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Grand Canyon was formed by exploring four possible hypotheses. Initially the superimposition mechanism intuitively seemed to be the more plausible explanation for Grand Canyon formation, but the weight of the evidence from the field trip lies with the overflow mechanism. Though it still leaves some questions unanswered, and some formations and processes unaccounted for, overall the overflow mechanism offers the best explanation.
To begin with, the first requirement for being able to explain Grand Canyon formation was determining whether the Colorado River was older or younger than the bedrock high of the Kaibab Plateau. Based on information provided at the various stops, one can conclude that the Colorado River is younger than the Kaibab Plateau.
At stop 2e there is evidence showing that the topography of the bedrock high controls the location of the transverse drainage incision into low elevation saddles. The topographical map shows how Marble Canyon is controlled by fractures. As for the mystery of Grand Canyon formation, yes, one would expect water to flow away from an upwarp, but the Colorado River and Little Colorado River, with the excess energy provided by the overflow transverse drainage mechanism, were able to flow up to their confluence.
One of the stops which shows evidence of sediments downstream of the bedrock high that record the rapid arrival of transverse drainage is stop 9d. The gravels in the two photos were deposited by the Colorado River above Lake Mead, which is downstream from the Kaibab Plateau. The gravels are termed exotic because they are unrelated to the rocks with which they are now found at the Lake Mead location, and have been moved from their place of origin by the drainage process. Stop 10a also refers to the existence of downstream sediments, which support the thesis that the Colorado River is younger than the bedrock high.
Stops 3a, 3c, 5c, and 7a show evidence of regional drainage prolongation or reorganization. The Mazatzal Mountains used to be connected with the nearby ranges to the east and west in a formation called the Mogollon Highlands, until extensional faulting occurred about 25 million years ago. Once the Mogollon Highlands stopped draining onto the Colorado Plateau, the drainage switched from flowing to the northeast to flowing southwest. Stop 3a also describes interior drainage, closed basins with lakes, as being common on the Colorado Plateau before an integrated Colorado River began to flow; this provides another indication that the overflow mechanism was responsible for Grand Canyon formation. Stop 7a shows the unusually large tributary canyon, Bright Angel Canyon, straight because Bright Angel Creek erodes headwards along the fault plane using weaknesses like faulted rock.
Stop 5c showing the Tapan Flow also discusses the rate at which the Little Colorado River has been cutting into its channel. According to studies by Holm (59-63), if the Little Colorado River originated by integration of the Bidahochi basin and the Colorado River, then average incision rates imply a 6 Ma age of eastern Grand Canyon where the Little Colorado River enters it. This estimate argues against both superimposition and antecedence, which require that the Colorado River be older than the Kaibab Plateau.
The evidence discovered at the preceding stops argues convincingly for the Colorado River being younger than uplift of the bedrock high, which in turn disqualifies superimposition and antecedence from further consideration. Only one stop, 5b, seemed to support the piracy mechanism being responsible for the transverse drainage. By contrast evidence at 10 stops describes processes that can be attributed to an overflow mechanism.
Beginning with stop 4a, there is a topographic indication of a paleo-basin upstream of the bedrock high, called Lake Bidahochi. Lake clay sediments, green in color in the image provided, were deposited roughly 6 million years ago, confirming the lake's existence. What remains uncertain is whether the Colorado River ever flowed into this lake, and whether it was large enough to spill over the Kaibab Plateau. For the overflow mechanism to be responsible for the transverse drainage, the answer to both questions must be yes.
Stops 4a, 4b, and 4d also show evidence of ponded deposits and/or paleo-shorelines upstream of the bedrock high that formed just prior to the transverse drainage formation. Stop 4d asks the question whether an erosion rate of 7.4 milliliters per thousand years is reasonable, the answer is yes it is. Also the presence of limey sand would seem to indicate a former lakebed. Stop 4b also provides evidence for an old lake basin, including the presence of fresh water mollusks, fish and fossilized minnows, along with stratigraphy indicating the size of the basin. It extended across the Little Colorado River and also covered most of the Little Colorado River drainage, including up to the Kaibab Plateau.
Stops 10a and 10c show evidence of rapid arrival of water then sediment downstream of bedrock of the bedrock high from upstream of the bedrock high. Based on the presence of sediment, it can be seen that a lake basin existed in the area where the addition of water by the Colorado River caused overtopping of the topographic high, then spillage into the next basin. Likewise, stop 10c shows green clays typical of a lake environment around the time of the arrival of the Colorado River. Sediments found in these areas indicate the Colorado River's sudden entrance at one of several closed basins that were associated with the Bouse Formation, followed by eventual overtopping, allowing the river to cuts a transverse gorge. After that the river moved farther downstream, washing the old lake sediment downstream to another closed lake basin, where the sequence repeated itself until the Colorado River eventually drained into the Gulf of California.
Stops 5d, 6a, 7c, and 9c show evidence of development of the transverse drainage across the lowest elevation topographic saddle in the bedrock high. The question comes up at stop 6a whether backward erosion of at the rate of 5 millimeters per year is reasonable; to the newly-initiated geomorphologist, it sounds entirely possible, as does the explanation as to how retreating circular scarp controlled the Colorado River's ability to just cut straight across the Kaibab Plateau. The discussion at stop 9c poses the question where did the lime come from that is found in the Hualapai Limestone? The previously offered hypothesis that the Colorado River overspilled a basin farther upstream in Utah, perhaps the Green River seems plausible.
To sum up, there is a large body of evidence indicating that the overflow mechanism is responsible for Grand Canyon formation. According to Spencer and Pearthree (1-6) recent studies support the hypothesis that the Bouse Formation was deposited by the Colorado River in a series of lakes that filled with water from the Colorado River, then spilled over, eventually linking the Colorado River with the Gulf of Mexico. In 1997 Patchett and Spencer determined that the strontium isotope composition of Bouse Formation limestone was similar to that of Colorado River water, which in fact was very different from sea water. The article also describes findings in 2002 by geologists House and Pearthree, who discovered evidence that the initial flooding of the northern section of the Colorado River trough was characterized by southward-transported flood deposits. These types of flood deposits would be unexpected for initial inundation from sea water derived far to the south, but rather, they are consistent with an upstream lake spillover that occurred during the initial influx of Colorado River water that originated from the north in the Lake Mead area (Spencer and Pearthree 1-6).
A third finding, this time by Dorsey and team in 2005, reported that Colorado River sands first arrived in the Salton Trough 5.3 million years ago. Had there been a large body of standing water, such as a lake or an estuary along the course of the Colorado River, such sands would have been deposited before reaching the Salton Trough. These sands are interpreted as revealing the existence of a through-going Colorado River by 5.3 million years ago, and show the beginning of a substantial influx of river sediment into the Salton Trough ever since (Spencer and Pearthree 1-6).
Spencer and Pearthree observe that these studies are consistent with the argument that the Bouse Formation was deposited in a geologically short-lived chain of lakes created by initial influx of water from the Colorado River into what had been previously closed basins. It should be noted that the article raises so-far unanswered questions about the presence of marine and estuarine organisms in the Bouse Formation, as well as the date of the uplifting of the Colorado Plateau. Nevertheless, according to Spencer and Pearthree, cutting of the modern Grand Canyon most likely 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. From there, the Colorado River sequentially filled, spilled over, and drained a series of formerly closed basins, and eventually linked with the Gulf of California by 5.3 million year ago (Spencer and Pearthree 1-6).
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