Mesozoic Earth History (final exam) Flashcards
What timeframe is the Mesozoic? What are the 3 periods in the Mesozoic era?
Mesozoic: 251-66 Ma
Triassic, Jurassic, Cretaceous
What’s the notable sea level trend in the Mesozoic?
Mostly around modern sea levels in the first half and then in the second half the sea levels were largely above current levels (in the Cretaceous)
What are some of the main Mesozoic geologic events?
- Pangaea broke up
- Atlantic ocean formed
- Rocky Mountains formed
- Accumulation of vast salt deposits > salt domes adjacent to which oil & gas were trapped (acting as a seal for a lot of our oil deposits)
- Gold and other ore deposits were emplaced in continental plutons/batholiths
(and all of these geologic impacts had a great effect on animal evolution)
Explain Stage 1 of the breakup of Pangaea
*Triassic
- rifting between Laurasia and Gondwana
- opening of Atlantic Ocean between North America and Africa in Triassic (beginning of Atlantic ocean)
Explain Stage 2 of the breakup of Pangaea
*Late Triassic / early Jurassic
- newly formed & expanding Atlantic Ocean > separated North America from Africa
- North America rifts from South America (this narrow sea between them ends up giving lots of evaporites)
Breakup of Gondwana:
- Antarctica and Australia separate from South America and Africa
- India rifts, moving north, separating from all
Explain why we had so many evaporite deposits with the breakup of Pangaea and why is this significant?
- Pangaea rifting apart and there was thinning crust along the rift zones
- with thinning comes depressions and water flows in to these shallow seas
- oftentimes there would be barriers from regular flows in from the ocean > therefore we have evaporites
- today we see evaporite deposits as evidence that coincides with the breakup - how and where the plates pulled apart
Explain Stage 3 of the breakup of Pangaea
*Jurassic
South America and Africa begin rifting apart
- narrow basin leads to thick evaporites
Eastern Tethys Sea begins closing due to rotation of Laurasia and Northward movement of Africa
- ancestor to modern Mediterranean sea
Explain Stage 4 of the breakup of Pangaea
*Cretaceous
- Australia and Antarctica rift apart
- India nears the equator > eventually flies up and hits land and forms Himalayas
- South America and Africa are widely separated (bc of seafloor spreading - divergent plate boundary)
- Greenland separates from Eurasia, then North America
- sea level rises > Cretaceous interior seaway > covers 1/3 of North American continent (Manitoba under water which is why we have the marine/reptile fossil creatures)
Why was the sea level rising so much during the Cretaceous?
- at the mid-ocean ridges there was lots of rifting happening - land masses forced apart - what comes up between them is lots of magma - as lava goes on to the surface we create new ocean floor - volume of lava displaces the water
*basically it’s because of the volcanic activity happening at mid-ocean ridges
What causes Earth’s Climates?
- caused by complex interaction between wind and ocean currents (between oceans and our atmosphere) > can explain the temperature gradient between tropics and poles
- location of continents
- topography (shape of the earth’s surface)
ex.
- arid, dry climates on large landmasses that are remote from sources of moisture or where barriers to moist air exist (ex. mountains)
- wet climates near large bodies of water or where wind carries moist air
How can we infer past climates from these climate specific rock deposits: evaporite deposits, sand dunes & red beds, and coal
Evaporite deposits: large area of thin water with no ready supply of more water > salt deposits in rock record = infer that the climate had greater evaporation than precipitation
Sand dunes & red beds: can infer arid regions typically
Coal: typically tropical, need plant material and moisture so we can infer humid conditions
How did Pangaea impact the climate of the Permian period?
Large size of Pangaea:
- arid conditions in low latitudes (centre of continent)
- humid conditions in higher latitudes (coasts of continent)
Vast expanse of Panthalassa ocean:
- small temperature gradient because of slow oceanic circulation
How did ocean circulation patterns change from Permian to Mesozoic and how did this impact climate?
*Greater oceanic circulation = less of a temperature gradient because it’s all moving around and mixing together
End Permian > arid climates across much of Pangaea
Mesozoic and during the breakup of Pangaea:
- change in circulation patterns - more humid
- more circulation of oceans in between land masses - more complicated circulation
- Temperature gradient from equator to poles increased (but still relatively warm at mid to high latitudes due to warm waters from Tethys ocean)
- This set up stronger seasonality and severity of weather changes with the season (still mild compared to what we see today though)
For the cratonic sequences of North America, what is the broad category of mountain-building episodes on the west margin and what are they on the east margin
west = cordilleran mountain-building episodes
east = appalachian-ouachita mountain-building
What are the notable Mesozoic Cratonic sequences?
- Absaroka sequence regression
- Zuni sequence
What are the main points about the Absaroka sequence and Zuni sequence from the Mesozoic?
Absaroka sequence regressing in beginning Mesozoic:
- lead to nearshore marine and nonmarine sediments
Zuni sequence:
- formation of the Sundance Sea in Jurassic
- Sedimentation from Cordilleran orogenies, Sundance Sea regressed northward
**Jurassic Morrison formation > famous for dinosaur fossils
- Mid Cretaceous sea level rise *Mowry Sea leading to Western Interior Seaway
What were the main events happening on the west and east margins of North America in the Triassic
West:
- Cordilleran mobile belt continuing to form a chain of volcanic islands (which began in the Permian)
East:
- exposed craton above sea level - uplift
- erosion, block faulting, and igneous activity in the Appalachians as North America separated from Africa
Explain in more detail the impacts of the Appalachian Mountain Erosion > what occurred from the separation of North America and Africa?
*late Triassic - North America separating from Africa
- fault-block basins formed from Nova Scotia to North Carolina
- extensive lava flows occurred on the floors of the fault-block basins > numerous dikes and sills intruded in the basins
*Erosion of fault-block mountains produced thick and poorly sorted non-marine detrital sediments of the mostly Late Triassic and partially Early Jurassic Newark Group (famous for dino footprints)
Explain why faults form and what are dikes and sills?
Faults:
- cold rocks behave differently from warm > more jarred movements, more brittle, will break and form faults (shown by thin lines on maps with arrows along them - 2 going opposite ways)
- Dikes:
- formed when faults get filled with lava > cross-cuts other cracks
Sill:
- sheet of igneous rock intruded between and PARALLEL with the existing strata
What are the main events in the Jurassic (regarding bodies of water)
- the Gulf of Mexico continued to widen and deposit extensive evaporites in the shallow sea as North America separated from South America
- Sundance sea flooded inland
What is the significance of the Gulf Coastal Region in the Jurassic and Cretaceous
It was above lea level until the separation of North and South America (became a newly flooded area)
- shallow and restricted leading to evaporites
- transitioned to normal marine by late Jurassic
REEFS during the Cretaceous
- bivalves = main reef building organism (rudists)
- reefs = major oil reservoirs in the gulf region>high porosity and permeability
Main events/sequences of North America during the Cretaceous
What’s going on with Manitoba?
- sea level rising steadily through the Cretaceous > **global transgressive sequences
- The North American continental interior, though inundated somewhat by seaways, was largely above sea level > Zuni Sequence
- Manitoba is completely underwater
Explain the stages of the Cretaceous Interior Seaway forming through the early, middle, and late Cretaceous
Early Cretaceous: Arctic waters spread southward over the North American craton to form a large inland sea in the Cordilleran region
Middle Cretaceous: global transgressions occurred due to expansion of active oceanic spreading ridges, rifting, and high heat flow > widespread deposition of black shales (anoxic conditions)
Late Cretaceous: the Cretaceous Interior Seaway formed, extended from the Arctic to the Gulf of Mexico
What are Terranes? How do they differ from each other and from continents?
Terranes: small, exotic lithospheric blocks
- accreted onto larger continental blocks
- include island arcs, oceanic crust, small fragments of continents (analogous to cratons)
(exotic means it was formed in a different geologic setting from the main north american continent)
Differ from each other and continents in:
- fossil content, stratigraphy, structural trends, paleomagnetic properties
What are Terranes and the accretion of Terranes evidence of?
Accretion of Terranes > they slam together and get sutured together
Evidence that we have for specific areas of continents not forming from the main continent
What role did accretion of Terranes play in the growth of Western North America?
What is an example of one?
More than 100 different sized terranes have been added to the western margin of North America during the last 200 million years
- as much as 25% of the western margin of North America is composed of accreted terranes
ex. Wrangellia > accreted to North America in Late Jurassic
Explain the resource potential from the Western North American Terranes
- the western accreted terrane rocks contain many resources > coal, petroleum, uranium, gold, iron, and copper
- metals are precipitated from and concentrated by cooling magma
- the richest uranium deposits in the US occur as carnotite in Mesozoic sandstones of the Colorado Plateau
List the Cordilleran Orogenies in order
*Cordilleran orogenies began in different places and at different times, but had zones of overlap
PRECORD-ORO: Sonoma orogeny (predates the Cordilleran orogenies : Permian-Triassic boundary)
- Nevadan orogeny
- Sevier orogeny
- Laramide orogeny
Explain the Sonoma Orogeny - how did the margins change?
Sonoma orogeny was a volcanic arc formed off western North America during Permian
- collided with North America during early Triassic > obduction - ocean rock thrust on top of continental rock (obduction is quite rare)
*Western margin transitioned from passive margin to active margin with active subduction under North American plate in Late Triassic
When was the Nevadan Orogeny and what did it lead to?
*late Jurassic-Cretaceous
- Subduction led to large volumes of granitic magma being created
- Migration of magmatism eastward >
- The subducting Farallon plate developed a shallower angle resulting in the more eastward migration of volcanism (from a high-angle to low-angle subduction)
Explain what the Sevier Orogeny resulted in
*starts after nevada orogeny started but they’re occurring concurrently
Subduction of the Farallon plate beneath the North American plate caused numerous overlapping, low-angle thrust faults in which blocks of older Paleozoic strata were thrust eastward on top of younger strata
> Keystone thrust fault
What is the significance of Mesozoic Sedimentation (where are they deposited and what are some famous landscapes as a result)
As with any orogenic episode, sediments that get eroded are redeposited somewhere else
During the Cordilleran orogenies (and those before), mountain-building led to great quantities of sediments that deposited on the continental shelf and fore-arc basins of western North America
Famous landscapes: Chimney rock in Utah, Painted desert and petrified forest in Arizona
What led to the deposition of Morrison Formation?
The retreat of Sundance Sea
Explain the geology of the Seaway (Cretaceous) - west and east
*thick western margin
- complex, shifting shoreline from fluctuating sediment supply
- higher weathering, erosion, and sedimentation rate
- uplift
*thinner eastern margin
- slowly subsided
- lower sedimentation rate
**basically the sedimentation rate above is going to dictate what gets deposited - it’s not uniform across the seaway
What is the significance of the Laramide orogeny?
- it is the final pulse of the Cordilleran Orogeny (starts in Mesozoic and finishes in Cenozoic)
- developed east of Sevier orogeny
*rock mountains! - most features of the present-day rocky mountains resulted from the Cenozoic phase of the Laramide orogeny
