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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
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Figure 7.2
- Extensional basin
- -different subsidence rates at A, B, C
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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!
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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!
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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
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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
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Figure 7.9
And that's how you form a sequence and its ST
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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
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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)
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Figure 7.14
Tilting changing bothy metro, but not necessarily particle size which depends on sedimentation and base level changes.
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Figure 7.15
RLS at shoreline for each time step
- figure 7.16
- shoreline between supratidal and intertidal
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Figure 7.15
waterdepth through time at locations 1 and 2
Figure 7.18
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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.
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Figure 7.19
Wheeler diagram for the succession and derived on lap curve
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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
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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
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Figure 7.22
- Surfaces redux...
- methods of defining surfaces
- isochronous and diachronous surfaces
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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
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SU
- Subaerial unconformity
- -forms during basinward shift of shoreline during forced regression
- -very diachronous scour
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RSME
- Regressive surface of marine erosion
- -forms during forced regression of the shoreline
- -very diachronous scour
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TRs
- Transgressive ravinement surfaces
- form during transgression of shoreline
- very diachronous
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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
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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
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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
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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
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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
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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
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Figure 7.25
- Vary rates of subsidence and sedimentation with same eustacy
- - could represent variability along strike
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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
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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
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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
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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
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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
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Figure 7.31
- Summary of controls on rates of diachroneity
- blue low(1)
- green intermediate (2)
- red high (3)
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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
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