Early Paleozoic Earth History

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Early Paleozoic Earth History
http//jan.ucc.nau.edu/rcb7/510NAt.jpg
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North American Paleogeography

• Broke away from supercontinent 600 Ma
• Configuration of supercontinent not
  well-understood
• How do we know?
• Evidence of rifting (divergent margin)
• Basalt
• Subsidence cooling and thinning of lithosphere

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Early Paleozoic Climate of NA

• North America in tropical location
• End of Snowball Earth
• Warmer in Cambrian and Ordovician

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Paleogeography

• Location of continents and ocean basins
  constantly changes
• Historical geology provides past geologic and
  paleogeographic reconstruction of the world
• Paleogeographic maps show
• distribution of land and sea
• possible climate regimes
• geographic features (mountain ranges, swamps,
  glaciers)

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Early Paleozoic Global History

• Cambrian world consisted of six major continents
  at low tropical latitudes
• Water circulated freely among ocean basins, and
  the polar regions were mostly ice free

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Paleogeography of the World

• For the Late Cambrian Period

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Paleogeography of the World

• For the Late Ordovician Period

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Paleogeography of the World

• For the Middle Silurian Period

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Early Paleozoic Evolution of North America

• The geologic history of the North American craton
  may be divided into two parts
• relatively stable continental interior over which
  epeiric seas transgressed and regressed
• mobile belts where mountain building occurred
• Sedimentary-rock record of NA subdivided into six
  cratonic sequences

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Paleozoic History

• The Paleozoic history of most continents
• major mountain-building activity along margins
• numerous shallow-water marine transgressions and
  regressions
• These transgressions and regressions
• caused by global changes in sea level
• related to plate activity and glaciation

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Cratonic Sequence

• A cratonic sequence is
• large-scale lithostratigraphic unit
• represents a major transgressive-regressive cycle
  
• bounded by unconformities
• The six unconformities extend across the North
  American craton

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Cratonic Sequences of N. America
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The Sauk Sequence

• Rocks of the Sauk Sequence
• Late Proterozoic-Early Ordovician
• record the first major transgression onto the
  North American craton
• Deposition of marine sediments limited to the
  shelf areas of the craton
• The craton itself was above sea level

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Transcontinental Arch

• By the Late Cambrian, Sauk Sea had covered most
  of North America, leaving above sea level only
• a portion of the Canadian Shield
• and a few large islands, the Transcontinental
  Arch,
• extended from New Mexico to Minnesota and the
  Lake Superior region

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Cambrian Paleogeography of North America

• During this time North America straddled the
  equator
• Trans-continental Arch

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Cambrian Facies
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Cambrian Facies

• Sediment derived from craton
• Sandy facies occur closest to craton
• Quartz-rich sand derived from craton
• craton surface weathered and eroded for half a
  billion years!
• no land plants yet erosion by wind more common
• Mature sandstone well-rounded, well-sorted, 99
  Qtz,

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Where were they deposited?

• Fossils suggest marine environment
• No land animals at this time
• How do we interpret environment?

Use sedimentary structures!
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Cross-stratification

• Formed by wind or water
• Wind dunes gt 1 m high
• Water typically smaller

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Cross-stratification
2 m
WIND!
www.env.duke.edu/eos/geo41/win2.htm and
homepage.ntlworld.com/donald.mcintyre/menu_files/s
tones_files/crossbeds_files/peddie_230.jpg
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End of Cambrian

• Most of North America underwater
• Sauk Transgression
• Epeiric sea
• shallow inland sea (over continent)
• lt 200 m deep

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Cambrian Transgression

• Cambrian strata exposed in the Grand Canyon

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Beginning of Ordovician

• Very little land remaining above SL
• Little terrigenous clastic sedimentation
• Shift to carbonate deposition
• limestones formed from shell debris
• clear, shallow water

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Stromatolites

• Constrain water depth
• need sunlight for photosynthesis
• Photic zone 150-200 m
• All evidence suggests very shallow Sauk Sea

Shark Bay, Australia
http//www.mlssa.asn.au/journals/1999Journal.htm
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Regression and Unconformity

• As the Sauk Sea regressed during the Early
  Ordovician, it revealed a landscape of low relief
• The rocks exposed were predominately limestones
• The resulting craton-wide unconformity marks the
  boundary between the Sauk and Tippecanoe sequences

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Ordovician Period

• Paleogeography of North America showing change in
  the position of the the equator

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What North American mountain range began to form
in the Ordovician?
Orogeny
Appalachians
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The Taconic Orogeny

• Named after present-day Taconic Mountains of
• eastern New York
• central Massachusetts
• and Vermont
• First of several orogenies to affect the
  Appalachian region

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Appalachian Area

• East coast was passive margin in Early Ordovician
  like modern Gulf of Mexico
• Changed to active margin in Middle Ordovician
• Collision with microcontinent or volcanic arc

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Appalachian Mobile Belt

• Middle Ordovician transition to convergence
  resulted in orogenic activity

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Ordovician Plate Tectonics
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Orogeny Timing

• Volcanic rocks from present-day Georgia to
  Newfoundland
• Clustering of radiometric ages between 440 to 480
  million years ago
• In addition, regional metamorphism coincides with
  the radiometric dates

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Clastic Wedge
Debris eroded from mountains into adjacent basin
Thick layer of sediment adjacent to source
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Silurian Period

• Reefs developed in the Michigan, Ohio, and
  Indiana-Illinois-Kentucky areas

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Silurian Sedimentation