Mesozoic Earth History

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