1. Geologic Evolution of Grand Canyon
Mitchell Jennings, Department of Geosciences, East Tennessee State University
.
Discussion
• Headward Erosion Theory requires a pre-incised canyon and a north
flowing Ancestral Colorado River.
• Problems with age dating of the pre-incised canyon and evidence of a
north flowing ancestral river.
• Insufficient headward erosion.
• Problems of Spillover Theory begin with lack of evidence that supports
a basin large enough to create the Grand Canyon.
• Lack of evidence for Lake Bidahochi’s water level.
• Topographic profiles in northern Arizona are not compatible with the
Spillover Theory.
Conclusion
• Miocene topographic inversion left the Colorado Plateau higher,
reversed some drainages, and created significant fault scarps.
• Drainages and faulted areas played a large role in transporting water to
the Colorado River but more information is needed to establish a
timeline as to when and how the Colorado River trough was formed.
• Unconformities within the canyon’s stratigraphy combined with
massive erosion of the landscape make determining the geologic
evolution of the Grand Canyon a near impossible task.
• Whether the Grand Canyon was formed due to the Headward Erosion
Theory, Spillover Theory, or possibly even a combination of the two, at
this time definitive evidence has not been found.
Abstract
Although many theories exist about how the canyon was cut, the timeline
of events that explain how it was formed is unknown. The stratigraphy and
geomorphology of the canyon preserves evidence of uplift and incision of
the Colorado River into the Grand Canyon. The pace and timing of plateau
uplift is debated and still unknown, but the occurrence of the modern
Colorado River marks a localized extinction event that occurred in what is
now Lake Mead.
The two competing theories for the evolution of the Grand Canyon are
Headward Erosion and Spillover. The Headward Erosion Theory involves
multiple drainage areas, a north flowing ancestral Colorado River, and a
pre-incised canyon that eroded headward and captured the Ancestral
Colorado River to form the Grand Canyon. The Spillover Theory proposes
that a large basin was created to the north, filled up due to a change in
flow direction and spilt over the Colorado Plateau to form the Colorado
River which incised the Grand Canyon.
Spillover Theory
Spillover Theory proposes that Kaibab uplift altered Ancestral Colorado
River flow, forcing it southeast toward the Gulf of Mexico. 12 Ma the
ancestral river’s path to the gulf was blocked. Blockage caused the river to
back up and form a large basin. The basin, thought to be Lake Bidahochi,
continued to fill until it overflowed across the plateau. The overflow
stream followed topographic low areas and combined with drainage
streams. Once the river reached what is now southern Nevada, it began
to incise into the sediment and work its way upstream towards the large
reservoir. It achieved this massive headward erosion via a series of water
falls. These high-energy waterfall areas began to erode and incise into the
landscape very rapidly to create the modern path of the Colorado River
and eventually the Grand Canyon.
Introduction
• The Grand Canyon is located in Northern Arizona between two large
man-made reservoirs: Lake Powell (north) and Lake Mead (south).
• Dimensions; 360 km long, 30 km wide (at South Rim), 1,830 m deep.
• Colorado River is driving force behind formation of Grand Canyon.
• Rock sequence preserves eight sea transgression events (Fig. 1).
• Laramide orogeny (70 – 80 Ma) caused by subduction of Farallon plate
under North American plate.
• Uplift unique to Colorado Plateau resulted in minimal tilting of strata.
• Inversion of topography reversed flow directions.
• New topographic valleys and drainage channels are created and form
flow path of modern Colorado River.
Headward Erosion Theory
The Headward Erosion Theory proposes that the modern day Colorado
River achieved its present course by a combination of headward erosion
and stream capture. In this model a pre-incised canyon deeper than
600 m, formed on the western Hualapai Plateau by headward erosion,
continued along a strike-valley drainage, and captured ancestral
Colorado River flow. (Young, 2008) At this time, thought to be late
Miocene, the ancestral Colorado River is projected to flow southeast
toward the Gulf of Mexico. Near modern day Little Colorado River, the
ancestral river turned northward toward the Gulf of California. This
northward flow of the ancestral river is key for the present Colorado
River’s interception of ancestral river flow.
Fig. 1 Stratigraphic Column of Grand Canyon.
Fig. 2 View of drainages and stratigraphy from Bright Angel Trail
Fig. 3 Vishnu Schist and Zoroaster Granite