Ch. 17 — Review

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 riseàhighest 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. Familialdiversityrecord 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 evidence Iridium layer near Drumheller in southern Alberta, Canada Earth History, Ch. 17

14. Mineral evidence microspherules shocked 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. Chicxulub crater Gravity survey data Earth History, Ch. 17

19. 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

20. 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

21. 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

22. Sevier Orogeny Earth History, Ch. 17

23. Cretaceous geology ofNorth 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

24. Late Cretaceous cyclic deposits Earth History, Ch. 17