Sedimentary Rocks and Depositional Environments

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Sedimentary Rocks and Depositional Environments

• Lab 1

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Rock Types - Background

• Igneous
• Metamorphic
• Sedimentary
• Broken down to form sediments
• Sediments fragments of rock, individual mineral
  grains (quartz), parts of plants or animals, clay
  minerals, and other minerals

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Minerals - Background

• Igneous, metamorphic, and sedimentary rocks are
  made of minerals
• Minerals substances that make up rock the
  building blocks of rocks
• Example minerals quartz, potassium feldspar,
  biotite mica, muscovite mica, calcite

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Sedimentary Rock Characteristics

• Grain size
• Rounding
• Minerals
• Matrix
• Color

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Grain Size

• The average diameter of the particles

Gravel gt 2 mm Visible to the unaided eye
Sand lt 2mm, gt1/8 mm Visible to the unaided eye, feels very gritty
Silt (mud) lt1/8 mm, gt 1/256 mm Invisible to the unaided eye, can see with a hand lens, feels gritty
Clay (mud) lt 1/256 mm Cannot be seen without a microscope, feels smooth, dull luster
Pg. 1-2 of lab manual
Ruler in Back of AGI Manual
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Minerals

• Building blocks of rocks

Mineral Hardness Cleavage Color Distinguishing Feature
Quartz 7 None (fractures) Milky to Colorless Hardness and looks glassy
Potassium feldspar 6 2 to 3 planes Salmon to Red Color
Biotite mica 2.5 1 plane Black to dark brown Black flakes
Muscovite mica 3 1 plane Colorless to light brown Light colored flakes shiny
Calcite 3 3 planes, rhomb White to Gray Reacts with acid (HCL)
Pg. 1-3 of lab manual
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Matrix

• Sedimentary rocks are composed of large particles
  (grains) surrounded by smaller particles
• The smaller particles are the matrix
• The fill in between larger grains

Nonethere are almost no open spaces
Some
All
Much
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Sedimentary Environments

• A rock will display certain characteristics
  depending on the environment in which it was
  formed
• The study of the composition and other
  characteristics of the sed. rocks can reveal
  info. about the conditions occurring during
  deposition (helps determine environment)

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Sedimentary Environments

• Alluvial Fans
• River Channels
• Glaciers
• Swamps
• Deltas
• Beaches and Barrier
• Islands
• Dunes

• Lagoons
• Tidal Flats
• Reefs
• Continental Shelf, Slope, and Rise
• Deep marine environments

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Refer to Figure 1.1 pg. 1-2
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Alluvial Fan

• Rivers in mountainous areas erode and transport
  sediment
• When meets flat plain deposits sediment in
  fan-like shape
• Common rocks sandstone, conglomerate, breccia,
  diamicton

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River Channels

• Rivers vary in size and energy level
• Contain various sediment types (gravel, sand)
• Feldspar less common
• Common rocks sandstone, conglomerate

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Glaciers

• Not able to sort sediment size
• Glacial sediments (till) are mixtures of gravel,
  sand, silt, and clay
• Rock types diamicton

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Swamps

• Lots of organic matter
• Slow decomposition
• Buried and compacted organic matter hardens to
  form coal
• Common rocks coal, shale

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Deltas

• Rivers flow into lakes or oceans
• Deposit sediment in a fan-like shape
• Mississippi River Delta, Nile Delta
• Large areas composed of various environments
  (channels, swamps, bays)
• Common rocks shale, siltstone, sandstone, coal

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Beaches and Barrier Islands

• Barrier island is like a beach separated from the
  mainland by a lagoon
• Constant wave action separates sand sized grains
  from others
• Sediment has traveled far from source, so well
  rounded
• Common rocks sandstone

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Dunes

• Form adjacent to beaches and barrier islands
• Contain non-marine fossils
• Common rocks sandstone

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Lagoons

• Behind a barrier island or reef
• Barrier island/reef act as breakwaters protecting
  the lagoon from wave action

• Low-energy environment
• Fine grain sizes settle and accumulate (silt,
  mud)
• Common rocks shale, siltstone

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Tidal Flats

• Broad, flat areas that are periodically covered
  in water when tide rises and dry when tide is low
• Variable energy levels
• Alternating layers of coarse and fine sediment
• Common rocks siltstone, shale, limestone

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Reefs

• Organisms build large, rocky accumulations by
  cementing their shells or other structures
• Common in tropical regions (warm water)
• Calcite precipitates
• Common rocks limestone

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Continental shelf, slope, and rise

• Shallow marine extend from beach to water
  depths of 100 m low energy fine grains settle
  common rocks are sandstone, siltstone, shale
• Deep marine extend down to thousands of meters
  low energy fine grains settle fossils are rare
  common rocks are shale, sandstone

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Todays Lab!!

• List properties of unknown sedimentary rocks
• Identify the unknown rock specimens (rock name)
• Determine possible depositional environment(s)
  where the rock formed

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Rock Names

• Shale
• Siltstone
• Sandstone
• Conglomerate
• Breccia
• Diamicton
• Limestone
• Coal