Sequence stratigraphy

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Angdredd
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Sequence stratigraphy
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2014-04-07 12:16:11
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Sequence models
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  1. Sequence models
    • Depositional sequences basically same as sloss, but added important concept of correlative conformities.
    • different sequence strat models based on position of sequence boundaries, especially the cc's.
    • but 6 varieties of models.
  2. Figure 6.1
    • Five different types, two main groups
    • cc's defined using base-level curve (SB's independent of sed rates - 2, 3, 4).
    • cc's defined using T-R curve (SB's dependent on sed rates - genetic, T-R sequence).
  3. Figure 6.3
    • Types of stratigraphic sequences
    • depositional sequences
    • whichever model, the concept of the bounding surface being SU-cc is over simplified.
    • at least part of the cc is reworked in shallow water into RSME.
    • key to interpretation are facies shift relationships and syndepositional shoreline shifts
    • deposequence models have SBs defined relative to base-level curve indept of sed rate - a good thing due to sed rate areal variability
    • But shallow water facies often undetectable even in outcrop
  4. Genetic stratigraphic sequence pros and cons
    • Has HST, LST, and TST - sequence defined on MFS
    • pro: overcomes problems of having to define the cc in shallow marine settings.
    • plus MFS relatively easy to map across a basin - often easier to map in well-logs and seismic lines than SUs due to widespread shales.
    • cons: ignoring SU within the sequence goes against main concept of seq strat; also -> strata genetically unrelated grouped together.
    • MFS depends on base level + sed rates -> diachronous  surface
  5. T-R sequence model
    figure 6.4
    • Bounded completely by SU + MRSs offshore
    • SUs seen as important but cc's recognized as problems in shallow marine, so avoid them apart from MRS which
    • pro: avoid shallow water cc problem
    • simplifies process by amalgamating STs
    • con: have deep water cc problem
    • simplificzgion creates problems in defining reservoirs
    • marine and non-marine parts of SB are temporally off-set with the duration of the low stand normal regression
    • may need transgressive ravinement surface to join the two parts of SB - a common situation
  6. Parasequences
    • A relatively conformable succession of genetically related beds or bedsets bounded by flooding surfaces
    • often seen in cu prograding lobes of coast-shallow marine settings
    • each lobe terminated by an abrupt deepening -> FS
    • catuneanu says more problems than value with them:
    • -confusion over defining FSs allows many meanings (TRS, MRS, MFS, within trend contacts)
    • -relationship between sequence and parasequence - paraseq. may not have full cycles of trends to define them
    • -paraseq. not "small" sequences as often suggested
  7. Figure 6.6
    Each parasequence in the we'll log corresponds to a stage of shoreline regression ended by abrupt water deepening
  8. figure 6.7
    • Different surfaces mark parasequences
    • A- incomplete T-R sequence as TST is below ravinement surface
    • B-larger than a T-R sequence as includes transgressive deposits of overlying T-R
    • C-bounded by 2 within trend facies contacts
  9. Fluvial sequences
    • Figure 6.8
    • marine base level not influence much of the fluvial system (influence range of 10's km)
    • beyond this they respond to climate and tectonics -> "upstream controls" and "downstream controls"
  10. Downstream fluvial systems
    Integrated with standard SS models
  11. Upstream systems
    Respond to changes in climate, source area tectonism and basin subsidence (high and low accommodation systems)
  12. First principles of fluvial SS (9)
    • 1. Fluvial incision may occur during periods of base level fall, increased discharge, or reduced sediment load.
    • 2. Fluvial aggradation may occur during periods of base-level rise, increased sediment load, or reduced discharge.
    • 3. Fluvial responses to base level shifts are mainly related to tectonism and eustatic fluctuations.  Fluvial responses to changes in sediment load and discharge are primarily climate related.
    • 4. Low sinuosity fluvial systems, such as braided rivers or rivers with alternate bars, are most likely to occur during times of low accommodation.
    • 5. Anastomosed fluvial systems are commonly associated with high rates of base level rise, such as during transgression.
    • 6. High-sinuosity (meandering) fluvial systems commonly characterize periods of low to moderate rates of base level rise.
    • 7. Straight fluvial systems that show little evidence of lateral migration are typical of areas of very low slope and low accommodation.
    • 8. Incised valleys may be filled by fluvial systems of all types.
    • 9. Evidence of marine influence within fluvial systems, such as tidal features, indicates flooding episodes (accommodation in excess of sedimentation).
  13. Fluvial cyclicity controlled by base level changes
    • Driven by impressed change in fluvial grade ->
    • migration of knickpoints, changes in stream energy, and changes in accommodation reflected in -> changes in fluvial deposystems and architecture, and changes in degrees of erosion and incision
    • High rate base-level rise -> floodplain aggradation (meandering -> anastomosing)
    • Low rate base-level rise -> channel amalgamation (braided)
  14. Figure 6.10
    • Models for downstream fluvial SS
    • channel fills change upward from braided to meandering as:
    • -coast aggrades
    • -gradients lower
    • -fluvial energy decreases
    • Uppermost HST channels often eroded by next FSST and LST -> steepening grade profiles -> SU + paleosol
  15. Figure 6.11
    • Downstream fluvial response to base level rise
    • Abrupt shifts I thresholds -> changes in braided-meandering systems
  16. Fluvial cyclicity independent of base level cyclicity
    • Unconformity bounded packages based on relative abundance of different fluvial architectural elements
    • Rely on:
    • -source area tectonism
    • -effects of climate on discharge and sediment supply
    • -amounts of fluvial accommodation available by basin subsidence
  17. Climate cycles
    • Primarily milankovitch scale forcing
    • influence discharge -> transport capacity and sed loads
    • Increase energy and transport capacity -> negative fluvial accommodation -> incision
    • Increase in sediment load relative to capacity -> positive accommodation -> aggradation
    • glaciation -> lower discharge relative to sediment load -> aggradation, then inverse
    • Especially evident in more stable areas like forelands
  18. Tectonic cycles
    • Higher frequency tectonic cycles may impact fluvial cyclicity.
    • Much variability in different settings - need to customize for each case.
  19. Figure 6.14
    • Low vs high accommodation settings
    • Syndepositional subsidence profound influence on architecture of unconformities and fluvial elements
  20. coastal to shallow marine clastic systems processes
    • (River mouths and open shorelines)
    • sediment supply and transport mechanisms
    • -tides - tidal rhythmites
    • -fairweather waves - offshore, parallel, oblique, onshore
    • -storm waves - high turbulence, gravitational reworking
    • -gravity flows - shear > internal strength, >0.3 slopes
    • -hypopycnal flows - river sed bypass delta as overflow
    • -hyperpycnal flows - during flood events
  21. Figure 6.16
    Energy fence with more onshore directed energy storing coarse sediment onshore
  22. Figure 6.17
    • Preservation of shoreface profile in equilibrium with wave energy
    • Key to forming
    • -wave-ravinement surfaces (TST)
    • -regressive surfaces of marine erosion (FSST)
  23. Figure 6.26
    • Sediment budgest
    • Fairweather: net onshore transport - fines deposited offshore below shallow wave-base, wave structures near shore
    • Storm: net offshore transport of coarse, washover fans back beach
  24. Cyclicity with shoreline shifts
    normal regressive
    • Aggradation and progradation
    • shoreline -> seaward and rises
    • Open shoreline and deltaic deposystems
    • sand trapped and accumulates nearshore
    • sed rates > accommodation increase
    • HST-normal shelf environments offshore -HCS sands and wave-rippled muds (Flaser, etc.)
    • LST-shoreface passes straight into deepwater sedimentation
  25. Cyclic it's with shoreline shifts
    Forced regressive
    • base level fall ->
    • fairweather transport lower to upper shoreface, plus added to from delta and alongshore
    • -sed supply > local energy
    • - -> RSME progressively downlapped by upper shoreface sand
    • sgorm nearshore sand redistributed to deep shelf
    • offshore systems shrink with Continued base level fall
    • last stage - all shelf exposed and sedn occurs at downstepping shelf edge
    • if base level not below shelf edge, shelf systems preserved
    •   -similar to normal regressive facies
  26. Cyclic it's with shoreline shifts
    Transgressive
    • Wave erosion of foreshore and shoreface as base level rises
    • -> highly diachronous wave ravinement - degree depends on rate of transgression
    • coarser part of eroded sediment transported onshore -> back stepping beaches or estuary mouth systems
    • finer sed moved offshore to heal lower shoreface by onlapping
    • may have sandy tidal macroforms on shelf -> sandy sheets and ridges
    • otherwise shelf mainly pelagic muds or condensed sections
  27. Figure 6.27
    • Deep water clastic sequences
    • allogenic controls on deep water gravity flows
  28. Progradation of shelf edge deltas
    figure 6.28
    • Late stage of base level fall
    • FR deltas have off lapping geometries truncated by SU
    • -> sandy high density t.c.s
    • NR deltas aggrade + prograde! have topsets
    • -> muddy low density t.c.s.
    • sediment suppliers to deep water
  29. Figure 6.30
    Deep water processes/products for systems tracts
  30. Figure 6.31
    Types of gravity flow deposits
  31. Main types of gravity flows relative to base level shifts
    • Figure 6.32
    • 1. Cohesive debris flows (mudflows) 
    • 2. High density t.c.s. And grain flows -> proximal frontal splays
    • 3. Low density t.c.s. -> leveed channels and distal frontal splays
  32. Ideal architecture of submarine fan complex during a base level change
    • Figure 6.37
    • fan progradation during 1-4
    • 1- mudflows (FSST)
    • 2- high density t.c.s. And grainflows (FSST)
    • 3&4- low density t.c.s. (LST +TST)
    • fan retrogradation during 5
    • t.c.s. Then mudflows (TST) 
    • starved/condensed section (HST)
  33. Carbonate sequence stratigraphy
    • Differ from clastics mainly by geometry of STs and spatial sediment budget
    • -influence accommodation
    • geometries
    • -ramp shelves similar geometry to clastics; others markedly
    • Sediment distribution
    • -carbonate environments dominated by intrabasinal sediment
    • -deep water clastics accumulate fast in LST, but carbonates in HST
    • figure 6.48
    • types of carbonate platforms based on geometry
    • -shelves, ramps, isolated platforms (banks) + basin
  34. Figure 6.49
    Simple carbonate sequence stratigraphy model
  35. Figure 6.50
    • Different sequence boundaries if use drowning surfaces
    • the type 2 sequence boundary has been abandoned
    • so now there are only:
    • -su+cc
    • -drowning (+MRS (deep)+MFS(shallow))
  36. Figure 6.51
    • Mixed carbonate-siliciclastic succession
    • hypothetical stratigraphic column of mixed carbonate-siliciclastic succession with drowning surfaces used as sequence boundaries

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