Early Paleozoic Events

1
Chapter 10

• Early Paleozoic Events

2
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)

3
Paleozoic Era

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

4
The Paleozoic Era

• The Paleozoic is characterized by long periods
  of sedimentation punctuated by mountain building.
  

5

• 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.

6
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.

7
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.

8
Orogenies

• In the Appalachian region, there were three
  Paleozoic mountain-building events (or
  orogenies)
• Taconic orogeny
• Acadian orogeny
• Alleghanian orogeny

9
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.

10
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).

11
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.

12
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.

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• Laurentia (North America, Greenland, Ireland, and
  Scotland)
• 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
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• 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
  sheets.
• Shallow epicontinental seas are unlikely during
  glaciations.

15

• 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.

16
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.

17
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.

18
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.

19
Unconformities

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

20
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)

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• North American cratonic sequences
• Green sedimentary deposits
• Yellow unconformities

22
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.

23
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.

24
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.

25
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

26
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.

27
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.

28
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.

29
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.

30
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.

31
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.

32
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.

33
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.

34
Comparison of Cambrian and Ordovician
Paleogeography

• LEFT Global paleogeography during the Cambrian
  PeriodRIGHT Global paleogeography during the
  Ordovician Period

35
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.

36
Ordovician Paleogeography

• The Taconic Orogenic Belt lies between Laurentia
  (North America) and Baltica (Europe and western
  Russia) in the Ordovician.

37
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.

38
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.

39
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.

40
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.

41
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).

42

• 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.

43

• 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).

44
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.

45
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.

46
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.

47
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.

48
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.

49
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.

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• 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.

51
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.

52
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.

53
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.

54
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.

55

• 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.

56
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.

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• 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.

58
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.