HISTORICAL GEOLOGY
LECTURE 10. LATE PALEOZOIC GEOLOGY I.
The Late Paleozoic
(Devonian-MississippianPennsylvanian-Permian)
is noted for a number of
geologically significant
events:
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- widespread COLONIZATION of the land by large
plants, reptiles and amphibians.
- a number of major OROGENIES, including the
ACADIAN (Devonian), the ANTLER (Mississippian),
the HERCYNIAN (Mississippian), the ALLEGHENY
(Pennsylvanian), OUACHITA (Pennsylvanian) and the
SONOMA (Permian).
- major non-marine deposits, including COAL beds
(related to the development of LAND PLANTS), and
sand dunes/evaporites (related to the widespread aridity
of many parts of the northern continents.
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DEVONIAN - MISSISSIPPIAN
1. Paleogeography
The North American craton had continued its counterclockwise
rotation relative to the Equator. Most of the craton was still
experiencing hot, tropical conditions. Carbonates were forming in
the quieter regions of epeiric (platform) seas; clastics were being
shed by the newly emergent mountainous areas along the orogenic
belts; coral reefs and evaporites were forming in the more restricted
shallow marine areas.
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2. Transgressions and Regressions
The beginning of the
Devonian is marked by the
TIPPECANOE REGRESSION,
causing widespread early
Devonian erosion and leaving a
major EROSIONAL
UNCONFORMITY, used to
identify the boundary between the
TIPPECANOE and the
KASKASKIA - the next major
transgressive sequence.
The rest of the Devonian is
characterized by THE
KASKASKIA
TRANSGRESSION, which
continued into the Mississippian.
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The Kaskaskia transgression
left coastal sands spread
across New York,
Pennsylvania and Virginia –
the Oriskany Sandstone.
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In deeper water, shales
formed (especially after
the Acadian orogeny –
which promoted
vigorous erosion and
mud production). A wellknown example is the
Chattanooga Shale of
Tennessee. 'Black shales'
are dark, as a result of
being especially rich in
unoxidized carbon
(stagnant water, little
oxygen).
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Exposure of Chattanooga Shale.
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3. Orogenies
The Late Paleozoic is noted for 3 major orogenic events:
1. THE ACADIAN OROGENY: the continental convergence that
formed the Taconic Uplands of the Early Paleozoic continued into
the Late Paleozoic as THE ACADIAN OROGENY. The Acadian
Mountains run from Newfoundland right down the east coast to
west Virginia. They consist of folded sedimentary and
metamorphic rocks containing igneous intrusions. A major
consequence of this mountain building was the formation of a very
large clastic wedge spreading westward across the craton - THE
CATSKILL WEDGE. Coarse terrestrial sediments in the east;
fine-grained clastics and carbonates in the west.
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The nonmarine sandstones and
shales in this wedge are often
deeply OXIDIZED (iron
combines with oxygen - similar
to rust), producing a red
coloration - giving these beds a
similar origin and appearance
to the OLD RED
SANDSTONES of Europe
(many older buildings in
eastern cities contain red
sandstone blocks from the
Catskill Wedge - “brown
stones”).
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Brownstones New York
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2. THE ANTLER OROGENY:
On the west coast, the island arc that formed back in the
Ordovician had now been pushed up against the continent
forming the highlands and volcanic activity of the ANTLER
OROGENY, running through (what is now) Nevada, Idaho and
into British Columbia. These mountains became source areas for
clastics that spread across the western states in the Pennsylvanian
and Permian.
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The mountainous west (Cordillera) was beginning to form.
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3. THE HERCYNIAN OROGENY: The collision between
Gondwanaland and the northern continents occurred slightly
earlier in Europe than in North America. The result was the
Late Mississippian HERCYNIAN OROGENY (roughly
equivalent to the Allegheny Orogeny of North America, which
occurred slightly later - in the Pennsylvanian). A great
mountain chain was pushed up along the margins of Southern
Europe. Some of the clastic wedges that formed around these
mountains were covered by forest and went on to become some
of the great European coal basins. Much of the Hercynian
Mountain chain was subsequently eroded away - only small
eroded stumps remain uncovered today.
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