Introduction
Sequence stratigraphy is a more specialised and modern refinement of ordinary stratigraphy (Chapter 2), analysing sedimentary rock sequences specifically in terms of relative sea-level change and the resulting depositional patterns — a framework of major practical importance in petroleum exploration (Chapter 8).
1. Depositional sequences and key bounding surfaces
A depositional sequence is a relatively conformable succession of related sedimentary layers, bounded above and below by specific, recognisable surfaces (most importantly, sequence boundaries, generally associated with a relative fall in sea level and consequent erosion or non-deposition) — this reframes the ordinary stratigraphic concept of an unconformity (Chapter 2) specifically in terms of the sea-level history that produced it, rather than treating it merely as a generic gap in the rock record.
2. Relative sea-level and accommodation concepts
Relative sea-level change (combining true global, eustatic sea-level change with local subsidence or uplift of the land itself) controls accommodation space — the available vertical space into which sediment can accumulate. When accommodation space increases (relative sea-level rise), sediment can accumulate and preserve more completely; when accommodation space decreases (relative sea-level fall), previously deposited sediment may instead be eroded — this accommodation concept is the fundamental control underlying every other concept in this chapter.
3. Systems tracts and stacking patterns
A systems tract is a linked set of depositional systems that formed together during a specific portion of a relative sea-level cycle, identified by characteristic internal stacking patterns of sediment through time: a lowstand systems tract forms during and just after a relative sea-level fall/lowstand; a transgressive systems tract forms during rising relative sea-level, characteristically showing a retrogradational stacking pattern (successive sediment layers shift progressively landward as the shoreline retreats); and a highstand systems tract forms as sea-level rise slows and accommodation growth decelerates, characteristically showing a progradational stacking pattern (successive layers shift progressively seaward as sediment supply outpaces the remaining accommodation space).
4. Facies shifts and basin interpretation
A facies is the overall characteristics (composition, texture, fossil content) of a rock unit, reflecting the specific environment in which it was deposited. As relative sea-level and accommodation space change through a depositional sequence, the location of specific facies shifts systematically across the basin (landward during transgression, seaward during progradation) — tracking these facies shifts through a sequence is precisely how systems tracts (point 3) are actually recognised and interpreted from real rock or well-log data in practice.
5. Use in stratigraphic correlation and exploration
Because major sequence boundaries and systems tracts often result from broad, sometimes regional-to-global relative sea-level changes, they can potentially be correlated across considerable distances and even between separate sedimentary basins — a powerful advantage over relying on individual, more locally variable rock units alone for correlation. This makes sequence stratigraphy an especially valuable practical tool for petroleum exploration, since specific systems tracts are strongly and predictably associated with characteristic reservoir, source, and seal rock types (developed fully in Chapter 8) at particular positions within a sequence.
Key Takeaways
- A depositional sequence is bounded by sequence boundaries, reframing ordinary unconformities in terms of the relative sea-level history that produced them.
- Relative sea-level change controls accommodation space, the fundamental concept underlying every other idea in sequence stratigraphy.
- Lowstand, transgressive, and highstand systems tracts are distinguished by characteristic retrogradational or progradational stacking patterns through a sea-level cycle.
- Facies shift systematically landward during transgression and seaward during progradation, and tracking these shifts is how systems tracts are actually identified in real data.
- Sequence stratigraphy’s potential for broad correlation across basins, combined with predictable systems-tract/reservoir-rock associations, makes it especially valuable for petroleum exploration.