HISTORICAL GEOLOGY
LECTURE 11. EARLY MESOZOIC GEOLOGY.
TRIASSIC-JURASSIC
(251-145 MYBP)
Introduction: The
Mesozoic lasted about
186 million years. The
first two of the three
periods that make up
the Mesozoic - the
Triassic and the Jurassic
lasted about 116 million
years, or about the first
half of the Mesozoic Era.
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The Triassic begun just after the suturing of N. america and
Gondwanaland, in the Allegheny and Oauchita orogenies.
Mountains now formed a continuous belt along these coastlines.
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The Triassic was
also a time of major
marine regression the ABSAROKA
REGRESSION,
which exposed a
land area over
about the eastern
3/4 of the Craton.
Much of the craton
become land. Only
on the west coast
was there
dominantly marine
conditions.
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A major difference in
geography between
the Paleozoic and
Mesozoic is that no
sooner had Pangaea
become assembled in
the Paleozoic, it
promptly begun to
break up again in the
Mesozoic.
Note: Tensional
faulting along
Appalachians;
continued growth of
Cordilleran mountain
belt.
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2. Tectonics and Sedimentation
Major tectonic activity was related to the breakup of
Pangaea, which begun in the Triassic. In N. America there
were 2 main effects:
1. the east coast became a divergent margin, characterised by
tensional forces.
2. subduction on the west coast was enhanced, causing
increased orogenic activity in the Cordillera.
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The East Coast
In the Late Triassic (ca. 200
mybp), Gondwanaland began
to separate from North
America, opening a new ocean
basin along the east coast part of the modern Atlantic
Ocean. This divergence caused
fault-bounded tensional
troughs to form throughout the
Appalachian region.
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b) Arkose - terrestrial;
derived from granitic
highlands, contain > 25%
feldspar grains (implies fairly
short transportation, because
feldspar is relatively soft and
erodes over long distances).
Commonly pink-red color.
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These troughs filled with arkosic red sandstones, from the erosion of
the adjacent granitic uplands, forming the NEWARK GROUP of the
east coast. In addition to the ARKOSE formation (which is
characteristic of inland fault basins), the rifting also released
numerous lava flows into the same area.
Arkose sandstone
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An example is the famous PALISADES of the Hudson River, a
basaltic sill, dated at about 190 mybp.
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The rest of the
Triassic/Jurassic
consists of erosion of
the fault-block
mountains to a much
more subdued surface
and the development of
the Atlantic Coastal
Plain formed from the
eroded mountains (this
continued into the
Cretaceous). Much of
this wedge of clastic
sediments is now
offshore and forms the
modern
CONTINENTAL
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SHELF.
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Cenozoic
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The South Coast
The early Mesozoic saw the
erosion of much of the Ouachita
Mountains. A new depositional
basin - THE GULF OF MEXICO
- begun to form at this time. The
Gulf probably formed a shallow
embayment between the
separating North and South
Americas. In the Triassic,
sedimentation consisted mainly of
evaporites - particularly SALTS
(shallow seas, restricted
circulations in Bay). In the
Jurassic, the seas deepened with
the Zuni transgression and
carbonates started to dominate.
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Mesozoic and Cenozoic
sedimentation in the Gulf of
Mexico and the relationship of
stratigraphy to oil deposits.
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Bolivar Peninsula High Island
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High Island
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High Island after Hurricane Ike.
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The West Coast
During the early Mesozoic, tectonic activity was most active
on the west coast. As the North American Craton was now moving
away from Gondwanaland, it was actively "pushing" into the
Pacific Plate, greatly increasing subduction and compression in
the west. 3 major tectonic events occurred in the early Mesozoic:
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1. Late in the Permian, this subduction had formed an offshore
island arc, surrounded by great thicknesses of greywackes and
volcanics, and the thrust belt mountains of the Sonoma orogeny
further inland. In the Mesozoic, this island arc and the surrounding
marine sediments became progressively pushed up against the edge
of the continent, becoming part of it.
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This Late Jurassic deformation took place in the west, resulting
in the NEVADAN OROGENY - the results of which (mainly
metamorphosed greywackes - Fig. 13.19) can be seen in
California and Nevada.
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2. The intensified
subduction also generated
vast pockets of magma,
which begun to be
emplaced as massive
GRANITIC
BATHOLITHS, starting
in the Jurassic and
continuing through the
Cretaceous. Examples
include the Sierra Nevada
(mainly California), Baja,
Idaho and Coast Range
(Washington and B.C.)
Batholiths (these
batholiths are a notable
feature of the Nevadan
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Orogeny).
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The Coast Range mountains, Vancouver, form local ski hills.
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Granite batholith exposed in the Sierra Nevadas.
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Many red beds in Utah,
Colorado and Arizona
are Triassic-Jurassic
terrestrial sandstones
and shales, formed in
deserts and river
floodplains.
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3. The other major tectonic
event in the west was the
accretion of
ALLOCTHONOUS
TERRANES microcontinents, island arcs
and submarine basaltic
plateaus were all scraped off
the subducting Pacific Plate
and thrust up onto the
continent. 50 of these exotic
terranes have been
recognized, making up over
70% of the Cordilleran
Region (especially in B.C.
and Alaska).
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