Transcript 4 Mesozoic Geology e.+

Last Time
• We ended our discussion of the Paleozoic
with the greatest recorded mass extinction to
affect Earth. This occurred at the end of the
Permian
– about 90% of all marine invertebrate species extinct
– Rugose and tabulate corals, many bryozoan and
brachiopod orders, and trilobites did not survive the
end of the Permian
– causes for this have been speculated to be:
• reduction in marine shelf as Pangaea formed
• global drop in sea level due to glaciation
• reduction in marine shelf due to regression
• climatic changes
HOWEVER
Stable Isotope ratios for Carbon d13C
• There are two stable isotopes of carbon that are
routinely measured in fossils.
• The standard carbon isotope ratios in calcite are
from belemnites in the Pee Dee Formation
(abbreviated as PDB “Pee Dee Belemnite”).
• The process of photosynthesis favors the lighter
form of carbon in plant tissue. AT THE
BOUNDARY:
• “… it appears that a significant portion of the
land plants burned; this would have released a
great deal of light carbon into the atmosphere”
• http://www.acad.carleton.edu/curricular/GEOL/D
aveSTELLA/Carbon/c_isotope_models.htm
65.7 mya another mass extinction
Cretaceous 144 – about 65 mya
Jurassic 208-144 mya
Triassic 248-208 mya
About 248 to 245 mya
Depends on method
Mesozoic Geology
Pangaea
Introduction
• The Mesozoic began
248 mya and ended
about 65 mya
– Three periods - Triassic,
Jurassic, Cretaceous
– breakup of Pangaea was
the major geologic event
– tectonism and
sedimentation are used
to classify the Mesozoic
in N. America
– Note the overlap in three
styles of Cordilleran
Orogeny
Tectonism and Sedimentation
Seaway drains
1. The Breakup of Pangaea
• The movement of
continents during and
after the breakup
affected global
climates
• Sea-level changes
due MOR heating and
growth, then cooling
Pangaea - Early Triassic
Pole to pole and
straddled equator
Panthalassa Ocean
E. coast indent is
“Tethys Sea”
Tethys
Panthalassa
Panthalassa
Late Triassic – Rifting E Orogeny W
We will consider mostly North America for this lecture
Orogeny
Rift
Equator to Pole
current
gyres
Better look at Tethys
Tethys
Pangaea – Early Jurassic
E Jurassic – Atlantic Rift Shallow
Note offshore Terranes
E Jurassic – Another Look
Orogeny
New Sea
Wrangellia
Pangaea: mid-Jurassic
North Atlantic opens, India and Antarctica-Australia leave Gondwana
Latest Jurassic – Early Cretaceous
Atlantic Connected with Tethys
Africa rotation closes Tethys
Tethys
Atlantic
Late K – Epeiric Sea until 70 mya
Mid-Atlantic Ridge huge and fast
NOTE the
Farallon and
Pacific plates
Mesozoic Global Climates
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•
•
•
•
•
Carbonates (via the stable isotope index d13C)
reveal large concentrations of carbon dioxide
present in the Mesozoic atmosphere.
This suggests a greenhouse climate.
No glaciers so CO2 abundant.
There is some coal due interior seas and
transgressions and regressions)
Greenhouse gasses pass sunlight which hits the
land and sea. Re-radiate heat (IR)
Greenhouse gasses hold the heat, not lost to
space as quickly. Warmer equilibrium.
Conifers, Cycads
Global Climates in the Mesozoic
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Mesozoic climates were
more equable than
today, lacked the strong
north-south climate
zones.
Mesozoic plant fossils
indicate subtropical
conditions in high
latitude locations
Seasonal differences
were monsoonal
Cycads
Next: Mesozoic Tectonics NA
• Cretaceous : global rise in sea level
until 75 -70 mya, vast MOR
• Jurassic:
–Atlantic opens E,
–began building the Cordillera W,
– Gulf of Mexico begins to form and
experiences evaporite deposition
• Late Triassic: Begin rifting in East
Late Triassic: Rifting opens the Atlantic
• The Newark Supergroup documents the rifting of
Pangaea to form the Atlantic
• Early Triassic saw coarse detrital sediments
deposited from the erosion of Appalachian
highlands
– fault-block basins developed as N. America separated
from Africa and filled with non-marine sediment plus
lava flows, dikes and sills
– East side eroded to a flat plane by
Cretaceous
Mesozoic rift basins
Kean University
Structure of the Newark basin
Recall radial cracks due swelling and
uplift. We live on the western side. Note
dominant block orientation. Other side is
in Morocco; most face toward the West
Note how faulting offsets sedimentation
Lake cycles, East Berlin formation
Laterites
Monsoonal
Dry
Wet cycle
Lake
Alternating wet and dry climate
due 21000y Milankovitch cycle of tilt axis wobble
Also 100,000 year cycles due to orbit eccentricity
E. Jurassic Gulf Coast Evaporites
200 mya is just outside our door
Restricted Basin
Lots of evaporation
Gulf Coastal Region
• First, as continents separate, restricted basin, thick
evaporites formed in the Gulf
• Normal marine deposition returned to the Gulf by Late
Jurassic, with transgressions and regressions
– thousand of meters of sediments were deposited over the salt
Remember: “The facies follow the shoreline shifts”
Does this cross-section show
a transgression or regression?
Gulf Coast continental margin
Rising Salt Domes tilt sediments
Concentrate petroleum
Next: Western North America Tectonics
• Building the western margin of North
America and the Cordillera
Displaced terranes – Western Cordillera
These terranes overlap in age but have
different rock types, paleolatitudes and
fossils. However, we can deduce when
they accreted from their order, and the
metamorphic ages of their suture
zones
Western Region
• Cordilleran Orogeny
– YOUNGEST
– Laramide – Vertical
blocks-built the
present day Rockies
late K-Tertiary
– Sevier – J thrust
faulting to the east
– Nevadan – mid J to
mid K batholith
intrusion in the Sierra
Nevada and
elsewhere on the
western edge
– OLDEST
Western Margin during Orogens
North America drifting west due opening of Atlantic
Barbs show dip of fault
Westward subduction zones
stopped when continental
crust arrived.
Sonoma began late Permian.
It continued into the Early Triassic
Sonomia docking Late Pm –Early Triassic
Late Triassic on, eastward
subduction of Farallon oceanic
crust continues Cordilleran Orogeny
Mid J to mid K Nevadan Batholiths
Nevadan Orogeny east subduction Farallon
Sierra Nevada Mountains
Nevadan Orogeny:
Subduction formed
batholith cores of
continental
volcanic arc, once
as tall as Andes
Mesozoic orogenic events
Thin-skinned tectonics
Cretaceous Sevier (due
Wrangellia docking?)
Later moved by
transform fault?
K-T Laramide Continental Overide
Bouyant Subduction
Sevier thin-skinned deformation
Competent (resistant to flow)
Sevier thrust belt
Precambrian and/or Paleozoic Sediments
thrust over younger Mesozoic rocks
Buoyant Subduction Laramide Orogeny
Vertical block uplift
Normal, thin-skinned
Subduction Zone overrun by fast drifting continental plate
Approaching Continent pushes
accretionary wedge sediments
into forearc sediments
Now let’s look down here
Look in detail at western plate margin
This area has much simpler geology
Franciscan Range, Great Valley Group, and Sierra Nevada Volcanics and Plutonics
Next: Mesozoic Sedimentation on the Craton
• Cretaceous
– extensive marine
deposition, thin to the
east
• Jurassic
Foreland Basin
– clean cross-bedded
sandstones (dunes)
– marine sediments in
the Sundance Sea
• Triassic
– shallow-water marine
clastics
– red beds
North America - Triassic
Marine deposition limited to western margin
Volcanic Arc sends frequent ashfalls eastward
Pollen
similar
Chinle
Note Equator
Newark
Late Triassic Chinle Fm.
Mudstones and Sandstones of stream deposits, volcanic ash,
with fossil trees (the Petrified Forest)
Texas, New Mexico, northern Arizona, Nevada, Utah, and western Colorado
Pollen studies show that the Chinle is the same age as early Newark Supergroup
http://en.wikipedia.org/wiki/Petrified_Forest_National_Park
Triassic caliche paleosol- Nova Sc.
Source of carbonates for d13C measurements. Results suggest high CO2 in atmosphere
Same Caliche found when our new gym was built
Similar in Newark Supergroup
Same Laterites found in our stream
North America - Jurassic period
Dry region
in the rain
shadow of the
beginning
Nevadans
Zuni Transgression
Sedimentation
Seaway drains
Evaporites
Jurassic Eolian sandstone
Navaho SS, S. Utah
http://rainbow.ldgo.columbia.edu/courses/v1001/morisson14.html
Jurassic Morrison Formation
Paul Olsen's Dinosaur Course
Stream Deposits, huge sauropods Camarasaurus, also Stegosaurus, carnivore Allosaurus
Fossils of Jurassic dinosaurs
Morrison Formation sandstones, DNM, Vernal, Utah
Late Cretaceous really big epeiric sea
Alaska dinosaurs
Dinosaurs on the North Slope
Alaska’s Jurassic Park
Land
Land
Did the Sevier Orogenic Belt form before or after the Navaho SS, purple, lowest left?
Did the Sevier Orogenic Belt form before or after the Fox Hills SS, red uppermost right?
Fox Hills SS
Western Interior Seaway Regression
Western Interior Seaway Transgression
Dakota Sandstone
Early Cretaceous shallow sea sediments gently folded by Sevier Orogeny.
Then, at 75-70 my, Regression
In Montana the sequence is similar. Above the marine Pierre Shale (ammonites)
and Claggett Sandstone (nearshore and beach) is the Late Cretaceous Judith
River Fm. containing dinosaur bones and conifers in stream deposits. Is this
sequence a transgression or a regression?
Western Interior Seaway Regression
Western Interior Seaway Transgression
75 mya Regression
Mesa Verde Sandstones over Mancos Shale: Coarsening Upward
K-T Boundary
Dust cloud
Plants need light
Herbivores eat plants
Carnivores eat herbivores
Survivors can sleep through it.
Uh, oh. That can’ t
be good.
End of Mesozoic Geology