FACIES MODELS

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FACIES MODELS
Pages 241-243
Sedimentology 340
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Previous Work

• Majority of the semester has been largely
  descriptive, identifying
• Lithology mineralogy
• Sedimentary fabric and texture
• Sedimentary structures
• Scales of observation
• Now a Story to tell and puzzles to solve
• Sediments are products of sedimentary processes
• Snapshot of the environment of deposition

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Sedimentary Facies

• Facies analysis
• The interpretation of rocks and sediments for the
  purpose of reconstructing the processes that were
  responsible for the original deposition
• Multi-pronged approach
• Physical
• Chemical
• Biological
• Lithofacies and Biofacies

Example of sedimentary facies modal Delta
distributary channel
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Pardon me, what are facies?

• In the field of geology, facies is often misused
  and misunderstood
• AGI (1984) Definition
• The aspect, appearance, and characteristics of a
  rock unit, usually reflecting the conditions of
  its origin especially as differentiating it from
  adjacent or associated units

Facies! Refers to the lithologic and biologic
characteristics of a sedimentary deposit imparted
by the processes collectively operating in the
depositional environment
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Facies defined

• Deposits that evolve in a specific depositional
  environment are considered to be sedimentary
  facies
• Ex. Arid desert lakes/salt flats

Bonneville Salt Flats, Utah
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Facies defined

• For this class and facies will be defined as
• A description of physical, chemical and
  biological properties of a sedimentary unit
• Main criteria in facies definition are
• Lithology, sedimentary structures, paleontology,
  and sediment body geometry

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Sediment body geometry is the 3-D shape of deposit
barrier island
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Question?
What are you looking at? How can you describe it
in the framework of its depositional environment?
This is a delta
How do we know?
?
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Delta
Delta is at the end of a river and is the
sedimentary record of deposition into deeper water
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Nile Delta Example
Modern examples are used to gather data on
processes and deposits (cores)
Facies model - a 3-D block diagram
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Facies model 2-D vertical succession diagram
Delta depositional environment?
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So question was.how do we know ? Answer facies
associations

• Depositional environments are generally composed
  of multiple subenvironments
• Ex. Delta
• Thus, we can expect that facies will vary
  throughout the environment to reflect the
  transition between subenvironments
• We must assume that adjacent facies represent
  adjacent environments

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Delta components and facies
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So how are they related?

• The association of certain facies near or
  adjacent to each other are characteristic of
  distinct depositional environments

Based on observations from modern environments of
deposition and parallels to past environments
By understanding these associations, we can
generate facies models
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4-D models add understanding of processes
3-D Block daigram facies associations in a delta
distributary channel environment
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Facies Models

• General summary given to a depositional system
• 2-D Vertical successions of sediment that
  represent facies relationships expected in the
  geologic record
• 3-D Block Diagrams
• 4-D Models detailing sedimentary processes

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Walthers Law of Correlation of Facies

• Facies that occur in comformable vertical
  successions of strata also occur in laterally
  adjacent environments
• Thus, facies boundaries may shift so that the
  deposits of an adjacent environment may lie
  directly atop those of a laterally related
  environment

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Takeaway Message For any sedimentary package
you can examine the facies and their
relationships to each other, apply a facies model
and assign a depositional environment
So, what is this?
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Catskill Delta (Devonian)
Catskill Delta builds westward through time with
sediment delivered by rivers
fluvial
delta
marine
Regressive- sediments coarsen upwards
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3-D Barrier Island Facies Model
barrier island
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Environments are dependent on sediment supply,
subsidence and changing sea level
Conditions (tectonics or climate) usually change
with time
With rise in sea level ..
The shoreline shifts landward and fine grained
marine sediments will overly coarser beach
sediments
Transgressive sediments fine upwards,
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Facies Changes

• Transgressive and Regressive systems can be
  identified by observing facies and lithological
  changes in rock/sediment
• Transgressive sediments fine upwards, represent
  deeper water environments
• Regressive- sediments coarsen upwards, represents
  shallowing effect

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Facies Distribution

• What factors control the nature and distribution
  of facies?
• 1. Sedimentary Processes
• 2. Sediment Supply
• 3. Climate
• 4. Tectonics (subsidence or uplift)
• 5. Sea level change

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Summary using an example

• What facies would you expect to find from this
  depositional environment?

• Fluvial Environment
• Meandering River
• Cross bedded sandstones
• Coarse gravel deposits
• Broad sheets of silty shale with root casts and
  plant debris
• Isolated Shale bodies
• Beds of peat and coal
• Facies models help solve the puzzle of
  depositional environment