Historical Geology
INTRODUCTION
THE BREAKUP OF PANGAEA
The Effects of the Breakup of Pangaea on Global Climates and Ocean Circulation Patterns
CONTINENTAL INTERIOR
EASTERN COASTAL REGION
GULF COASTAL REGION
WESTERN REGION
Mesozoic Tectonics
Mesozoic Sedimentation
¬PerspectivE Petrified Forest National Park
WHAT ROLE DID ACCRETION OF TERRANES PLAY IN THE GROWTH OF
WESTERN NORTH AMERICA?
MESOZOIC MINERAL RESOURCES
The following content objectives are presented in Chapter 14:
¬ The Mesozoic breakup of Pangaea profoundly affected geologic and biologic events.
¬ During the Triassic Period most of North America was above sea level.
¬ During the Jurassic Period a seaway flooded the interior of western North America.
¬ A global rise in sea level during the Cretaceous Period resulted in an enormous interior seaway that extended from the Gulf of Mexico to the Arctic Ocean and divided North America into two large landmasses.
¬ Western North America was affected by four interrelated orogenies that took place along an oceanic-continental plate boundary.
¬ Terrane accretion played an important role in the Mesozoic geologic history of western North America.
¬ Coal, petroleum, uranium, and copper deposits are major Mesozoic mineral resources.
To exhibit
mastery of this chapter, students should be able to demonstrate comprehension
of the following:
¬ the causes and evidence for continental break-up in both modern environments and in the geological record
¬ the breakup of Pangaea and its effect on global climates and oceanic circulation patterns
¬ the general geologic history of North AmericaÕs continental interior, eastern coastal region, and Gulf coastal region
¬ the general geologic history for western North America, particularly the orogenic events of the Cordilleran mobile belt
¬ terrane tectonics in the history of North America
¬ the importance of Mesozoic mineral deposits
1. The breakup of Pangaea can be summarized as follows: (a) During the Late Triassic, North America began separating from Africa. This was followed by the rifting of North America from South America; (b) During the Late Triassic and Jurassic periods, Antarctica and Australia—which remained sutured together—began separating from South America and Africa, and India began rifting from Gondwana; (c) South American and Africa began separating during the Jurassic, and Europe and Africa began converging during this time; (d) The final stage in PangaeaÕs breakup occurred during the Cenozoic when Greenland completely separated from Europe and North America.
Figure 14.1 Mesozoic Paleogeography
2. 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.
Figure 14.2 Evaporite Formation
Figure 14.3 Mesozoic Oceanic Circulation Pattern
Although global temperatures were equable in the Mesozoic, researchers have found a record of two greenhouse episodes in the latest Cretaceous through analysis of carbon and oxygen isotopes from paleosol carbonates. There appears to be a strong correlation between terrestrial climates and ocean temperatures.
A possible cause for the events is extensive volcanic
degassing in the Deccan Traps, India, leading to global climate change. Nordt,
Atchley, and Dworkin, ÒTerrestrial Evidnece for Two Greenhouse Events in the
Latest Cretaceous,Ó GSA Today,
Dec.2003, v.13 n.12 p.4-9.
3. An increased rate of sea-floor spreading during the Cretaceous Period caused sea level to rise and transgressions to occur.
4. 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.
Figure 14.4 Triassic Paleogeography of North America
Figure 14.5 Jurassic Paleogeography of North America
Figure 14.6 Cretaceous Paleogeography of North America
5. The Eastern Coastal Plain was the initial site of the separation of North America from Africa that began during the Late Triassic. During the Cretaceous Period, it was inundated by marine transgressions.
Figure 14.7 North American Triassic Fault-Block Basins
Figure 14.8 Triassic Newark Group Reptile Footprints
6. 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 transgressing from the north to create the Cretaceous Interior Seaway.
Figure 14.9 Early Cretaceous
Shelf-Margin Sedimentary Facies
A hot and
dry climate caused extensive evaporation of seawaters during the formation of
the Gulf of Mexico. Water salinity
rose higher and higher until salts began to precipitate, with eventually as
much as 4000 meters of salt accumulated in a deposit known as the Louann
Salt. Salt has significant
properties, including a low density. Under pressure, it is capable of plastic
flow; when it is buried beneath denser sediments, it starts to flow and move
upward, lifting or pushing aside the overlying sediments. The Five Islands salt domes of Louisiana
are simple domes that extend to very near the surface, and produce topographic
highs above their surrounding margins.
The salt is not only an economic resource to be mined, but the salt
intrusions create a variety of stratigraphic traps that are capable of trapping
oil and gas. (Excerpt from ÒGeology
of Louisiana,Ó special chapter by Clary (editor), Duex, Kinsland, and Lock,
2005.)
7. 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.
Figure 14.10 Sonoma Orogeny
Figure 14.11 Mesozoic Cordilleran Orogenies
8. 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 was related to seafloor spreading and opening of the Atlantic Ocean.
Figure 14.12 Cordilleran Batholiths
Figure 14.13 The Franciscan Complex
Figure 14.14 Cretaceous Subduction in the Cordilleran Mobile Belt
Figure 14.15 Sevier Orogeny
9. Mesozoic rocks of the western region of North America were deposited in a variety of continental and marine environments.
Figure 14.16 Triassic and Jurassic Formations in the Western United States
Figure 14.17 Navajo Sandstone
Figure 14.18 Morrison Formation
Figure 14.19 Cretaceous Facies of the Western Cretaceous Interior Seaway
In July 2006, the Quarry Visitor Center at Dinosaur National Monument was closed because of safety issues. Unfortunately, the building was constructed on expansive soils in 1957, and over the years the Quarry Visitor Center experienced extreme foundation movement. Warped door frames, tilted walls, weakened ceiling supports, and shattered glass all contributed to the closing of the building. Although visitors cannot physically view the wall of embedded fossils, the National Park Service has made a Virtual Tour of the Douglass Quarry available. A Temporary Quarry Visitor Center is open, and it is also possible to take a virtual tour inside the old Quarry Visitor Center. Access the virtual tours and learn more about Dinosaur National Monument at the official National Park Service website: http://www.nps.gov/dino/
10. 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 America. It is believed, however, that more than 25% of
the North American western margin originated from the accretion of terranes.
Figure 14.20 Mesozoic Terranes
.
11. Mesozoic rocks contain a variety of mineral resources, including coal, petroleum, uranium, gold, and copper.
The Mesozoic
Cordilleran Orogeny
1. To illustrate how magmas rise in the crust to form large igneous intrusions, demonstrate this simple analogy in the class. Put equal amounts of two differently colored immiscible liquids, such as colored Karo syrup and colored lamp oil, into a long cylindrical jar. After pouring the two liquids into the jar, allow them to separate and then invert the jar.
Students should see teardrop-shaped blobs of the lower density liquid rise to the surface. Point out that many granite plutons, such as those associated with the Sierra Nevada batholiths, have this shape.
2. Accreted and suspect terranes, and Cordilleran tectonics, have been the subjects of several books by John McPhee. All of the following provide useful information and anecdotes for lectures about the tectonics of western North America: Basin and Range, Assembling California, and Rising from the Plain are primarily about the western states.
3. You can demonstrate folding or thrust faulting by using structural models. Compressional tectonics can be effectively demonstrated using stacks of paper or layered clay. Remind students that compressional tectonic actions will greatly thicken and shorten the continental crust.
Salt Accumulation and
Diapirism
1. To encourage students to understand the magnitude of salt accumulation in the Gulf of Mexico during the Mesozoic, calculate the amount of seawater necessary to produce 1000 meters of deposited sodium chloride (halite) in part of the Louann Salt mother lode. A generous modern salinity of 4% for ocean water can be used, and students can estimate how much water must evaporate in order to produce a 1000 meter thick salt layer the size of the state of Louisiana (an estimated 43,566 square miles, or roughly 1.14 x 105 square kilometers).
2. Salt diapirism can be demonstrated using a lava lamp. The heated wax rises to the top of the lamp in much the same fashion as low-density salt flows upward beneath the overlying sedimentary layers.
1. Why are the best dinosaur fossil localities where they are? Why aren't dinosaur fossils found in other areas of the country in locales with Mesozoic age rocks, such as Idaho, Georgia, and Mississippi?
2. How would the climate of western North America have varied because of terrane accretion? Discuss possible oceanic influences on climate.
3. As India begins its independent track northward in the Mesozoic—and continuing into the Cenozoic—will it experience climate changes?
4. What would the rifting Gulf of Mexico have been like in the Mesozoic when thousands of meters of salt were accumulating in the basin? Would the environment have been conducive to plant and/or animal life?
|
Cordilleran orogeny |
Nevadan orogeny |
Sundance Sea |
|
Cretaceous Interior Seaway |
Sevier orogeny |
terrane |
|
Laramide orogeny |
Sonoma orogeny |
|
1. Fossils, Dinosaurs, and their World, slide set, Educational Images, Ltd.
2. Satellite Imagery – Earth from
Space, slide set, Educational Images, Ltd.
3. Interpretation of Roadside Geology,
slide set, Educational Images, Ltd.
|
1. c |
5. e |
9. c |
|
2. a |
6. b |
10. e |
|
3. c |
7. e |
|
|
4. a |
8. c |
|
11. The magnetic anomalies from the Mesozoic Era are preserved in the oceanic crust. Geologists have a good record of the history of PangaeaÕs breakup, and the direction of movement of the various continents during the Mesozoic and Cenozoic eras.
12. Past climatic conditions can be inferred from the distribution of climate-sensitive deposits. Evaporites form when evaporation exceeds precipitation. Sand dunes and red beds may form locally in humid regions, but are more characteristic of arid regions. Coal forms in warm and cool humid climates.
13. High-grade bituminous and anthracite coals have experienced greater metamorphism (through heat and/or pressure). It is logical that Paleozoic coals have experienced greater metamorphism because of their location along the eastern coast of North America. This was the site of continental-continental collision during the formation of the supercontinent, Pangaea. The area was subjected to intense deformation. The Mesozoic coals formed in similar humid environments, but have not been subjected to the intense pressures that the Paleozoic coals have.
14. The Cordilleran orogeny
was one of continental-oceanic convergence, which included the emplacement of
granitic batholiths, some andesitic volcanism, back-arc thrusting, and uplift
and basin formation in the Rocky Mountains. The progressive eastward shift of
orogenic activity and the change in style from batholith production to
structural deformation with some volcanism was the result of flattening of the
subduction angle of the oceanic plate with time. The orogeny that affected the
Appalachians began as continental-oceanic convergence but evolved into
continental-continental. There were large sedimentary basins adjacent to the
mountain range resulting from erosion of the high peaks.
15. The opening of the Atlantic pushed North America to the west and established an oceanic-continental convergence style of plate boundary that exists with modifications to the present time. During the first pulse of the Cordilleran orogeny, the Nevadan, the opening of the Atlantic Ocean was partially responsible for the North American continent overriding a westerly dipping subduction zone, leaving only the easterly dipping subduction zone along the western periphery.
16. Gulf of Mexico evaporites were deposited in an elongate, newly rifted ocean basin segment that had restricted water flow, similar to the modern Red Sea. Paleozoic evaporites collected in circular or oval intracratonic basins where restricted water flow was caused and controlled by fringing reefs. Both settings were at low latitudes.
17. Oceanic circulation patterns became more complex and segmented. The global temperature gradient from equator to pole became much steeper, and oceanic circulation became much steeper as a consequence. Widespread Triassic evaporates, red beds, and desert dunes in the low and middle latitudes of North and South American, Europe, and Africa indicate dry climates in those regions. The increase of the global temperature gradient, caused by the breakup of Pangaea during the Late Triassic, resulted in the acceleration of oceanic and atmospheric ciruculation patterns.
Middle- and higher-latitude oceans were still quite warm, though, 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.
18. Because the west coast of North America was active with a converging oceanic plate, terranes were being delivered during the Mesozoic (and later) time. The growth along this active margin was faster than along a passive margin, and the accreted terranes were new additions to the continent as opposed to being reworked, older continental material.
19. Both orogenies were the result of a collision between an island arc with the western side of the North American craton. The Sonoma occurred later, at the boundary between the Permian and Triassic.
20. Diagrams will vary. Hot seafloor is buoyant and less dense, and takes up more volume than cold seafloor. The more seafloor spreading, the hotter the seafloor and the more volume it occupies. This buoyant elevated seafloor displaces more water and causes sea level to rise.
Apply Your Knowledge
1. Student
answers will vary. If Pangaea had broken up differently, geologic
processes would still have operated in the same way but in different places.
For example, the leading edge of an overriding plate will be the site of a
volcanic arc, along with any natural resources that might form there. Ocean
circulation would be the same, with gyres moving water from warmer to cooler
regions, but the oceans would be between different continents. Evolution would
also operate in the same way. For example, marsupial mammals would probably
survive on the first continents to split from Pangaea, only to go extinct if
they came into contact with placental mammals because of a land connection.
Coal and petroleum would form wherever conditions were suitable.
2. Yes, there have been other gold discoveries associated with igneous intrusions outside the state of California. There have been smaller gold mining operations in Nevada, Idaho, Montana, and South Dakota. Gold is associated with batholithic intrusions because magma generated by the partial melting of a subducting plate rises toward the surface. As the magma cools, it precipitates and concentrates various metallic ores. This is because water will percolate into fractures in the rock and come in contact with hot magma. The water dissolves normally stable minerals—like gold—and brings it up through fractures to higher levels as it cools. CaliforniaÕs Mother Lode has such as high concentration of gold because there is a major fault zone that acted as a conduit for the rising mineral-laden water.
3. Student responses will vary.
However, students should point out that the large reservoirs in the Persian
Gulf region formed during the Mesozoic when that region was a broad passive
continental margin conducive for petroleum reservoir formation. Geologists do
not expect petroleum reservoirs to occur without regard to past tectonics, and
spread evenly over the North American continent.
The
Gulf Coast region of the United States formed on a similar broad shelf over
which transgressions and regressions occurred, and geologists might expect to
find reservoir rocks in this area.
However, the very diverse National Park system contains areas in which
petroleum reservoirs would be unfeasible.
In addition, students should also address environment issues involved in
petroleum exploration and production.
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