Geology 104 - Spring 2007
Lecture 3 - Grand Canyon NP

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Rocks of the Grand Canyon

Stratigraphy = history of an area as interpreted from rock layers

Formation = a distinctive rock unit

Formation	Rock	Age
Sand and gravel		Quaternary
Lava flows	igneous (basalt)	Quaternary
Kaibab	limestone	Permian
Coconino	sandstone	Permian
Hermit	shale	Permian
Supai	sandstone & shale	Permian
UNCONFORMITY
Redwall	limestone	Mississippian
UNCONFORMITY
Temple Butte	limestone	Devonian
UNCONFORMITY
Muav	limestone	Cambrian
Bright Angel	shale	Cambrian
Tapeats	sandstone	Cambrian
THE GREAT UNCONFORMITY
Grand Canyon	sandstone, limestone & shale	Proterozoic
UNCONFORMITY
Zoroaster	igneous (granite)	Archean
Vishnu	metamorphic (schist)	Archean

Alternation of marine and terrestrial deposits implies that sea level changes:

• When sea level rises, water floods over land, new sediment layers deposited = transgression
• When sea level falls, land emerges from water, old layers eroded = regression

Differential weatherability = variable resistance to erosion. Different rocks weather at different rates. More resistant layers form cliffs or steep slopes, less resistant layers form shelves.

Grand Canyon History

Three oroginies = major times of mountain-building, characterized by uplift and deformation

• Earliest one formed the Vishnu Schist & Zoroaster Granite (Early Precambrian)
• Next one formed the tilted GC formations (Late Precambrian)
• Latest one (Laramide orogeny, 70-50 m.y.a.) led to development of the GC itself
  

The cutting of the Grand Canyon

• The Laramide orogeny raised the Rocky Mountains to the east of the Colorado Plateau. Associated uplift continued even more recently.
• Uplift of mountains caused streams (including the Colorado River) to cut their valleys deeper.
• The Gulf of California (a divergent boundary) began spreading < 2 m.y.a.
• Opening of a new outlet to the ocean also caused streams to cut their valleys deeper. The Grand Canyon was eroded to its present depth at this time.
  

Effects of Glen Canyon Dam

• Provides hydroelectric power
• Provides drinking water
• Changed many characteristics of the river:

Characteristic	Before dam	After dam
flow	variable	regular
color	red	clear green
temperature	warm	cold
beaches	stable	eroding
rapids	boulders removed during floods	river choked with boulders

• Controlled flood (March - April 1996)
  • 45,000 cfs for seven days
  • Restored some characteristics to their pre-dam condition
    
    
• Description of controlled flood
• Pictures of controlled flood

Grand Canyon slides

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