The Grand Canyon of the Colorado River in Arizona is one of the world’s most awe-inspiring places. The canyon is 277 miles (446 km) long, 6,000 feet (1829 m ) deep from rim to river, and 18 miles (29 km) wide at its widest point.
The canyon cuts through the Colorado Plateau, a highland that has existed since the Laramide orogeny which built the Rocky Mountains beginning about 70 million years ago. The canyon exposes rocks which range in age from almost 2 billion years to 200 million years. The earlier sediments represent deposits in warm, shallow seas, while the younger layers represent desert sand dunes.
The origin of the canyon has been the subject of geological debate. The following story, pieced together from several sources, most notably, the Arizona Geological Survey, is based on the most recent research and age dating.
The Colorado Plateau was uplifted to an elevation of about 2 miles (3.2km) above sea level. One theoretical study suggests multiple stages of subsidence and uplift related to plate tectonic movement.
The Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction. Deposition of evaporites and the Green River formation in Utah and Colorado, indicate that these northeast flowing rivers emptied into a series of lakes which were mountain bound, similar to the Great Salt Lake in Utah. (See paleomaps here.) (Some researchers suggest that there was an outlet to the Gulf of Mexico.) Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest.
Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago. The evidence and details of the story are continued now by the Arizona Geological Survey “Arizona Geology, Winter 2005.”
The Colorado River [now] leaves the Colorado Plateau at the Grand Wash Cliffs where it flows into Lake Mead. A long, north-south trending valley at the foot of the Grand Wash Cliffs, known as Grand Wash trough, contains lake sediments that were deposited between about 11 and 6 million years ago. While the lake sediments were being deposited, alluvial fans derived from highlands on both sides of the valley were deposited on the flanks of the valley. There is no evidence in these sedimentary rocks of a large river like the modern Colorado entering the valley. Indeed, if the modern Colorado had entered the valley, it would have quickly filled the lake with sand and gravel. A volcanic ash bed near the stratigraphic top of the limestone is 6 million years old, so the Colorado River must have arrived in this area after this time.
South of Lake Mead the Colorado River traverses 400 km (250 mi) of low desert before arriving at th head of the Gulf of California south of Yuma. Deposit of the Bouse Formation record a brief but deep inundation of basins along the course of the river. The Bouse Formation consists of a basal layer of silt limestone that is typically one to several meters thick and is commonly overlain by up to hundreds of meters of siltstone and minor sandstone. These sediments were thought to have been deposited in an estuary of the developing Gulf of California after a U. S. Geological Survey (USGS) paleontologist first reported the presence of marine invertebrates in 1960. Estuaries generally have variable salinity conditions because, during times of high river flow, fresh water dilutes the salt water while sea water is dominant in times when river input is low. This leads to a mix of marine, brackish and fresh water organisms like those represented by the fossils in the Bouse Formation.
The Bouse Formation is exposed at elevations of up to 536 m (1760 ft), with elevations generally increasing from south to north. If in fact the Bouse Formation was deposited in an arm of the sea, then the lower Colorado River trough must have been uplifted by up to 536 m in the past 5 million years. USGS geologist Ivo Lucchitta proposed in 1979 that not only was the lower Colorado River trough uplifted in the past 5 million years, but that this uplift was part of a more regional tectonic uplift that carried the Colorado Plateau up to its current high elevation. Three discoveries in the past 10 years have cast doubt on the interpretation that the Bouse Formation was deposited in an estuary. In a paper published in 1997, Professor P. Jonathan Patchett of the University of Arizona and Jon Spencer of the AZGS showed that the strontium isotope composition of Bouse Formation limestone was similar to that of Colorado River water, and quite different from sea water. They proposed that the Bouse Formation was deposited by the Colorado River in a series of lakes that filled with river water and spilled over, eventually linking the river with the Gulf of California.
In 2002, during the course of a field mapping project in the Bullhead City–Laughlin area (Mohave Valley), Kyle House of the Nevada Bureau of Geology and Mines and Phil Pearthree of the AZGS discovered evidence that initial inundation of the northern part of the Colorado River trough was marked by southward-transported flood deposits. Such flood deposits would not be expected for initial inundation from sea water derived from far to the south, but they are consistent with an upstream lake spillover and initial influx of Colorado River water derived from the north in the Lake Mead area. These flood deposits are directly overlain by the basal limestone of the Bouse Formation. On the flank of the valley, they found a volcanic ash layer just below the Bouse Formation that was determined by Mike Perkins of the University of Utah to be 5.5 million years old. Bouse deposition was succeeded by a period of erosion, which was followed by deposition of at least 250 m (800 ft) of sand and gravel that is clearly associated with the Colorado River. This period of massive river aggradation ended about 4 million years ago, as another volcanic ash was found near the top of the river deposits.
Yet another volcanic ash indicates that the Colorado River had downcut at least 60 m (200 ft) below the level of maximum aggradation by 3.3 million years ago. Most recently, Rebecca Dorsey of the University of Oregon said that Colorado River sands first arrived in the Salton Trough south of Yuma 5.3 million years ago. Such sands would have been deposited before reaching the Salton Trough if any large body of standing water such as a lake or an estuary existed along the course of the Colorado River. The sand deposits thus reveal the existence of a through-going Colorado River by 5.3 million years ago and mark the beginning of an enormous influx of river sediment that has filled the Salton Trough since that time.
All of these recent studies are consistent with the concept that the Bouse Formation was deposited in a geologically short-lived chain of lakes that were created by initial influx of Colorado River water into previously closed basins. At the mouth of the Grand Canyon, the Colorado River first arrived less than 6 million years ago. Along the course of the lower Colorado River, it appears that all of the Bouse Formation was deposited between 5.5 and 5.3 million years ago and Colorado River sands arrived in the Salton Trough less than two hundred thousand years after deep flooding of Mohave Valley. The influx of Colorado River water and sediment at this time marks the inception of the modern Colorado River.
Tens to possibly hundreds of meters of river sand and gravel were deposited in Mohave Valley and elsewhere in the lower Colorado River trough in the subsequent 1- to 1.5 million years, as a tremendous volume of river sediment accumulated in the Salton trough. This massive aggradation probably resulted from rapid erosion in the Grand Canyon as the Colorado River downcut through that region.
The studies mentioned above have created a new problem for geologists. What are we supposed to make of the marine and estuarine organisms in the Bouse Formation? These organisms require salty water, but have been found only in sediments of the southernmost of the basins in which the Bouse Formation was deposited (the large Blythe sub-basin). Recent calculations show that evaporation in the southernmost lake as it was filling with river water could have elevated salinity to sea-water levels. The organisms, however, would have to be carried from the sea to the lake and delivered in sufficient numbers to provide a reproducing population. This seems like an impossible task, but long distance transport of aquatic organisms by birds has been documented in a number of places.
In conclusion, the abrupt arrival of the Colorado River to the low western desert region as a series of lakes roughly coincides with the beginning of incision of the Grand Canyon. Before this time,
water that flowed off of the west slope of the Rocky Mountains and into the interior of the Colorado Plateau must have terminated in a lake on the Plateau or exited the Plateau along an unknown route. We think it most likely that cutting of the modern Grand Canyon began with spillover of a very large lake in northeastern Arizona and rapid incision of the lake outflow point about 5.5 million years ago. Details of the spillover and development of the Colorado River through Grand Canyon are not known, but we suspect that it involved catastrophic flooding and rapid erosion. As the Colorado River propagated downstream through the Basin and Range province, it sequentially filled, spilled over, and drained a series of formerly closed basins, eventually linking with the Gulf of California by 5.3 million years ago.
But, the Colorado River has not always been free flowing to the Gulf of California. Volcanic flows have dammed the river several times in the last 1 million years (see that story here.)
For a broader view of Arizona geological history, see my seven-part series beginning here.
