SEQUENCE STRATIGRAPHY - TRANSGRESSION, REGRESSION AND SYSTEM TRACTS

 SEQUENCE STRATIGRAPHY 

TRANSGRESSION

• Shoreline shift landward

• Retrogradational pattern- fining upward

• Feature name - Estuaries

• Sea level rise

• Deepening upward (Deep facies found at the top whereas shallow facies found at the bottom)

• Transgressive system tract found - due to rapid increase in sea level

  

REGRESSION

• Shoreline shifts seaward

• either due to sedimentation - Normal regression

• due to sea level fall - Forced regression

  Regression is of two types

1) NORMAL REGRESSION

• Shoreline shift seaward due to sedimentation

• Low stand system tract, High stand system tract

2) FORCED REGRESSION

• Shoreline shift seaward due to rapid fall in sea level

• Falling stage system tract - rapid decrease in sea level

• Coarsening upward - Progradational pattern

SYSTEM TRACTS

1) FSST - Rapid fall in sea level : Forced regression

2) LST - Sea level just start rising (initial stage when sea level start rises)

3) TST - rapid rise in sea level : Transgression (retrogradation pattern)

4) HST - Last stages of rise in sea level (progradation pattern)

BOUNDARIES 

• Maximum flooding surface : TST and HST

• Sequence boundary : FSST and HST

• 

• Maximum flooding surface is present between Transgressive system tract and High stand system tract

• The mfs often mark the bounding surface between coarsening and/or fining upward cycles

OVERALL FIGURE

RELATION OF STRATA

• Upper boundary :

  • Erosional truncation

  • Toplap : Progradation

  • Concordance

• Lower boundary

  • Onlap : Transgression

  • Downlap : Forced regression

  • Concordance

    

• ONLAP:

  • Termination of low angle strata against a steep stratigraphic surface

  • Indicates Transgression

  • Inclined or horizontal strata terminate against a surface of greater inclination

• DOWNLAP:

  • Termination of steep inclined strata against a low lying surface

  • Indicates forced regression

  • Onlap and downlap indicates non-depositional hiatuses

• TOPLAP:

  • Upper boundary of depositional sequence

  • Termination of inclined strata against a low lying surface

  • Indicates progradation

• OFFLAP:

  • a surface where stratigraphic terminations downstep basinward during forced regression.