Sequence Stratigraphy

What is Sequence Stratigraphy?

Sequence stratigraphy is a branch of stratigraphy that interprets sedimentary deposits within a framework of relative sea-level changes, basin subsidence, and sediment supply. It focuses on recognizing and analyzing sedimentary sequences formed by these dynamic interactions over time.

Study involved in Sequence Stratigraphy

Depositional Sequences

Definition: A depositional sequence is a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities.

Formation: These sequences are formed due to cyclic changes in sea level, which control sediment deposition patterns.

Systems Tracts

Depositional sequences are subdivided into systems tracts based on their position within the sequence and the associated sea-level changes:

source: http://sepmstrata.org/page.aspx?pageid=15

1. Lowstand Systems Tract (LST): Deposited during the lowest relative sea level when sediment accumulates in deep basins or shelf edges.
2. Transgressive Systems Tract (TST): Deposited during rising sea level, characterized by retrogradational (landward-shifting) deposits
3. Highstand Systems Tract (HST): Deposited during the highest relative sea level, with sediment progradation (seaward-shifting).
4. Falling Stage Systems Tract (FSST): Deposited during falling sea levels after a highstand.

Surfaces in Sequence Stratigraphy

1. Unconformity: A surface representing a gap in the geological record due to erosion or non-deposition.
2. Sequence Boundary (SB): The boundary between sequences, typically an unconformity.
3. Maximum Flooding Surface (MFS): Represents the point of maximum marine transgression, often associated with condensed sections rich in fossils or organic material.
4. Transgressive Surface (TS): Marks the onset of transgression when marine conditions begin to dominate over terrestrial deposits.

Sea-Level Changes

Relative Sea Level: Changes in sea level relative to the land, influenced by tectonics and sediment supply.

Eustatic Sea Level: Global changes in sea level due to factors such as glaciation and deglaciation.

Applications of Sequence Stratigraphy

Hydrocarbon Exploration:

Identifies potential reservoirs, source rocks, and seals. Predicts the spatial distribution of sand bodies and other reservoir-quality rocks.

Paleogeography Reconstruction:

Provides insights into ancient landscapes, depositional environments, and basin evolution.

Stratigraphic Correlation:

Facilitates correlation across basins by using sequence boundaries and key surfaces.

Mineral Exploration:

Aids in the understanding of depositional environments associated with placer deposits, coal, or evaporites.

Stratigraphic Models in Sequence Stratigraphy

Exxon Model:

Focuses on global eustatic sea-level changes as the primary driver of sequence formation.

Galloway Model:

Emphasizes sediment supply and accommodation space rather than strictly sea-level changes.

Practice Questions

1. Multiple-Choice Question (MCQ) Which of the following represents the maximum transgression in sequence stratigraphy? a) Sequence Boundary b) Transgressive Surface c) Maximum Flooding Surface d) Falling Stage Surface

Answer: c) Maximum Flooding Surface

2. Multiple-Select Question (MSQ) Which of the following statements about Lowstand Systems Tract (LST) are correct? a) It forms during the lowest relative sea level. b) It is deposited above the maximum flooding surface. c) It consists of basin-floor fans and slope aprons. d) It is always associated with transgressive conditions.

Answer: a) It forms during the lowest relative sea level. c) It consists of basin-floor fans and slope aprons.

3. Subjective Question Explain the significance of systems tracts in sequence stratigraphy and describe the depositional characteristics of Highstand Systems Tract (HST).

Answer Outline:

Define systems tracts and their role in sequence stratigraphy.

Discuss HST formation during high relative sea level.

Describe progradational deposits, including shelf sediments and coastal facies, and their implications for resource exploration.