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Transcript
Chapter 10
Early Paleozoic Events
The Phanerozoic Eon
• Phanerozoic = "visible life".
• 542 million years ago to the present
• Consists of three eras (from oldest to
youngest):
– Paleozoic = "ancient life" (542-251 m.y. ago)
– Mesozoic = "middle life" (251-65.5 m.y. ago)
– Cenozoic = "recent life" (65.5 m.y. ago present)
Paleozoic Era
The Paleozoic Era can be divided into:
• Early Paleozoic = Cambrian, Ordovician
and Silurian
• Late Paleozoic = Devonian, Mississippian,
Pennsylvanian, and Permian
Paleozoic
overview
The Paleozoic Era
The Paleozoic is characterized by long
periods of sedimentation punctuated by
mountain building.
• Paleozoic rocks of the
platform are relatively
flat-lying to gently
dipping.
• Paleozoic rocks in the
Ouachita-Appalachian
orogenic belt are
folded, faulted,
metamorphosed, and
intruded by granitic
rocks.
Paleozoic Rocks on the Platform
Across the platform,
in the continental
interior, Paleozoic
strata are relatively
flat-lying to gently
dipping, and warped
into basins, domes,
arches, and broad
synclines.
Orogenic Belts
Orogenic belts are present along the edges
of the continent.
In the orogenic belts, strata are intensely
deformed, with folding, faulting,
metamorphism, and igneous intrusions.
Deformation occurred as a result of
continental collision.
Orogenies
In the Appalachian region, there were three
Paleozoic mountain-building events (or
orogenies):
– Taconic orogeny
– Acadian orogeny
– Alleghanian orogeny
Paleozoic Paleogeography
• Paleogeography = "ancient geography".
The ancient geographic arrangement of
the continents.
• Reconstructing the paleogeography
requires paleomagnetic, paleoclimatic,
geochronologic, tectonic, sedimentologic,
and biogeographic fossil data.
Paleozoic Paleogeography
• Paleomagnetic evidence provides information on
the latitude at which the rocks formed.
• The orientation of the continent can be
determined from the direction to the
paleomagnetic pole, as recorded by bits of iron
in the rock.
• Longitudes, however, cannot be determined
(which accounts for some of the differences in
the paleogeographic reconstructions).
Paleozoic Paleoclimates
• Paleoclimatic evidence comes from
environmentally-sensitive sedimentary
rocks (glacial deposits, coal swamp
deposits, reef carbonates, evaporites).
• The early Paleozoic climate was affected
by several factors:
– The Earth spun faster and had shorter days.
– Tidal effects were stronger because the Moon
was closer to Earth.
– No vascular plants were present on the land.
Neoproterozoic (Late Precambrian)
Paleogeography
Just before the Paleozoic
began, the Precambrian
supercontinent, Rodinia,
had rifted apart to form
six large continents and
several smaller
continents.
The
Continents
1. Laurentia (North America,
Greenland, Ireland, and
Scotland)
2. Baltica (Northern Europe and
western Russia)
3. Kazakhstania (between the
Caspian Sea and China)
4. Siberia (Russia east of the
Ural Mtns and north of
Mongolia)
5. China (China, Indochina, and
the Malay Peninsula)
6. Gondwana (Africa, South
America, India, Australia,
Antarctica)
When continents are
located on a pole, if
conditions are right,
glaciers will form.
During glaciations, sea
level is lowered
worldwide because the
water is tied up in the ice
sheets.
Shallow epicontinental
seas are unlikely during
glaciations.
By the Cambrian Period,
the continents moved off
the pole. Some
continents lie on the
equator.
Glaciers melted, sea
levels rose, and shallow
epicontinental seas
flooded the continents.
Transgressions and Regressions
Shallow epicontinental seas transgressed
across the Laurentian (North American)
craton during the Early Paleozoic as the
glaciers melted and sea level rose. The
seas regressed as the glaciers enlarged
and sea level dropped.
Transgressive-Regressive
Sequences
The transgression and regression of the
seas deposited sequences of sedimentary
rocks that reflect the deepening and
shallowing of the waters. These are called
transgressive-regressive sequences.
Epicontinental Seas
Wave-washed sands, muds, and carbonates
were deposited in the shallow
epicontinental seas.
The epicontinental seas were sites of major
diversification of marine life.
Unconformities
During regressions, the former seafloor
was exposed to erosion, creating
extensive unconformities that mark the
boundaries between the transgressiveregressive sequences.
Cratonic Sequences
• The unconformities can be used to
correlate particular sequences from one
region to another.
• The unconformity-bounded sequences are
sometimes called cratonic sequences.
• Two major transgressions occurred during
the Early Paleozoic in North America:
– Sauk sequence (older - primarily Cambrian)
– Tippecanoe sequence (Ordovician-Silurian)
North
American
cratonic
sequences
Green =
sedimentary
deposits
Yellow =
unconformities
Unconformities
• Unconformities cover a greater time range near
the center of the craton.
• Unconformities near the edge of the craton
span less time, if they are present at all.
• This is because the edges of the craton are most
likely to remain flooded.
• The center of the craton is flooded only during
times of major sea level high stands or
transgressions.
Worldwide Sea Level Change
• Similar transgressive-regressive sequences are
found on other continents, suggesting that
worldwide sea level change caused the
transgressions and regressions.
• Worldwide sea level changes were probably
related to glaciations and/or sea floor
spreading.
• During times of rapid sea floor spreading, midocean ridge volcanism displaces sea water onto
the continents.
Vail Curves
• Cratonic sequences correspond to Vail
curves of global sea level change.
• Vail curves are derived from seismic
stratigraphic profiles, which permit tracing
of unconformities across the craton and
into thick continental margin sedimentary
rocks.
Vail Curves
showing global
sea level change
Cambrian Paleogeography
• No continents at poles. Continents are on equator.
• Shallow seas cover many of the continents.
• Evaporites within 30o N and S of equator - the
latitude at which deserts are present today.
• Iapetus Ocean (or Proto-Atlantic) formed as
Laurentia drifted away from South America.
Cambrian
Paleogeography
• Laurentia is nearly
covered by shallow
epicontinental seas.
• Laurentia lies on the
equator, so water is
warm.
• Deposition of sand &
carbonate sediments
• Water deepens
toward edges of
continent, where
shale is deposited
The Base of the Cambrian
• The base of the Cambrian was formerly
identified by the first-occurrence of shell-bearing
organisms such as trilobites.
• In the 1970's, small shelly fossils were found
below the first trilobites, and dated at 544 m.y.
The small shelly fauna includes
sponge spicules, brachiopods,
molluscs, and possibly annelids.
The Base of the Cambrian
• The base of the Cambrian is now placed at the
oldest occurrence of feeding burrows of the
trace fossil Phycodes pedum, and dated
radiometrically at 542 m.y. using uranium-lead
isotope dates from rocks in Oman coinciding
with a chemical anomaly known as the "negative
carbon-isotope excursion”.
Cambrian Sedimentary Deposits The Sauk Sequence
• During the Cambrian, there were no vascular
plants on the land, so the landscape was barren.
Erosion was active and severe without plant
roots to hold the soil.
• After the Neoproterozoic glaciation, the sea
transgressed onto the craton.
• Shoreline (beach) deposition produced a vast
apron of clean quartz sand.
• Carbonate deposition occurred farther from land.
Cambrian Deposits of the
Grand Canyon Region
In the Grand Canyon region, the Lower Cambrian
Tapeats Sandstone is an example of the sandy
beach deposits unconformably overlying
Precambrian rocks.
Cambrian Deposits of the
Grand Canyon Region
Tapeats Sandstone is overlain by Bright Angel
Shale, an offshore deposit. Bright Angel Shale is
overlain by Muav Limestone, deposited farther from
land.
These rocks form a transgressive sequence.
Cambrian Deposits of the
Grand Canyon Region
These sedimentary units are diachronous (i.e.,
they cut across time lines). In each case, the
sedimentary units are older in the west than in the
east. The red lines are trilobite zones, which
approximate time lines.
Cambrian Deposits of the
Grand Canyon Region
The three facies (sandstone, shale, and limestone)
coexisted and migrated laterally as sea level rose.
The Bright Angel Shale is Early Cambrian in the
west, and Middle Cambrian in the east.
Cambrian Deposits of the
Grand Canyon Region
Near the end of the Early Ordovician, the seas
regressed (due to glaciation).
The Muav Limestone was exposed to subaerial
erosion and a widespread unconformity developed.
Comparison of Cambrian
and Ordovician Paleogeography
LEFT = Global paleogeography during the
Cambrian Period
RIGHT = Global paleogeography during the
Ordovician Period
Ouachita Terrane
The Ouachita Terrane or "Ouachita embayment
microcontinent" has broken off from Laurentia/North
America, and is headed for a collision with South
America in the Andes region. This is the missing part
of the Appalachian Mountain chain between Alabama
and Arkansas.
Ordovician Paleogeography
The Taconic Orogenic
Belt lies between
Laurentia (North
America) and Baltica
(Europe and western
Russia) in the
Ordovician.
Ordovician Paleogeography
• Global sea levels were high. Shallow seas
cover large areas of some of the
continents, particularly North America
(Sauk epicontinental sea) and Siberia.
Ordovician Carbonate Rocks
• In the Appalachian area, shallow water
carbonate rocks were deposited during the
Cambrian and early Ordovician.
• Shallow water deposition is indicated by
the presence of mudcracks and
stromatolites.
End of Carbonate Deposition
• The depositional setting changed
dramatically during the Middle Ordovician.
• Carbonate sedimentation ended.
• The carbonate platform in eastern North
America collapsed or was downwarped.
• This was caused by the partial closure of
the Iapetus Ocean along a subduction
zone.
Volcanic Island Arc Collides with
Eastern North America
• As the Iapetus Ocean narrowed, a
volcanic island arc approached and
collided with the North American craton,
causing folding, faulting, metamorphism,
and mountain building.
• This mountain-building event in the
Appalachian region is called the Taconic
orogeny. 480 - 460 m.y. ago.
Ordovician
Paleogeography
Note the mountains
and volcanoes in the
Appalachian region.
Volcanic ash deposits
are found in
Ordovician rocks
throughout the eastern
U.S. (Now altered to a
clay called bentonite).
Ordovician Glaciation
• By Middle Ordovician, Gondwanaland moved
toward the South Pole, leading to glaciation in
Africa at the end of the Ordovician.
• Glacial deposits are present in NW Africa
(Sahara desert region), indicating that this
region was located in the South Pole region.
Ordovician Glaciation
Sea levels fluctuated
during the Ordovician, and
dropped sharply at the
end of the Ordovician,
coinciding with the
glaciation.
Plate tectonic cross-section showing forces
that caused the Taconic Orogeny.
A - Eastern North America in the Cambrian and early
Ordovician, following the breakup of Rodinia.
B - Large volcanic island arc nears eastern North America.
C - Volcanic island arc collides with eastern North America
causing Taconic orogeny.
• The area in eastern North America that
had been deep water shales during the
Cambrian was deformed and uplifted to
form the Taconic mountain belt.
• The shales were altered to metamorphic
and igneous rocks by the high
temperatures and pressures associated
with mountain building (orogeny).
Upper Ordovician
Sedimentary Deposits
As the Taconic mountain
belt eroded, Upper
Ordovician to Lower
Silurian red sandstones and
shales were deposited to
the west in huge delta
systems.
Upper Ordovician
Sedimentary Deposits
These sediments formed a
wedge-shaped deposit
known as the Queenston
clastic wedge, or the
Queenston delta. Red
deltaic sediments coarsen
and thicken to the east
(toward the mountainous
source area), and become
thinner and finer grained to
the west.
Upper Ordovician
Sedimentary Deposits
• The size of the clastic
wedge suggests that the
mountains may have
been more than 4000 m
(13,100 ft) high.
• There were two main
highland areas; the
higher of the two was in
the northern
Appalachians.
Caledonian Orogenic Belt
The Caledonian orogenic belt (which extends
along the northwestern edge of Europe) is part
of the same trend as the Taconic orogenic belt.
The Caledonian orogeny reached its climax
slightly later, in the Late Silurian to early
Devonian.
The Caledonian event is recognized in the
Canadian Maritime Provinces, northeastern
Greenland, northwestern Great Britain, and
Norway.
Comparison of Ordovician
and Silurian Paleogeography
• Laurentia (North America) still sits on the equator
• The Iapetus Ocean is beginning to close as
Laurentia and Baltica converge.
• Gondwanaland moves toward the South Pole.
• Silurian sea levels were high worldwide.
• In Laurentia (North America), much of the
craton was flooded, indicating melting of
the Late Ordovician glaciers.
• This was the second major transgression
of the Paleozoic, which deposited the
Tippecanoe Sequence.
Silurian
Paleogeography
• Mountains in eastern N.
America are eroding.
• Sandstone &
conglomerate deposits.
• Widespread carbonate
deposition.
• Deep marine deposits in
NW and SE U.S.
• Reefs and evaporites.
Silurian Sedimentary Deposits
• As the Tippecanoe Sea flooded North
America, deposition began with nearshore
sands.
• These include the famous
St. Peter Sandstone, an
unusually pure, well sorted,
well rounded quartz sandstone.
• The Silurian Tuscarora Sandstone was
deposited in the central Appalachian
region.
Silurian Sedimentary Deposits
• Sandstone is overlain by extensive limestone
deposits, locally replaced by dolomite.
• In eastern U.S., limestones are overlain by and
interbedded with shales along the periphery of
the Queenston delta. Niagara Falls is a classic
locality where these rocks are exposed.
Silurian Michigan Basin Evaporites
• Near the end of the Tippecanoe sequence, reeffringed basins developed, such as the Michigan
Basin.
• Evaporation led to the precipitation of immense
quantities of rock salt and gypsum within the
basin, indicating an arid paleoclimate.
• Evaporite minerals total 750 m thick in the
Michigan Basin.
• Accumulation of thick evaporites requires
continual addition and evaporation of sea water,
indicating that the basin was connected to the sea.
• This restricted basin is called a barred basin
because it has a bar or sill between it and the sea.
• Sea water flows into the basin over the bar.
• Evaporation produces dense brines, which sink to
the bottom. When the brine becomes sufficiently
concentrated, evaporite minerals are precipitated.
Silurian Iron Ore
• Economically important sedimentary iron ore
deposits accumulated during the Silurian in the
southern Appalachians, particularly around
Birmingham, Alabama.
• Steel was produced for many years in Birmingham
from this iron ore.
• Fuel was supplied by nearby Late Paleozoic coal
deposits.
• Limestone, also found nearby, was used as flux in
the blast furnace.
• In the Middle Silurian, shallow seas
appear to have covered more of the
continents than at any other time.
• The epicontinental seas withdrew
(regressed) toward the end of the Silurian.
Silurian Orogenic Activity
• Orogenic activity (mountain building) was
more or less continuous at one place or
another during the Silurian and Devonian.
• The Caledonian orogeny was most intense
in Norway, as the Iapetus Ocean closed.
• The folded rocks of the Caledonians end
in Ireland, but can be traced to NE
Greenland, Newfoundland, and Nova
Scotia, Canada.