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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

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Chapter 14

Mesozoic Earth History


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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


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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


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Gold Mining

  • By 1852,

    • mining operations were well underway

    • on the American River near Sacramento


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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


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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


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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


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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


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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


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$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


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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


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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


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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


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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


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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


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Paleogeography of the World

  • During the Triassic Period


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Paleogeography of the World

  • During the Jurassic Period


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Paleogeography of the World

  • During the Late Cretaceous Period


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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


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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


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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


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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


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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


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Paleogeography of the World

  • During the Jurassic Period


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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


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Thick Southern Ocean Evaporites

  • Marine water flowed into the southern Atlantic Ocean from the south


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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


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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


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Paleogeography of the World

  • During the Late Cretaceous Period


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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


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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


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Paleogeography of the World

  • During the Late Cretaceous Period


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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


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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


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Paleogeography of the World Patterns

  • For the Late Permian Period


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Ocean Currents and Continents Patterns

  • 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


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Climate-Sensitive Deposits Patterns

  • 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


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Evaporites, Red Beds, Dunes, Coal Patterns

  • 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


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Bordering the Tethys Sea Patterns

  • 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


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Faster Circulation Patterns

  • 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


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Areas Dominated by Seas Patternsare 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


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Oceans Still Quite Warm Patterns

  • 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


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Global Temperature Gradient Patterns

  • 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


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Equable Worldwide Climate Patterns

  • 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


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Oceanic Circulation Evolved Patterns

  • From a simple pattern in a single ocean (Panthalassa) with a single continent (Pangaea)


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Oceanic Circulation Evolved Patterns

  • to a more complex pattern in the newly formed oceans of the Cretaceous Period


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The Mesozoic History Patternsof 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


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Permian Period Patterns

  • Paleogeography of North America during the Permian Period


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Triassic Period Patterns

  • Paleogeography of North America during the Triassic Period


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Gulf of Mexico Patterns

  • The newly forming Gulf of Mexico

    • experienced extensive evaporite deposition

    • during the Late Triassic and Jurassic

    • as North America separated from South America


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Jurassic Period Patterns

  • Paleogeography of North America during the Jurassic Period


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Global Sea-Level Rise Patterns

  • 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


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Volcanic Island Arc at the Western Edge of the Craton Patterns

  • 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


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Cordilleran Area Patterns

  • 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


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Next, Specific Regions Patterns

  • Keeping in mind this simplified overview

    • of the Mesozoic history of North America,

    • we will now examine the specific regions of the continent


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Continental Interior Patterns

  • 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


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Cratonic Sequences of North America Patterns

  • White areas represent sequences of rocks

  • that are separated by large-scale uncon-formities

  • shown in brown


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Continental Interior With Inundation Patterns

  • 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


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Cratonic Sequences of North America Patterns

  • Zuni sequence

  • Absaroka sequence


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Eastern Coastal Region Patterns

  • 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


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Fault-block Basins Patterns

  • 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


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Triassic Fault Basins Patterns

  • Areas where Triassic fault-block basin deposits

    • crop out in eastern North America


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Fault-Block Basins Patterns

  • After the Appalachians were eroded to a low-lying plain

    • by the Middle Triassic,

  • fault-block basins formed

    • as a result of Late Triassic rifting

    • between North America and Africa


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Newark Group Patterns

  • 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


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Down-dropped valleys accumulated sediments Patterns

  • Down-dropped valleys accumulated tremendous thickness of sediments

    • and were themselves broken

    • by a complex of normal faults during rifting


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Reptile Footprints Patterns

  • 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


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Reptile Tracks Patterns

  • 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


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Reptile Tracks Patterns


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Igneous Activity Patterns

  • 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


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Palisades Sill of the Hudson River Patterns

  • 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


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Passive Continental Margin Patterns

  • 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


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Eastern Continental Shelf Patterns

  • The sediments produced

    • by this erosion

    • contributed to the growing eastern continental shelf

  • During the Cretaceous Period,

    • the Appalachian region was reelevated

    • and once again shed sediments

    • onto the continental shelf,

    • forming a gently dipping,

    • seaward-thickening wedge of rocks

    • up to 3000 m thick


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Seaward-Thickening Wedge Patterns

  • The seaward-thickening wedge of rocks

    • is currently exposed

    • in a belt extending from

    • Long Island, New York,

    • to Georgia


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Gulf Coastal Region Patterns

  • Paleogeographic Map of North America during the Triassic Period

  • The Gulf Coastal region was above sea level until the Late Triassic

    -


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Evaporites in Gulf of Mexico Patterns

  • 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


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Jurassic Period Patterns

  • Paleogeographic reconstruction for the Jurassic Period

  • The Gulf of Mexico began to form

    • with the precipitation of evaporites



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Evaporite Deposition Ended Patterns

  • By the Late Jurassic,

    • circulation in the Gulf of Mexico

    • was less restricted,

    • and evaporite deposition ended


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Normal Marine Conditions Patterns

  • 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


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Cretaceous Period Patterns

  • Paleogeography of North America during the Cretaceous Period

    • with its northward-transgressing seas


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Transgressions and Regression Patterns

  • 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


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Cretaceous Period Patterns

  • Paleogeography of North America during the Cretaceous Period

  • Cretaceous Interior Seaway


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Cretaceous Bivalve Reefs Patterns

  • 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


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Gulf Shelf-Margin Facies Patterns

  • Early Cretaceous shelf-margin facies around the Gulf of Mexico Basin

  • The reef trend shows as a black line


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Reef Environments Patterns

  • Depositional environment and facies changes across the Stuart City reef trend, South Texas


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Rudist Reef Facies Patterns 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


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Western Region— PatternsMesozoic 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


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Cordilleran Orogenic Activity Patterns

  • 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


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Sonoma Orogeny Patterns

  • 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


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Sonoma Orogeny Patterns

  • 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.


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Triassic Period Patterns

  • Paleogeography of North America during the Triassic Period

    • with a volcanic island arc in the west


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Sonoma Orogeny Patterns

  • in western North America

  • was the result of a collision

  • between the southwestern margin of North America

  • and an island arc system

    • Tectonic activity that culminated

      • in the Permian-Triassic Sonoma orogeny


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    Oceanic-Continental Convergent Plate Boundary Patterns

    • 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


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    Steeply Dipping Subduction Zone Patterns

    • 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


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    Cordilleran Orogeny Patterns

    • 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, mountain-building events

      • that occurred in different regions at different times

      • but overlapped to some extent


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    Cordilleran PatternsMobile Belt

    • Mesozoic orogenies

      • occurring in the Cordilleran mobile belt


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    Cordilleran Orogeny Patterns

    • 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


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    Nevadan Orogeny Patterns

    • 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


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    Cordilleran PatternsMobile Belt

    • Mesozoic orogenies

      • occurring in the Cordilleran mobile belt


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    Nevadan Orogeny Patterns

    • 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 ComplexandGreat Valley Group were deposited and deformed


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    Batholiths Patterns

    • Location of Jurassic and Cretaceous batholiths

      • in western North America


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    Franciscan Complex Patterns

    • 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


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    Franciscan Complex Patterns

    • 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


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    Franciscan Complex Patterns

    • Map showing the location of the Franciscan Complex


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    Depositional Environment Patterns

    • Reconstruction of the depositional environment

      • of the Franciscan Complex

      • during the Late Jurassic and Cretaceous periods


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    Franciscan Complex Patterns

    • Bedded chert exposed in Marin County, California

    • Most of the layers are about 5 cm thick.


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    Great Valley Group Patterns

    • 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


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    Great Valley Group Patterns

    • 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


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    Great Valley Group Environment Patterns

    • Environments of the Great Valley Group

      • in relation to the Franciscan Complex


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    Plutonic Activity PatternsMigrated 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


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    Eastward Migrating Patterns

    • A possible cause

      • for the eastward migration

      • of Cordilleran igneous activity

      • during the Cretaceous

      • was a change from high angle subduction to


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    Lower-Angle Subduction Patterns

    • to low-angle subduction

    • As the subducting plate

      • moved downward

      • at a lower angle,

      • its melting depth

      • moved farther to the east


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    Sevier Orogeny Patterns

    • 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


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    Cordilleran PatternsMobile Belt

    • Mesozoic orogenies

      • occurring in the Cordilleran mobile belt


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    Thrust Faults Patterns

    • 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


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    Sevier Orogeny Patterns

    • of the Late Cretaceous Sevier orogeny

    • caused by subduction of the Farallon plate

    • under the North American plate

    • Associated tectonic features



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    Keystone Thrust Fault Patterns

    • The Keystone thrust fault is a major fault in the Sevier overthrust belt

      • It is exposed west of Las Vagas, 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



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    Laramide orogeny Patterns

    • 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


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    Cordilleran PatternsMobile Belt

    • Mesozoic orogenies

      • occurring in the Cordilleran mobile belt


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    Present-Day Rocky Mountains Patterns

    • Most of the features

      • of the present-day Rocky Mountains

      • resulted from the Cenozoic phase

      • of the Laramide orogeny


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    Mesozoic Sedimentation Patterns

    • 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


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    Marine and Nonmarine PatternsTriassic 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


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    Triassic and Jurassic Formations Patterns

    • Triassic and Jurassic formations in the western United States


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    Sedimentary Structures Patterns

    • 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


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    Shinarump and Chinle Patterns

    • 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


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    Triassic and Jurassic Formations Patterns

    • Triassic and Jurassic formations in the western United States


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    Petrified Wood and Plants Fossils Patterns

    • 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


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    Cycads Patterns


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    Fossilization Patterns

    • 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


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    Other Fossils Patterns

    • 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


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    Upward in the Stratigraphy Patterns

    • 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


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    Triassic and Jurassic Formations Patterns

    • Triassic and Jurassic formations in the western United States


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    Early Jurassic Sandstones Patterns

    • 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



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    Navajo Sandstone, Zion Canyon Patterns

    • 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



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    Navajo PatternsSandstone's Large-Scale Cross-Beds

    • The NavajoSandstone's most distinguishing feature

      • is its large-scale cross-beds,

      • some of which are more than 25 m high


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    Navajo PatternsSandstone

    • Large cross-beds of the Jurassic Navajo Sandstone in Zion National Park, Utah


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    Sundance Sea Patterns

    • 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


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    Sundance Sea Patterns

    • The resulting deposits,

      • the Sundance Formation,

      • were produced from erosion

      • of tectonic highlands to the west

      • that paralleled the shoreline


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    Sundance Sea Retreated Northward Patterns

    • 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


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    Morrison Formation Patterns

    • 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


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    Morrison Formation Patterns

    • View of the Jurassic Morrison Formation

      • from the Visitors’ center

      • at Dinosaur National Monument, Utah


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    Skeletons Deposited on Sandbars Patterns

    • 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


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    Dinosaur National Monument Patterns

    • 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


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    North Wall Patterns


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    Mid-Cretaceous Transgressions Patterns

    • 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


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    Black Shale Deposition Patterns

    • 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.


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    Cretaceous Interior Seaway Patterns

    • 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


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    Cretaceous Interior Seaway Patterns

    • 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


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    Cretaceous Interior Seaway Patterns

    • Paleogeography of North America during the Cretaceous Period

    • Cretaceous Interior Seaway


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    Cretaceous Deposits Patterns

    • 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


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    Facies Relationships Patterns

    • 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


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    Cretaceous Facies Related to Sevier Patterns

    • This restored west-east cross section

      • of Cretaceous facies of the western Cretaceous Interior Seaway

      • shows the facies relationship to the Sevier orogenic belt


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    Cretaceous Interior Seaway Patterns

    • 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.


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    Accretion of Terranes Patterns

    • 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


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    Accretion of Terranes Patterns

    • 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


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    Terranes Patterns

    • 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


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    Accretion of Terranes Patterns

    • 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


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    Accretion of Terranes Patterns

    • 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


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    More Than 100 Terranes Patterns

    • It is estimated that more than 100 different-sized 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


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    Terranes of Western North America Patterns

    • 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


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    Terranes of Western North America Patterns

    • The light green blocks

      • are possibly displaced parts of North America

    • Dark green

      • represents the North American craton


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    Growth along Active Margins Patterns

    • 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


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    New Additions Patterns

    • 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


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    Terranes Patterns

    • Thus, terranes

      • provide another way

      • of viewing Earth

      • and gaining a better understanding

      • of the geologic history of the continents


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    Mesozoic Mineral Resources Patterns

    • 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


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    Petroleum in Gulfs Patterns

    • 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


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    Gulf Coast Region Patterns

    • 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


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    Louann Salt Patterns

    • 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


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    Salt Patterns

    • Salt deposits in the Gulf of Mexico

    • formed during the initial opening of the Atlantic


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    Uranium Ores Patterns

    • 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-, vanadium-bearing mineral

        • called carnotite,

      • are associated with plant remains in sandstones

      • that were deposited in ancient stream channels


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    Mesozoic Iron Ores Patterns

    • 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


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    Kimberlite Pipes Patterns

    • 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


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    Cretaceous Kimberlite Pipes Patterns

    • 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


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    Mother Lode Patterns

    • 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


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    Porphyry Copper Patterns

    • 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


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    Origin of Porphyry Copper Patterns

    • 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


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    Plate Tectonics and the Distribution of Natural Resources Patterns

    • Bingham Mine in Utah is a huge open-pit copper mine

    • Magma generated by subduction can create this activity

      • Example: copper deposits in western Americas


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    Summary Patterns

    • Summary of the breakup of Pangaea

      1. During the Late Triassic,

      • North America began separating from Africa

      • This was followed by the rifting

      • of North America from South America

        2. During the Late Triassic and Jurassic periods,

      • Antarctica and Australia,

        • which remained sutured together

      • began separating from South America and Africa,

      • while India began rifting from Gondwana


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    Summary Patterns

    3. South America and Africa

    • began separating

    • during the Jurassic

    • and Europe and Africa

    • began converging during this time

      4. The final stage in Pangaea's breakup

    • occurred during the Cenozoic

    • when Greenland completely separated

    • from Europe and North America


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    Summary Patterns

    • The breakup of Pangaea influenced

      • global climatic

      • and atmospheric circulation patterns

    • Although the temperature gradient

      • from the tropics to the poles

      • gradually increased during the Mesozoic

      • overall global temperatures remained equable

    • An increased rate of seafloor spreading

      • during the Cretaceous Period

      • caused sea level to rise

      • and transgressions to occur


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    Summary Patterns

    • Except for incursions along the continental margin and two major transgressions

      • the Sundance Sea and the Cretaceous Interior Seaway

      • the North American craton was above sea level during the Mesozoic Era

    • The Eastern Coastal Plain

      • was the initial site of the separation of North America from Africa

      • that began during the Late Triassic


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    Summary Patterns

    • During the Cretaceous Period,

      • the Coastal Plain was inundated by marine transgressions

    • The Gulf Coastal region

      • was the site of major evaporite accumulation

      • during the Jurassic

      • as North America rifted from South America

    • During the Cretaceous, it was inundated

      • by a transgressing sea, which, at its maximum,

      • connected with a sea transgression from the north

      • to create the Cretaceous Interior Seaway


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    Summary Patterns

    • Mesozoic rocks of the western region of North America

      • were deposited in a variety of continental and marine environments

      • One of the major controls of sediment distribution patterns was tectonism

    • Western North America was affected by four interrelated orogenies

      • the Sonoma, Nevadan, Sevier, and Laramide

      • Each involved igneous intrusions,

      • as well as eastward thrust faulting and folding


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    Summary Patterns

    • The cause of the Sonoma, Nevadan, Sevier, and Laramide orogenies

      • was the changing angle of subduction of the oceanic Farallon plate

      • under the continental North American plate

    • The timing, rate, and, to some degree, the direction of plate movement

      • were related to seafloor spreading

      • and the opening of the Atlantic Ocean


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    Summary Patterns

    • Orogenic activity

      • associated with the oceanic-continental convergent plate boundary

      • in the Cordilleran mobile belt

      • explains the structural features

      • of the western margin of North American

    • It is thought, however,

      • that more than 25% of the North American western margin

      • originated from accretion of terranes


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    Summary Patterns

    • Mesozoic rocks

      • contain a variety of mineral resources,

      • including

        • coal,

        • petroleum,

        • uranium,

        • gold,

        • and copper


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