Introduction to Geology GEO101004 Class 10: Sedimentary Rocks

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Introduction to GeologyGEO-101-004Class 10
Sedimentary Rocks
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Exam 1

• Four items on each of your exams
• MC/TF Multiple choice and true/false section
  (graded out of 45)
• SA Short answer (graded out of 12)
• Essay Essay section (graded out of 10)
• Final letter grade (curve adjusted for a C
  average).
• This exam represented 15 of your final grade
• If you got 0 on this exam theoretically you can
  still get a B in the class
• If you got 100 you can still fail!
• Main piece of advice for success in this class --
  READ

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• I will be away on the 16th and 22nd of February
  Dr. Delores Robinson and Dr. Geoffrey Tick will
  talking my place on those days

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Question of the Day
____________, in the north Atlantic Ocean is a
volcanic island formed over a hot spot on a
divergent plate boundary
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In the News..
The next few slides are courtesy of the Royal
Navy..
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From E.J.W. Jones, Marine Geophysics
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Why did it happen?

• Indian plate is being subducted beneath Eurasia /
  Burma plate
• Rate 50-60mm/yr but very oblique
• Two large earthquakes during C19th further to
  south (Figure from studies by Kerry Sieh, CalTech)

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Generic subduction zone cross section
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Lines Run
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OVERLAY OF DATA COLLECTED AND CURRENT CHART
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Channel and ridge
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Examples of Landslide Scars
Slide Scar Complex
Slide Scar
Canyon leading on to abyssal plain
Length of image approx. 25km
Processed by Caris
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Slide Scars and Thrust Faults
Slide scar propagating southwards
Ridges from thrust faulting
Processed by Caris
Canyon
Plate boundary
(Approx. 40km across)
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Youthful Slide with Slide Blocks
Youthful Slide Scar
Slide Scar
Canyon
Youthful Slide Scar
Slide Scar
Canyon
Abyssal Plain
Abyssal Plain
Processed by Caris
Slide Blocks
Slide Blocks
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Example of Slide Scar
Mature slide scar area with cross cutting canyons
Slide Scar
Canyon
Processed by Caris
Abyssal Plain
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Diversion of Canyon
Canyon
Ridge causing diversion of canyon as it is
forced upwards
Processed by Caris
Abyssal Plain
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Area of Active Erosion
Slide scars
Processed by Caris
Canyons
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Older slide on newer ridge
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What is a sedimentary rock?

• Sedimentary rocks are products of mechanical and
  chemical weathering
• They account for about 5 (by volume) of Earths
  outer 10 miles
• Contain evidence of past environments
• Provide information about sediment transport
• Often contain fossils

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What is a sedimentary rock?

• Sedimentary rocks are important for economic
  considerations because they may contain
• Coal
• Petroleum and natural gas
• Sources of iron, aluminum, and manganese
• What else??

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Turning sediment into rock

• Many changes occur to sediment after it is
  deposited
• Diagenesis chemical, physical, and biological
  changes that take place after sediments are
  deposited
• Burial promote diagenesis
• Occurs within the upper few kilometers of Earths
  crust at temperatures less than 150-200oC

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Turning sediment into rock

• Diagenesis
• Includes
• Recrystallization development of more stable
  minerals from less stable ones
• Aragonite (a form of calcium carbonate secreted
  by marine animals to form shells) recrystalizes
  to form the more stable mineral calcite as burial
  takes place
• The end result is limestone

http//www.palaeos.com
http//mineral.galleries.com/
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Turning sediment into rock

• Diagenesis
• Includes
• Lithification sediments are transformed into
  solid rock by
• Compaction the most common diagenetic change
• The weight of the overlying material compresses
  the sediment pore space is reduced
• Burial of a clay may result in a 40 reduction in
  volume
• Sands do not compact as much
• The squeezing out of pore water may promote other
  processes????

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Turning sediment into rock

• Diagenesis
• Includes
• Lithification sediments are transformed into
  solid rock by
• Cementation the most important process for
  turning sediments to rock
• Cement is carried by fluids circulating in the
  pore spaces
• In time the cement precipitates onto the sediment
  grains and eventually fills the pore spaces and
  joins the particles
• Natural cements include calcite (reacts with
  HCl), silica (the hardest), and iron oxide
  (reddish color)

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Question of the Day
Why are these rocks different colors? What can
you say about the source of these beach rocks?
Describe and explain the shape of the rocks
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Types of sedimentary rocks

• Sediment originates from mechanical and/or
  chemical weathering
• Rock types are based on the source of the
  material
• Detrital sedimentary rocks transported sediment
  as solid particles
• Chemical sedimentary rocks sediment that was
  once in solution

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Detrital sedimentary rocks

• The chief constituents of detrital rocks include
• Clay minerals the most abundant product of
  chemical weathering of silicate minerals e.g.
  feldspars
• Quartz very durable and resistant
• Feldspars
• Micas
• Chemical weathering will rapidly destroy
  feldspars and micas, so if they are present in a
  rock it indicates they might not have traveled
  far from their source
• Particle size is used to distinguish among the
  various rock types

http//mineral.galleries.com
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Detrital sedimentary rocks

• Common detrital sedimentary rocks
• Shale
• Silt- and clay-sized (microscopic) particles in
  thin layers that are commonly referred to as
  laminea
• Most common sedimentary rock
• Forms in non-turbulent settings flood plains,
  lagoons.

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Detrital sedimentary rocks

• Common detrital sedimentary rocks
• Shale
• Black shale organic rich
• Must have formed in swamp like environment
• A swamp is an oxygen poor environment where
  organic material does not readily oxidize and
  decay
• Exhibits an ability to split into layers -
  fissility

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If shale is such a common rock in the sedimentary
rock world, why isn't it as prominently exposed
at the surface as sandstone?
07.04

• Shale crumbles easily, causing increased
  mechanical weathering, whereas sandstone resists
  weathering more effectively.
• Shale resists weathering more effectively,
  whereas sandstone crumbles easily, causing
  increased mechanical weathering.
• Shale has easily broken, highly reactive
  minerals, whereas sandstone has harder-to-break
  minerals.
• Shale dissolves in water, whereas sandstone
  dissolves only if enough carbonic acid is present
  in the water.

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Detrital sedimentary rocks

• Common detrital sedimentary rocks
• Sandstone
• Composed of sand-sized particles (1/16 - 2 mm)
• Forms in a variety of environments
• Quartz is the predominant mineral

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Sorting the degree of similarity is particle
size in a sedimentary rock If all the grains are
about the same size it is well sorted If all
grain size varies widely it is poorly sorted
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A quartz sandstone
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Detrital sedimentary rocks

• Conglomerate and breccia
• Both are composed of particles greater than 2mm
  in diameter large enough to be recognized as
  distinctive rock types
• Very valuable for determining the source of the
  rock
• Conglomerate consists largely of rounded gravels
• Mountain rivers, rapidly eroding coasts, glacial
  and landslide deposity
• Breccia is composed mainly of large angular
  particles not transported as far

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Chemical sedimentary rocks

• Consist of precipitated material that was once in
  solution
• Salt
• Calcite
• Precipitation of material occurs by
• Inorganic processes
• Organic processes (biochemical origin)

http//www.palaeos.com
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Chemical sedimentary rocks

• Common chemical sedimentary rocks
• Limestone
• Most abundant chemical rock
• Composed chiefly of the mineral calcite
• Marine biochemical limestones form as coral
  reefs, coquina (broken shells), and chalk
  (microscopic organisms)

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Coquina
Figure 7.9
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Fossiliferous limestone
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Chalk