Prepared by Mark R' Noll

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Prepared by Mark R. Noll SUNY College at Brockport
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The Nature of Sedimentary Rocks

• Sedimentary rocks are composed of
• Fragments of other rocks
• Chemical precipitates
• Organic matter or biochemically produced materials

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The Nature of Sedimentary Rocks

• Sedimentary rocks are common at the Earths
  surface
• Cover 75 of the continents
• Cover nearly all of the ocean floor
• Easily eroded
• Occur in distinct layers (strata)

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The Nature of Sedimentary Rocks

• Layers are easily identified
• Majors layers (formations) easily recognized over
  large distances
• Smaller layers within a formation are separated
  by bedding planes
• Gradation in grain size, composition or physical
  features may vary

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Fig. 5.1. Layered sedimentary rocks exposed
in the Grand Canyon, AZ
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The Nature of Sedimentary Rocks

• Sedimentary rocks contain evidence of their
  environment of formation
• Animal and plant fossils
• Bedding planes indicate environment and mode of
  transport of sediment
• Texture consists of particle aggregates or
  crystalline precipitate minerals

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Classification of Sedimentary Rocks

• Sedimentary rocks are classified by texture and
  composition
• Texture defines 2 major categories
• Clastic
• Chemical and Biochemical

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Clastic Sedimentary Rocks

• Made of rock mineral fragments or clasts
• Clasts are broken and worn particles transported
  by water, wind or ice
• Clastic rocks are subdivided by grain size

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Fig. 5.4. Grain size ranges for classification
of common clastic sedimentary rocks
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Clastic Sedimentary Rocks

• Grain size is controlled by
• Size of grains in source rock
• Carrying capacity of transport process
• Weathering and erosion that occurs during
  transportation

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Clastic Sedimentary Rocks

• Common clastic sedimentary rocks
• Conglomerate
• Sandstone
• Mudrock or Shale
• Siltstone
• Claystone

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Chemical/Biochemical Sedimentary Rocks

• Formed by a process that takes ions from solution
  to form a solid
• Chemical Sediments
• Precipitates from water by an inorganic process
• Biochemical Sediments
• Formed during the growth of some organism

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Chemical/Biochemical Sedimentary Rocks

• Subdivided by composition and mode of formation
• e.g., Limestone
• Biochemical formation by algae, coral, etc.
• Direct chemical precipitate from warm sea water -
  oolites
• Chemical precipitate from springs and in caves

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Chemical/Biochemical Sedimentary Rocks

• Common Chemical/Biochemical rocks
• Dolostone - composed of dolomite
• Chert - microcrystalline quartz
• Various modes of formation
• Evaporites
• Rock salt - halite
• Gypsum

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Sedimentary Structures

• Strata or beds
• Distinct layers having variations in texture,
  color or physical properties
• Formations
• Large scale stratification
• Contain numerous individual strata

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Sedimentary Structures

• Cross-bedding
• Individual layers are inclined in reference to
  the top or bottom of the larger bed
• Formed by movement of sand waves or dunes
• Inclination shows flow direction and right-side-up

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TOP
BOTTOM
Fig. 5.9. Cross-bedding formed by dune
migration. Flow direction and top of bed
indicated.
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Sedimentary Structures

• Graded bedding
• Progressive change in grain size upward through a
  bed
• Fining upward
• Commonly formed by turbidites
• Subaqueous flows of muddy water
• Coarsest particles settle first

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Sedimentary Structures

• Surface impressions
• Indicate the top of a sedimentary bed
• Preserve features indicating past environment
• Ripple marks
• Mud cracks

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Sedimentary Structures

• Fossils
• Fossil remains or trace fossils
• Indicate paleo-environment
• Detailed study may provide
• Depth of water
• Temperature and salinity
• Relative age of rock

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Sedimentary Systems

• Systems work on the Earths crust
• Energy drives the system to change
• Solar
• Chemical Potential
• Gravity

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Sedimentary Systems

• Systems include
• Weathering
• Transportation
• Deposition
• Lithification
• Final sedimentary rock records the processes that
  produced it

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Fig. 5.13. Idealized diagram of
major sedimentary systems.
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Stratigraphic Sequences

• Sequences - Groups of formations bounded by
  erosional surfaces
• 17,000 formations within the U.S.
• Each formation is distinct in time, place and
  rock type
• Sequences of formations relate changes in
  depositional environments

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Stratigraphic Sequences

• Advance and retreat of shallow seas
• Common example of a stratigraphic sequence
• One cycle consists of
• Sandstone-shale-limestone-shale-sandstone
• Sequence base is an unconformity

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Fig 5.26. Sequence of sediments deposited
by transgression and regression of shallow sea
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Sequence Stratigraphy

• Sedimentary rock formations classified by rock
  type
• Sequences of formations may be grouped together
• Sequences separated by unconformity
• Relate global scale events
• e.g., Change in sea level

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Fig. 5.27. Changes in sea level throughout
geologic history
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Sedimentary Systems

• Plate tectonics plays a major role
• Sediment deposition geographic distribution
• Sediment source areas
• Formation of sequences

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Continental Environments

• Sediment produced by weathering erosion
• Transported by streams, wind or glaciers
• Thin deposits formed
• Most sediment carried to the sea

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Continental Rift Valleys

• Sequences of conglomerates sandstones
• Proximity to source area
• Wedge shaped deposits
• Lake or evaporite deposits may form
• Dependent on climate

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Convergent Plate Boundaries

• Subsiding basins associated with folded mountains
  accumulate clastic sediments
• Shallow seas may form in basins
• Marine limestones, shales or evaporites
• Turbidites associated with trenches
• Sediment from ocean floor is scraped off to form
  a melange

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Tectonic settings and their associated
sedimentary sequences