Transcript Late Mesozoic Geology.

HISTORICAL GEOLOGY
LECTURE 12. LATE MESOZOIC GEOLOGY.
CRETACEOUS (145-65
MYBP)
Paleogeography: The
Cretaceous begun with
the continuation of the
ZUNI
TRANSGRESSION,
which resulted in the
largest marine invasion
since the end of the
Pennsylvanian, 200
million years before.
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In north Texas, Cretaceous
rocks cover the surface and
dip gently to the southeast
(the Gulf) and grade into
Cenozoic rocks farther
south.
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If you travel west (which we do
on the field trip) you come to
the edge of the Cretaceous
bedrock and move onto the
underlying Pennsylvanian and
then Permian rocks, which dip
to the northwest.
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Here’s a smaller scale map
showing the bigger picture.
Arrows show dip, the red lines
show the locations of the
following upper and lower
cross sections. Also notice the
Llano Uplift (LU) and the
outliers of Cretaceous rocks
(OL).
OL
upper
lower
LU
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Permian
Pennsylvanian
Crtetaceous
Uplifted craton
Ouachita Orogenic Belt
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Permian Basin
Llano Uplift
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The result, in North America and elsewhere, was the
establishment of large EPEIRIC SEAS, characterized by
carbonate deposits especially CHALK, from the remains of
golden-brown algae (earlier blue-green + green algae did not
secret shells)("Creta" = Latin for chalk).
Coccoliths
(golden-brown
algae shells).
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There are many
cretaceous chalk
deposits around the
world. One example is
the famous WHITE
CLIFFS OF DOVER.
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Tectonics and Sedimentation
The east coast continued to be a passive divergent
margin in the Cretaceous; the Appalachians continued to
erode. The greatest tectonic activity was on the west coast.
Orogenic activity, resulting from plate convergence,
subduction and terrane accretion, had been going on since
the Early Paleozoic - the peak periods being the Antler,
Sonoma and Nevadan Orogenies.
By the Late Cretaceous, the mountainous Cordillera
of the west coast of North America was well established. Two
new orogenies (periods of intense mountain building)
occurred in the Cretaceous:
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THE SEVIER OROGENY (mid-Cretaceous)
The "crumpling up" of the west coast, which intensified
with the break up of Pangaea, was transmitted from west to
east through the Mesozoic. The Sevier Orogeny is notable for
low angle thrust faults which formed further inland (e.g.
Nevada, Utah, Montana, B.C. and Alberta), as compressional
stress from the west coast was transmitted eastwards. In many
cases Proterozoic rocks were thrust up over Mesozoic rocks in the case of the famous Lewis Thrust (Montana, B.C.,
Alberta) over a distance of 65 km (40 miles). The structure
produced is known as a decollement (old rocks pushed up on
top of younger rocks).
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The Sevier orogeny thrust belts
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THE LARAMIDE OROGENY (Late Cretaceous)
Orogenic activity continued into the Late Cretaceous,
again further eastward than previously. This resulted mostly
in folding of strata in the site of the present-day Rocky
Mountains of New Mexico, Colorado and Wyoming,
producing uplifted domes and anticlines, and basins. MOST
OF THE UNDERLYING STRUCTURES OF THE PRESENT
ROCKY MOUNTAINS WERE THE RESULT OF THE
LARAMIDE OROGENY; HOWEVER, THE ACTUAL
LANDSCAPE WE SEE TODAY RESULTED FROM
EPISODES OF UPLIFT AND EROSION IN THE LATER
CENOZOIC ERA.
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SEDIMENTATION
Apart from the chalk deposition already mentioned,
much of the sedimentation in North America occurred in the
backarc basin of the growing Cordillera region. This basin
stretched from Utah to Kansas. It contains a gradation of
clastics eroded from the Cordillera, from proximal
conglomerates and coarse sandstones to distal marine shales
A famous example is the Dakota sandstone, a beach deposit
formed along the eastern front of the Rockies
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This is similar to the situation in the east during the Pennsylvanian – a
clastic wedge created lowlands that were periodically submerged by
fluctuating sea levels. The result was cyclotherms and coal in states such
as Colorado.
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COAL
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EURASIA AND THE TETHYS SEAWAY
The Tethys seaway was a deep marine depositional
trough between Gondwanaland and Eurasia during the
Mesozoic. The trough was a site mainly of limestone
deposition. Soon after the break up of Pangaea, Africa begun
to converge on Eurasia. The result, mainly in the Cretaceous,
was the closing and compression of the Tethys Sea, along
what is now southern Europe. This caused orogenic activity
and volcanism along the southern margin of Europe, which
continued and intensified in the later Cenozoic. At the same
time, India had broken free from Gondwanaland and was
heading towards the site of the present-day Himalayas.
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Tethys
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