Identification and Classification of Sedimentary Rocks

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                 Stratigraphy

          Concepts Related to Subdivision
               of the Rock Record



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     Inextricably Related Issues in Geology
          (that must be distinguished!)
• Material Units: The Rock Record
   – Have physical extent and consist of tangible material
   – The material record of events that have occurred during
     the passage of geological time
• Geological Time: The Abstraction of Time Since
  the Advent of the Earth
   – Measured in years before present
   – Divided into discrete intervals
         • Ma; millions of years before present
         • Ga; billions of years before present

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    Subdivision of the Rock Record
• Lithostratigraphy –
   – study of the physical relationship among rock
     units;
         • no time connotation other than superposition
         • Otbo physical properties and stratigraphic position
           relative to other lithostrat units
• Chronostratigraphy –
   • integrated approach to establishing the time
     relationships among geologic units
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                 Chronostratigraphy:
   Integrated Approach to Establishing the Time
       Relationships Among Geologic Units
• Biostratigraphy
    – Study of the fossil record with emphasis on faunal succession to
      establish relative time relationships
    – The correlation web
• Magnetostratigraphy
    – Study of the magnetic properties of rock units for the purpose of
      correlation otbo magnetic polarity reversals
• Allostratigraphy
    – Study of rock units defined by unconformities and other features
      generated by base level change
• Geochronology
    – various techniques, especially isotope geochemistry, to establish the
      absolute age of rock units
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              M or e L a t e r



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              Stratigraphic Correlation




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                       Lithostratigraphy
• Formations, Members, Groups, etc.
    – Material Units and "Classical Layer Cake Stratigraphy“
    – catastrophism: continuous layering = time equivalence
    – NO!
• Further organization and subdivision of the rock record otbo
    – Relative age
          • Superposition, unconformities, cross cutting relationships, included
            fragments
    – Original Horizontality
    – Lateral Continuity


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Objective Subdivision of the Stratigraphic Record Into
           Distinct Lithostratigraphic Units
  • Formations
      – with a type section, geographic or lithologic name, and
        definition based on
            • limited and distinctive lithologic variability
            • consistent stratigraphic context
            • “extensive” map distribution in the surface or subsurface
  • Groups and Supergroups
      – Are formations lumped otbo stratigraphic association
  • Members and beds
      – Subdivisions of Formations
      – lithostrat units with less areal extent defined as it is useful

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Actualism and "Genetic Stratigraphy"

• Recognition of Uniformitarianism
   – the relationship between modern processes of
     sedimentation and the rock record




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      Actualism and "Genetic Stratigraphy"

• Sediments with distinct lithologic
  aspect are deposited in only
  limited areas in a given time
  period.
• This results from the limited
  lateral extent of contiguous
  depositional environments.
• Distinct depositional
  environments migrate in space,
  through time so that lithofacies
  units are inherently diachronous
  (variable in age throughout)



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              Diachronous Stratigraphic Units




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              Diachronous Stratigraphic Units


                                       Basement




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      Actualism and "Genetic Stratigraphy"
• Sedimentary (litho)Facies:
    – lithologic aspect; a result of depositional process not age.
• Lithologically distinct sediment bodies deposited synchronously in adjacent
  sites (distinct lithofacies) must be separated across some kind of boundary.
• Such contemporaneous and physically associated strata are said to be in
  Facies Relationship.
• Lithologic aspect in space is not
  directly equitable to time but
  rather depositional process at
  that place at some time.




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Geometry of Facies/Lithostratigraphic
Relationships




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        Actualism and "Genetic Stratigraphy"

• Depositional Processes -----> Geological Product
Sedimentary Depositional Environments                 Sedimentary Lithofacies


• Defined by
    –   Lithology
    –   Sedimentary structures (physical, chemical, deformational, etc)
    –   Geometry: bedding scale and outcrop scale, vertical trends
    –   Paleocurrent indicators
    –   Fossils



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        Walther's Law of Correlation of
                    Facies
  • “Only those lithofacies which are a product of sedimentary
    environments found adjacent to one another in the modern can
    be occur superimposed in continuous, uninterrupted
    stratigraphic succession.”


Also, lith units in
conformable vertical
relationship were in lateral
relationship during
deposition




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               Autostratigraphic Variation
                     Autocyclicity
• Lithologic variability, within distinct geomorphic settings, in space
  and time may be the result of variations in depositional
  conditions inherent to that depositional environment----->




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External Controls on Stratigraphic Architecture:
                        Allocyclicity
• Variation in lithologic character due to regional
  scale variations in Base Level
   – level at the earth's surface above which permanent
     sediment accumulation does not occur
   – Closely related to local sea level
         • Basin Subsidence (tectonic)
         • Eustacy (global sea level)
         • Sediment input (climate and topography/tectonics)

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              Unconformity




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      Transgression and Regression
              Geometric relationship of idealized
              "graded, shore parallel facies belts“
• deepening upwards, shoreline moves landward
  through time:
                Transgression
   – Basin Subsidence, +/- Sea Level Rise, +/- Sediment
     Starvation (With Subsidence)
   – May (rarely) leave a distinct material record--->
         • Marine flooding surfaces
         • Ravinement surfaces
         • Landward shift in sedimentary facies up section
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                       Transgression
• Geometric relationship of idealized "graded, shore
  parallel facies belts“
    – Fining Upwards Sequence:       FUS
    – More basin-ward facies overlie more landward facies
          • Compared to depositional systems models




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       Transgression and Regression
• Shallowing upwards, shoreline moves basinward through
  time
                       Regression
  – sea level drop, +/- uplift, +/- sediment supply
        • Progradation; Normal Regression
               – excess sediment supply relative to accommodation space
        • Forced Regression: Relative sea level drop and formation of erosion
          surfaces: Unconformity (surface of subaerial exposure)
               – Soils; kaolinitized, clay-rich layers
               – Angular discordance with underlying units (disconformity)
               – Plant remains, rooted zones
               – Non-genetic stratal relationships:
               Abrupt Basinward Shift In Sedimentary Facies Upsection
               – Strata across lithologic boundaries NOT in accordance with Walther’s
                 law

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                          Regression
• Geometric relationship of "graded, shore parallel facies
  belts“
    – Coarsening Upwards Sequence: CUS
    – More landward facies overlie more basin-ward facies
          • Compared to depositional systems models




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Relative         Causes of Base Level
Change                 Change




Eustatic
Change




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                 Sea Level Cycles
• 1st Order Cycles
   – 100’s my
   – 100’s of meters
                            Falling

• 2nd Order Cycles
   – 10’s my                Rising


   – 100’s of meters
• 3rd Order Cycles
   – 1-10 my
   – 10’s of meters


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       Transgression/Regression vs.
            Relative Sea Level
• Relative Sea Level Curves




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