Sequence Stratigraphy

• Linked contemporaneous depositional systems
• Subdivisions of a Sequence
• Interpreted based on:
• -stratal stacking patterns
• -position within the sequence
• -types of bounding surfaces
• Assigned particular positions along an inferred curve of base level changes at the shoreline

widespread erosion, with subaerial unconformities developed across the entire continental shelf.

minimal erosion, with subaerial unconformities restricted to the basin margin.

• Big weakness is that they ignored the idea that sediment could be deposited on the shelf during sea level fall (reflected here by apparent instant base-level fall)
• Just treated as coastal onlap, but in fact systems includes fluvial deposits too
• Also didn't consider forced regressive deposits

• Then put forced regressive shelf deposits into LST, above the SB
• Alternative-place the SB at the subaerial erosion surface above the falling stage marine deposits

low rate progradation and aggradation (base-level rise at the shoreline and normal regression)

High rate progradation and offlap (base-level fall at the shoreline and forced regression)

Low rate progradation and aggradation (base-level rise at the shoreline and normal regression)

retrogradation and aggradation (base-level rise at the shoreline and transgression)

Wheeler diagram of the systems tracts and surfaces

• Forms during late phase of base-level rise
• Rise rates < sedn rates
• Normal shoreline regression
• Aggradation+progradation
• Base - MFS
• Top - SU+BSFR+RSME
• All deposystems

Grading trends

Delta styles on the shelf

Petroleum significance

General trends of peat

• Strata deposited during FR of the shoreline
• Varisou previous names
• Mainly SU=shallow+deep marine strata
• Base - BSFR+RSME
• Top - SU+cc+RSME

• By-pass and incised fluvial systems during base level fall
• IVs have V shaped profiles
• Incision increases as base level gets to shelf edge
• Note knickpoint migration

Compare IV with by-passing valleys

IV facies successions

• Key features:
• fluvial incision and/or bypass
• delta plain bypass or erosion (no topset)
• delta front progradation and offlap
• erosion in the lower shoreface
• outer shelf and shelf edge instability
• dominant gravity flows; debris flows/mudflows

• Key features:
• fluvial bypass and/or incision
• delta plain bypass or erosion (no topset)
• delta front progradation and offlap
• Dominant gravity flows: high density turbidites

• River dominated nearshore system with falling base level 
• character of river dominated deltas where clinoforms steeper than wave equilibrium profile have no regressive ravinements

• Seismic profile showing a cu trend
• A-mudflow deposits=early FR
• B-turbidites=late FR

find first off-lap->youngest clinoform-> correlative conformity red dash) -> coarsest deep-water facies -> likely best deep-water reservoir

• TC channel with leve in time slice and cross section
• => high density tc with sed/water ratio -> aggradation even on steep continental slope.

• Turbidite systems with sandy frontal splays (strong reflections) aggrading near continental slope
• =>high density tc's with high sand/mud ratios in late FSST

• Sed deposits formed during early rise normal regression
• base - SU+marine cc
• top - MRS
• low rates of prog/aggrad
• if shelf remains submerged:
• base includes youngest RSME
• later phase LS wedge has all deposystems
• Sed evenly distributed in fluvial-nearshore-offshore
• Sand in fluvial, beach, delta front, deep-sea fans
• With time prism expands landward with fluvial aggradation and onlap 
• -> onshore storage of sediment slowly cutting off sediment offshore
• -> low density tc's - entrench on outer slope and depositing on very low slope of abyssal plain
• coarsest of all fluvial and shallow-marine but in the deep-marine, finer than FSST below

Basin floor leveed channels high in mud on low slopes but entrenched channels on higher angles of continental slope

• TST
• base level rise > sedimentation rates at shoreline
• includes all deposystems in 2 main wedges
• regrogradational stacking
• coastal onlap
• fu profiles
• potentially thick
• condensed sections deep marine
• base - MRS
• top - MFS

TST rive mouth environments and deposystems

Pictorial depiction of previous table showing shoreline types and lithofacies successions during base level rise

• TST coastal shallow marine systems
• -open shoreline example
• back stepping beaches and estuary mouth deposits and transgressive lags overlie wave ravinement that has removed the MRS locally
• offshore- onlap of healing phase shelf wedge of finer sediment

• Early phase TST 
• rapid base level rise -> retrogradation
• most sed trapped nearshore
• wave ravinement erosion supplies turbidite sands -> lo density tc (under load of sediment -> entrenchment on slope and distal deposition on low slopes)

• Late phase TST
• most sed trapped in fluvial and nearshore
• estuaries or deltas - depends on accommodation vs sedimentation
• wave-ravinement + broad shelf -> best preservation of shelf Bedford (storm or tidal forms)
• shelf edge instability+rising water load -> mudflows off shore
• starved outer shelf MFS can rework MRS

• Basin floor submarine fan succession during full base level change
• 1-cu, 2-fu, 3-prograde, 4-retrograde, 5-incr blr ->fu, 6-shore transgr and retrograde ->fu, 7-shoreline regress and prograde -> cu, 8-lower grade -> decrease sed rates and fu, 9-increase grade -> incision and cu, 10-all sizes export to deep water, 11-coarse sizes trapped on shelf -> lower sand/mud -> mudflows

• Transgressive transparent shale can be used as a marker
• bounded by MFS at top and a flooding surface at bottom

• Part of the T-R sequence model
• all strata deposited during shoreline regression I.e. Undifferentiated HST+FSST+LST
• progradational stacking patterns
• good when stratal terminations and stacking are poorly known
• internal surfaces hard to define
• base - MFS
• top - cu+MRS

E.g. RST in wave dominated shallow marine system

Figure 5.66

Fair weather wave base

Within trend facies contact

Within trend normal regressive surface

• To define all TR systems (HST, FSST, LST, and TST) you need evidence of syndepositional shoreline shifts and need good control on marine and non-marine portions of a basin.
• but where:
• -progradational and retrogradational packages are poorly constrained, or
• -basins dominated by non-marine processes (e.g. Overfilled), or
• -non-marine facies are the only ones preserved or available to study,...you need something else
• Then consider mainly fulvial architectureal elements, and have:

• Low accommodation systems tracts (LAST)
• IV fills, underlain by IV topography
• Multi-storied channel-fills

• High accommodation systems tracts (HAST)
• Simpler fluvial architecture

• Main features of LAST and HAST
• 1. => gradual spill over of fluvial seds into basin. Once fluvial reestablished change to fu
• 2. Depends on morphology at start of fluvial accommodation created by past incision processes.
• 3. depends on mechanism of accommodation -sea level (tabular), different subsidence (wedge) 
• 4. Valid for veg flood plains. PreC-E. Paleoz sheet wash rather than veg floodplain
• 5. Depends on rate of accommodation and duration of system
• 6. Commonly compound coals
• 7. Simpler (fewer hiatuses), more numerous and thicker
• 8. Commonly multiple and compound
• 9. Thinner, widely spaced and organic rich

• LAST fluvial and lacustrine above HAST lacustrine deposits
• LAST deposits of fine grained lacustrine overlain by amalgamated fluvial channel fills characteristics of distal LAST systems reflecting time required for fluvial to reach distal parts. Lake/river contact conformable but diachronous and NOT an s.u. boundary.

• Overall cu -> basin filling and prograding orogenic front -> overall change from HAST to LAST
• each sequence -> fu (high to low energy systems) -> LAST up to HAST with tectonic pulses