Sedimentary Rocks

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Sedimentary Rocks
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Terminology

• Sedimentary Rock - a rock formed through the
  deposition of sediments from weathering or
  biologic activity.
• Diagenesis- Refers to all the chemical, physical,
  and biological changes that occur after sediment
  is deposited and before and after lithification
• Lithification - The process by which
  unconsolidated sediments are transformed into
  sedimentary rocks.

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Lithification

• Two Processes control Lithification Compaction
  and Cementation
• Compaction- As sediment accumulates the weight of
  the overlying material compresses the lower
  levels.
• This leads to a reduction in pore space (Imagine
  a garbage compressor)
• The size and shape of the grains that make up the
  sediment will directly determine the amount of
  compaction that occurs.
• Sands and other course-grained sediment are less
  compressible than fine grained sediments such as
  clay.

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Cementation

• Cementation- Refers to the glue (cement) that
  holds the sediments together)
• For the most part, cementing materials are the by
  product of chemical weathering (They are the
  dissolved components of other rocks).
• Important Cementing agents
• Calcite (CaCO3)
• Silica (SiO4)
• Iron Oxide (Fe2O3, FeO)
• These materials precipitate out of solution and
  become deposited within the pore spaces between
  the sediment grains

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Types of Sedimentary Rocks

• Detrital- Accumulated material that is
  transported as solid particles that originated
  from chemical or mechanical weathering.
• Chemical- Formed as previously dissolved minerals
  precipitate out of solution.

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Identification of Sedimentary Rocks

• Detrital sedimentary rocks are identified by both
  their composition and their grain size
• Chemical Sedimentary rocks are classified by
  their chemical composition

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Sedimentary Environments

• Sedimentary rocks form in many different
  depositional environments ie. beaches, oceans,
  mountains
• Each area depositing a similar material is
  considered a sedimentary facies.
• As we move from the Continental Environment to
  the Marine Environment there will be a gradual
  transition between the facies due to the matter
  of transport and other factors affecting
  deposition.

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Grain Size vs. Sedimentary Environment

• Detrital grain size gives some indication of the
  energy conditions during transport and deposition
• High-energy processes such as swift-flowing
  streams and waves are needed to transport large
  grains such as gravel
• Conglomerate (composed mostly of gravel) must
  have been deposited in areas of high energy
• Sand transport also requires vigorous currents
• Silt and clay are transported by weak currents
  and and therefore only accumulate under
  low-energy conditions such as in lakes and lagoons

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Examples of Sed. Facies
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Continental Environments

• Dominated by erosion and deposition associated
  with streams
• Channel Deposits
• Alluvial Fans
• Flood Plains
• In colder areas glacial movement takes the place
  of rivers and streams
• Morrains
• In more arid areas wind is the more dominant
  factor
• Dunes
• Playa Lakes

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Transitional Environments (the shore line)

• Beaches
• Tidal flats
• Lagoons
• Spits, bars and barrier islands

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Marine

• Divided according to depth
• Shallow Marine-
• reaches to depths of about 200 meters (700ft)
• Extends from the shore to the outer edge of the
  continental shelf
• In some areas can reach up to 1500km, however,
  average is about 80 km
• Types of sediment deposited depend on several
  factors
• Distance from shore
• Elevation of adjacent land
• Water depth
• Water temperature
• Climate

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Deep Marine

• Seaward of the continental shelf and reaches
  depth greater than 200 m
• Deposition in these environments are composed of
  tiny particles that drift in the ocean current as
  well as skeletal remains of microscopic animals
  such as diatoms.

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Sorting and Rounding

• Sorting and rounding are two textural features of
  detrital sedimentary rocks that aid in
  determining depositional processes
• Sorting refers to the variation in size of
  particles making up sediment or sedimentary rocks
• It results from processes that selectively
  transport and deposit sediments of particular
  sizes

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Sorting

• If the size range is not very great, the sediment
  or rock is well sorted
• If they have a wide range of sizes, they are
  poorly sorted
• For example
• Wind has a limited ability to transport sediment
  so dune sand tends to be well sorted
• Glaciers can carry any sized particles, because
  of their transport power, so glacier deposits are
  poorly sorted

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Rounding

• Rounding is the degree to which detrital
  particles have their sharp corners and edges warn
  away by abrasion
• Gravel in transport is rounded very quickly as
  the particles collide with one another
• Sand becomes rounded with considerably more
  transport

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Rounding and Sorting

• A deposit of well rounded and well sorted gravel

• Angular, poorly sorted gravel

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Sedimentary Structures

• Sedimentary structures are visible features that
  formed at the time of deposition or shortly
  thereafter
• Represent manifestations of the physical and
  biological processes that operated in
  depositional environments
• Structures seen in present-day environments or
  produced in experiments help provide information
  about depositional environments of rocks with
  similar structures

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Bedding

• Sedimentary rocks generally have bedding or
  stratification

• Individual layers less than 1 cm thick are
  laminations
• common in mudrocks

• Beds are thicker than 1 cm
• common in rocks with coarser grains

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Graded Bedding

• Some beds show an upward gradual decrease in
  grain size, known as graded bedding

• Graded bedding is common in turbidity current
  deposits
• form when sediment-water mixtures flow along the
  seafloor

• As they slow, the largest particles settle out
  then smaller ones

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Cross-Bedding

• Cross-bedding forms when layers come to rest at
  an angle to the surface upon which they
  accumulate as on the downwind side of a sand dune
• Cross-beds result from transport by either water
  or wind
• The beds are inclined or dip downward in the
  direction of the prevailing current
• They indicate ancient current directions, or
  paleocurrents
• useful for relative dating of deformed
  sedimentary rocks

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Ripple Marks

• Small-scale alternating ridges and troughs known
  as ripple marks are common on bedding planes,
  especially in sandstone
• Current ripple marks form in response to water or
  wind currents flowing in one direction
• have asymmetric profiles allowing geologists to
  determine paleocurrent directions
• Wave-formed ripple marks result from the
  to-and-fro motion of waves
• tend to be symmetrical
• Useful for relative dating of deformed
  sedimentary rocks

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Current Ripple Marks

• Ripples with an asymmetrical shape
• In the close-up of one ripple, the internal
  structure shows small-scale cross-bedding
• The photo shows current ripples that formed in a
  small stream channel with flow from right to left

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Wave-Formed Ripples

• As the waves wash back and forth, symmetrical
  ripples form
• The photo shows wave-formed ripple marks in
  shallow seawater

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Cross-Bedding

• Tabular cross-bedding forms by deposition on sand
  waves
• Typically produce where streams enter large
  bodies of water (AKA Deltas)

• Tabular cross-bedding in the Upper Cretaceous

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• Animation of crossbedding

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Cross-Bedding

• Trough cross-bedding formed by migrating dunes

• Trough cross-beds in the Pliocene Six Mile Creek
  Formation, Montana

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Mud Cracks

• When clay-rich sediments dry, they shrink and
  crack into polygonal patterns bounded by
  fractures called mud cracks
• Mud cracks require wetting and drying to form,

• lakeshore
• river flood plain
• or where mud is exposed at low tide along a
  seashore

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Ancient Mud Cracks

• Mud cracks in ancient rocks in Glacier National
  Park, Montana
• Mud cracks typically fill in with sediment when
  they are preserved as seen here

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Biogenic Sedimentary Structures

• Biogenic sedimentary structures include
• tracks
• burrows
• trails
• Also called trace fossils
• Extensive burrowing by organisms is called
  bioturbation
• may alter sediments so thoroughly that other
  structures are disrupted or destroyed

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Sedimentary Structures

• Sedimentary rocks form as layer upon layer of
• sediment accumulates in various environments.
• These layers are known as Strata or Beds.

• Each Stratum or bed of a sedimentary rock is
  unique
• Texture and composition of the bed reflects the
  different conditions under which each layer was
  deposited.

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Between the Lines

• Separating each strata are bedding planes.
• Bedding Planes are flat surfaces along which the
  rock tends to break.
• Caused by changes in grain size.
• May also occur after pauses in deposition.
• In general most deposition occurs due to
  sediments settling out of water therefore most
  bedding is horizontal.
• In wind driven deposition, layers are not always
  horizontal. Sand dunes are a prime example. In
  these cases the bedding is described as Cross
  Bedding

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Bedding Planes