GEOL 102 Historical Geology
Spring Semester 2011
The Mesozoic Era II: Cretaceous Geology
Paleogeography and Geology of the Cretaceous:
Continued breakup of Gondwana
(which has been intact since the breakup of Pannotia!). Isolated into Africa and a South
America + Indomadagascar + Antarctica/Australia unit connected by now-submerged ridges.
As South Atlantic widens, beginning of
Andean Orogeny in western South America (a long ongoing mountain building
process, still active today).
Equatorial current allows continued warming of tropical seas.
The mid-Cretaceous saw some major worldwide events:
- Increased rate of sea-floor spreading along mid-ocean ridges
- Decreased rate of magnetic reversals (mid-Cretaceous is marked by the Long
Cretaceous Normal, 40 Myr without reversals)
- Highest transgressions of the post-Pangaean world, rivaling the highest Cambrian
- Many continents divided up by epeiric seas: North America divided in three by
Western Interior Seaway; European Archipelago isolated from Asian mainland; Africa
is divided into three; etc.)
- Black shales and anoxic waters periodically flood continental shelves and
epeiric seas: black shales may be generated in part by eutrophication from continental runoff of nutrients, and in
part by sluggish oceanic circulation (and thus more anoxia)
- Globally warm and mild climates
During Late Cretaceous, beginnings of regression, but main regression event not until the
Maastrichtian (last Age).
In warm Cretaceous seas, important new types of deposits:
- CHALK!!! --
coccolithophorids appear in Jurassic, but take off in producing chalk during
mid-to Late Cretaceous. The Chalk of Europe, Niobrara Chalk of the Western
Interior Seaway, etc..
diatoms first appear and become common in Cretaceous
- Foraminiferal ooze: planktonic
foraminiferans arise in Cretaceous
- Rudist reefs (particularly in Tethys and along equatorial regions):
rudists are reef-forming bivalves.
In Early Cretaceous, continuation of Nevadan Orogeny.
During the mid-Cretaceous:
- Increased speed of sea-floor spreading means subduction along Pacific margin of
North America at a lower angle
- Various small microplates swept up by western margin of North America
- Subducting Farallon Plate reaches melting point are regions further eastward
- Eastward migration of mountain range from Washington/Oregon to Idaho
- This new style is called Sevier
Orogeny: lasts until near the end of the Late Cretaceous
- Within forearc basin, many regional transgression-regression events
- Slowdown of seafloor spreading rate
- Huge Maastrichtian Regression: Western Interior Seaway drains to near
modern Gulf Coast
- Beginning of Laramide
Orogeny in Cordilleran system: foundering Farallon Plate brings uplift of region,
some volcanism as far east as Colorado, Wyoming, New Mexico: continues well into Tertiary
Deccan Traps: massive flood basalt episode in western India, third largest in
Phanerozoic (begins during last magnetic normal (C30n) interval of Cretaceous, so no closer than
350 kyr from 65.51 Ma (which is during C29r, a reversed chron)
- Cooling, more continental climates
Cretaceous-Paleogene Boundary: impact of a huge (10 km diameter) asteroid at Chicxulub
in the Yucatan Peninsula of Mexico: more on this later!
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Last modified: 14 January 2011