The Geology of the Mesozoic Era

201 Chapter 23 The Geology of the Mesozoic Era GUIDED STUDY The text chapter should be studied one section at a time. Before you read, preview each section by skimming it, noting headings and boldface items. Then read the appropriate section objectives from the following outline. Keep these objectives in mind and, as you read the chapter section, search for the information that will enable you to meet each objective. Once you have finished a section, write out answers for its objectives. 4. Describe the events that occurred with the breakup of Gondwana during the Jurassic and Early Cretaceous periods. The Divisions of the Mesozoic (pp. 478-479) 1. Explain the events that both open and close the Mesozoic Era. 5. What features indicate that Mesozoic climates were both warmer and more stable than in the preceding Paleozoic? A Global Perspective (pp. 479-481) 2. Describe the geography of Pangaea at the beginning of the Mesozoic Era. 6. What evidence suggests the occurrence of monsoon climates in northern and southern Pangaea during the Triassic and early Jurassic periods? 3. What happened in the initial stage of breakup of the continent of Pangaea? North America in the Mesozoic (pp. 482-498) 7. Describe the tectonic activity that occurred along the eastern margin of North America following the formation of the Appalachian Mountains.

202 8. Why did the Gulf Coast margin accumulate such a great thickness of sediments during the Mesozoic Era? 9. Describe the events of the Nevadan orogeny. 10. Describe the effects of the Sevier orogeny on the foreland basin sediments of the western margin of North America. 11. Discuss how deformation of the Laramide orogeny differed significantly from that of the thinskinned Sevier tectonics. 12. Describe the depositional environments represented Triassic sedimentary rocks of western North America. CHAPTER REVIEW When you have finished reading the chapter, work through the material that follows to review it. Complete the sentences. As you proceed, evaluate your performance for each section by consulting the answers on pages 209-210. Do not continue with the next section until you understand each answer. If you need to, review or reread the appropriate section in the textbook before continuing. Introduction (pp. 477-478) 1. The Ancestral Puebloans built their dwellings beneath cliffs of exposed in southwestern Colorado. 2. Their structures were placed inside caves and alcoves formed by the of calcite cement in the porous rock cliffs. 3. The greatest change that occurred in North America during the Mesozoic Era was the of Pangaea. 4. North America grew substantially larger in the Mesozoic, primarily because of an abundance of accreting. Divisions of the Mesozoic (pp. 478-479) 13. What is the significance of the Jurassic Morrison Formation of western North America? 14. What factors caused the Western Interior Seaway to form, and flood the North American continent to its greatest coverage since the Paleozoic? 15. What are the tectonic implications of the Great Valley and Franciscan groups of western California? 5. The beginning of the Mesozoic is defined by a boundary at 248 million years, which marks an abrupt change in the. 6. The end of the Mesozoic is marked by the mass extinction of many animals including, which occurred 66 million years ago. 7. The Period was defined in 1834 from Germany, where three distinct rock formations are found. 8. The Jurassic Period was named from the Jura Mountains of and, where this system was first described in 1799.

203 9. The last period of the Mesozoic is called the, and was named for the chalk exposures found in France and southern England. A Global Perspective (pp. 479-481) 10. At the beginning of the Mesozoic, the continent of stretched from nearly pole to pole. 11. The worldwide ocean called had an eastern indentation called the sea. 12. The breakup of Pangaea occurred in four stages that lasted from the Late Period until the early Era. 13. The first stage in the breakup of Pangaea involved between Laurasia and Gondwana. 14. Faulting and stretching of the crust between eastern North America and Africa produced a series of along the margins of the separating continents. 15. The separation of North and South America allowed seawater to invade the new rift zone, producing the. 16. The second stage of the breakup of Pangaea involved the fragmentation of. 17. The landmass of Africa-South America was separated from the landmass of Antarctica-. 18. The continent of moved northward and was completely separated from the other land masses throughout the Mesozoic Era. 19. The Ocean was formed between the Late Jurassic and Early Cretaceous periods, by the rifting of South America and. 20. Separation of the two above continents involve the formation of a three-way rift or. 21. The westward movement of South America caused a to form on its western margin. 22. The rotation of Eurasia caused to move away from North America. Northward movement of Africa narrowed the Tethys Sea. 23. The final stage of the breakup of Pangaea involved separation of North America, Greenland, and, and also the separation of Australia from. 24. During the Triassic, free of the Panthalassa Ocean produced deposits in the high latitudes of Antarctica and Australia. 25. The unusually warm climate of the Jurassic and Cretaceous periods was caused by carrying warmer waters of the cratons and from the elevated levels of in the atmosphere. 26. Mesozoic climates have been investigated with the use of oxygen isotopes, aeolian sandstones, and the examination of sedimentary structures and. North America in the Mesozoic (pp. 482-498) 27. The geology of eastern North America was profoundly affected by rifting form the African continent in the early. The chain of rift basins that formed were eventually filled in by sediments to form the modern. 28. The chain of rift basins that formed were eventually filled in by sediments to form the modern. 29. Erosion of the left a broad region of gentle topography. 30. Rifting between North America and Africa began at the start of the Period. Slivers of African underline the coastal plain of Florida, Georgia, and South Carolina. 31. Slivers of African underline the coastal plain of Florida, Georgia, and South Carolina.

204 32. Normal faulting in the crust of the Appalachian orogen produced down-dropped valleys called and half-grabens. 33. The normal faults that border many of the rift basins were actually and faults formed during Paleozoic plate collisions. 34. Many of the rift basins have been buried underneath the coastal plain, or lie submerged on the. 35. The sedimentary rocks that accumulated in the Mesozoic rift basins of eastern North America are collectively known as the. 36. Sedimentation in the rift basins was temporarily interrupted by volcanic activity early in the Jurassic Period. This activity included basalt flows, shallow, and dikes. 37. Basement rocks of the Atlantic continental shelf were by Mesozoic rifting and by numerous normal faults. 38. The newly formed basins were filled with detrital sediments, overlain by, then by shallow water, and finally detrital sediments shed from the erosion of the Appalachian Mountains. 39. The Gulf Coast of North America formed through a sequence of events in an almost identical fashion to that of the Atlantic margin, except that sediment thickness in the Gulf Coast was far. 40. After the initial deposition of redbeds in the rift between North and South America, thick deposits of formed by evaporation of marine waters that flowed in between the new Pacific and Atlantic oceans. 41. Gulf Coast sediments were supplied from the continental interior, the western end of the Appalachian-Ouachita fold belt, and the rising mountains of the. 42. Cretaceous sediments of the Gulf Coast included shallow water shelf edge reefs made of clams, and deeper water facies containing shales and. 43. Petroleum deposits in the Gulf Coast region were formed as salt squeezed upward, forming many dome-shaped for oil and gas by bending and breaking overlying rock layers. 44. Much of western North America is composed of numerous, which were accreted to the continent at subduction zones. 45. In some places, younger igneous rocks have older accreted terranes, confusing the timing of geologic events. 46. Some terranes have not remained in place where they, but have been displaced along faults. 47. From the end of the Permian into the beginning of the Triassic, the accretion of a volcanic arc terrane that now comprises western Nevada and northern California and subsequent deformation was called the. 48. Although the accretion of Sonomia ended westward-dipping subduction of the North American plate, oceanic crust of the plate was subducted underneath the continent from the Late Triassic onward. 49. The sequence of Mesozoic mountain-building events in western North America is called the orogeny. 50. The formation of a large igneous arc parallel to the coastline of North America from Late Triassic to Early Cretaceous time is called the orogeny. 51. The deeply-eroded remnants of this magmatic arc are seen as numerous composed of granite and granodiorite. 52. Compression east of the magmatic belt caused sedimentary rocks to be folded and thrust tens of kilometers eastward. This deformation is called the orogeny. 53. Compression east of the magmatic belt may have been caused by the docking of a series of crustal blocks known as. 54. Vertical and lateral uplift of Precambrian basement rocks, forming broad uplifts, steep-sided folds, and

205 deep basins is assigned to a period of deformation called the orogeny. 55. Igneous activity in the Sevier orogeny ended at the end of the Cretaceous, suggesting a change in the process. 56. The North American plate may have overidden the oceanic Farallon plate, thereby causing it to flatten out in a process called. 57. The drag of the underlying ocean plate on the base of the overriding continental plate may have caused it to pile up in blocks that were pushed. 58. Much deposition along the margin of western North America occurred in the that formed inland from the orogenic belt. 59. River channels, floodplains and lakes located on an alluvial plain are the depositional environments characteristic of the of late Triassic age in the Four Corners area. 60. The eolian deposits of the represent sand dunes that migrated across a coastal desert. 61. Braided and meandering stream deposits, as well as intervening floodplain sediments gradually filled the Sundance Sea as a clastic wedge called the. 62. Early to Late Cretaceous worldwide transgression, caused by rapid rates of sea-floor spreading flooded North America with the. 63. Sediments of the were deposited in the forearc basin seaward of the magmatic arc. 64. Sediments of the represent oceanic sediments scraped off the subducted oceanic plates, mixed with slivers of oceanic plates and volcanic arc sediments. PRACTICE TESTS After you thoroughly understand the correct answers of the Chapter Review, answer the following questions and check them with the answers on pages 210-211. If your answer is incorrect, consult the appropriate pages of the text. Multiple Choice Questions 1. The sandstone cliffs that support the top of Mesa Verde represent a sedimentary environment of: a. coastal sand dunes. b. lowland river floodplains. c. barrier islands. d. alluvial fans. 2. The three periods of the Mesozoic were all defined from exposures in: a. North America. b. Europe. c. Africa. d. Australia. 3. The Wilson cycle that marked the assembly of the supercontinent of Pangaea continued to operate during the: a. Cretaceous Period. b. Jurassic Period. c. Triassic Period. d. Permian Period. 4. The polar regions of the supercontinent Pangaea were unusually warm because of: a. oceanic circulation. b. large landmasses over the poles. c. large mountain ranges near the poles. d. large freshwater lakes near the poles. 5. The initial rifting of Pangaea involved separation between North America and: a. Europe. b. Asia. c. South America. d. Africa. 6. The initial Atlantic Ocean was much like today s: a. Gulf of Mexico. b. Red Sea. c. Mediterranean Sea. d. Atlantic Ocean. 7. The Jurassic breakup of Gondwana completely isolated the continent of: a. Antarctica. b. Australia. c. India. d. Africa. 8. The aulacogen occupied by the Niger River Valley of Africa formed during its rifting from a. South America. b. India. c. Australia. d. Antarctica. Circle your answers to the following questions.

206 9. The final stage of Pangaean breakup involved the rifting from both Europe and North America of : a. Laurentia. b. Laurasia. c. Africa. d. Greenland. 10. The seasonal climate of the Pangaean interior is indicated by: a. coal deposits. b. paleosols. c. subtropical plants. d. chalk deposits. 11. Jurassic rifting in eastern North America ultimately produced a: a. foreland basin. b. forearc basin. c. passive plate margin. d. backarc basin. 12. The sediments of the Newark Supergroup filled: a. the subsiding foreland basin. b. the subsiding backarc basin. c. the subsiding forearc basin. d. the down-dropped rift basins. 13. Igneous activity along the eastern margin of North America erupted into: a. the subsiding foreland basin. b. the subsiding backarc basin. c. the subsiding forearc basin. d. the down-dropped rift basins. 14. A Mesozoic sedimentary deposit unique to the Gulf Coast is: a. salt. c. oil. b. coal. d. gas. 15. The great thickness of Gulf Coast sediments accumulated in the Mesozoic is attributed to: a. rifting. b. subsidence. c. sea level change. d. volcanism. 16. The oil deposits of the Gulf Coast can largely be attributed to traps involving: a. faulting. b. stratigraphic change. c. salt domes. d. compressional deformation. 17. North America s western margin was enlarged by: a. accretion of displaced terranes. b. continental collision. c. rifting. d. sea-floor spreading. 18. The formation of a magmatic arc along the western margin of North America that began in the Late Triassic resulted from : a. rifting of the Farallon plate. b. rifting of the Sonoma plate. c. subduction of the Sonoma plate. d. subduction of the Farallon plate. 19. Activity of the Nevadan orogeny was responsible for the formation of the present-day: a. Cascade Range. b. Sierra Nevada. c. Ouachita Mountains. d. Appalachian Mountains. 20. The cause of the Sevier compressional deformation may be with: a. Sonomia. b. Avalonia. c. Wrangellia. d. Klamathia. 21. The major effects of the Laramide orogeny were seen in: a. thin-skinned thrusts. b. rifting. c. broad uplifts and steep-sided folds. d. intrusions of granite and granodiorite. 22. The major site of deposition at the western margin of North America was the: a. foreland basin. b. forearc basin. c. backarc basin. d. rift basins. 23. The Chinle Group of the Four Corners area represents: a. arid tidal flats. b. a coastal desert. c. a highland region. d. a broad alluvial plain. 24. The Navajo Sandstone of Jurassic age represents: a. arid tidal flats. b. a coastal desert. c. a highland region. d. a broad alluvial plain. 25. The Sundance Sea represents the: a. Absaroka transgression. b. Kaskaskia transgression. c. Zuni transgression. d. Western Interior transgression.

207 26. The Western Interior Seaway resulted from : a. global warming. b. glacier formation. c. slowed rates of sea-floor spreading. d. accelerated rates of sea-floor spreading. 27. Sediments of the Great Valley Group were deposited in the: a. forearc basin b. foreland basin. c. backarc basin. d. rift basins. True or False Items Write true or false on the line in front of each statement. 1. The sandstones of the Mesa Verde area were deposited 75 million years ago. 2. All three of the Mesozoic periods were defined from outcrops in Germany. 3. The Pangaean supercontinent was surrounded by the Tethys global ocean. 4. The mid-atlantic ridge system had formed in by the Middle of the Jurassic Period. 5. Rifting of North America from Africa caused North and South America to collide. 6. Antarctica and Australia remained together throughout the Mesozoic Era. 7. The northward migration of India was tectonically quiet. 8. Africa separated from Arabia in the Late Cretaceous Period. 9. The northward drift of Pangaea during the early Mesozoic warmed the southern polar regions once covered by glaciers. 10. Fossils from the polar regions of Pangaea indicate a severe climate. 11. Crustal extension that formed the Mesozoic rift basins in eastern North America moved along previously formed thrust and reverse faults. 12. Initial sedimentation in the Gulf of Mexico involved deposition of evaporites. 13. Rapid filling of the Gulf of Mexico rift zone by sediments produced a shallow shelf by Cretaceous time. 14. The Nevadan orogeny included the formation of a magmatic arc parallel to the coastline. 15. The thin-skinned tectonics of the Sevier orogeny were probably a result of collision with the displaced terrane called Sonomia. 16. Broad uplifts, steep-sided faults, and deep basins were formed as a result of the Laramide orogeny. 17. Mesozoic sedimentation on the western margin of North America occurred mainly in the foreland basin inland from the orogenic belt. 18. The Western Interior Seaway was a result of the greatest transgression since the early Paleozoic. Essay Questions Write a brief essay on a separate sheet of paper answering each of the following questions. 1. Describe the breakup of the Pangaean World. 2. Discuss the three phases of the Cordilleran orogeny. 3. Explain the tectonic significance of the Great Valley and Franciscan Groups of California. CHALLENGE TEST Answer these questions the day before an exam as a final check on your understanding of the chapter s terms and concepts. Check your responses with the answers on pages 211-212. If your answer is incorrect, consult the appropriate pages of the text. Completion Fill in the correct answers. 1. Cretaceous sandstones of the Mesa Verde region represent and depositional environments. 2. The series of collisions between North America and accreted terranes produced vast. 3. The Triassic Period began million years ago, and the Cretaceous Period ended million years ago. 4. The assembly of Pangaea ended the that began in the Late Proterozoic with the breakup of Rodinia. 5. Rifting between Laurasia and Gondwana began in the. 6. Fragmentation of Gondwana began in the Period.

208 7. The opening of the south Atlantic Ocean between Africa and South America occurred between Late Jurassic and time. 8. Sedimentary structures such as and various types of paleosols indicate that Triassic climates in the interior of Pangaea were and seasonal. 9. The eastern edge of North America is called the modern. It was the site of several orogenies during the Paleozoic Era. 10. Thrust and reverses faults that were active in the Allegeheny orogeny were reactivated by forces associated with the rifting of continents. 11. Sediments of the Newark Supergroup occupied Mesozoic in eastern North America. 12. The greater thickness of sediments in the Gulf Coast region of North America was due to the rapid of the shelf following rifting. 13. In western North America, displaced terranes accreted to the continental margin at. 14. The volcanic arc terrane that was added to the western margin of North America between the end of the Permian and the beginning of the Triassic Period was called. 15. The Nevadan orogeny formed a large, parallel to the coastline. 16. In the Laramide orogeny, large blocks of Precambrian rocks were pushed vertically and laterally. 17. The Late Jurassic landscape of western North America contained a low-relief alluvial plain that bordered the. 18. Rocks of the Franciscan Group represent an formed above the trench to the west of the volcanic arc. Multiple-Choice Questions Circle the correct answer. 1. The sandstone cliffs that support the top of Mesa Verde are of : a. Cretaceous age. b. Jurassic age. c. Triassic age. d. Permian age. 2. Which Mesozoic period s upper boundary marked the greatest extinction event of the era? a. Permian. c. Jurassic. b. Triassic. d. Cretaceous. 3. A new Wilson cycle of rifting and drifting began in the: a. Permian Period c. Jurassic Period. b. Triassic Period. d. Cretaceous Period. 4. The large indentation in the eastern region of the Pangaea coastline was occupied by: a. The Mediterranean Sea c. The Tethys Sea. b. The Atlantic Ocean. d. The Black Sea. 5. Late Triassic separation of North America from Africa produced: a. a foreland basin. c. a mountain belt. b. a volcanic arc. d. a line of rift basins. 6. The Gulf of Mexico formed between North and South America as a: a. rift zone. c. foreland basin. b. backarc basin. d. trench. 7. The two Gondwana continents that remained attached throughout the Mesozoic were: a. Africa and South America. b. Antarctica and Australia. c. Africa and India. d. South America and Antarctica. 8. The Tethys Sea was narrowed in the Cretaceous by the northward movement of: a. India. b. Australia. c. Antarctica. d. Africa. 9. Warm climates that existed in the high latitudes of Mesozoic Pangaea are indicated by polar: a. coal beds. b. fossil subtropical plants c. fossil dinosaur remains. d. All of the above.

209 10. Mountain building ended on the eastern margin of North America with the Pennsylvanian-Permian: a. Sonoma orogeny. b. Nevadan orogeny. c. Allegeheny orogeny. d. Antler orogeny. 11. The actual separation of continental crust between North America and Africa occurred: a. far to the east of the rift basins. b. far to the west of the rift basins. c. right along the rift basins themselves. d. None of the above. 12. The source of sediments shed onto the Gulf Coast shelf was from: a. the western Appalachian-Ouachita fold belt. b. the continental interior. c. the rising western Cordillera. d. All of the above. 13. The accreted terrane of Sonomia includes parts of: a. Montana and Wyoming. b. California and Nevada. c. Oregon and Washington. d. Utah and Idaho. 14. Activity of the Sevier orogeny in western North America was mainly: a. normal faulting. c. thrust faulting. b. volcanism. d. rifting. 15. The cause of Laramide mountain building may be a process called: a. a arid tidal flats. b. a coastal desert. c. a highland region. d. a broad alluvial plain. 16. The Morrison Formation of Late Jurassic age represents: a. arid tidal flats. b. a coastal desert. c. a highland region. d. a broad alluvial plain. 17. Regression during latest Cretaceous time was due in part to: a. rapid deposition from eroding mountains. b. global warming. c. rapid coastal subsidence. d. accelerated sea-floor spreading. 18. Sediments of the Franciscan Group represent deposits of the: a. foreland basin. b. accretionary wedge beyond the volcanic arc. c. backarc basin. d. rift basins. True or False Items Write true or false on the line in front of each statement. 1. The greatest geologic change that North America experienced was the buildup of the supercontinent Pangaea. 2. The Triassic Period was the first of the three Mesozoic periods to be established. 3. The land surface of Pangaea was unusually high because of numerous mountain ranges. 4. Pangaea was not completely assembled until the Late Triassic Period. 5. Rifting began between North America and Africa. 6. Central America was a crustal fragment that remained between North and South America. 7. The Niger River aulacogen formed as Greenland pulled away from North America. 8. Australia finally separated from Antarctica in the early Cenozoic Era. 9. Mesozoic paleotemperatures can be determined from an analysis of the oxygen isotopes contained in fossil shells. 10. Sedimentary structures such as mudcracks indicate an arid climate in the interior of Pangaea throughout the year. 11. Erosion of the Appalachian Mountains by the Jurassic Period had reduced the eastern margin of North America to a broad region with a gentle topography. 12. Mesozoic rift basins in eastern North America formed west of where North America and Africa rifted apart. 13. Salt beds in the Gulf of Mexico contain vast quantities of oil. 14. The Mesozoic mountains of the western margin of North America were formed by continental collisions. 15. The Cordilleran orogeny has been divided into three separate events. 16. The dinosaur-bearing sediments of the Morrison Formation represent coastal deserts and shallow-shelf environments. 17. The late Mesozoic plate margin of western North America is represented by an accretionary wedge, a forearc basin, and a magmatic arc. ANSWERS CHAPTER REVIEW 1. Cretaceous sandstones 2. dissolution 3. breakup 4. terranes 5. fossil biota

210 6. dinosaurs 7. Triassic 8. France; Switzerland 9. Cretaceous 10. Pangaea 11. Panthalassa; Tethys 12. Triassic; Cenozoic 13. rifting 14. sedimentary basins 15. Gulf of Mexico 16. Gondwana 17. Australia 18. India 19. South Atlantic; Africa 20. aulacogen 21. subduction zone 22. Greenland 23. Europe; Antarctica 24. circulation; coal 25. transgressions;greenhouse gases 26. paleosols 27. Mesozoic 28. passive continental margins 29. Appalachian mountain belt 30. Jurassic 31. terrane 32. grabens 33. thrust; reverse 34. continental shelf 35. Newark Supergroup 36. sills 37. stretched; broken 38. evaporites; carbonates 39. greater 40. salt 41. Cordillera 42. rudists; chalk 43. traps 44. displaced terranes 45. intruded 46. docked; transform 47. Sonoma orogeny 48. Farallon 49. Cordilleran 50. Nevadan 51. batholiths 52. Sevier 53. Wrangellia 54. Laramide 55. subduction 56. buoyant subduction 57. upward 58. foreland basin 59. Chinle Group 60. Navajo Sandstone 61. Morrison Formation 62. western Interior Seaway 63. Great Valley Group 64. Franciscan Group PRACTICE TESTS Multiple-Choice Questions 1. c 2. b 3. c 4. a 5. d 6. b 7. c 8. a 9. d 10. b 11. c 12. d 13. d 14. a 15. b 16. c 17. a 18. d 19. b 20. c 21. c 22. a 23. d 24. b 25. c 26. d 27. a True or False Items 1. True. 2. False. Only the Triassic Period was defined from outcrops in Germany. 3. False. Panthalassa was the global ocean surrounding Pangaea. 4. True. 5. False. Rifting between North America and Africa caused separation between North and South America as well. 6. True. 7. False. India moved northward over a hot spot in the Cretaceous, producing tremendous outpourings of basalt. 8. False. Africa separated from Arabia early in the Cenozoic. 9. True. 10. False. Fossils from the polar latitudes of Pangaea indicate a mild climate. 11. True. 12. False. Initial sedimentation in the Gulf of Mexico involved continental redbeds. 13. True. 14. True.

211 15. False. The displaced terrane involved in the Sevier orogeny was probably Wrangellia. 16. True. 17. True. 18. True. Essay Questions 1. The breakup of the Pangaean world occurred in a series of four stages: a. The initial stage of separation involved rifting between the eastern margin of North America and Africa. The modern Atlantic began as a narrow sea, but by the Middle Jurassic, had developed a midocean ridge system, and each continental edge had become a passive plate margin. The initial rifting also caused North America to pull away from South America, opening the Gulf of Mexico. Fragments of crust between the two continents became parts of Cuba and Central America. b. The second stage of Pangaean fragmentation involved the rifting of Gondwana. Africa and South America split apart from Antarctica and Australia, and India was left to move northward by itself. Mid-ocean ridges to its south, and subduction zones to its north caused India to move towards Asia. As it moved over a hot spot, large volumes of basalts were erupted during the Cretaceous Period. c. The third stage of Pangaean breakup came from the opening of the South Atlantic Ocean between South America and Africa. This separation involved at least one three-way rift, or aulacogen. As South America drifted westward, it experienced a subducted oceanic plate to its west, forming the Andes magmatic arc. d. The final stage of rifting caused separation between North America, Greenland, and Europe. This allowed a further widening of the North Atlantic Ocean. Also, Australia separated from Antarctica, and Arabia separated from Africa. These events occurred during the early Cenozoic Era, and have given the world its modern appearance. 2. Mountain-building events along the western margin of North America during the Mesozoic Era have been described as the Cordilleran orogeny. The collision and accretion of various terranes derived from an eastward-subducting oceanic plate resulted in large-scale intrusions of granite and granodiorite, which caused uplift and eastward-directed thrust faulting. The events of the Nevadan orogeny also produced a foreland basin developed inland from the region of uplift. Intense crustal compression resulting from extensive intrusions cause rocks of the foreland basin to be folded and thrust many miles eastward. The Sevier orogeny is an example of thin-skinned tectonics, where deeper crustal rocks were not invoved in the deformation. Docking of the displaced terrane called Wrangellia also may have played a part. The Laramide orogeny involved uplift and faulting of basement Precambrian rocks into steep-sided folds, deep basins, and broad uplifts. Because of the deep-seated movement involved, a process called buoyant subduction may have caused oceanic crust to pile up underneath the continental crust, resulting in upward pressure and uplift. 3. The sediments deposited on the seaward side of the magmatic arc provide valuable information about Mesozoic tectonics on the western margin of North America. The Great Valley sediments represent over 16, 000 meters of sediments shed into the forearc basin west of the magmatic arc. Farther west, over 7, 000 meters of highly deformed sediments of the Franciscan Group represent a mélange of oceanic sediments scraped off the oceanic plate, slivers of oceanic crust, and volcanic sediments from the magmatic arc to the east. This sediment formed an accretionary wedge above the trench to the west of the magmatic arc. The Late Mesozoic plate margin of the western United States consisted of a trench, accretionary wedge, forearc basin, and magmatic arc closed to shore. CHALLENGE TEST Completion 1. shoreline; barrier islands 2. mountain ranges 3. 248; 66 4. Wilson cycle 5. Late Triassic 6. Jurassic 7. Early Cretaceous 8. mudcracks; dry 9. Atlantic margin 10. extensional 11. rift basins 12. subsidence 13. subduction zones 14. Sonomia 15. igneous arc 16. basement 17. Sundance Sea 18. accretionary wedge Multiple-Choice Questions 1. a 2. d 3. b 4. c 5. d 6. a 7. b

212 8. d 9. d 10. c 11. a 12. d 13. b 14. c 15. d 16. d 17. a 18. b True or False Items 1. False. The greatest geologic change to North America in the Mesozoic was the breakup of the supercontinent of Pangaea. 2. False. The Triassic Period was the last of the three Mesozoic time periods to be established. 3. True. 4. False. Pangaea began to rift apart in the Late Triassic Period. 5. True. 6. True. 7 False. The Niger River aulacogen formed as Africa pulled away from South America. 8. True. 9. True. 10. False. Mudcracks indicate a seasonal climate with alternating wet and dry periods. 11. True. 12. True. 13. False. Salt domes of the Gulf Coast trap large quantities of oil and gas contained in other rocks. 14. False. The Mesozoic mountains of the western margin of North America were formed by accretion of displaced terranes at subduction zones. 15. True. 16. False. Dinosaur-bearing sediments of the Morrison Formation represent stream channel and floodplain deposits. 17. True.