Early Paleozoic Events

1
Early Paleozoic Events

• CHAPTER 8

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In the Beginning (of the Paleozoic)

• Phanerozoic "visible life".
• Refers to rocks younger than the Precambrian.
  540 m.y. ago to the presentConsists of three
  eras (from oldest to youngest)
• Paleozoic "ancient life
• Mesozoic "middle life
• Cenozoic "recent life"
• Early Paleozoic Cambrian, Ordovician and
  Silurian
• Late Paleozoic Devonian, Mississippian,
  Pennsylvanian, and Permian

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Figure 3-38 (p. 85)Sedimentary facies
(lithofacies) developed in the sea adjacent to a
land area. The upper surface of the diagram shows
present-day facies, whereas the front face shows
the shifting of facies through time. Notice that
bottom-dwelling organisms also differ in
environments having different bottom sediment and
water depth.
4
Figure 3-41 (p. 87)An illustration of Walthers
Principle, which states that vertical facies
changes correspond to lateral facies changes.
(After Brice, J. C., Levin, H. L., and Smith, M.
S. 1993. Laboratory Studies in Earth History, 5th
ed., Dubuque, IA William C. Brown.)
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Marine Transgression

• The rocks of each facies become younger
• in a landward direction during a marine
  transgression

• One body of rock with the same attributes
• (a facies) was deposited gradually at different
  times
• in different places so it is time transgressive
• meaning the ages vary from place to place

younger shale
older shale
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Marine Regression

• During a marine regression,
• sea level falls with respect to the continent

• and the environments paralleling the shoreline
  migrate seaward

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Marine Regression

• A marine regression
• is the opposite of a marine transgression
• It yields a vertical sequence
• with nearshore facies overlying offshore facies
• and rock units become younger in the seaward
  direction

older shale
younger shale
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Sea Level Changes
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Paleogeography

• The ancient geographic arrangement of the
  continents is referred to as paleogeography
  ("ancient geography").
• Reconstructing the ancient arrangement of the
  continents requires paleomagnetic, paleoclimatic,
  geochronologic, tectonic, sedimentologic, and
  biogeographic fossil data.
• 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.

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Paleogeography

• Longitudes, however, cannot be determined (which
  accounts for some of the differences in the
  paleogeographic reconstructions).
• Paleoclimatic evidence comes from
  environmentally-sensitive sedimentary rock types
  (glacial deposits, coal swamp deposits, reef
  carbonates, evaporites).

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Late Neoproterozoic Paleogeography

• Global paleogeography during the Late
  Neoproterozoic, about 750 mya. Note that the
  continents are joined, forming the supercontinent
  Rodinia (or Proto-Pangea). Rodinia had begun to
  rift apart. The surrounding ocean is Mirovia.

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Late Neoproterozoic Paleogeography
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Cambrian Paleogeography

• continents have moved off the South Pole
• Iapetus Ocean (sometimes called the
  Proto-Atlantic) formed as Laurentia drifted away
  from South America.
• Note the Ouachita trough segment (green on the
  map above), which separates off as a
  microcontinent (see Middle Ordovician map), and
  eventually converges with South America (orange).

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Cambrian Paleogeography

• Note that shallow seas cover many of the
  continents. The evaporite deposits are clustered
  within about 30 degrees north and south of the
  equator. This is the latitude at which the great
  deserts of the world occur today.

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Figure 8-2 (p. 269)Paleogeographic and tectonic
elements of North America during the Cambrian
Period, showing position of the Cambrian
paleoequator.

• Warm ocean waters
• The presence of stromatolites and mudcracks in
  these carbonate rocks indicate deposition in
  shallow water.
• The water deepens toward the edges of the
  continent, where deep water shale is deposited
  (blue-green). Along the edge of the exposed land
  mass (yellow), sand is deposited.

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Ordovician Paleogeography

• Continents distributed along the equator.
• Note the Ouachita Terrane
• Also note the Taconic Orogenic Belt between
  Laurentia (North America) and Baltica (Europe and
  western Russia).

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Ordovician Paleogeography

• global sea levels were high. Shallow seas nearly
  cover many of the continents,
• By the Middle Ordovician, Gondwanaland was moving
  toward the south pole, which led to glaciation in
  Africa at the end of the Ordovician.

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Figure 7-9 (p. 248)In its long history, the
Earth has experienced several major episodes of
widespread continental glaciation.
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Figure 8-27 (p. 289) Paleogeographic map of
Ordovician North America.

• In North America during the Ordovician, sea
  levels were high, and the craton was flooded.
• In the Appalachian area, the Cambrian and early
  Ordovician were dominated by shallow water
  carbonate deposition, as indicated by the
  presence of mudcracks and stromatolites.

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Change in Ordovician Depositional Setting

• The depositional setting changed dramatically
  during the Middle Ordovician. Carbonate
  sedimentation ended. The carbonate platform
  collapsed or was downwarped.
• This was caused by the narrowing of the Iapetus
  Ocean along a subduction zone. Large volumes of
  black shales with graptolites, and immature
  sandstones spread westward over the carbonate
  rocks. As the Iapetus Ocean narrowed, a volcanic
  island arc approached the North American craton,
  leading to deformation and orogenic activity.

21
Figure 8-24 (p. 286)Plate tectonic forces that
resulted in the Taconic orogeny. (Adapted from
Rowley, D. B. and Kidd, S. F. 1981. J. Geol. 89
199-218.)
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Compare Cambrian paleography to Ordovician
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Taconic Orogeny

• There are two main highlands areas, the higher of
  the two is in the northern Appalachians.
• Erosion of Taconic mountains created the
  Queenston Delta (or clastic wedge)

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Silurian Paleogeography

• Note that the Iapetus Ocean is beginning to close
  as Laurentia and Baltica converge.

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Figure 8-30 (p. 291) Paleogeographic map of
Silurian North America.
Shallow marine deposits formed in the
epicontinental sea, including carbonates with
reefs, and the Michigan basin evaporites. Note
the Silurian Tuscarora Sandstone in the central
Appalachian region.
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Early Paleozoic Sedimentary Deposits

• The Cambrian-Precambrian Boundary
• The base of the Cambrian was long identified by
  the first-occurrence of shell-bearing organisms
  such as trilobites.
• In the 1970's, a distinctive group of small
  shelly fossils was found below the first
  trilobites in Siberia and elsewhere, and dated at
  544 my. This small shelly fauna includes sponge
  spicules, brachiopods, molluscs, and possibly
  annelids.

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Tiny Early Cambrian fossils with shells from
Siberia.
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New classification of the Cambrian base

• The new classification of Lower Cambrian
  chronostratigraphic units is based on the
  discoveries of the small shelly fauna and a
  variety of trace fossils. 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 544 my using
  uranium-lead isotope dates from rocks in NE
  Siberia .

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Figure 8-7 (p. 271)The trace fossil Phycodes.
(After Crimes, T. P. 1989. Trace Fossils, in The
Cambrian- Precambrian Boundary. Oxford Monographs
on Geology and Geophysics 12. Oxford, England
Clarendon Press.)
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Early Paleozoic Sedimentary Sequences

• Transgressions and regressions of seas occurred
  across North America in the Early Paleozoic as
  the glaciers melted and enlarged. These sequences
  are bounded by (or separated by) unconformities.
• Lawrence Sloss used the sedimentary record across
  the North American craton to divide the Paleozoic
  rock record into unconformity-bounded sequences,
  sometimes called cratonic sequences or Sloss
  sequences.

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Early Pz Cratonic Sequences

• Two major transgressions occurred in North
  America in the Early Paleozoic, which Sloss
  named
• Sauk sequence (older - primarily Cambrian)
• Tippecanoe (Ordovician-Silurian)

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Cratonic Sequences

• Similar sequences are found on other continents,
  suggesting that worldwide (or eustatic) sea level
  change was responsible for the transgressions and
  regressions.
• These eustatic sea level changes were probably
  related to glaciations and sea floor spreading.
  During times of rapid sea floor spreading, ridge
  formation displaces sea water onto the
  continents.

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Figure 3-42 (p. 88)Sloss 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.
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Cambrian Sedimentary Deposits - Sauk Sequence

• During the Cambrian, there were no vascular
  plants on the land, so the landscape was barren.
  Erosion would have been active and severe without
  plant roots to hold the soil.
• Transgression of the sea onto the craton followed
  the Neoproterozoic glaciaton.
• Shoreline (beach) deposition produced a vast
  apron of clean quartz sand.
• Carbonate deposition occurred farther from land.

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Figure 8-9 (p. 274)Upper Cambrian lithofacies
map. (Simplified and adapted from Stratigraphic
Atlas of North America and Central America. Shell
Oil Company, Exploration Department.)
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Grand Canyon Region

• The Lower Cambrian Tapeats Sandstone is an
  example of the sandy beach deposits unconformably
  overlying Precambrian rocks
• The Tapeats Sandstone is overlain by the Bright
  Angel Shale, an offshore deposit.
• The Bright Angel Shale is overlain by the Muav
  Limestone, indicating deposition farther from the
  land.
• These rocks form a transgressive sequence.

37
Cross section of Cambrian strata exposed in the
Grand Canyon. The red lines are trilobite zones,
which approximate time lines. Note that these
sedimentary units are diachronous (cut across
time lines). In each case, the sedimentary units
are older in the west than in the east.
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A Marine Transgression in the Grand Canyon

• Three formations deposited
• in a widespread marine transgression
• exposed in the walls of the Grand Canyon, Arizona

39
Ordovician-Silurian Deposits - Tippecanoe
Sequence

• When the Tippecanoe Sea flooded North America, it
  deposited the famous St. Peter Sandstone, an
  unusually pure, well sorted, well rounded quartz
  sandstone.
• The sandstone is overlain by extensive limestone
  deposits, locally replaced by dolomite.
• In the eastern US, the limestones are overlain by
  and interbedded with shales along the periphery
  of the Queenston delta or clastic wedge. The
  Niagara Falls area is a classic locality where
  these rocks are exposed.

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Figure 8-15 (p. 278)Stratigraphic section (A)
and block diagram (B) of Niagara Falls. (Map
after E. T. Raisz.)
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Niagara Falls
42
Tippecanoe Evaporites

• Near the end of the Tippecanoe sequence,
  landlocked reef-fringed basins developed in the
  Great Lakes area.
• Michigan Basin.
• During the Silurian evaporation led to the
  precipitation of immense quantities of rock salt
  and gypsum within the basin, indicating an arid
  paleoclimate.

43
Figure 8-17 (p. 279) Isopach map showing
thickness and area of evaporite basins during the
late Silurian. Areas of evaporite precipitation
were surrounded by carbonate banks and reefs.
(Adapted from Alling, H. L., and Biggs, L. I.
1961. Bull. Am. Assoc. Petrol. Geol. 45 515-547.)
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Figure 8-18 (p. 280)Cross-section illustrating
a model for the deposition of evaporites in a
barred basin.
Sea water flows into the basin over the
partially-submerged barrier. Evaporation produces
dense brines, which sink to the bottom of the
basin and are unable to mix with open sea water.
When the brine becomes sufficiently concentrated,
evaporite minerals are precipitated.
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Early Paleozoic Tectonics

• Plate Tectonic Events (in order)
• Breakup of Rodinia (Proto-Pangaea)
• Oceanic closing and orogeny to form Pangaea
• Taconic orogeny
• Caledonian orogeny
• Acadian orogeny
• Alleghenian orogeny
• Hercynian orogeny

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Taconic Orogeny

• Taconic orogeny occurred in three phases
• Appalachian Mountains built in collision with
  part of western Europe
• compression folded shelf sediments into mountains
  and Logan's thrust formed (48 km displacement)
• Giant granite batholiths produced by Taconic
  melting

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Appalachian Mobile Belt

• Evolution of the Appalachian mobile belt
• Late Proterozoic opening of Iapetus Ocean

• with passive continental margins
• and large carbonate platforms

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Appalachian Mobile Belt

• Middle Ordovician transition to convergence
  resulted in orogenic activity

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Early Paleozoic Climate

• Transgressions generally mild climate
• Regressions harsher, more diverse climates