Sequence stratigraphy

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Sequence stratigraphy
2014-04-07 11:48:21
Eight Nine

Hierarchy of sequences and sequence boundaries
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  1. Figure 8.1
    • Hierarchy of sequences and sequence boundaries
    • diagram demonstrating hierarchy based on frequency
  2. Figure 8.2
    • Ordra of shoreline shifts with time - 3rd=lowest, 1st=highest
    • 3rd represents true shoreline shift
    • higher orders -> larger scales of space and time
    • note - second degree MRS=MFS first degree
    • shows low rank (higher frequency) surfaces on higher rank surfaces doesn't change the latter's significance
  3. Figure 8.5
    • Hierarchy of system based on magnitude of base level changes
    • (independent of cycle duration)
    • 5 orders of sequence boundaries based on boundary character that reflects base level changes
    • principles to determine order:
    • 1. A sequence cannot contain a SB>/= lowest magnitude boundary within it
    • 2. Sequence order = order of lowest magnitude boundary
  4. Six attributes to establish boundary magnitude and hierarchy
    • 1. Areal extent of the whole boundary
    • 2. Areal extent of boundary unconformity
    • 3. Degree of deformation of strata below boundary
    • 4. Magnitude of deepening and flooding (nature and extent of transgressive strata above boundary)
    • 5. Degree of change in sedimentary regime across boundary (degree of facies dislocation)
    • 6. Degree of change in tectonic setting across the boundary

    • Boundaries showing greatest magnitude
    • -> highest amount of base level shift
    • -> are higher rank and therefore are higher in the hierarchy

    • Two potential issues:
    • 1. Implies there is always tectonic control in sequencegeneration
    •    Major glacio-eustatic sequences (e.g. Permo-Carb, Late Cenozoic
    •   -primarily based on areal extent, but lower order on nature of SB
    • 2. requires good preservation of basin margin to assess areal extent and degree of deformation
  5. Figure 8.3
    • Original hierarchy was based on duration time
    • - after vail et. al. 1977
    • - assumes eustacy is driver of sequences (meaning they're global)
    •    - controlled by plate tectonics and climate
    • - but ranges may not be mutually exclusive
    • - how like are basin records to be global?
    • - need good chronology to do this
    • there are some similarities between the two schrmes
  6. Figure 8.4
    • So far most sequence strat models based on Phanerozoic
    • what of precambrian?
  7. Figure 9.1
    • Final note - reminder on differences in shelf gradients
    • low gradient va high gradient systems
  8. Standardizing sequence stratigraphy
    • If this is to be done, some standard principles need to be considered: 
    • 1. Surfaces can serve as systems tract and sequence boundaries (std. 7)
    • 2. Subsidence and magnitude/timing of base level change vary spatially
    • 3. 4 main events in base level (FR, 2x NR, Tr) cycle mark changes in direction or type of shoreline shift (except inland fluvial systems, carbonates, glacial?)
    • 4. Recognizing surfaces is data dependent (outcrop, well logs, seismics,...)
    • 5. Difficulties in recognizing surfaces don't negate their existence or validity
    • 6. Different genetic types of deposits need to be separated into STs
    • 7. Surfaces formed independently of sedimentation (the cc's) are closer to time lines than those that mark end of R or T
    • 8. Highest frequency (lowest order/rank) reflect true changes in depositional trends.  Higher ranks represent overall trends.
    • 9. Where 2 or more orders of stratigraphic surfaces are superimposed always use names of youngest surface