myrmecophaga tridactyla in the deltaic deposits in the southern Gran Desierto. Van Devender notes that the specimen was found in association with fossils of mammoths, sloths and boa constrictors; a tropical faunal assemblage which supports a contention that the Colorado River delta of a previous interglacial period (>120,000 years ago) was much warmer and wetter than the present interglacial. Paleo-deltaic deposits near Salina Grande correlate with a ubiquitous indurated shell deposit dated by Io/U radiometric methods at 146,000 + 13,000/-11, 000 years of age .

Slate obtained K-Ar ages for basalt flows in the western Pinacates. Based on this work, some aeolian activity may have been present as early as 700,000 years ago, as evidenced by the dated accretionary mantles on basalt flows of the Pinacate volcanic field.

Blount and Lancaster proposed that by late Pleistocene time, the Colorado River was a highly competent stream flowing through the area which is occupied today by the massive western star dune zone. The seashore at this time was at least 45 km south of its present-day location. Primary bed loads of poorly sorted gravel were deposited from present-day Yuma, Arizona to an area south of the present-day Sierra del Rosario mountains.

As rifting of the Gulf of California progressed to the northwest, and uplift along the coast began, the river channel shifted westward, leaving primary bedload deposits in the former channel and floodplain. Deltaic sediments beneath the Gran Desierto may be as much as 6,000 meters deep (Biehler et al. ; Colleta and Ortlieb .)

Annual sediment loads prior to the damming of the Colorado River were prodigious. Sykes reported a single flood event which deposited an estimated 100,000,000 m3 of coarse to medium sand as a sheet deposit on the modern delta just south of the international boundary. Events like this, even if rare, could fill up the Gran Desierto in only a few millennia.