Earth History, Ch. 17 1

Ch. 17 — Review

• Life in the Cretaceous

– Diversification of diatoms, planktonic forams, calcareous nannoplankton

– Diversification of mobile predators (especially mollusks and teleost fishes)

– Origin of the angiosperms and co-evolution with insects

– Dinosaur glory days!! Complex dinosaur communities that mimic modern mammal communities

– Mammals still small and inconspicuous (in the dark?)

Earth History, Ch. 17 2

Today’s outline

• Cretaceous paleogeography

• End-Cretaceous mass extinction

• Cretaceous geology of North America

Earth History, Ch. 17 3

Cretaceous paleogeography

• 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 4

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

Earth History, Ch. 17 5

Earth History, Ch. 17 6

Cretaceous paleogeography

• Large Tethys ocean was tropical and

probably accounted for warm climate and

gentle latitudinal climatic gradients

– Dinosaurs and warm-adapted plants lived

within 15º of the south pole

• High rates of seafloor spreading caused

mid-ocean ridges to rise � highest sea

level in Phanerozoic history

Earth History, Ch. 17 7

Earth History, Ch. 17 8

End-Cretaceous mass extinction

• 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

Earth History, Ch. 17 9

Familial

diversity

record

Earth History, Ch. 17 10

End-Cretaceous mass extinction

• Possible causes include asteroid impact, volcanism, climate change, or combination of all

• 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 fractures due to enormous pressure

– Microspherules

• Liquefied droplets of molten rock that cool rapidly

– Microscopic diamonds

• Again, high pressure minerals

Earth History, Ch. 17 11

Earth History, Ch. 17 12

Iridium layer at Gubbio, Italy

Earth History, Ch. 17 13

Mineral evidenceIridium layer near

Drumheller in

southern Alberta,

Canada

Earth History, Ch. 17 14

Mineral evidence

microspherulesshocked quartz

Earth History, Ch. 17 15

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 16

Chicxulub crater

Impact

trajectory

Earth History, Ch. 17 17

Chicxulub crater

• Trajectory of asteroid was at a low angle

(20-30°) and from southeast to northwest

– Fiery vapor cloud was driven across west-

central North America

• Western North American floras were hardest his

– Microspherule layers are thickest in Mexico

(~ 1m), thinner in Texas (~10cm), thinner still

in New Jersey (~5cm)

Earth History, Ch. 17 18

Radar image of Chicxulub crater

Earth History, Ch. 17 19

Chicxulub crater

Gravity survey

data

Earth History, Ch. 17 20

Impact of the impact

• Perpetual darkness from atmospheric dust

– Months in duration? No photosynthesis?

• 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 21

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 22

Cretaceous geology of

North 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 23

Sevier Orogeny

Earth History, Ch. 17 24

Cretaceous geology of

North America

• Interior seaway developed when continent was

flooded: northern 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 25

Late Cretaceous cyclic deposits