Allostratigraphy/Sequence Stratigraphy

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Allostratigraphy/Sequence Stratigraphy

• Subdivision of the stratigraphic record otbo
  bounding discontinuities
• Bounding discontinuities define Depositional
  Sequences
• relatively conformable successions of genetically
  related strata bounded by unconformity or
  correlative conformities

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Utility of Depositional Sequences

• If bounding discontinuities are produced by
  isochronous events (at least basin-wide) then
  these material (or non-material) features can be
  used for correlation since they are isochronous.
• How are sequence bounding unconformities
  recognized and correlated in different places?

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Depositional Sequences/Sequence Boundaries

• Importance and utility first recognized in the
  analysis of seismic reflection profile date and
  reflection terminations

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Basin Fill and Accommodation Space

• Sedimentary Basins
• Topographic/Bathymetric versus
• Geological
• Sites of Thick (1000's of meters) sediment
  accumulation over Long Time Periods (Ma--gt
  mega-annum millions of years).
• Sedimentary basins accumulate sedimentary fill
  due to available
• Accommodation Space
• Space available below base level for sediment
  accumulation
• The thickness and type of sedimentary deposit
  formed in a sedimentary basin is controlled by
• Sediment supply
• Accommodation space for that sediment

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First Principles and ControlsSequence
Stratigraphy

• Accommodation is defined by
• The Equilibrium Profile
• The theoretical, steady state (no gain or loss of
  sediment) depositional surface in any environment
• Controlled by various factors, including
• Hydrodynamic gradient of the system
• Substrate and sediment load characteristics
• and

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First Principles and ControlsSequence
Stratigraphy

• Base Level
• Points coinciding with theoretical equilibrium
  profile along an actual depositional profile

• Erosion occurs above and deposition occurs below
  base level

Posamentier and Allen, 1999
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Accommodation Space

• Produced (in an idealized shallow marine
  environment) by
• Rise in sea level relative to the depositional
  surface
• Sediment accumulates to sea level (or base level
  for the more general case) if sufficient supply
  exists

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Creation of Accommodation Space Eustatic vs.
Relative Sea Level Change

• Eustacy and Accommodation
• World wide change in the elevation of sea level
  relative to a fixed point in the earths interior

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Creation of Accommodation Space Eustatic vs.
Relative Sea Level Change

• Relative Sea Level Change and Accommodation
• Tectonic Accommodation
• Local thermal or mechanical processes resulting
  in
• Up or down motion of the depositional surface
  relative to a fixed point in the earths interior
  in local areas effected by those tectonic events
• Sediment Supply Accommodation
• Sediment input in excess of space made available
  by tectonic or eustatic change

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Aggradation, Progradation, and Retrogradation

• Stacking patterns of sedimentary basin fill due
  to relative sea level and sediment input
  influence on accommodation
• Aggradation
• Progradation
• Retrogradation

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Aggradation

• Sediment input accommodation
• Not common for long periods

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Progradation/Regression

• Sediment input gt accommodation
• Facies prograde and shift offshore
• Regression

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Forced Regression

• Rapid Relative Sea Level Fall
• Negative accommodation
• Facies shift basinward
• Erosion occurs in landward areas
• Regression and Unconformity Surface

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Transgression/Retrogradation

• Relative Sea Level Rise
• Accommodation created in excess of sediment
  input
• Facies shift landward

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Effects of Changing Accommodation on the
Stratigraphic Record

• Transgression/Regression
• Water depth/shoreline changes interpreted from
  vertical changes in grain size (CUS/FUS) or
  sedimentary facies in local sections
• Onlap/Offlap
• Landward/basinward shift in shoreline due to
  relative sea level change
• The fundamental geometric aspect of the
  sedimentary record upon which relative sea level
  change is interpreted

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Wheeler DiagramsTime/Space relationships of
Unconformity

• Space/Space Stratigraphic Cross Section
• Time/Space Chronostratigraphic Chart

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Data Types for Sequence Analysis

• Geological
• From bore holes
• (Typically from offshore, subsurface areas)
• Geophysical
• Seismic reflection profile data

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Cycles of Relative Sea Level Change

Fichter and Poche, 2001
Nichols, 1999
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The Geologic Record, Relative Sea Level, and
Eustacy

• Eustatic Sea Level
• The Holey Grail of stratigraphic studies

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Cycles of Relative Sea Level, Base Level, and
Accommodation Space

• Cycles of relative sea level,
• Migration of base level up and down the
  depositional surface, and
• Variations in accommodation space can be directly
  related to the variation in

Strata Geometry and Sedimentary Facies
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In Fichter and Poche, 2001 After Mitchum and Van
Wagoner, 1990
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In Fichter and Poche, 2001 After Mitchum and Van
Wagoner, 1990
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Depositional Sequences

• Relatively Conformable
• Genetically Related (abide by Walther's Law)
• Bounded by Unconformity or Correlative Conformity
• isochronous with respect to unconformity

• Depositional Sequences are Chronostratigraphic
  Units
• the defining Sequence Boundary defines older
  (below) and younger (above) strata
• Chronostratigraphic Units
• geological time significant

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Depositional Systems Tracts

• Use of Depositional Sequence concepts to predict
  stratal geometry

• Three dimensional assemblage of sedimentary
  facies genetically linked by their origin in
  related depositional environments
• Basically Waltherian World

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Low Stand Systems Tract

• Forced Regression
• Subaerial exposure and formation of a
• Sequence Boundary
• Offlap
• Basinward shift in facies belts

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Transgressive Systems Tract

• Retrogradation
• Flooding of the shelf and onlap
• Landward shift in facies belts

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Maximum Flooding SurfaceCondensed Section

• Between TST and HST (highstand systems tract)
• Represented by a change from retrogradation to
  aggradation/progradation
• The surface or thin succession characterized by
  evidence for slow rates of sedimentation

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Highstand Systems Tract

• Progradation (accommodation lt sediment input)
• Regressive successions
• Onlap and offlap/down-lap

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Basic Depositional Sequence/Systems Tract Model
for a Generic, Clastics-Dominated Continental
Margin
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Stratigraphic Expression of Relative Sea Level
Cycles

• The Sequence Model
• A tool for predicting stratigraphic architecture
• Methodology for basin wide correlation
• May be used for interregional correlation through
  recognition of Eustatic Cycles