Earth History, Ch. 16 1

Early Mesozoic of Iowa• No Triassic rocks in Iowa (surface or

subsurface)• Jurassic rocks crop out in vicinity of Ft.

Dodge (Webster County)– Ft. Dodge Formation is gypsum deposit

(evaporite)– Iowa is 2nd largest gypsum producing state in

U.S. (1.65 million tons/year)– Same age as dinosaur-bearing Morrison

Formation to the west

Earth History, Ch. 17 2

MiddleJurassic

Earth History, Ch. 16 3

Ft. Dodge Formation

Earth History, Ch. 16 4

Jurassic rocksof midcontinent

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Cardiff Giant

• Man-like figure carved from Ft. Dodge gypsum

• 10’ 4.5” long, 3’ wide, 2990 pounds

• Cooked up as a hoax in 1866 by George Hull (visiting his sister in Ackley, Iowa)

• Now preserved in Cooperstown, New York

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Earth History, Ch. 16 8

Cardiff Giant

Sold for $37,500 and then put on display

Earth History, Ch. 17 9P.T. Barnum’s fake of a fake

“There’s a sucker born every minute” (David Hannum)

Earth History, Ch. 17 10

Cretaceous World• Cretaceous System

was named in 1822 for deposits that are “chalky” in many parts of the world

• Duration of Cretaceous Period is longer than any other

142

206

65

251

CretaceousSystem

TriassicSystem

JurassicSystemM

esoz

oic

Era

Earth History, Ch. 17 11

Rise of the angiosperms

• Gymnosperms continued to dominate terrestrial floras in Cretaceous (just as in Triassic and Jurassic)

• Angiosperms initial appearance was in middle Cretaceous time– Double-fertilization and enclosed seeds

– Flowers (for attracting insects)

– Co-evolution wih insects

– Faster reproductive cycles

Earth History, Ch. 17 12

Angiosperm diversification

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Rudist reef

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Tyrannosaurus rex

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Warm-blooded Dinosaurs?

• Evidence in support of endothermy– Cretaceous dinos lived in high latitudes– Birds are endothermic, and birds are highly

specialized dinosaurs– Dinosaur bone structure resembles that in

endothermic mammals• Haversian canals

• But………

Earth History, Ch. 17 18

Dinosaur bone structure

Earth History, Ch. 17 19

Sue!!

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End-Cretaceous mass extinction

• Geologically instantaneous • Non-avian Dinosaurs, mosasaurs,

plesiosaurs, rudists and ammonoids were totally eliminated

• Angiosperms and gymnosperms suffered big hits

• 90% of the species of calcareous nannoplankton and planktonic foraminifers were wiped out

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End-Cretaceous mass extinction

• Cause is controversial– Asteroid impact– Volcanism (Deccan Traps, India)

Earth History, Ch. 17 25

End-Cretaceous mass extinction

• Mineral evidence for asteroid impact:– Iridium anomaly at top of Cretaceous in both marine

and terrestrial rocks• Iridium is rare on Earth, but abundant in meteorites

– Shocked quartz grains• Welded planar fractures due to enormous pressure

– Microspherules• Liquefied droplets of molten rock that cool rapidly

– Microscopic diamonds• Again, high pressure minerals

Earth History, Ch. 17 26

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Iridium layer at Gubbio, Italy

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Mineral evidenceIridium layer nearDrumheller insouthern Alberta,Canada

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Mineral evidence

microspherulesshocked quartz

Earth History, Ch. 17 30

End-Cretaceous mass extinction

• Further evidence for asteroid impact:– The crater itself has been discovered in the Gulf

of Mexico, just offshore Yucatan Peninsula

• Chicxulub crater– Central cavity (60 miles in diameter)– Outer ring (120 miles in diameter)– Magma that cooled after impact is dated at 65 ±

0.4 Ma, exactly same as end-Cretaceous boundary

Earth History, Ch. 17 31

Lab exam“What is it?”• Plants

– Ferns– Sphenopsids– Lycopsids

• Protists– Fusulinids

• Echinoderms– Crinoids– Blastoids

• Mollusks– Pelecypods

– Gastropods

– Ammonoids• Goniatitic suture

• Ceratitic suture

• Ammonitic suture

– Belemnites

• Cnidarians (corals)– Rugose

– Tabulate

– Scleractinian

Earth History, Ch. 17 32

Chicxulub crater

Earth History, Ch. 17 33

Radar image of Chicxulub crater

Earth History, Ch. 17 34

Chicxulub crater

Gravity surveydata

trajectory

Earth History, Ch. 17 35

Effects of the impact

• Perpetual darkness from atmospheric dust• Short-term global refrigeration from dust

and aerosol particles (like “nuclear winter”)• Acid rain from sulfur dioxide and water in

atmosphere• Wildfires, especially in North America• Long-term global warming from aerosols

that stayed in atmosphere

Earth History, Ch. 17 36

Aftermath

• Although angiosperms suffered loss of diversity, they recovered to become the dominant flora

• With dinosaurs out of the way, mammals diversified spectacularly in post-extinction Cenozoic Era

Earth History, Ch. 17 37

Continued break-up of Pangaea

• Remember: Pangaea began to break up during early Mesozoic– Triassic rifting between N. Africa and S.

Europe – Jurassic rifting between N. America and S.

America; between N. America and Africa– But, Gondwanaland remained intact

Earth History, Ch. 17 38

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Cretaceous paleogeography

• By late Cretaceous time:– South America, Africa and India had become

discrete entities– Only Australia and Antarctica remained

attached to one another– Greenland split from North America– Sea-level nearly at an all-time high

Earth History, Ch. 17 40

Early Triassic

Earth History, Ch. 17 41

Late Triassic

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Early Jurassic

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Middle Jurassic

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Late Jurassic

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Early Cretaceous

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Late early Cretaceous

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Late Cretaceous

Earth History, Ch. 17 48

Cretaceous geology ofNorth America

• East coast, now a passive continental margin, was mostly quiet

• West coast, a convergent margin, continued to experience mountain building– Sevier orogeny produced folding and thrusting

as far east as Wyoming; igneous activity in California, Nevada, Idaho, and farther north

Earth History, Ch. 17 49

Western U.S.orogenies

Earth History, Ch. 17 50

Sevier Orogeny

Nevadan Orogeny

Earth History, Ch. 17 51

Cretaceous geology ofNorth America

• Interior seaway developed when continent was flooded: Arctic Ocean joined with Gulf of Mexico

• Late Cretaceous rocks of interior seaway are cyclic deposits produced by oscillation of shoreline– Nearshore sand facies

– Shallow marine shale facies

– Offshore chalk facies

Earth History, Ch. 17 52

Late Cretaceous cyclic deposits

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Rememberthis?

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