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Early Paleozoic Events


Mountains and volcanoes were present in the Appalachian region. ... These include the famous St. Peter Sandstone, an unusually pure, well sorted, ... – PowerPoint PPT presentation

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Title: Early Paleozoic Events

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 -

Paleozoic Era
  • The Paleozoic Era can be divided into
  • Early Paleozoic Cambrian, Ordovician and
  • Late Paleozoic Devonian, Carboniferous
    (Mississippian Pennsylvanian in N. Amer.), and

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
  • In the orogenic belts, strata are intensely
    deformed, with folding, faulting, metamorphism,
    and igneous intrusions.
  • Deformation occurred as a result of continental

  • In the Appalachian region, there were three
    Paleozoic mountain-building events (or
  • 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

  • Laurentia (North America, Greenland, Ireland, and
  • Baltica (Northern Europe and western Russia)
  • Kazakhstania (between the Caspian Sea and China)
  • Siberia (Russia east of the Ural Mtns and north
    of Mongolia)
  • China (China, Indochina, and the Malay Peninsula)
  • Gondwana (Africa, South America, India,
    Australia, Antarctica)

The Continents
  • When continents are located on a pole, and
    conditions are right, glaciers will form.
  • During glaciations, sea level is lowered
    worldwide because the water is tied up in the ice
  • Shallow epicontinental seas are unlikely during

  • 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.

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.

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

  • During regressions, the former seafloor was
    exposed to erosion, creating extensive
    unconformities that mark the boundaries between
    the transgressive-regressive 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 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

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,
    mid-ocean ridge volcanism displaces sea water
    onto the continents.

Cambrian Paleogeography
  • No continents at poles. Continents are on
  • 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

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

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
  • LEFT Global paleogeography during the Cambrian
    PeriodRIGHT 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
  • 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
  • Mountains and volcanoes were present in the
    Appalachian region.
  • Volcanic ash deposits are found in Ordovician
    rocks throughout the eastern U.S. (Now altered to
    a clay called bentonite).

  • 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

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

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
  • The Caledonian event is recognized in the
    Canadian Maritime Provinces, northeastern
    Greenland, northwestern Great Britain, and

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.

Comparison of Ordovician and Silurian
  • Laurentia (North America) still sits on the
  • 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

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,
    reef-fringed 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 (2325 ft) 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
  • This restricted basin is called a barred basin
    because it has a bar or sill between it and the
  • 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

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
  • 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.
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