Transcript Chapter 23 The Geology of the Mesozoic Era

Mesozoic Geology
Introduction
• The Mesozoic began
248 mya and ended 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
Pangaea ran from
pole to pole and
straddled the
equator.
The Panthalassa
Ocean allowed
equatorial waters to
warm polar lands.
The East coast
indent is called the
“Tethys Sea”
Tethys
Panthalassa
Panthalassa
Late Triassic – Rifting E Orogeny W
We will consider mostly North America for this lecture
Orogeny
Rift
Better look at Tethys
Pangaea – Early Jurassic
E Jurassic – Atlantic Rift Shallow
Note offshore Terranes
E Jurassic – Another Look
Orogeny
New Sea
Wrangellia
Pangaea: mid-Jurassic
Latest Jurassic – Early Cretaceous
Atlantic Connected with Tethys
Africa rotation closes Tethys
Tethys
Atlantic
Late K – Epeiric Sea until 70 mya
Mesozoic Global Climates
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Carbonates (for example the stable
isotope ratio d13C) reveal large
concentrations of carbon dioxide
present in the Mesozoic atmosphere.
This suggests a greenhouse climate.
No glaciers, no coal, so CO2 abundant.
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.
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
Start here
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 nonmarine sediment plus
dikes and sills
– eroded to a flat plain by the Cretaceous
Mesozoic rift basins
Kean University
Structure of the Newark basin
Note how faulting offsets sedimentation
Lake cycles, East Berlin formation
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
Does this cross-section show
a transgression or regression?
Gulf Coast continental margin
Rising Salt Domes tilt sediments
Concentrate petroleum
Discussion: Petroleum exploration around salt domes
Squint: The Petroleum is in the bright spots, at the boundary of the salt and pushed
up sediment. Tells us where to look, saves money, lowers cost of fuel.
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 this map.
Exercise: Arrange the following
terranes by oldest to youngest time of
accretion onto the west coast:
Alexander, Cache Creek, Chugach,
Eastern, Stikine, Taku, Tracy Arm,
Wrangellia, Yukon-Takana
Western Region
• Cordilleran
Orogeny
– Laramide –
Vertical blocksbuilt the present
day Rockies KTertiary
– Sevier – J-K thrust
faulting to the east
– Nevadan Jurassic batholith
intrusion in the
Sierra Nevada and
elsewhere on the
western edge
Western Margin during Orogens
North America drifting west due opening of Atlantic
Westward subduction zones
stopped when continental
crust arrived.
Remember the late Permian Sonoma?
It continued into the Early Triassic
Sonomia docking Late Pm –Early Triassic
Late Triassic on, eastward
subduction of Farallon oceanic
crust continues Cordilleran Orogeny
Late J Early 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 Wrangellia docking?
Later moved by transform fault?
K-T Laramide Continental Overide
Bouyant Subduction
Buoyant Subduction Laramide Orogeny
Vertical block uplift
Normal, thin-skinned
Now we understand weird looking Tetons
Approaching Continent pushes
accretionary wedge sediments
into forearc sediments
Sevier thin-skinned deformation
Using the layer colored sky blue, look at the faults.
Is the hanging wall mostly up or down? What kind of faults
are these?
Sevier thrust belt
Precambrian and/or Paleozoic Sediments
thrust over younger Mesozoic rocks
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 until
70mya, thins to the
east
• Jurassic
Foreland Basin!
– clean cross-bedded
sandstones
– 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
Similar in Newark Supergroup
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 Apatosaurus, also Stegosaurus, carnivore Allosaurus
Fossils of Jurassic dinosaurs
Morrison Formation sandstones, DNM, Vernal, Utah
Giant sauropod and Allosaurus bones,
Mossison Fm., DNM, Vernal Utah
Late Cretaceous really big epeiric sea
Dinosaurs on the North Slope
Until 70 mya
Land
Land
Did the Sevier Orogenic Belt form before or after the Navaho SS, lower left?
Did the Sevier Orogenic Belt form before or after the Fox Hills SS, upper right?
Fox Hills SS
Western Interior Seaway Regression
Western Interior Seaway Transgression
Dakota SS is bottom right
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
Pierre
Shale
75 mya Regression
Mesa Verde Sandstones over Mancos Shale: Coarsening Upward
K-T Boundary
End of Mesozoic Geology