Transcript No Slide Title

Chapter 14
Mesozoic Earth History
Nevadan Orogeny and Gold
• Approximately 150 to 210 million years after
– the emplacement of massive plutons created the
Sierra Nevada
• Nevadan orogeny
– gold was discovered at Sutter's Mill
• on the South Fork of the American River at Coloma,
California
• On January 24, 1848, James Marshall,
– a carpenter building a sawmill for John Sutter,
– found bits of the glittering metal in the mill's
tailrace
Gold Rush
• Soon, settlements throughout the state
– were completely abandoned as word
– of the chance for instant riches
– spread throughout California
• Within a year after
– the news of the gold discovery reached the East
Coast,
– the Sutter's Mill area was swarming with more than
80,000 prospectors,
– all hoping to make their fortune
Gold Mining
• By 1852,
– mining
operations were
well underway
– on the
American
River near
Sacramento
Prospecting Was Very Hard Work
• At least 250,000 gold seekers
– worked the Sutter's Mill area,
• and although most were Americans,
– they came from all over the world,
• even as far away as China
• Most of them thought
– the gold was simply waiting to be taken,
– and didn't realize that prospecting
– was very hard work
Shop Owners Made More Money
• No one gave any thought
– to the consequences of so many people converging
on the Sutter's Mill area,
– all intent on making easy money
• In fact, life in the mining camps
– was extremely hard and expensive
• Frequently, the shop owners and traders
– made more money than the prospectors
Abandoning Their Dream
• In reality, only a small percentage of
prospectors
– ever hit it big
– or were even moderately successful
• The rest barely eked out a living
– until they eventually abandoned their dream and
went home
Placer Gold
• Although many prospectors searched for the
mother lode,
– the gold they recovered was mostly in the form of
placer deposits
• deposits of sand and gravel containing gold particles
• large enough to be recovered by panning
• Placer deposits form
– when gold-bearing igneous rocks weather
– and stream transport mechanically separates
minerals
• by density
Gold Panning
• Panning is a common method for recovering
placer deposits
• In this method,
– a shallow pan is dipped into a streambed,
– the material is swirled around
– and the lighter material is poured off
• Gold, being about six times heavier
– than most sand grains and rock chips,
– concentrates on the bottom of the pan
– and can then be picked out
$200 million in gold
• Although some prospectors
–
–
–
–
–
dug $30,000 worth of gold dust a week
out of a single claim
and some gold was found sitting on the surface
most of this easy gold was recovered
very early during the gold rush
• Most prospectors made only a living wage
working their claims
• Nevertheless, during the five years
• from 1848 to 1853
• that constituted the gold rush proper,
– miners extracted more than $200 million in gold
Mesozoic Era
• The Mesozoic Era
– 251 to 66 million years ago
– was an important time in Earth history
• The major geologic event
– was the breakup of Pangaea,
– which affected oceanic and climatic circulation
patterns
– and influenced the evolution of the terrestrial and
marine biotas
Other Mesozoic Events
• Other important Mesozoic geologic events
– resulting from plate movement
• include
– the origin of the Atlantic Ocean basin
– and the Rocky Mountains
– accumulation of vast salt deposits
• that eventually formed salt domes
• adjacent to which oil and natural gas were trapped
– and the emplacement of huge batholiths
• accounting for the origin of various mineral resources
The Breakup of Pangaea
• Just as the formation of Pangaea
– influenced geologic and biologic events
• during the Paleozoic,
– the breakup of this supercontinent
– profoundly affected geologic and biologic events
• during the Mesozoic
• The movement of continents
– affected the global climatic and oceanic regimes
– as well as the climates of the individual continents
Effect of the Breakup
• Populations became isolated
– or were brought into contact
– with other populations,
– leading to evolutionary changes in the biota
• So great was the effect of this breakup
– on the world,
– that it forms the central theme of the Mesozoic
Progress of the Breakup
• The breakup of Pangaea
– began with rifting
– between Laurasia and Gondwana during the
Triassic
• By the end of the Triassic,
– the expanding Atlantic Ocean
– separated North America from Africa
• This change was followed
– by the rifting of North America from South
America
• sometime during the Late Triassic and Early Jurassic
Paleogeography of the World
• During the Triassic Period
Paleogeography of the World
• During the Jurassic Period
Paleogeography of the World
• During the Late Cretaceous Period
Oceans Responded to
Continental Separation
• Separation of the continents
– allowed water from the Tethys Sea
– to flow into the expanding central Atlantic Ocean,
• while Pacific Ocean waters
– flowed into the newly formed Gulf of Mexico,
– which at that time was little more than a restricted
bay
• Thick evaporite deposits formed in these areas
Early Mesozoic Evaporites
• Evaporites
accumulated in
shallow basins
– as Pangaea
broke apart
during the Early
Mesozoic
– Water from the
Tethys Sea
flowed into the
Central Atlantic
Ocean
Early Mesozoic Evaporites
• Water from the
Pacific Ocean
flowed into the
the newly
formed Gulf of
Mexico
• Marine water
from the south
flowed into the
area that would
eventually
become the
southern
Atlantic Ocean
Evaporite Deposits
• During that time, these areas were located
–
–
–
–
–
in the low tropical latitudes
where high temperatures
and high rates of evaporation
were ideal for the formation
of thick evaporite deposits
Further Breakup
• During the Late Triassic and Jurassic periods,
– Antarctica and Australia,
• which remained sutured together,
– began separating from South America and Africa
• Also during this time,
– India began rifting from the Gondwana continent
– and moved nothward
• During the Jurassic,
– South America and Africa began separating
Paleogeography of the World
• During the Jurassic Period
Thick Evaporites from the
Southern Ocean
• The subsequent separation of South America
and Africa
–
–
–
–
formed a narrow basin
where thick evaporite deposits
accumulated from the evaporation
of southern ocean waters
Thick Southern Ocean Evaporites
• Marine
water
flowed into
the
southern
Atlantic
Ocean
from the
south
Tethys Sea
• During this time, the eastern end of the Tethys
Sea
– began closing
– as a result of the clockwise rotation
– of Laurasia and the northward movement of Africa
• This narrow Late Jurassic and Cretaceous
seaway
– between Africa and Europe
– was the forerunner
– of the present Mediterranean Sea
End of the Cretaceous
• By the end of the Cretaceous,
–
–
–
–
Australia and Antarctica had separated,
India was nearly to the equator,
South America and Africa were widely separated,
and Greenland was essentially an independent
landmass
Paleogeography of the World
• During the Late Cretaceous Period
Higher Heat Flow Caused
Sea Level Rise
• A global rise in sea level
– during the Cretaceous
– resulted in worldwide transgressions
– onto the continents
• These transgressions were caused
– by higher heat flow along the oceanic ridges
– caused by increased rifting
– and rapid expansion of oceanic ridges
Middle Cretaceous Sea Level
Was High
• By the Middle Cretaceous,
– sea level was probably as high
– as at any time since the Ordovician,
– and approximately one-third of the present land
area
– was inundated by epeiric seas
Paleogeography of the World
• During the Late Cretaceous Period
Final Stage in Pangaea's Breakup
• The final stage in Pangaea's breakup
– occurred during the Cenozoic
• During this time,
– Australia continued moving northward,
• and Greenland completely separated
– from Europe and North America
– and formed a separate landmass
The Effects on Global Climates
and Ocean Circulation Patterns
• By the end of the Permian Period,
– Pangaea extended from pole to pole,
– covered about one-fourth of Earth's surface,
– and was surrounded by Panthalassa,
• a global ocean that encompassed about 300 degrees of
longitude
• Such a configuration exerted tremendous
influence
– on the world's climate
– and resulted in generally arid conditions
– over large parts of Pangaea's interior
Paleogeography of the World
• For the Late Permian Period
Ocean Currents and Continents
• The world's climates result from the complex
interaction between
– wind and ocean currents
– and the location and topography of the continents
• In general, dry climates occur
–
–
–
–
on large landmasses
in areas remote from sources of moisture
and where barriers to moist air exist,
such as mountain ranges
• Wet climates occur
– near large bodies of water
– or where winds can carry moist air over land
Climate-Sensitive Deposits
• Past climatic conditions can be inferred from
– the distribution of climate-sensitive deposits
• Evaporites are deposited
– where evaporation exceeds precipitation
• While dunes and red beds
– may form locally in humid regions,
– they are characteristic of arid regions
• Coal forms in both warm and cool humid
climates
– Vegetation that is eventually converted into coal
– requires at least a good seasonal water supply
• Thus, coal deposits are indicative of humid
conditions
Evaporites, Red Beds, Dunes, Coal
• Widespread Triassic evaporites, red beds, and
desert dunes
– in the low and middle latitudes
– of North and South America, Europe, and Africa
– indicate dry climates in those regions,
• while coal deposits
– are found mainly in the high latitudes,
– indicating humid conditions
• These high-latitude coals are analogous to
– today's Scottish peat bog
– or Canadian muskeg
Bordering the Tethys Sea
• The lands bordering the Tethys Sea
– were probably dominated by seasonal monsoon
rains
– resulting from the warm, moist winds
– and warm oceanic currents
– impinging against the east-facing coast of
Pangaea
Faster Circulation
• The temperature gradient
– between the tropics and the poles
– also affects oceanic and atmospheric circulation
• The greater the temperature difference
– between the tropics and the poles,
– the steeper the temperature gradient
– and the faster the circulation of the oceans and
atmosphere
Areas Dominated by Seas are Warmer
• Oceans absorb about 90% of the solar radiation
they receive,
– while continents absorb only about 50%,
– even less if they are snow covered
• The rest of the solar radiation is reflected back
into space
• Therefore, areas dominated by seas are warmer
– than those dominated by continents
Oceans Still Quite Warm
• By knowing the distribution of continents and ocean
basins,
–
–
–
–
geologists can generally estimate
the average annual temperature
for any region on Earth,
as well as determining a temperature gradient
• The breakup of Pangaea
–
–
–
–
–
during the Late Triassic
caused the global temperature gradient to increase
because the Northern Hemisphere continents
moved farther northward,
displacing higher-latitude ocean waters
Global Temperature Gradient
• Decrease in temperature in the high latitudes
– and the changing positions of the continents,
– caused the steeper global temperature gradient
• Thus, oceanic and atmospheric circulation
patterns
– greatly accelerated during the Mesozoic
• Though the temperature gradient and
seasonality on land
– were increasing during the Jurassic and Cretaceous,
– the middle- and higher-latitude oceans
– were still quite warm
Equable Worldwide Climate
• Higher-latitude oceans remained warm
– because warm waters from the Tethys Sea
– were circulating to the higher latitudes
• The result was a relatively equable worldwide
climate
– through the end of the Cretaceous
Oceanic Circulation Evolved
• From a simple pattern in a single ocean
(Panthalassa) with a single continent (Pangaea)
Oceanic Circulation Evolved
• to a more complex pattern in the newly formed
oceans of the Cretaceous Period
The Mesozoic History
of North America
• In North America, the beginning of the
Mesozoic Era
– was essentially the same in terms of tectonism and
sedimentation
– as the preceding Permian Period
• Terrestrial sedimentation continued over much
of the craton,
– while block faulting and igneous activity
– began in the Appalachian region
– as North America and Africa began separating
Permian Period
• Paleogeography
of North
America during
the Permian
Period
Triassic Period
• Paleogeography
of North
America during
the Triassic
Period
Gulf of Mexico
• The newly forming Gulf of Mexico
– experienced extensive evaporite deposition
– during the Late Triassic and Jurassic
– as North America separated from South America
Jurassic Period
• Paleogeography
of North
America during
the Jurassic
Period
Global Sea-Level Rise
• A global rise in sea level
– during the Cretaceous
– resulted in worldwide transgressions
– onto the continents such that marine deposition
– was continuous over much of the North American
Cordilleran
Volcanic Island Arc at the
Western Edge of the Craton
• A volcanic island arc system
– that formed off the western edge of the craton
– during the Permian
• was sutured to North America
– sometime during the Permian or Triassic
• This event is referred to as the Sonoma
orogeny
Cordilleran Area
• During the Jurassic,
–
–
–
–
–
–
the entire Cordilleran area
was involved in a series
of major mountain-building episodes
that result in the formation of the Sierra Nevada,
the Rocky Mountains,
and other lesser mountain ranges
• Although each orogenic episode
– has its own name,
– the entire mountain-building event
– is simply called the Cordilleran orogeny
Next, Specific Regions
• Keeping in mind this simplified overview
– of the Mesozoic history of North America,
– we will now examine the specific regions of the
continent
Continental Interior
• Recall that the history of the North American
craton
– can be divided into unconformity-bound sequences
– reflecting advances and retreats of epeiric seas
– over the craton
• Although these transgressions and regressions
– played a major role in the Paleozoic geologic history
of the continent,
– they were not as important during the Mesozoic
Cratonic Sequences of North America
• White areas represent sequences of rocks
• that are
separated
by largescale
unconformities
• shown in
brown
Continental Interior With Inundation
• Cratonic sequences are less important because
–
–
–
–
most of the continental interior
during the Mesozoic
was well above sea level
and did not experience epeiric sea inundation
• As we examine the Mesozoic history
– of the continental margin regions of North America
• we will combine the two cratonic sequences,
– the Absaroka Sequence
• Late Mississippian to Early Jurassic
– and Zuni Sequence
• Early Jurassic to Early Paleocene
Cratonic Sequences of North America
• Absaroka sequence
• Zuni
sequence
Eastern Coastal Region
• During the Early and Middle Triassic,
– coarse detrital sediments derived from the erosion
of the recently uplifted Appalachians
• Alleghenian Orogeny
– filled the various intermontane basins
– and spread over the surrounding areas
• As weathering and erosion continued during
the Mesozoic,
– this once lofty mountain system was reduced to a
low-lying plain
Fault-block Basins
• During the Late Triassic,
– the first stage in the breakup of Pangaea began
– with North America separating from Africa
• Fault-block basins developed
–
–
–
–
in response to upwelling magma
beneath Pangaea
in a zone stretching
from present-day Nova Scotia to North Carolina
Triassic Fault Basins
• Areas where
Triassic faultblock basin
deposits
– crop out in
eastern
North
America
Fault-Block Basins
• After the Appalachians were eroded to a lowlying plain
– by the Middle Triassic,
• fault-block basins formed
– as a result of Late Triassic rifting
– between North America and Africa
Newark Group
• Erosion of the adjacent fault-block mountains
– filled these basins with great quantities
• up to 6000 m
– of poorly sorted red nonmarine detrital sediments
– known as the Newark Group
Down-dropped valleys
accumulated sediments
• Down-dropped valleys accumulated
tremendous thickness of sediments
– and were themselves broken
– by a complex of normal faults during rifting
Reptile Footprints
• Reptiles roamed along the margins
–
–
–
–
of the various lakes and streams
that formed in these basins,
leaving their footprints and trackways
in the soft sediments
• Although the Newark Group rocks contain
numerous dinosaur footprints,
– they are almost completely devoid of dinosaur
bones!
• The Newark Group is mostly Late Triassic,
– but in some areas deposition began in the Early
Jurassic
Reptile Tracks
• Reptile tracks in the Triassic Newark
Group
– were uncovered during the excavation
– for a new state building in Hartford,
Connecticut
• Because the tracks were so spectacular,
– the building side was moved
– and the excavation was designated as a state
park
Reptile Tracks
Igneous Activity
• Concurrent with sedimentation
–
–
–
–
in the fault-block basins
were extensive lava flows
that blanketed the basin floors
as well as intrusions of numerous dikes and sills
• The most famous intrusion
– is the prominent Palisades sill
– along the Hudson River
– in the New York-New Jersey area
Palisades Sill of the Hudson River
• This sill was one of many that were intruded
into the Newark sediments
– during the Late Triassic rifting
– that marked the separation
– of North America from Africa
Passive Continental Margin
• As the Atlantic Ocean grew,
–
–
–
–
rifting ceased along the eastern margin
of North America,
and this once active plate margin
became a passive, trailing continental margin
• The fault-block mountains
– that were produced by this rifting
– continued eroding
• during the Jurassic and Early Cretaceous
– until all that was left was a large low-relief area
Eastern Continental Shelf
• The sediments produced
– by this erosion
– contributed to the growing eastern continental shelf
• During the Cretaceous Period,
–
–
–
–
–
–
the Appalachian region was re-elevated
and once again shed sediments
onto the continental shelf,
forming a gently dipping,
seaward-thickening wedge of rocks
up to 3000 m thick
Seaward-Thickening Wedge
• The seaward-thickening wedge of rocks
–
–
–
–
is currently exposed
in a belt extending from
Long Island, New York,
to Georgia
Gulf Coastal Region
• Paleogeographic
Map of North
America during
the Triassic
Period
• The Gulf Coastal
region was above
sea level until the
Late Triassic
-
Evaporites in Gulf of Mexico
• As North America separated from South
America
– during the Late Triassic and Early Jurassic,
– the Gulf of Mexico began to form
• With oceanic waters flowing into
– this newly formed, shallow, restricted basin,
– conditions were ideal for evaporite formation
• More than 1000 m of evaporites were
precipitated, and
– these Jurassic evaporites are thought to be the source
– for the Paleogene salt domes
– found today in the Gulf of Mexico and southern
Louisiana
Jurassic Period
• Paleogeographic
reconstruction for
the Jurassic
Period
• The Gulf of
Mexico began to
form
– with the
precipitation of
evaporites
Paleogene Salt Domes
Evaporite Deposition Ended
• By the Late Jurassic,
– circulation in the Gulf of Mexico
– was less restricted,
– and evaporite deposition ended
Normal Marine Conditions
• Normal marine conditions
– returned to the area
– with alternating transgressing and regressing seas
• The resulting sediments were
– covered and buried by thousands of meters
– of Cretaceous and Cenozoic sediments
• During the Cretaceous,
– the Gulf Coastal region,
– like the rest of the continental margin,
– was flooded by northward-transgressing seas
Cretaceous Period
• Paleogeography
of North
America during
the Cretaceous
Period
– with its
northwardtransgressing
seas
Transgressions and Regression
• As a result of the transgression,
– nearshore sandstones
– are overlain by finer sediments
– characteristic of deeper waters
• Following an extensive regression
–
–
–
–
at the end of the Early Cretaceous,
a major transgression began
during which a wide seaway extended
from the Arctic Ocean to the Gulf of Mexico
• Sediments that were deposited in the Gulf
Coastal region
– formed a seaward-thickening wedge
Cretaceous Period
• Paleogeography
of North
America during
the Cretaceous
Period
• Cretaceous
Interior Seaway
Cretaceous Bivalve Reefs
• Reefs were also widespread
– in the Gulf Coastal region during the Cretaceous
• Bivalves called rudists
– were the main constituent
– of many of these reefs
• Because of their high porosity and
permeability,
– rudistoid reefs make excellent petroleum reservoirs
– A good example of a Cretaceous reef complex
occurs in Texas
Gulf Shelf-Margin Facies
• Early Cretaceous
shelf-margin
facies around the
Gulf of Mexico
Basin
• The reef trend
shows as a black
line
Reef Environments
• Depositional environment and facies changes
across the Stuart City reef trend, South Texas
Rudist Reef Facies Patterns
• Here the reef trend
– had a strong influence
– on the carbonate platform deposition of the region
• The facies patterns of these carbonate rocks
– are as complex as those found
– in the major barrier-reef systems
– of the Paleozoic Era
Western Region—
Mesozoic Tectonics
• The Mesozoic geologic history
–
–
–
–
–
of the North American Cordilleran mobile belt
is very complex,
involving the eastward subduction
of the oceanic Farallon plate
under the continental North American plate
• Activity along this oceanic-continental
convergent plate boundary,
– resulted in an eastward movement of deformation
Cordilleran Orogenic Activity
• This orogenic activity
– progressively affected
– the trench and continental slope, the continental shelf,
and the cratonic margin,
– causing a thickening of the continental crust
• The accretion of terranes and microplates
– played a significant role in this area
Sonoma Orogeny
• Except for the Late Devonian-Early Mississippian
Antler orogeny,
– the Cordilleran region of North America experienced
little tectonism during the Paleozoic
• During the Permian, however, an island arc and
ocean basin formed
– off the western North American craton
– followed by subduction of an oceanic plate
• beneath the island arc
– and the thrusting of oceanic and island arc rocks
– eastward against the craton margin
Sonoma Orogeny
• This event, known as the Sonoma orogeny,
– occurred at or near the Permian-Triassic boundary
• Like the Antler orogeny,
– it resulted in the suturing of island-arc terranes
– along the western edge of North America.
Triassic Period
• Paleogeography
of North
America during
the Triassic
Period
– with a
volcanic
island arc in
the west
Sonoma Orogeny
• Tectonic activity that culminated
– in the Permian-Triassic Sonoma orogeny
• in western
North
America
– was the result
of a collision
– between the
southwestern
margin of
North America
– and an island
arc system
Oceanic-Continental Convergent
Plate Boundary
• Following the Late Paleozoic-Early Mesozoic
–
–
–
–
destruction of the volcanic island arc
during the Sonoma orogeny,
the western margin of North America
became an oceanic-continental convergent plate
boundary
Steeply Dipping Subduction Zone
• During the Late Triassic,
–
–
–
–
a steeply dipping subduction zone developed
along the western margin of North America
in response to the westward movement
of North America over the Farallon plate
• This newly created oceanic-continental plate
boundary
–
–
–
–
–
controlled Cordilleran tectonics
for the rest of the Mesozoic Era
and for most of the Cenozoic Era
This subduction zone marks the beginning
of the modern circum-Pacific orogenic system
Cordilleran Orogeny
• The general term Cordilleran orogeny
– is applied to the mountain-building activity
– that began during the Jurassic
– and continued into the Cenozoic
• The Cordilleran orogeny
– consisted of a series
– of individual named, but interrelated, mountainbuilding events
– that occurred in different regions at different times
– but overlapped to some extent
Cordilleran
Mobile Belt
• Mesozoic orogenies
– occurring in the
Cordilleran mobile belt
Cordilleran Orogeny
• Most of this Cordilleran orogenic activity
– is related to the continued westward movement of
the North American plate
– as it overrode the Farallon plate
– and its history is highly complex
Nevadan Orogeny
• The first phase of the Cordilleran orogeny,
– the Nevadan orogeny,
– began during the Mid to Late Jurassic
– and continued into the Cretaceous
• During the Middle to Late Jurassic,
– two subduction zones, dipping in opposite directions,
– formed at the western margin of North America.
• As the North American plate moved westward,
–
–
–
–
as a result of the opening of the Atlantic Ocean,
it soon overrode the westerly subduction zone
leaving only the easterly dipping subduction zone
along its western periphery
Cordilleran
Mobile Belt
• Mesozoic orogenies
– occurring in the
Cordilleran mobile belt
Nevadan Orogeny
• As the easterly dipping ocean crust
–
–
–
–
continued to be subducted,
large volumes of granitic magma
were generated at depth
beneath the western edge of North America
• These granitic masses
– ascended as huge batholiths
– that are now recognized as
– the Sierra Nevada, Southern California, Idaho, and Coast
Range batholiths
• At this time, the Franciscan Complex and Great
Valley Group were deposited and deformed
Batholiths
• Location of Jurassic and
Cretaceous batholiths
– in western North
America
Franciscan Complex
• The Franciscan Complex,
–
–
–
–
which is up to 7000 m thick,
is an unusual rock unit
consisting of a chaotic mixture of rocks
that accumulated during the Late Jurassic and
Cretaceous
• The various rock types include
– graywacke, volcanic breccia, siltstone, black shale,
– chert, pillow basalt, and blueschist metamorphic rocks
Franciscan Complex
• The rock types suggest
– that continental shelf, slope, and deep-sea
environments
– were brought together
– in a submarine trench
– when North America overrode the subducting
Farallon plate
Franciscan Complex
• Map showing the
location of the
Franciscan Complex
Depositional Environment
• Reconstruction of the depositional environment
– of the Franciscan Complex
– during the Late Jurassic and Cretaceous periods
Franciscan Complex
• Bedded chert exposed in Marin County,
California
• Most of the layers are about 5 cm thick.
Great Valley Group
• The Franciscan Complex and Great Valley
Group
– that lies east of it
– were both squeezed against the edge of the North
American craton
– as a result of subduction of the Farallon plate
– beneath the North America plate.
• The Franciscan Complex and the Great Valley
Group
– are currently separated
– by a major thrust fault
Great Valley Group
• The Great Valley Group consists of
– more than 16,000 m
– of conglomerates, sandstones, siltstones, and shales
• These sediments were deposited
– on the continental shelf and slope
– at the same time the Franciscan deposits
– were accumulating in the submarine trench
Great Valley Group Environment
• Environments of the Great Valley Group
– in relation to the Franciscan Complex
Plutonic Activity
Migrated Eastward
• By the Late Cretaceous,
– most of the volcanic and plutonic activity
– had migrated eastward into Nevada and Idaho
• This migration was probably caused
– by a change from high-angle to low-angle
subduction,
– resulting in the subducting oceanic plate
– reaching its melting depth farther east
Eastward Migrating
• A possible
cause
– for the
eastward
migration
– of Cordilleran
igneous
activity
– during the
Cretaceous
– was a change
from high
angle
subduction to
Lower-Angle Subduction
• to low-angle
subduction
• As the
subducting
plate
– moved
downward
– at a lower
angle,
– its melting
depth
– moved farther
to the east
Sevier Orogeny
• Thrusting occurred progressively farther east
– so that by the Late Cretaceous,
– it extended all the way
– to the Idaho-Washington border
• The second phase of the Cordilleran orogeny,
–
–
–
–
–
the Sevier orogeny,
was mostly a Cretaceous event
although it began in the Late Jurassic
and is associated with the tectonic activity
of the Nevadan orogeny
Cordilleran
Mobile Belt
• Mesozoic orogenies
– occurring in the
Cordilleran mobile belt
Thrust Faults
• Subduction of the Farallon plate
– beneath the North American plate during this time
– resulted in numerous overlapping,
– low-angle thrust faults
• As compressional forces generated in the subduction
zone
–
–
–
–
were transmitted eastward,
numerous blocks of older Paleozoic strata
were thrust eastward
on top of younger strata
• This deformation resulted in crustal shortening
– and produced generally north-south-trending mountain
ranges
Sevier Orogeny
• Associated tectonic features
– of the Late
Cretaceous
Sevier orogeny
– caused by
subduction of
the Farallon
plate
– under the North
American plate
Keystone Thrust Fault
Keystone Thrust Fault
• The Keystone thrust fault is a major fault
in the Sevier overthrust belt
– It is exposed west of Las Vegas, Nevada
• The sharp boundary
– between the light-colored Mesozoic rocks
– and the overlying dark-colored Paleozoic
rocks
• marks the trace of the Keystone thrust
fault
Keystone Thrust Fault
Laramide orogeny
• During the Late Cretaceous to Early Cenozoic,
– the final pulse of the Cordilleran orogeny occurred
• The Laramide orogeny
– developed east of the Sevier orogenic belt
– in the present-day Rocky Mountain areas
– of New Mexico, Colorado, and Wyoming
Cordilleran
Mobile Belt
• Mesozoic orogenies
– occurring in the
Cordilleran mobile belt
Present-Day Rocky Mountains
• Most of the features
– of the present-day Rocky Mountains
– resulted from the Cenozoic phase
– of the Laramide orogeny
Mesozoic Sedimentation
• Concurrent with the tectonism
– in the Cordilleran mobile belt,
• Early Triassic sedimentation
– on the western continental shelf
– consisted of shallow-water marine
– sandstones, shales, and limestones
• During the Middle and Late Triassic,
– the western shallow seas
– regressed farther west,
– exposing large areas of former seafloor to erosion
Marine and Nonmarine
Triassic Rocks
• Marginal marine and nonmarine Triassic rocks,
– particularly red beds,
– contribute to the spectacular
– and colorful scenery of the region
• The Lower Triassic Moenkopi Formation
– of the southwestern United States
– consists of a succession of brick-red
– and chocolate-colored mudstones
Triassic and Jurassic Formations
• Triassic and
Jurassic formations
in the western
United States
Sedimentary Structures
• Such sedimentary structures
– as desiccation cracks and ripple marks,
• as well as fossil amphibians and reptiles and their tracks,
– indicate deposition in a variety of continental
environments,
• including stream channels, floodplains, and fresh and
brackish water ponds
• Thin tongues of marine limestones
– indicate brief incursions of the sea,
– while local beds with gypsum and halite crystal
casts
– attest to a rather arid climate
Shinarump and Chinle
• Unconformably overlying the Moenkopi
– is the Upper Triassic Shinarump Conglomerate,
– a widespread unit generally less than 50 m thick
• Above the Shinarump
– are the multicolored shales, siltstones, and
sandstones
– of the Upper Triassic Chinle Formation
• This formation is widely exposed
– throughout the Colorado Plateau
– and is probably most famous for its petrified wood,
– spectacularly exposed in Petrified Forest National
Park, Arizona
Triassic and Jurassic Formations
• Triassic and
Jurassic formations
in the western
United States
Petrified Wood and Plants Fossils
• Whereas fossil ferns are found here,
–
–
–
–
the park is best known for
its abundant and beautifully preserved logs
of gymnosperms, especially conifers
and plants called cycads
• Fossilization resulted from the silicification of
the plant tissues
• Weathering of volcanic ash beds
– interbedded with fluvial and deltaic Chinle
sediments
– provided most of the silica for silicification
Cycads
Fossilization
• Some trees were preserved in place,
– but most were transported during floods
– and deposited on sandbars
– and on floodplains,
– where fossilization took place
• After burial, silica-rich groundwater
– percolated through the sediments
– and silicified the wood
Other Fossils
• Although best known for its petrified wood, the
Chinle Formation has also yielded fossils of
– labyrinthodont amphibians,
– phytosaurs,
– and small dinosaurs
• Palynologic studies show similar assemblages
of pollen
– from the Chinle and Lower Newark Group
– indicating that they are the same age
Upward in the Stratigraphy
• Early Jurassic-age deposits in large part of the western
region
– consist mostly of clean, cross-bedded sandstones
– indicative of windblown deposits
• The lowermost unit is the Wingate Sandstone,
– a desert dune deposit,
– which if overlain by the Kayenta Formation,
– a stream and lake deposit,
• These two formations are well exposed
– in southwestern Utah
Triassic and Jurassic Formations
• Triassic and
Jurassic formations
in the western
United States
Early Jurassic Sandstones
• The thickest and most prominent of the
Jurassic cross-bedded sandstones
– is the Navajo Sandstone,
• a widespread formation
– that accumulated in a coastal dune environment
– along the southwestern margin of the craton
Navajo Sandstone, Zion Canyon
Navajo Sandstone, Zion Canyon
• View of East Entrance of Zion Canyon, Zion
National Park, Utah
• The light-colored massive rocks
– are the Jurassic Navajo Sandstone
• while the slope-forming rocks below the
Navajo
– are the Lower Jurassic Kayenta Formation
Navajo Sandstone, Zion Canyon
Navajo Sandstone's
Large-Scale Cross-Beds
• The Navajo Sandstone's most distinguishing
feature
– is its large-scale cross-beds,
– some of which are more than 25 m high
Navajo Sandstone
• Large cross-beds of the Jurassic Navajo
Sandstone in Zion National Park, Utah
Sundance Sea
• The upper part of the Navajo
– contains smaller cross-beds
– as well as dinosaur and crocodilian fossils
• Marine conditions returned to the region
– during the Middle Jurassic
– when a seaway called the Sundance Sea
– twice flooded the interior of western North
America
Sundance Sea
• The resulting
deposits,
– the Sundance
Formation,
– were produced
from erosion
– of tectonic
highlands to
the west
– that paralleled
the shoreline
Sundance Sea Retreated Northward
• These highlands
– resulted from intrusive igneous activity
– and associated volcanism
– that began during the Triassic
• During the Late Jurassic,
–
–
–
–
a mountain chain formed
in Nevada, Utah, and Idaho
as a result of the deformation
produced by the Nevadan orogeny
• As the mountain chain grew
– and shed sediments eastward,
– the Sundance Sea began retreating northward
Morrison Formation
• A large part of the area
– formerly occupied by the Sundance Sea
– was then covered
– by multicolored sandstones, mudstones, shales, and
occasional lenses of conglomerates
– that comprise the world-famous Morrison
Formation
• The Morrison Formation
– contains the world's richest assemblage
– of Jurassic dinosaur remains
Morrison Formation
• View of the Jurassic Morrison Formation
– from the Visitors’ center
– at Dinosaur National Monument, Utah
Skeletons Deposited on Sandbars
• Although most of the dinosaur skeletons
– are broken up,
– as many as 50 individuals
– have been found together in a small area
• Such a concentration indicates
– that the skeletons were brought together
– during times of flooding and deposited on sandbars
• in stream channels
• Soils in the Morrison indicate
– that the climate was seasonably dry
Dinosaur National Monument
• Although most major museums have either
– complete dinosaur skeletons
– or at least bones from the Morrison Formation,
– the best place to see the bones still embedded in the
rocks
– is the visitors' center at Dinosaur National
Monument near Vernal, Utah
• The north wall of the visitors’ center
– shows dinosaur bones in bas relief
– just as they were deposited 140 million years ago
North Wall
Mid-Cretaceous Transgressions
• Shortly before the end of the Early Cretaceous,
– Arctic waters spread southward
– over the craton, forming a large inland sea
– in the Cordilleran foreland basin area
• Mid-Cretaceous transgressions
–
–
–
–
–
also occurred on other continents,
and all were part of the global mid-Cretaceous
rise in sea level
that resulted from accelerated seafloor spreading
as Pangaea continued to fragment
Black Shale Deposition
• Middle Cretaceous transgressions are marked
– by widespread black shale deposition
– within oceanic areas,
– the shallow sea shelf areas,
– and the continental regions
• that were inundated by the transgressions.
Cretaceous Interior Seaway
• By the beginning of the Late Cretaceous,
– this incursion
– joined the northward-transgressing waters from the
Gulf area
– to create an enormous Cretaceous Interior Seaway
– that occupied the area east of the Sevier orogenic
belt
Cretaceous Interior Seaway
• Extending from the Gulf of Mexico
– to the Arctic Ocean
– and more than 1500 km wide at its maximum
extent,
• this seaway
– effectively divided North America
– into two large landmasses
– until just before the end of the Late Cretaceous
Cretaceous Interior Seaway
• Paleogeography
of North
America during
the Cretaceous
Period
• Cretaceous
Interior Seaway
Cretaceous Deposits
• Cretaceous deposits
– less than 100 m thick indicate
– that the eastern margin of the Cretaceous Interior
Seaway
– subsided slowly
– and received little sediment
– from the emergent, low-relief craton to the east
• The western shoreline, however,
–
–
–
–
shifted back and forth,
primarily in response to fluctuations
in the supply of sediment
from the Cordilleran Sevier orogenic belt to the
west
Facies Relationships
• The facies relationships
– show lateral changes
– from conglomerate and coarse sandstone adjacent
to the mountain belt
– through finer sandstones, siltstones, shales,
– and even limestones and chalks in the east
• During times of particularly active mountain
building,
– these coarse clastic wedges of gravel and sand
– prograded even further east
Cretaceous Facies Related to Sevier
• This restored west-east cross section
– of Cretaceous facies of the western Cretaceous
Interior Seaway
– shows the facies relationship to the Sevier orogenic
belt
Cretaceous Interior Seaway
• As the Mesozoic Era ended,
• the Cretaceous Interior Seaway
– withdrew from the craton.
• During the regression,
–
–
–
–
marine waters retreated to the north and south,
and marginal marine and continental deposition
formed widespread coal-bearing deposits
on the coastal plain.
Accretion of Terranes
• Orogenies along convergent plate boundaries
– resulted in continental accretion
• Much of the material accreted to continents
– during such events is simply eroded older
continental crust,
• but a significant amount of new material
– is added to continents
– such as igneous rocks that formed as a consequence
– of subduction and partial melting
Accretion of Terranes
• Although subduction
– is the predominant influence
– on the tectonic history
– in many regions of orogenesis,
• other processes are also involved
– in mountain building
– and continental accretion,
– especially the accretion of terranes
Terranes
• Geologists now know that portions of many
mountain systems
– are composed of small accreted lithospheric blocks
– that are clearly of foreign origin
• These terranes
–
–
–
–
–
–
differ completely in their fossil content,
stratigraphy, structural trends,
and paleomagnetic properties
from the rocks
of the surrounding mountain system
and adjacent craton
Accretion of Terranes
• In fact, terranes are so different from adjacent
rocks
–
–
–
–
–
that most geologists think they formed elsewhere
and were carried great distances
as parts of other plates
until they collided
with other terranes or continents
• Geologic evidence indicates
–
–
–
–
that more than 25%
of the entire Pacific Coast
from Alaska to Baja California
consists of accreted terranes
Accretion of Terranes
• The accreting terranes
–
–
–
–
–
–
are composed of volcanic island arcs,
oceanic ridges,
seamounts,
volcanic plateaus,
hot spot tracks,
and small fragments of continents
• that were scraped off and accreted
– to the continent's margin
– as the oceanic plate with which they were carried
– was subducted under the continent
More Than 100 Terranes
• It is estimated that more than 100 differentsized terranes
– have been added to the western margin
– of North America
– during the last 200 million years
• Good examples of this
– are the Wrangellian terranes
– which have been accreted
– to North America's western margin
Terranes of Western
North America
• Some of the accreted
lithospheric blocks
– called terranes
– that form the western margin
– of the North American Craton
• The dark brown blocks
– probably originated as terranes
– and were accreted to North
America
Terranes of Western
North America
• The light green blocks
– are possibly displaced parts of
North America
• Dark green
– represents the North
American craton
Growth along Active Margins
• The basic plate tectonic reconstruction
– of orogenies and continental accretion
– remains unchanged,
• but the details of such reconstructions
– are decidedly different
– in view of terrane tectonics
• For example, growth along active continental
margins
– is faster than along passive continental margins
– because of the accretion of terranes
New Additions
• Furthermore, these accreted microplates
– are often new additions to a continent,
– rather than reworked older continental material
• So far, most terranes
–
–
–
–
have been identified in mountains
of the North American Pacific Coast region,
but a number of such plates are suspected
to be present in other mountain systems as well
• They are more difficult to recognize in older
mountain systems,
– such as the Appalachians, however,
– because of greater deformation and erosion
Terranes
• Thus, terranes
–
–
–
–
provide another way
of viewing Earth
and gaining a better understanding
of the geologic history of the continents
Mesozoic Mineral Resources
• Although much of the coal in North America
– is Pennsylvanian or Paleogene in age,
– important Mesozoic coals
– occur in the Rocky Mountains states
• These are mostly lignite and bituminous coals,
– but some local anthracites are present as well
• Particularly widespread in western North
American
– are coals of Cretaceous age
• Mesozoic coals are also known
– from Australia, Russia, and China
Petroleum in Gulfs
• Large concentrations of petroleum
– occur in many areas of the world,
– but more than 50% of all proven reserves
– are in the Persian Gulf region
• During the Mesozoic Era,
– what is now the Gulf region
– was a broad passive continental margin
– conducive for the formation of petroleum
• Similar conditions existed in what is now the
Gulf Coast region
– of the United States and Central America
Gulf Coast Region
• Here, petroleum and natural gas
– also formed on a broad shelf
– over which transgressions and regressions occurred
• In this region, the hydrocarbons
–
–
–
–
are largely in reservoir rocks
that were deposited
as distributary channels on deltas
and as barrier-island and beach sands
• Some of these hydrocarbons are associated
– with structures formed adjacent to rising salt domes
Louann Salt
• The salt, called the Louann Salt,
– initially formed in a long, narrow sea
– when North America separated from Europe and
North Africa
– during the fragmentation of Pangaea
Salt
• Salt
deposits
in the
Gulf of
Mexico
• formed
during the
initial
opening
of the
Atlantic
Uranium Ores
• The richest uranium ores in the United States
– are widespread in Mesozoic rocks
– of the Colorado Plateau area of Colorado
– and adjoining parts of Wyoming, Utah, Arizona,
and New Mexico
• These ores, consisting of fairly pure masses
– of a complex potassium-, uranium-, vanadiumbearing mineral
• called carnotite,
– are associated with plant remains in sandstones
– that were deposited in ancient stream channels
Mesozoic Iron Ores
• Proterozoic banded iron formations
– are the main sources of iron ores
• Exceptions exist such as
– the Jurassic-age "Minette" iron ores of Western
Europe,
– which are composed of oolitic limonite and hematite,
– and are important ores in France, Germany, Belgium,
and Luxembourg
• In Great Britain, low-grade Jurassic iron ores
– consist of oolitic siderite, which is an iron carbonate
• In Spain, Cretaceous rocks are the host rocks for
iron minerals
Kimberlite Pipes
• South Africa,
– the world's leading producer of gem-quality
diamonds
– and among the leaders in industrial diamond
production,
– mines these minerals from conical igneous
intrusions
• called kimberlite pipes
– Kimberlite pipes
• are composed of dark gray or blue igneous rock known
as kimberlite
Cretaceous Kimberlite Pipes
• Diamonds,
• which form at great depth where pressure and
temperature are high,
– are brought to the surface
• during the explosive volcanism
• that forms kimberlite pipes
• Although kimberlite pipes have formed
throughout geologic time,
–
–
–
–
the most intense episode
of such activity in South Africa
and adjacent countries
was during the Cretaceous Period
Mother Lode
• Emplacement of Triassic and Jurassic
– diamond-bearing kimberlites
– also occurred in Siberia
• The mother lode
• or source for the placer deposits mined during the
California gold rush
– is in Jurassic-age intrusive rocks of the Sierra
Nevada
• Gold placers are also known in Cretaceous-age
conglomerates
– of the Klamath Mountains of California and
Oregon
Porphyry Copper
• Porphyry copper was originally named
– for copper deposits in the western United States
– mined from porphyritic granodiorite,
– but the term now applies to large, low-grade copper
deposits
– disseminated in a variety of rocks
• These porphyry copper deposits
– are an excellent example of the relationship
– between convergent plate boundaries
– and the distribution, concentration, and exploitation
of valuable metallic ores
Origin of Porphyry Copper
• Magma generated by partial melting
– of a subducting plate
– rises toward the surface,
– and as it cools, it precipitates and concentrates
various metallic ores
• The world's largest copper deposits
– were formed during the Mesozoic and Cenozoic
– in a belt along the western margins
– of North and South America
Plate Tectonics and the Distribution
of Natural Resources
• Magma generated by subduction can create this
activity
– Bingham Mine in Utah is a
– Example: copper
huge open-pit copper mine
deposits in
western Americas