HISTORICAL GEOLOGY
LECTURE 11. LATE PALEOZOIC GEOLOGY II.
PENNSYLVANIAN - PERMIAN.
1. Paleogeography
The Pennsylvanian opened with the Kaskaskia Regression,
which left a widespread erosional unconformity - the basis for
the division of the CARBONIFEROUS into the
MISSISSIPPIAN and PENNSYLVANIAN in North America.
The regression, combined with continuing orogenic uplift in
the east, and the continuing deposition of clastics, resulted in
a large land area forming down the eastern half of the North
American Craton. Africa was now joined to the north east of
the North American Craton, the SUTURE ZONE marked by
a vast mountain chain (the Appalachians).
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The Craton continued its counterclockwise rotation and
was now moving towards the north pole - bringing the
Equator into a WSW - ENE alignment across the Craton.
The climate was still, however, Tropical. The Antler
Mountains were continuing to grow in the west and some
other small upland areas resulting from plate convergence
were developing further north along the west coast.
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2. Transgressions and Regressions
The KASKASKIA REGRESSION continued until midPennsylvanian time; the rest of the Pennsylvanian is
characterized by a marine transgression - THE ABSAROKA
TRANSGRESSION.
An important part of Pennsylvanian deposits were the COAL
BEDS that formed in low-lying areas to the west of the eastern
mountains. These deposits probably originated as thickly
vegetated swampy areas near the coast. Frequent marine
inundation would have killed off the vegetation and ensured
rapid burial under swampland muds. This would have
prevented decomposition of the organic material by
OXIDATION; resulting in the formation of carbon-rich peatlike deposits. Continuing burial and compaction, eventually
transformed this to COAL.
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Features often associated with the coal beds are
CYCLOTHERMS, which record frequent and rapid sealevel changes in the form of a cycle of land and sea deposits.
Typically, the cyclotherm consists of a basal layer of
terrestrial swampy sands, shales and clays, usually capped
by coal; aburptly overlain by shallow marine shales and
limestones, which gradually graded up into terrestrial
deposits once again. Each cyclotherm is typically about 30 50 feet thick; in Missouri and Kansas, stacked cyclotherms
are up to about 2500 feet in thickness. The cause of the
repeated sea-level fluctuations isn't certain, but it may have
been periodic regional uplift or subsidence related to the
continuing orogeny in the east; or eustatic (global) sea-level
changes related, perhaps, to glaciations in Gondwanaland.
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By the Early Permian
the sea was regressing,
resulting in restricted
shallow inland seas and,
conequently, the
development of
evaporite deposits;
especially notable are
the large Permian salt
beds in Kansas.
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The Permian Basin of Texas and the Guadalupe Mountains - a Permian reef.
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El Capitan - part of the Permian reef.
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3. Orogenies
The major orogenic episode occurred during Early
Pennsylvanian time - THE ALLEGHENY OROGENY. This was
essentially the continuation of the (slightly) earlier
HERCYNIAN OROGENY in Europe. Northwest Africa collided
with eastern North America, producing mountain building,
mainly in the Southern Appalachians. It was this compression
that caused widespread thrust-faulting throughout the
Appalachians, resulting in the "valley and ridge" province.
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Satellite image of severe
folding recording the
Allegheny Orogeny.
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At about the same time, South America collided with North
America crumpling up the Ouachita Trough and forming a
great mountain chain along the southern margin of North
America - THE OUACHITA OROGENY.
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View of
balcones
escarpment.
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Erosion has since removed much
of these mountains leaving only
the Ouachita Mountains of
Oklahoma/Arkansas, the
Arbuckles of Oklahoma and the
Marathon Mountains of Texas.
This collision between
Gondwanaland and Luarussia
not only threw up mountain belts
around the edge of the Craton,
but also transmitted stress into
the interior of the Craton,
resulting in INTRACRATONIC
OROGENESIS - the results were
the COLORADO MOUNTAINS
(the Ancestral Rockies).
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Arbuckles pix
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Turner Falls
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In the west, the ANTLER OROGENY continued into the
Pennsylvanian. Subduction also continued, building an island
arc down the entire west coast. These uplands provided large
amounts of clastic and volcanic deposits to this region. Many of
these volcanic uplands continued moving eastward, pushing up
onto the North American Craton and becoming part of it in the
late Permian SONOMA OROGENY. The mountainous
CORDILLERA region was clearly beginning to take form.
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Permian sand dunes in Arizona
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Pennsylvanian in north Texas: variety of coastal environments
– black shelf muds, patch reefs, deltas, limestones (we see
these, and more, on the field trip)
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PANGAEA
With the suturing of Baltica and Laurentia by the Caledonians
" Laurussia and Gondwana " Hercynians, Appalachians,
Ouachitas"
Laurussia and Siberia " Urals.....
Pangaea was now assembled.
Of course, it promptly breaks up
again…
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