Sequence stratigraphy

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Sequence stratigraphy
2014-04-07 11:59:17

Time attributes of stratigraphic surfaces and sequence boundaries (revisited)
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  1. Are surfaces time lines?
    • Within dating resolution
    • generally a subaerial unconformity is counted as a time line but it does take time to form.
    • figure 7.1
  2. Figure 7.2
    • Extensional basin
    • -different subsidence rates at A, B, C
  3. So how does this look with conceptual forward modeling? Figure 7.5
    • Accommodation at shoreline decreases with time -> increasing progradation rates
    • Accommodation=interpolation between RSL curves at A and B relative to shoreline
    • -each section tilted according to rates of subsidence

    • forced regression starts and gets erosion at shoreline
    • -amount calculated from interpolation bet RSL curves at A&B according to shoreline 
    • Each tilted at appropriate rates
    • No offlapping shallow marine facies shown for simplicity!
  4. Figure 7.6
    • Forced regression starts and get erosion at shoreline
    • -amount calculated from interpolation bet RSL curves at A and B according to shoreline
    • Each tilted at appropriate rates
    • No offlapping shallow-marine facies shown for simplicity!
  5. Figure 7.7
    • Change from NR to Tr depends on accommodation at shoreline and sediment supply (unconstrained here but taken at step 10 because accelerated base level rise then) 
    • Accommodation amount calculated from interpolation bet RSL curves at A B and C according to shoreline
    • Rates of prograde or retrograd <-rates of accommodation at shoreline
  6. Figure 7.8
    • Change from Tr to NR depends on sedimentation and accommodation at the shoreline
    • -start selected when lower rate base level rise (at 13) 
    • Accommodation amount calculated from interpolation bet RSL curves at A B and C according to shoreline
    • Rates of prograde or retrograde <- rates of accommodation at shoreline
  7. Figure 7.9
    And that's how you form a sequence and its ST
  8. Figure 7.10
    RSL at shoreline - now with sediment added in to figure 7.4 from figure 7.9 curve -> accommodation at shoreline -> a dynamic curve -> sedimentation dictates timing on fall STs and bounding surfaces
  9. Figure 7.11
    • Now two different water depth changes (nearshore and offshore) for same situation
    • Determined as = sea level - sea floor depth
    • Tr shoreline association with basinal deepening requires increased subsidence + decreased sedimentation (common on divergent margins, but not everywhere)
  10. Figure 7.14
    Tilting changing bothy metro, but not necessarily particle size which depends on sedimentation and base level changes.
  11. Figure 7.15
    RLS at shoreline for each time step
    • figure 7.16
    • shoreline between supratidal and intertidal
  12. Figure 7.15
    waterdepth through time at locations 1 and 2
    Figure 7.18
  13. Figure 7.18
    • Model for low gradient ramp in intracratonic basin
    • main STs distinguished on facies dislocations. Here max water depth is later than MFS due to sedimentation rate changes and differences nearshore and offshore.
  14. Figure 7.19
    Wheeler diagram for the succession and derived on lap curve
  15. Figure 7.20
    • Normal regression (HST and LST)
    • base level, water depth and grain size trends at the shoreline (no subsidence, loading or consolidation)
    • T is locus of water depth change =0
    • note reverse trends between HST and LST states
  16. Figure 7.21
    • Controls on water deepening during progradation of coarsening upward successions
    • emphasizing that cu or fu trends can be dependent on a mix of variables and not just base level change
  17. Figure 7.22
    • Surfaces redux...
    • methods of defining surfaces
    • isochronous and diachronous surfaces
  18. Definitions of surfaces used in sequence stratigraphy
    • Seven total
    • three surfaces formed during particular stages of shoreline shifts
    • 1. Subaerial unconformity
    • 2. Regressive surface of marine erosion
    • 3. Transgressive ravinement surfaces
    • four surfaces linked to events of base level change
    • 4. And 5. Correlative conformity
    • 6. Maximum regressive surface
    • 7. Maximum flooding surface
  19. SU
    • Subaerial unconformity
    • -forms during basinward shift of shoreline during forced regression
    • -very diachronous scour
  20. RSME
    • Regressive surface of marine erosion
    • -forms during forced regression of the shoreline
    • -very diachronous scour
  21. TRs
    • Transgressive ravinement surfaces
    • form during transgression of shoreline
    • very diachronous
  22. cc
    • Correlative conformities
    • - both defined by facies stacking patterns
    • - timing depends on interplay of subsidence and eustacy at shoreline
    • - most likely different degrees of diachroneity spatially
  23. Onset of fall cc
    • (Posamentier and Allen)
    • cc represents sea floor at change from HS-NR to FR
    • - onset of FR and base level fall at the shoreline
    • - =basal surface of forced regression of hunt and tucker
    • -low diachroneity offshore or down dip
    • -may be higher along strike depending on subsidence rates
  24. End of fall cc
    • (Hunt and Tucker)
    • cc represents sea floor at change from FR to LS-NR
    • - end of FR and base level fall at the shoreline
    • - low diachroneity offshore or down dip
    • - surface you going basin ward <- -> sedimentation rate offshore
    • - may be higher along strike depending on subsidence rates
  25. MRS and MFS
    • Maximum regressive surface
    • at change from regression to transgression
    • Maximum flooding surface
    • at change from transgression to regression
    • - both depend on interplay of subsidence, eustacy + sedimentation
    • - more diachronius than the cc's, but hopefully below biostrat resolution
    • - main issues are at the marine end
    • - defined by either i. Grading or stratal stacking patterns, or ii. Water depth changes
    • - these two are not = because grading may not change with water depth
  26. Figure 7.23
    2D model to assess effect of subsidence and sedimentation on timing of MrS and MFS define by bathymetric

    • Assumptions:
    • 1. Eustacy varies sinusoid ally at ten meters amplitude

    2. Basin 200km across

    3. Sedimentation rates change linearly down dip

    4. Model in 0.125my incremental time steps
  27. Figure 7.24
    • Timing and locus of formation of surfaces marking peaks of deepest and shallowest water
    • - a function of
    • eustacy (E) 
    • subsidence (T)
    • Sedimentation (S)
    • - rate of vertical shifts in sea floor relative to center of earth (D) 
    • - rate of water depth change (W)
    • - D=T-S
    • W=E+D
    • W=E+T-S
    • -MRS and MFS where W=0, which varies with time laterally down dip
  28. Figure 7.25
    • Vary rates of subsidence and sedimentation with same eustacy
    • - could represent variability along strike
  29. Figure 7.26
    If profiles B and C do represent variability along strike, then timing of water depth min and max at MRS and MFS (respectively) varies
  30. Figure 7.27
    • Diachronous formation of surfaces separating deposits
    • - under relative sea level fall and rise (left)
    • - under water deepening and ah allowing (right)
    • conclude:
    • model shows that MFS and MRS defined on water depth changes are not suitable for seq. strat. Because:
    • - limited laterally 
    • - maybe highly diachronous, even biostratigraphically
  31. Figure 7.28
    • Differences in surfaces: 
    • type A defined on stratal stacking patterns
    • - timing independent of offshore variation in subsidence and sedimentation
    • - dependent in base level change +/- sedimentation at shoreline
    • type B defined on water depth changes
    • - timing dependent on offshore variation in subsidence and sedimentation
    • - makes surface very diachronous
    • grading does not necessarily reflect water depth changes
  32. Figure 7.29
    • Subaerial unconformity
    • assume strata under the SU are older than those above
    • landward extent of fluvial erosion depends on:
    • - size of base level changes
    • - land surface gradients
    • - river size
  33. Figure 7.30
    • Example of FR fluvial sediments are younger than oldest marine FR deposits
    • fluvial #1 is older than shoreline #3
    • - they lie on either side of the SU so it's crossed by time lines
  34. Figure 7.31
    • Summary of controls on rates of diachroneity
    • blue low(1)
    • green intermediate (2)
    • red high (3)
  35. Summary: sequence strat surfaces grouped into two
    • 1. "Event significant"
    • - 4 - onset of fall (BSFR), end of fall (cc), end of R (MRS), end of T (MFS)
    • - near time lines along dip
    • - young basinward at low rates dependent on offshore sediment transport
    • 2. "stage significant'
    • - 3 - form during stages of shoreline shift SU, TRS, RSME
    • -potentially more diachronous down dip than along strike
    • Within trend surfaces are similar to these