EVOLUTION OF THE CONTINENTS
Evolution of the Continents
- Continents are made up of
stable interior, orogenic belts, and coastal plain
- orogeny - mountain building
- Orogenic belts consist of
deformed sedimentary, igneous and metamorphic rock
(Fig. 21-1)
- Stable interior - Craton
- no tectonic activity for
a long time
- Composed of:
- Shield
- Precambrian rocks at or near
surface
- form nuclei of continents
(see transparency)
- Platform
- veneer of relatively undeformed
sedimentary rocks covering shield
- Platform deposits record
sea level change
- flooding of continent - transgression
- marine sandstones, limestones,
shales
- epicontinental seas
- retreat of sea - regression
- fluvial (river) sandstones,
shales; coal
- Orogenic Belts
- younger deformed rock nearer
to margins of continent
- form from continental collision
- folding, faulting
- stacking of thrust sheets (see
transparency)
Major Structural Features of North
America
- Canadian Shield (Fig.
21-1)
- Canadian shield dominated
by granitic and high-grade metamorphic rocks
- evidence for ancient orogeny
- composite of many smaller
blocks (former plates)
(see transparency)
- Central stable region
- platform covered with thick
accumulations of relatively undeformed Paleozoic sedimentary rock
- underlain by shield rocks
- sedimentary rocks - cycles
of shales, limestones, sandstones, coal
- rocks reflect transgression
and regression
- Appalachian mountain belt
- Orogenic belt
- Late Precambrian and Paleozoic
mountain-building associated with subduction and continental collision
- Cordilleran mountain belt
(Fig. 21-10)
- extends from Alaska to Guatemala
- includes mountain belts:
Coast Ranges, Sierra Nevadas, Rocky Mountains
- Includes Basin-Range Province
(Nevada)
- Later Mesozoic and early
Tertiary orogeny plus later rejuvenation
- Colorado Plateau
- "Island" of central
stable region relatively unaffected by Cordilleran mountain-building
except for uplift
- includes Grand Canyon
- Gulf and Atlantic Coastal
Plain
- Gently dipping platform
- Relatively thick accumulations
of later Mesozoic and Tertiary sediments
- Alternating shales and sandstones
with some lignite