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Mesozoic Earth History

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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Mesozoic Earth History

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  1. Chapter 14 Mesozoic Earth History

  2. 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

  3. 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

  4. Gold Mining • By 1852, • mining operations were well underway • on the American River near Sacramento

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. $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

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Paleogeography of the World • During the Triassic Period

  17. Paleogeography of the World • During the Jurassic Period

  18. Paleogeography of the World • During the Late Cretaceous Period

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. Paleogeography of the World • During the Jurassic Period

  25. 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

  26. Thick Southern Ocean Evaporites • Marine water flowed into the southern Atlantic Ocean from the south

  27. 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

  28. 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

  29. Paleogeography of the World • During the Late Cretaceous Period

  30. 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

  31. 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

  32. Paleogeography of the World • During the Late Cretaceous Period

  33. 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

  34. 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

  35. Paleogeography of the World • For the Late Permian Period

  36. 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

  37. 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

  38. 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

  39. 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

  40. 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

  41. 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

  42. 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

  43. 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

  44. 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

  45. Oceanic Circulation Evolved • From a simple pattern in a single ocean (Panthalassa) with a single continent (Pangaea)

  46. Oceanic Circulation Evolved • to a more complex pattern in the newly formed oceans of the Cretaceous Period

  47. 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

  48. Permian Period • Paleogeography of North America during the Permian Period

  49. Triassic Period • Paleogeography of North America during the Triassic Period

  50. 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

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