Sequence Stratigraphy

Sequence stratigraphy interprets the layered rock record as a series of packages bounded by surfaces of erosion or non-deposition, each reflecting the interplay of sea-level change, subsidence, and sediment supply.

Definition

Sequence stratigraphy is the study of sedimentary strata as genetically related successions bounded by unconformities and their correlative conformities, organized into systems tracts that record cycles of relative sea-level change.

Scope

This topic covers the analysis of strata in terms of depositional sequences and systems tracts controlled by changes in relative sea level. It addresses the recognition of key bounding surfaces, the predictive stacking patterns they produce, and the application of these concepts in seismic and subsurface stratigraphy.

Core questions

• How do changes in relative sea level organize strata into sequences?
• What surfaces and stacking patterns define systems tracts?
• How is sequence stratigraphy applied to seismic and well data in exploration?

Key theories

Seismic and sequence stratigraphy

Vail and colleagues showed that reflection patterns in seismic data record depositional sequences bounded by unconformities, and interpreted coastal onlap as a record of relative sea-level change, founding the sequence-stratigraphic method.

Accommodation and systems tracts

The balance between accommodation, the space available for sediment created by sea-level and subsidence changes, and sediment supply controls whether strata prograde, aggrade, or retrograde, defining a predictable succession of systems tracts within each sequence.

Mechanisms

Relative sea level, set by global sea level plus local subsidence or uplift, creates or removes accommodation space. When supply outpaces accommodation, the shoreline progrades seaward; when accommodation grows faster, it retreats landward. These shifts produce characteristic stacking patterns and bounding surfaces, including sequence boundaries, maximum flooding surfaces, and transgressive surfaces, that allow sequences and systems tracts to be mapped.

Clinical relevance

Sequence stratigraphy is a primary predictive tool in petroleum exploration, locating reservoir, source, and seal rocks within a sea-level framework, and it provides the basis for distinguishing global from local controls in reconstructing past sea-level change.

History

Sloss introduced large-scale stratigraphic sequences in the 1960s. The approach was transformed in 1977 when Vail and colleagues at Exxon published seismic stratigraphy and the coastal-onlap sea-level curves, and it was later systematized and partly standardized in the 1990s and 2000s through works such as Catuneanu's synthesis.

Debates

Eustatic versus tectonic control on sequences

Whether the global sea-level curves derived from seismic stratigraphy reflect truly eustatic changes or are dominated by regional tectonic and sediment-supply effects has been a long-running controversy in stratigraphy.

Key figures

• Peter Vail
• Robert Mitchum
• Octavian Catuneanu
• Lawrence Sloss

Seminal works

• vail1977
• catuneanu2006

Frequently asked questions

What is a depositional sequence?

A depositional sequence is a package of genetically related sedimentary strata bounded above and below by unconformities or their correlative conformities, representing one cycle of relative sea-level change.