Gran Desierto de Altar
Encyclopedia
The Gran Desierto de Altar is one of the major portions of the Sonoran Desert
of Mexico, including the only active erg
dune region in North America
. It extends across much of the northern border of the Gulf of California
, reaching more than 100 kilometers east to west, and over 50 km north to south, and constitutes the largest continuous wilderness area within the Sonoran Desert. The eastern portion of the area is a biosphere reserve
(the full title is Reserva de la Biosfera el Pinacate y Gran Desierto de Altar
)
The Gran Desierto covers approximately 5,700 km2, most of it in the Mexican state of Sonora. The northernmost edges overlap the border into southwestern Arizona
. The dominant sand sheets and dunes range in thickness from less than one meter to greater than 120 meters. The total volume of sand in the Gran Desierto is about 60 km3. Most of that volume was delivered by the Pleistocene
Colorado River
which flowed through the present-day Gran Desierto area ~120,000 years before present. This Pleistocene delta
migrated westward concomitant with strike-slip faulting and rift
ing associated with the opening of the Salton Trough and the Gulf of California
.
volcanic complex of the Sierra Pinacate, a composite volcanic field covering more than 1,800 km2 with a summit elevation of 1,206 m. Aeolian sands have climbed onto many of the western slopes of the Sierras Pinacate, defining the eastern limit of the dune field. To the north, the sands thin out against the distal margins of alluvial fan
s 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 Enternada is a smaller inselberg almost completely buried by the sand near the boundary of the Gran Desierto and the Pinacate volcanic complex
.
) 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 arroyo
s and washes.
Felger [1980] estimated total vegetation cover at 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 [Seely and Louw, 1980].
Several teams (e.g. Van Devender et al. [1987] and Hall et al. [1988]) have examined the nests (middens) built by pack rat
s as a proxy for ancient vegetation regimes. All have concluded that the Gran Desierto has been a refugia 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. Van Devender has also reported C14 dates for a midden from the Tinajas Altas Mountains showing 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
.
, Sonora (located on the southeastern margin of the sand sea) and decreases northward toward Yuma, Arizona
(on the northwestern edge) to 62 mm per year [Lancaster et al., 1987]. Mid-summer highs in excess of 45 °C are common in the central sand sea. Mid-winter lows of less than 10 °C are rare. Winds are controlled in part by the position and strength of the Sonoran Low in summer, creating southerly winds, and by the Great Basin High in winter, with north-to-northeasterly winds.
epochs which occurred over much of the southwestern United States during the most-recent (Wisconsin
) ice age
may not have extended as far south as the Gran Desierto. It appears that the climatic regime of the past 150,000 years at this site has been one of gradually increasing aridity with current hyperarid conditions being firmly in place by at least 43,000 years ago.
As a minimum, it may be assumed that onshore coastal winds from the south were less important to sand movement when the Wisconsin shoreline was located 45 km seaward of its current position.
, which is a northern extension of the Gulf of California, an embayment created by rifting initiated during the Pliocene
along the East Pacific Rise
and the San Andreas fault
system [Rusnak and Fisher, 1964; Larson et al., 1968; Angelier et al., 1981]. Regional subsidence has propagated to the northwest as rifting and strike-slip faulting continues into the present-day. The central portion of the nearby Salton Trough is more than 70 m below sea level
; it is protected from marine embayment only by the natural dike of the Colorado River Delta
.
Ongoing tectonic activity modifies the Gran Desierto today. The southernmost extension of the San Andreas fault system, the Cerro Prieto Fault
, passes directly through the area before continuing offshore into the Gulf of California [Merriam, 1965]. Strike-slip movement in the area is as high as 60 mm/yr. [Gastil et al., 1975; Curray and Moore, 1984].
Since 1900, one magnitude 6.3 and two magnitude 7.1 earthquake
s have originated within the erg. Most seismicity within the Gran Desierto originates at depths of five to six kilometers, corresponding to the transition between deltaic deposits and basement
crystalline rocks (Von der Haar and Howard, 1981]. Local uplift
is still occurring along the Mesa Arenosa, a drag folded fault block forming the coastal boundary.
The Gran Desierto sand sheets and dunes are located atop deltaic deposits of the Pleistocene Colorado River. Lucchitta [1988] 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 [1984] and dated at between 700,000 and 120,000 years before present. Vertebrate
fossil
s found by Merriam [1965] 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 [1987] 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 mammoth
s, sloth
s and boa constrictor
s; 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 [Colletta and Ortlieb, 1984].
Slate [1985] 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 [1990] 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. [1964]; Colleta and Ortlieb [1984].)
Annual sediment loads prior to the damming of the Colorado River were prodigious. Sykes [1937b] 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.
and the associated Gran Desierto sand sink continues offshore into the Gulf of California. Reports on the submarine topography of the Gulf of California by van Andel [1964], describe three former river channels on the seafloor: one originating at the present-day Colorado delta, another from the area of the paleo-delta between El Golfo and Salina Grande, and a third to the area of present-day Puerto Penasco
. Rusnak et al. [1964] reported on sonar soundings which discovered the valleys and also describe two elongate depressions, each about 40 km in length, into which the valley networks terminate at a depth of ~180 m below sea level. Those incised valley systems were also interpreted as fluvial
in origin.
Sonoran Desert
The Sonoran Desert is a North American desert which straddles part of the United States-Mexico border and covers large parts of the U.S. states of Arizona and California and the northwest Mexican states of Sonora, Baja California, and Baja California Sur. It is one of the largest and hottest...
of Mexico, including the only active erg
Erg (landform)
An erg is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The term takes its name from the Arabic word ʿarq , meaning "dune field"...
dune region in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. It extends across much of the northern border of the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
, reaching more than 100 kilometers east to west, and over 50 km north to south, and constitutes the largest continuous wilderness area within the Sonoran Desert. The eastern portion of the area is a biosphere reserve
Biosphere reserve
The Man and the Biosphere Programme of UNESCO was established in 1971 to promote interdisciplinary approaches to management, research and education in ecosystem conservation and sustainable use of natural resources.-Development:...
(the full title is Reserva de la Biosfera el Pinacate y Gran Desierto de Altar
Pinacate Peaks
The Pinacate Peaks are a volcanic group of volcanic peaks and cinder cones located mostly in the Mexican state of Sonora along the international border adjacent to the U.S...
)
The Gran Desierto covers approximately 5,700 km2, most of it in the Mexican state of Sonora. The northernmost edges overlap the border into southwestern Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
. The dominant sand sheets and dunes range in thickness from less than one meter to greater than 120 meters. The total volume of sand in the Gran Desierto is about 60 km3. Most of that volume was delivered by the Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
Colorado River
Colorado River
The Colorado River , is a river in the Southwestern United States and northwestern Mexico, approximately long, draining a part of the arid regions on the western slope of the Rocky Mountains. The watershed of the Colorado River covers in parts of seven U.S. states and two Mexican states...
which flowed through the present-day Gran Desierto area ~120,000 years before present. This Pleistocene delta
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
migrated westward concomitant with strike-slip faulting and rift
Rift
In geology, a rift or chasm is a place where the Earth's crust and lithosphere are being pulled apart and is an example of extensional tectonics....
ing associated with the opening of the Salton Trough and the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
.
Physiography
The eastern margin of the Gran Desierto abuts the CenozoicCenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
volcanic complex of the Sierra Pinacate, a composite volcanic field covering more than 1,800 km2 with a summit elevation of 1,206 m. Aeolian sands have climbed onto many of the western slopes of the Sierras Pinacate, defining the eastern limit of the dune field. To the north, the sands thin out against the distal margins of alluvial fan
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
s from the Tinajas Altas and Tule Mountains
Tule Mountains
The Tule Mountains is a mountain landform in Yuma County, Arizona. There is a diverse flora and fauna population within the Tule Mountains; one of the notable trees found in this mountain range is the Elephant tree, Bursera microphylla.- References :...
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
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
cuts across the area and lies beneath several prominent granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
inselbergs, most notably the Sierra del Rosario mountains which are surrounded by the erg on all sides. The Sierra Enternada is a smaller inselberg almost completely buried by the sand near the boundary of the Gran Desierto and the Pinacate volcanic complex
Pinacate Peaks
The Pinacate Peaks are a volcanic group of volcanic peaks and cinder cones located mostly in the Mexican state of Sonora along the international border adjacent to the U.S...
.
Sand dune distribution
The Gran Desierto is best-known for its magnificent Star Dunes, many in excess of 100 meters 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 morphologic 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 dumosaAmbrosia dumosa
Ambrosia dumosa, the burro-weed or white bursage, is a common constituent of the creosote-bush scrub community throughout the Mojave desert of California, Nevada, and Utah and the Sonoran Desert of Arizona and northwestern Mexico....
) and longleaf jointfir (Ephedra trifurca
Ephedra trifurca
Ephedra trifurca is a species of Ephedra known by the common names longleaf jointfir and Mexican tea. It is native to many of the northern states of Mexico and the southwestern United States, where it is a resident of desert scrub plant communities.This sprawling shrub may approach two meters in...
) with creosote bush (Larrea tridentata) in areas of thin sand cover. Palo Verde
Parkinsonia
Parkinsonia , also Cercidium , is a genus of flowering plants in the pea family, Fabaceae. It contains about 12 species that are native to semi-desert regions of Africa and the Americas...
/Acacia
Acacia
Acacia is a genus of shrubs and trees belonging to the subfamily Mimosoideae of the family Fabaceae, first described in Africa by the Swedish botanist Carl Linnaeus in 1773. Many non-Australian species tend to be thorny, whereas the majority of Australian acacias are not...
/Ocotillo communities occur on alluvial slopes on the northern side of the sand sea, particularly in arroyo
Arroyo (creek)
An arroyo , a Spanish word translated as brook, and also called a wash is usually a dry creek or stream bed—gulch that temporarily or seasonally fills and flows after sufficient rain. Wadi is a similar term in Africa. In Spain, a rambla has a similar meaning to arroyo.-Types and processes:Arroyos...
s and washes.
Felger [1980] estimated total vegetation cover at 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 [Seely and Louw, 1980].
Several teams (e.g. Van Devender et al. [1987] and Hall et al. [1988]) have examined the nests (middens) built by pack rat
Pack rat
A packrat, also called a woodrat, can be any of the species in the rodent genus Neotoma. Packrats have a rat-like appearance, with long tails, large ears and large black eyes. Compared to deer mice, harvest mice and grasshopper mice, packrats are noticeably larger and are usually somewhat larger...
s as a proxy for ancient vegetation regimes. All have concluded that the Gran Desierto has been a refugia for desert plants since at least the late Pleistocene. The Gran Desierto has served as a refuge for most dominant Mojave Desert
Mojave Desert
The Mojave Desert occupies a significant portion of southeastern California and smaller parts of central California, southern Nevada, southwestern Utah and northwestern Arizona, in the United States...
plant species during cooler pluvial epochs as well. Van Devender has also reported C14 dates for a midden from the Tinajas Altas Mountains showing assemblages of Juniper
Juniper
Junipers are coniferous plants in the genus Juniperus of the cypress family Cupressaceae. Depending on taxonomic viewpoint, there are between 50-67 species of juniper, widely distributed throughout the northern hemisphere, from the Arctic, south to tropical Africa in the Old World, and to the...
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
Wisconsinan glaciation
The Wisconsin Glacial Episode was the most recent major advance of the North American Laurentide ice sheet. Globally, this advance is known as the last glacial period. The Wisconsin glaciation extended from approximately 110,000 to 10,000 years ago, between the Eemian interglacial and the current...
.
Climate
The Gran Desierto has a warm-to-hot arid climate. Mean annual rainfall, most of which occurs between September and December, is 73 mm at Puerto PeñascoPuerto Peñasco
Puerto Peñasco is a city and municipality located in the northwest of the state of Sonora about 100 km from the Arizona border. It is located on the small strip of land that joins the peninsula of Baja California with the rest of Mexico. The area is part of the Altar Desert, one of the driest...
, Sonora (located on the southeastern margin of the sand sea) and decreases northward toward Yuma, Arizona
Yuma, Arizona
Yuma is a city in and the county seat of Yuma County, Arizona, United States. It is located in the southwestern corner of the state, and the population of the city was 77,515 at the 2000 census, with a 2008 Census Bureau estimated population of 90,041....
(on the northwestern edge) to 62 mm per year [Lancaster et al., 1987]. Mid-summer highs in excess of 45 °C are common in the central sand sea. Mid-winter lows of less than 10 °C are rare. Winds are controlled in part by the position and strength of the Sonoran Low in summer, creating southerly winds, and by the Great Basin High in winter, with north-to-northeasterly winds.
Paleoclimate
Van Devender and Spaulding [1979] and Spaulding et al. [1983] report that the well-documented pluvialPluvial
In geology and climatology, a pluvial was an extended period of abundant rainfall lasting many thousands of years. Pluvial is also applied to the sediments of these periods . The term is especially applied to such periods during the Pleistocene Epoch...
epochs which occurred over much of the southwestern United States during the most-recent (Wisconsin
Wisconsin glaciation
The last glacial period was the most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago....
) ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
may not have extended as far south as the Gran Desierto. It appears that the climatic regime of the past 150,000 years at this site has been one of gradually increasing aridity with current hyperarid conditions being firmly in place by at least 43,000 years ago.
As a minimum, it may be assumed that onshore coastal winds from the south were less important to sand movement when the Wisconsin shoreline was located 45 km seaward of its current position.
Tectonics
The Gran Desierto is located adjacent to a rapidly subsiding tectonic basin, the Salton TroughSalton Sink
The Salton Sink is a geographic sink in the Coachella and Imperial valleys of southeastern California. It is in the Colorado Desert subregion of the Sonoran Desert ecoregion...
, which is a northern extension of the Gulf of California, an embayment created by rifting initiated during the Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
along the East Pacific Rise
East Pacific Rise
The East Pacific Rise is a mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate...
and the San Andreas fault
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
system [Rusnak and Fisher, 1964; Larson et al., 1968; Angelier et al., 1981]. Regional subsidence has propagated to the northwest as rifting and strike-slip faulting continues into the present-day. The central portion of the nearby Salton Trough is more than 70 m below sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...
; it is protected from marine embayment only by the natural dike of the Colorado River Delta
Colorado River Delta
The Colorado River Delta is the region where the Colorado River flows into the Gulf of California . The delta is part of a larger geologic region called the Salton Trough. Historically, the interaction of the river’s flow and the ocean’s tide created a dynamic environment, supporting freshwater,...
.
Ongoing tectonic activity modifies the Gran Desierto today. The southernmost extension of the San Andreas fault system, the Cerro Prieto Fault
Cerro Prieto Fault
The Cerro Prieto Fault is a transform fault located in far northern Baja California. It runs between the Cerro Prieto spreading center located southwest of Mexicali, and the Wagner Basin, another spreading center which lies under the Gulf of California...
, passes directly through the area before continuing offshore into the Gulf of California [Merriam, 1965]. Strike-slip movement in the area is as high as 60 mm/yr. [Gastil et al., 1975; Curray and Moore, 1984].
Since 1900, one magnitude 6.3 and two magnitude 7.1 earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s have originated within the erg. Most seismicity within the Gran Desierto originates at depths of five to six kilometers, corresponding to the transition between deltaic deposits and basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
crystalline rocks (Von der Haar and Howard, 1981]. Local uplift
Tectonic uplift
Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...
is still occurring along the Mesa Arenosa, a drag folded fault block forming the coastal boundary.
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 [1988] 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
Conglomerate (geology)
A conglomerate is a rock consisting of individual clasts within a finer-grained matrix that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts...
sands and silts beneath the Mesa Arenosa were examined by Colletta and Ortlieb [1984] and dated at between 700,000 and 120,000 years before present. Vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s found by Merriam [1965] within the deltaic deposits include Equus
Equus
Equus may refer to:* Equus , a genus of animals including horses, donkeys, zebras and onagers* Equus , a play by Peter Shaffer* Equus , a film adaptation of the Peter Shaffer play...
, Gomphotherium
Gomphotherium
Gomphotherium is an extinct genus of proboscid which evolved in the Early Miocene of North America from 13.650—3.6 Ma, living about .The genus emigrated into Asia, Europe and Africa after a drop in sea level allowed them to cross over...
and Bison
Bison
Members of the genus Bison are large, even-toed ungulates within the subfamily Bovinae. Two extant and four extinct species are recognized...
and were assigned to Irvingtonian
Irvingtonian
The Irvingtonian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology , typically set from 1,800,000 to 300,000 years BP, a period of . It is usually considered to overlap the Lower Pleistocene and Middle...
age (0.5 to 1.8 million years before present); dates consistent with the aforementioned capture of the lower Colorado River. Shaw and McDonald [1987] documented evidence of a giant anteater
Giant Anteater
The Giant Anteater, Myrmecophaga tridactyla, is the largest species of anteater. It is the only species in the genus Myrmecophaga. It is found in Central and South America from Honduras to northern Argentina...
Myrmecophaga tridactyla in the deltaic deposits in the southern Gran Desierto. Van Devender notes that the specimen was found in association with fossils of mammoth
Mammoth
A mammoth is any species of the extinct genus Mammuthus. These proboscideans are members of Elephantidae, the family of elephants and mammoths, and close relatives of modern elephants. They were often equipped with long curved tusks and, in northern species, a covering of long hair...
s, sloth
Sloth
Sloths are the six species of medium-sized mammals belonging to the families Megalonychidae and Bradypodidae , part of the order Pilosa and therefore related to armadillos and anteaters, which sport a similar set of specialized claws.They are arboreal residents of the jungles of Central and South...
s and boa constrictor
Boa constrictor
The Boa constrictor is a large, heavy-bodied species of snake. It is a member of the family Boidae found in North, Central, and South America, as well as some islands in the Caribbean. A staple of private collections and public displays, its color pattern is highly variable yet distinctive...
s; a tropical faunal assemblage
Faunal assemblage
Faunal Assemblage is the archaeological or paleontological term for a group of associated animal fossils found together in a given stratum.The principle of faunal succession is used in biostratigraphy to determine each biostratigraphic unit, or biozone...
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
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
at 146,000 + 13,000/-11, 000 years of age [Colletta and Ortlieb, 1984].
Slate [1985] obtained K-Ar ages
Potassium-argon dating
Potassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
for basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
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 [1990] 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. [1964]; Colleta and Ortlieb [1984].)
Annual sediment loads prior to the damming of the Colorado River were prodigious. Sykes [1937b] 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.
Offshore features
The synchronous development of the Colorado River DeltaColorado River Delta
The Colorado River Delta is the region where the Colorado River flows into the Gulf of California . The delta is part of a larger geologic region called the Salton Trough. Historically, the interaction of the river’s flow and the ocean’s tide created a dynamic environment, supporting freshwater,...
and the associated Gran Desierto sand sink continues offshore into the Gulf of California. Reports on the submarine topography of the Gulf of California by van Andel [1964], describe three former river channels on the seafloor: one originating at the present-day Colorado delta, another from the area of the paleo-delta between El Golfo and Salina Grande, and a third to the area of present-day Puerto Penasco
Puerto Peñasco
Puerto Peñasco is a city and municipality located in the northwest of the state of Sonora about 100 km from the Arizona border. It is located on the small strip of land that joins the peninsula of Baja California with the rest of Mexico. The area is part of the Altar Desert, one of the driest...
. Rusnak et al. [1964] reported on sonar soundings which discovered the valleys and also describe two elongate depressions, each about 40 km in length, into which the valley networks terminate at a depth of ~180 m below sea level. Those incised valley systems were also interpreted as fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
in origin.
See also
- Pinacate PeaksPinacate PeaksThe Pinacate Peaks are a volcanic group of volcanic peaks and cinder cones located mostly in the Mexican state of Sonora along the international border adjacent to the U.S...
- Sonoran DesertSonoran DesertThe Sonoran Desert is a North American desert which straddles part of the United States-Mexico border and covers large parts of the U.S. states of Arizona and California and the northwest Mexican states of Sonora, Baja California, and Baja California Sur. It is one of the largest and hottest...
- Chihuahuan DesertChihuahuan DesertThe Chihuahuan Desert is a desert, and an ecoregion designation, that straddles the U.S.-Mexico border in the central and northern portions of the Mexican Plateau, bordered on the west by the extensive Sierra Madre Occidental range, and overlaying northern portions of the east range, the Sierra...