Sequence Stratigraphy

Sequence Stratigraphy > Lithostratigraphy

• Lithostratigraphy: deals with lithology of strata and their organization into units based on lithological characteristics

• Sequence Stratigraphy: considers depositional environments, systems tracts, and relative sea-level changes, offering a comprehensive framework for understanding the lateral and vertical variability of sedimentary packages.

  • Reconstructing depositional systems, predicting subsurface reservoirs, and linking stratigraphy to tectonic and climatic influences

Sedimentary Accommodation vs. Water Depth

• Accommodation: Amount of available space for sediment to fill in a basin, from seafloor to base-level

• Water Depth: Vertical distance from water surface to the ocean/lake floor at a given point

• Accommodation is a measure of potential sediment storage space, whereas water depth is a direct measurement of current water levels

Walther’s Law

• The same succession that is present vertically also is present horizontally unless there is a break in sedimentation (Posamentier and Allen, 1999)

Forced Regression vs. Normal Regression

• Forced Regression: Shoreline migrates seaward due to a drop in base-level, regardless of sediment supply

• Normal Regression: Seaward migration of a shoreline due to the oversupply of sediment

Systems Tract w/ well-developed, mature Paleosols

• Falling Stage Systems Tract

  • Valleys creep updip and expand laterally, allowing for best paleosols

Sequence

• A relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities.

Parasequence

• Stratigraphic unit used to describe a relatively conformable succession of genetically related beds and/or bedsets bounded by flooding surfaces and may coincide with other systems tract boundaries.

What problems does the author have with the parasequence concept?

• Main problem → Flooding surfaces

• Flooding surfaces are poorly defined term that allows for multiple meanings

• Depending on what type of stratigraphic surface the flooding surface actually is, parasequences may be anything from transgressive-regressive sequences to genetic stratigraphic sequences and allostratigraphic units.

When does the author suggest that the term parasequence may be used?

• Restricted to prograding successions in coastal to shallow-water settings, where evidence of flooding surfaces can be produced

  • And where transgressive ravinement surfaces (erosional surfaces) are unlikely to occur

Systems Tracts

• A linkage of contemporaneous depositional systems, forming the subdivision of a sequence, and depositing at a deepening or swallowing rate

Depositional Sequence

• Three-dimensional assemblages of lithofacies, genetically linked by active processes of inferred processes and environments. Sediments are distributed and accumulated within a specific bathometric or climate

Systems Tracts and Sequence Stratigraphic Succession

• Basal Surface of Forced Regression (BSFR): Separates HST from FSST

• Falling Stage Systems Tract (FSST): Absolute drop in sea-level; Forced Regression

• Sequence Boundary (SB)

• Correlative Conformity (C.C): Separates FSST from LST

• Lowstand Systems Tract (LST): Regression; rise in sea-level

• Maximum Regression Surface (MRS): aka “Transgressive Surface (TS);” transition from regression to transgression

• Transgressive Systems Tract (TST)

• Maximum Flooding Surface (MFS): aka “Condensed Section (C.S);” for very long periods of time we’re adding very little sediment

• Highstand Systems Tract (HST)

Can a Transgressive and a Regressive Systems Tracts be located on the same shoreline?

Yes, they represent different stages of sea level change and shoreline movement within a single depositional sequence. Can be expressed in Transgressive-Regressive Sequence (T-R). A T-R sequence can be divided into a transgressive systems tract below and a regressive systems tract above by using the maximum flooding surface (MFS) as a mutual boundary.