Time and the by OVBPXT

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									Time and the
Stratigraphic record
•   Lithostratigraphy revisited
•   Asymetric cyclicity
•   The imperfect recorder
•   Facies and temporal correlation


                     Lithostratigraphic concepts, 1
•   Facies
•   Walther’s Law
•   Time-transgressive
•   Sharp contact
•   Pinch-out and intertounging
•   “to grade” or intergrade
                     Lithostratigraphic concepts, 2
•   Transgression and the fining upward sequence
•   Regression and the coarsening upward sequence
•   Prograde vs. retrograde
•   Aggrade vs. degrade
Symetric or asymetric sea level change?
•   Fining upward sequences are stratigraphically rare
•   Coarsening upward sequences are more common
•   Why?
•   Reworking of fining upward sequences by high energy nearshore processes during SL rise
•   Sediment character results in differential preservation
Example:
Coal Cyclothem
•   Traditional model implied gradual transgression (Right curve)
•   Modern model assumes rapid transgression (Left curve)
•   Discontinuity is seen as a function of erosion and reworking during transgression, not regression

                                     Paradigm shift?
• The asymetric model has gained widespread acceptance, but is it
  the final word?
• Remaining questions-
         • Is the rapid transgression an apparent one due to preservational differences?
         • Strata from rapidly subsiding margins preserve thick transgressional sequences
         • Asymmetry may also arise from variable sediment supply (Example: Devonian
           Hamilton Group)


Base level and sediment accumulation
•   Marine sediments compose the majority of the stratigraphic record due to their position relative to
•   Base level of erosion [Powell, 1875; Gilbert, 1914]
•   Base level of aggradation [Barrell 1917; Dunbar & Rogers 1957]
•   Profound influence on the nature of the record

                                 Stratigraphic model
                                       of Ager
Gaps due to
• Erosion
• Non-deposition
• More rock is missing than accumulating
• Gradual biotic transitions may appear abrupt or continuous
     Examples: Facies models plotted versus time
“More gaps than record”
[Ager, 1980]
• Grand Canyon strata are almost 1 mile thick, but remarkably incomplete
• Represent less than 10% of Paleozoic time (590 to 250 Ma)
The Inuit have 44 words for “snow”;
Stratigraphers have almost as many for time and
missing time!
•   Angular unconformity
•   Nonconformity
•   Disconformity
•   Paraconformity
•   Hiatus
•   Degradational vacuity
•   Lacuna
•   Diastems
•   Diacronous (time-transgressive)
Time and place from a geologic Perspective
Time in geochronologic units
Time-rock in chronostratigraphic units
        Major unconformaties (Angular, non- and
      disconformities) are associated with lacuna
            Paraconformities and diastems
• A minor unconformity (Paraconformity) is the expression of a diastem
• Barrell diagram shows diastems in respose to changes in base level
Example barrell diagram for Cedar Mesa SS, UT
             Estimating lost time
•Lacuna - time gap in the sequence resulting from a hiatus and a degradational vacuity.

•Potential response to the transgression/regression cycle

•Marine realm we talk of
        Non-depositional hiatus
        Erosional hiatus

                              Measuring the lacuna
• The depth section (blue box) must be reploted as a time section (red box)

• Requires identification of time lines by biostrat or some other method
                            Regression and erosion
• Unconformity develops in response to regression
• A degradational vacuity or erosional hiatus (blue wedge) develops as base
  level moves seaward (basinward), removing sediment deposited from T0
  to T7
                      Transgression and the hiatus
• At the maximum extent of the regression, much of the section lies above
  base level and thus accumulates no sediment.
• The sea then transgresses over the unconformity surface.
• The time associated with the hiatus (non-deposition) is outlined in red.

								
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