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Allostratigraphy/Sequence Stratigraphy

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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Allostratigraphy/Sequence Stratigraphy

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  1. Allostratigraphy/Sequence Stratigraphy • Subdivision of the stratigraphic record otbo bounding discontinuities • Bounding discontinuitiesdefine Depositional Sequences • relatively conformable successions of genetically related strata bounded by unconformity or correlative conformities

  2. 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?

  3. Depositional Sequences/Sequence Boundaries • Importance and utility first recognized in the analysis of seismic reflection profile date and reflection terminations

  4. 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--> 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

  5. 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

  6. Base Level Points coinciding with theoretical equilibrium profile along an actual depositional profile Erosion occurs above and deposition occurs below base level First Principles and ControlsSequence Stratigraphy Posamentier and Allen, 1999

  7. 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

  8. 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 earth’s interior

  9. 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 earth’s 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

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

  11. Aggradation • Sediment input ~ accommodation • Not common for long periods

  12. Progradation/Regression • Sediment input > accommodation • Facies prograde and shift offshore • Regression

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

  14. Transgression/Retrogradation • Relative Sea Level Rise • Accommodation created in excess of sediment input • Facies shift landward

  15. 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

  16. Wheeler DiagramsTime/Space relationships of Unconformity • Space/Space Stratigraphic Cross Section • Time/Space Chronostratigraphic Chart

  17. Data Types for Sequence Analysis • Geological • From bore holes • (Typically from offshore, subsurface areas) • Geophysical • Seismic reflection profile data

  18. Cycles of Relative Sea Level Change Fichter and Poche, 2001 Nichols, 1999

  19. The Geologic Record, Relative Sea Level, and Eustacy • Eustatic Sea Level • The “Holey Grail” of stratigraphic studies

  20. 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

  21. In Fichter and Poche, 2001 After Mitchum and Van Wagoner, 1990

  22. In Fichter and Poche, 2001 After Mitchum and Van Wagoner, 1990

  23. 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 Depositional Sequences

  24. 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 Depositional Systems Tracts

  25. Low Stand Systems Tract • Forced Regression • Subaerial exposure and formation of a Sequence Boundary • Offlap • Basinward shift in facies belts

  26. Transgressive Systems Tract • Retrogradation • Flooding of the shelf and onlap • Landward shift in facies belts

  27. 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

  28. Highstand Systems Tract • Progradation (accommodation < sediment input) • Regressive successions • Onlap and offlap/down-lap

  29. Basic Depositional Sequence/Systems Tract Model for a Generic, Clastics-Dominated Continental Margin

  30. Short Term Cycles Long Term Cycles 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

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