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Geological cross-section of eastern North Dakota
Sheyenne meltwater trench
Moraines
Moraines
Lake Agassiz beds
Glacial drift
Pierre Formation
PreCambrian metamorphic rocks
Niobrara Formation
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• OLDEST ROCKS IN THE REGION AT THE SURFACE
• CRETACEOUS PERIOD ( ABOUT 90 Ma)
• TWO FORMATIONS
• Pierre Formation
• dark grey and black shales with bentonites
• Niobrara Formation
• pale yellow limestones
• Fossils include fish remains (teeth and scales),
  mosasaur
• remains (bones and teeth), ammonites (shells),
  gastropods
• (shells), bivalves (shells).
• Deposited in a shallow sea (Cretaceous Interior
  Seaway). The
• eastern shoreline was in central Minnesota and
  the western
• shoreline was in western North Dakota

GEOLOGY 304 Presentation 1
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bentonite
Cretaceous - Pierre Shale with bentonites
Buffalo Gap, South Dakota
Pierre Shale with layers of bentonite
Bentonite is weathered volcanic ash
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Cretaceous - Niobrara Formation - chalk
Fort Ransom, North Dakota
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Types of Cretaceous marine invertebrate fossils
Planktonic foraminifera (single-celled organisms)
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Cretaceous - Inoceramus - bivalve (clam)
Inoceramus Cretaceous bivalve (clam) - mollusc
Gregory member, Pierre Shale, near Kathryn, North
Dakota
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5 cm
Coiled ammonite swimming mollusc similar to a
snail in shape but possess septal sutures
Cretaceous Pierre Shale Didymocerus A
differently coiled ammonite
Late Cretaceous Ammonite - Acanthoscaphites
Pierre Shale, nr. Glendive, Montana.
Photo by A.C.Ashworth
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Squid - Fishermans Wharf, San Francisco
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Cretaceous sea floor - ammonites and other
molluscs
The cone-like shells are the ammonite Baculites
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Cretaceous mosasaur - major predator
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Western North Dakota in the Upper
Cretaceous Tyrranosaurus terrorizing giant sea
turtles Archelon
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Cretaceous Interior Seaway
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• ICE AGES ON EARTH
• There have been five major ice ages during the
• Earths 4.65 billion year history

Causes of ice ages

• Position of continents
• Can only have ice ages when there are continents
  near
• the Earths North and South poles (plate
  tectonics)

• During glaciations carbon dioxide levels are 30
  less
• than during interglaciations. Dust particles
  in the
• atmosphere are much higher

• Changes in carbon dioxide levels
• Mountain

PRIMARY CAUSES

• Mountain building (orogeny)
• Uplift of mountains e.g. Himalayas and the
  Tibetan Plateau
• produces change atmospheric and oceanic
  circulation
• patterns

• Orbital changes James Croll and Milan
  Milankovic
• Changes in Earth Sun relationships
  (Milankovitch cycles)

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Fossil air is trapped by ice as it accumulates in
ice sheets e.g. Greenland and Antarctica.
Studies of ice cores provide direct evidence of
the changes in composition of the atmosphere
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The difference in a Greenland ice core between
glacial (dirty - stronger circulation) and
interglacial ice (clearer - less circulation).
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Eccentricity of the Earths orbit

• Changes in orbital eccentricity affect the
  Earth-sun distance.
• The shape of the Earths orbit changes from
  being elliptical to
• being nearly circular in a cycle that takes
  about 100,000 years.
• When the orbit is highly elliptical, the amount
  of insolation received
• at perihelion would be on the order of 20 to
  30 percent greater than
• at aphelion resulting in a substantially
  different climate from what
• we experience today

http//earthobservatory.nasa.gov/Library/Giants/Mi
lankovitch/milankovitch_2.html
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Obliquity (change in axial tilt)

• Today, the Earth's axis
• is tilted 23.5º from the plane
• of its orbit around the sun

• During a cycle that averages
• about 40,000 years, the tilt of
• the axis varies between 22.1º
• and 24.5º

• As the axial tilt increases, the seasonal
  contrast increases so
• that winters are colder and summers are warmer
  in both
• hemispheres

• It's the cool summers that are thought to allow
  snow and ice to
• last from year-to-year in high latitudes,
  eventually building up
• into massive ice sheets. Further cooling
  occurs as heat is
• reflected from the white surfaces (Albedo).

..
http//earthobservatory.nasa.gov/Library/Giants/Mi
lankovitch/milankovitch_2.html
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Precession or Wobble of the Earths axis

• The axis returns to the same
• position about every
• 21,000 years.

• Changes in axial precession alter the dates of
  perihelion
• and aphelion, and therefore increase the
  seasonal contrast
• in one hemisphere and decrease the seasonal
  contrast in
• the other hemisphere.

http//earthobservatory.nasa.gov/Library/Giants/Mi
lankovitch/milankovitch_2.html