I. Overview of Mesozoic (251 to 65.5 million years ago) A. Spans Three Periods 1. Triassic: spans 51 million years
2. Jurassic: spans 55 million years
3. Cretaceous: spans 80 million years
B. Evolution 1. Many new families of plants and animals
2. Two new vertebrate classes (birds and mammals)
C. Supercontinent of Pangaea: rifted apart over a span of 150 million years. II. Breakup of Pangaea: 4 stages (three of those 3 during Mesozoic) A. Stage One: Triassic 1. Rifting and volcanism, normal faulting
a. tensional stresses separated North America from Gondwanaland
b. similarly Mexico from South America
c. similarly eastern North America from northern Africa
2. Sea-floor generation during opening of oceans (basaltic volcanism)
B. Stage Two: Triassic-Jurassic 1. Rifting of narrow oceans between South Africa and Antarctica, Africa and India
2. Massive outpouring of basaltic lavas (covering 7 million km2)
C. Stage Three: Jurassic-Cretaceous 1. Atlantic Ocean rift extended northward
2. Clockwise rotation of Eurasia (Europe)
3. Closing of eastern Tethys Sea (pre-Mediterranean)
4. South America-Africa split apart
5. Australia-Antarctica remained intact
6. Eastern North America-Greenland-Baltica remained intact
D. Stage Four: post-Mesozoic 1. Complete North America-Eurasia (Laurentia-Baltica) split
2. Antarctica-Australia split
III. Mesozoic History of North America A. Triassic and Jurassic (eastern and southern areas) 1. Normal fault-bounded basins developed due to rifting: Nova Scotia to North Carolina
a. troughs filled with terrestrial sediments and volcanics
b. Newark Group (Upper Triassic-Lower Jurassic)
c. Palisades basalts of NJ and NY (190 million years ago)
a. occupied areas opening south of Appalachian-Ouachita folded mountains
b. filled with Upper Triassic-Lower Jurassic salts and evaporites (indicating aridity); over 1000 m deposited; origin of Gulf coast salt domes of today
B. Cretaceous (eastern and southern areas) 1. Flooding of coastal lowlands due to high sea levels
a. Atlantic and Gulf Coastal Plains inundated as they acted as subsiding shelves at this time
b. thick deposits of deltas, barrier islands, shelves, reefs, etc. formed
c. Florida: shallow submarine bank for limestones
d. reefs made of rudistid bivalves rimmed Gulf Coastal area during Early Cretaceous
e. extensive chalk deposits of Cretaceous sea due to massive production of microscopic calcareous plankton (coccoliths); creta = chalk
2. Rifting and ocean opening on eastern side led to closure and compression on the western side of continent; subduction resulted
C. Triassic (western areas) 1. Accretionary tectonics - characteristic of west coast subduction at this time
a. steeply dipping subduction zone
b. volcanic arcs and micro-continents carried to western margin (displaced, exotic, or alien terrains—as many as 50 now known)
c. massive accretion by subduction (including volcanism), obduction, and tectonic accretion of displaced terrains
d. tectonic collage of displaced terrains may be 70% of total western accretion; termed accretionary tectonics
2. Cordilleran region divisions
a. western belt: volcanics and siliceous deposits (800 m)
b. eastern belt: stable interior sediments
3. Sonoma orogeny: Permian-Triassic, Nevada
a. island arc collided with west coast
b. then a west-dipping subduction zone
c. added 300 km new area to west
d. massive thrust faulting
e. Sonoma terrain: additional rocks added during accretion
b. eastern tract: deeper water carbonates and shales
a. widening split of South America and Africa
b. Andean belt deformation and volcanism
c. great outpourings of andesite in volcanic chain
d. emplacement of large batholiths
B. Africa 1. Early Triassic: Africa still joined to South America
2. Triassic-Jurassic: Karoo basin filled with fossiliferous continental clastics, then
1000 m of basalt flows due to rifting
3. Jurassic-Cretaceous: Africa is a stable continent
C. India 1. Triassic-Cretaceous: moved continually north
2. Jurassic: interior sedimentation (dinosaurs well represented)
3. Cretaceous: vast outpourings of basaltic lava in northern regions (Deccan Traps covering 500,000 km2, as much as 1 million km3 of basalt makes it Earth’s greatest known outpouring)
Answers to Discussion Questions
1. Rifting generally precedes the separation of continental crustal segments. Examples of Mesozoic rifting include Triassic rifting of eastern North America and Gondwanaland and the Jurassic-Cretaceous rifting of South America and Africa.
2. The Triassic Newark Group of the east coast of North America was formed in a tensional stress environment that caused rifting with associated normal faulting and basaltic volcanism. The rifts filled with alluvial and lacustrine sediments shed from adjacent fault-block mountains. The normal faults originated from gravity effects after crustal stretching and fracturing. Reverse faults were not produced for lack of requisite compression. Older lateral strike-slip faults and reverse faults provided sites for reactivation of crustal movement during rifting.
3. The Sevier orogeny involved basement crystalline rocks which were displaced laterally along imbricate thrust faults. These low-angle features represent a décollement where extensive amounts of crust become “unstuck” and are displaced laterally many kilometers. The most well known structure is the Lewis Thrust with over 65 km displacement.
4. Arid conditions are accountable for producing extensive evaporation to produce 1000 m of salt and gypsum. The tectonic setting also contributed such that a limited amount of seawater could enter the basin, evaporate, and then be recharged by another influx of seawater. The lighter specific gravity (density) of salt and gypsum and salt’s tendency to be plastically deformed under great pressure help create conditions so that deeply buried salt flows upward in the form of salt domes. Gulf Coast salt domes deform overlying strata as they migrate upward and thus create structural conditions for accumulation of migrating hydrocarbons in “petroleum traps.”
5. Accretionary tectonics probably accounts for most of the increase in size of North America during Mesozoic. Accretionary tectonics encompasses all active margin processes which add continental crust via subduction (including volcanism), obduction (including mélange accretion), and welding of displaced terrains (both micro-continents and island arcs). In North America during Mesozoic, accretionary tectonics characterizes the western belt of the Cordilleran area during Triassic. The Sonoma Orogeny specifically is an example of such tectonics. Likewise, with Jurassic the Nevadan Orogeny represents accretionary tectonics of western North America. Allochthonous terrains contribute displaced continental crust (perhaps 70% of total western Mesozoic accretion) and the energy of accretionary collision produces structural effects and regional metamorphism. Passive margin sedimentation accounts for the other part of North American size growth during Mesozoic.
6. Deccan volcanism would have contributed significant amounts of greenhouse gasses (especially CO2) in a relatively short period of time. Such an imbalance in atmospheric gas mixture could have temporarily strongly elevated global temperatures.
7. Obduction involves off-scraping and thrusting of lighter rock, especially sediments, and in some places, pieces of denser seafloor (ophiolites) onto continental crust as a process in tectonic accretion. Subduction involves decent and re-melting of denser seafloor at a convergent boundary. Examples of Triassic obduction are common. Examples of subduction abound: the entire Mesozoic history of the western U.S. is dictated by continual subduction as the Pacific Ocean closes.
8. Chalk is a sedimentary rock composed of microscopic calcareous plankton compressed together. Cretaceous strata are especially rich in chalk deposits. The marine plankton-chalk connection dictates composition and physical properties of the rock. The profusion of coccoliths during Cretaceous is key to chalk’s domination of the marine sediment record of that period.
9. Mesozoic Era lasted approximately 179 million years. Cretaceous Period lasted the longest of the Mesozoic periods, 78 million years.
10. Cordilleran highlands adjacent to the retreating Sundance Sea were the likely sources of clastic sediments comprising the Morrison Formation of the Rocky Mountain region. The Morrison Formation shows evidence of fluvial (river) deposition and includes abundant traces and bones from terrestrial vertebrates, especially dinosaurs.
11. Epicontinental (epeiric) seas were most extensive during Cretaceous and least extensive during Triassic according to Mesozoic paleogeographic maps.
12. When the breakup of Pangaea began about 180 million years ago, the Tethys seaway was located between Gondwanaland and Laurasia on the eastern side of Pangaea. Later, the Tethys occupied the region between Africa-India-Australia (as a southern continent) and Eurasia or Europe (as a northern continent). The Tethys was closed by northward movement of Africa-India-Australia and subduction of Tethys seafloor beneath Eurasia (Europe). The Mediterranean Sea, Adriatic Sea, Aegean Sea, and Black Sea occupy the former position of the Tethys, and thus may be part of the former Tethys. Rocks deposited within the Tethys, which extended from Japan to the western Mediterranean, may be formed in the Alps and Himalayan Mountains.
13. The Late Cretaceous development of a “great embayment” of the Tethys resulted in a connection of the Tethys and the northern Atlantic Ocean. Using the Western Interior Seaway of North America as an example, the distribution of many sea creatures, for example the abundant oysters of that time, could be used as evidence of a connection during that time.
14. c 15. B 16. d 17. b
Chapter Activities Student activities for in-depth learning. 1. Some of the best-known and best-preserved early dinosaurs come from a locality in South America at Ischigualasto, Argentina. Look at the University of California, Berkeley, Museum of Paleontology web page on this famous locality and describe the age and types of dinosaur remains that are found here http://www.ucmp.berkeley.edu/mesozoic/triassic/ischigualasto.html. Why do you think that this is such a good place to look for dinosaur bones?
2. The Palisades basalts of the Hudson River area are remarkable volcanic features. Look at the background information on these basalts at http://en.wikipedia.org/wiki/Palisades_Sill and answer these questions. When were the Palisades basalts formed? Where do they lie relative to the Hudson River today? How were they formed? What was going on in the geological history of Pangaea that contributed to the outpouring of basalts that became the Palisades?