Sequence stratigraphy

General principles

Sequence stratigraphy is concerned with the large-scale, three-dimensional arrangement of sedimentary strata, and the major factors that influence their geometries – such as sea-level change, contemporaneous fault movements, basin subsidence and sediment supply (cf. reviews by Emery & Myers 1996; Miall 1997).  The observational basis of sequence stratigraphy is the ubiquitous arrangement of strata into units bounded above and below by unconformities that can be traced out into conformable surfaces in a basinward direction.  These surfaces are defined as the sequence boundaries and the strata between them constitute a depositional sequence.

The geometrical relationships are observable from seismic reflection profiles, extensive (e.g. hillside) field exposures, or are inferred by correlations from smaller locations. A generalized model of a depositional sequence, including details of the internal geometries, is shown in Fig. 6.  No temporal or thickness scale is given in this figure because sequences develop in a hierarchical fashion at a great range of scales (e.g. Van Wagoner et al. 1990).  The principal factor thought to govern the genesis of a depositional sequence is relative sea-level change.  Relative sea-level change is the net result of global sea-level change combined with local subsidence or uplift of the depositional area (Posamentier & Vail 1988).

An important practical aspect of sequence stratigraphy is the recognition of key surfaces. In the Exxon model of sequence stratigraphy (e.g. Haq et al. 1988) the unconformity surface represents the proximal area of a sequence boundary and passes into expanded sedimentary successions in more basinal settings; it develops when proximal accommodation is no longer available. Accommodation is a loosely-defined term meaning space available for sediment to accumulate: this space is capped by a dynamic 'accommodation limit', a surface which passes through the shoreline and is itself dependent on sediment supply and transport processes. The proximal region of the sequence boundary is characterized by erosion of the underlying strata and may include such features as river valleys cut into previously deposited marine strata of the underlying surface.  Conversely, the distal region of the sequence boundary may be represented by the increased volume of sedimentary debris eroded from the more landward sites.

In addition to the sequence boundary, two other important surfaces occur within a sequence.  The more distal portion of the maximum flooding surface represents, like the sequence boundary, a break in deposition or very slow sedimentation, but unlike the sequence boundary it develops at the far end of the sediment transport path as a result of sediment starvation, and may be characterised by condensed marine deposits containing an abundant pelagic fauna and well-developed early (sea-floor) authigenic mineralisation, especially with glauconite and phosphate.  The more landward portions of the maximum flooding surface may, by contrast, be hidden within a thick succession of relatively shallow marine or non-marine strata. (It is the maximum flooding surface that normally defines the limits of Galloway's (1989) genetic stratigraphical sequences: see below).  A third important surface is the transgressive surface, which is generally taken to be the first significant marine flooding surface within the sequence.

Within sequences, further, more subtle geometrical and facies relationships have been used to define systems tracts (Van Wagoner et al. 1988; cf. Helland-Hansen & Gjelberg 1994; Helland-Hansen & Martinsen 1996).  Geometrical arrangements of facies or smaller-scale sedimentary cycles (`parasequences') may be such that systems tracts can be recognized in single vertical sections at outcrop or within a borehole (Van Wagoner et al. 1990).  A four-systems-tract subdivision of depositional sequences is now commonly employed (Hunt & Tucker 1992, 1995).  Overlying the sequence boundary is the lowstand systems tract, characterized by inferred rising relative sea-level and shoreline regression (the latter continuous from the preceding systems tract).  The transgressive systems tract comprises strata whose depositional environments migrate overall in a landward direction (i.e. are transgressive) and whose component stratal surfaces onlap pre-existing deposits;  the base is defined by the transgressive surface and relative sea-level at the shoreline is also inferred to have been rising.  The transgressive systems tract is terminated at its top at the maximum flooding surface, above which strata of the highstand systems tract shift basinward again, with successive stratal surfaces terminating in progressively more distal locations, forming a geometrical pattern known as downlap (note the general similarities with the lowstand systems tract).  The final, forced regressive systems tract is represented by an arrangement of strata whose shoreline positions migrated progressively downwards as well as basinwards and so is produced during falling relative sea-level and regression (the surface defining the base had been termed the ‘basal surface of forced regression’).

The timing and nature of gravity-flow deposits within a depositional sequence is a matter of some discussion.  Early models (e.g. Posamentier & Vail 1988) placed major phases of debris flow and turbidity current deposition as occurring during relative sea-level fall and lowstand.  More recent work (e.g. Helland-Hansen & Gjelberg 1994) has shown that, conceptually, gravity-driven deposition may occur at any stage of relative sea-level change, depending on the development of steep and unstable slopes.  In Fig. 6 ‘fan’ systems are shown in the more conventional position corresponding to relative sea-level fall.

Carbonate sediments respond in a different manner to siliciclastic sediments in response to relative sea-level change because the sedimentary grains are produced in situ rather than transported from a hinterland.  Many carbonate-producing processes require warmth and light in shallow-water settings and are particularly sensistive to changes in nutrient supply. Thus the large-scale geometries of carbonate systems are very different from those of siliciclastic systems (Schlager 1992).  During relative sea-level rise carbonate production may be extremely effective, such that large volumes of shallow water carbonate accumulate, and there is sufficient production for export of carbonate grains to deep water settings.  During relative sea-level falls, large regions of potentially productive shelf may be exposed and effectively shut down, so that little sediment can accumulate in either shallow-water or deep-water settings.  However, it is probable that in the past some sea-level rises have been associated with increases in nutrient supply and/or siliciclastic sediment that have led to the demise of the carbonate platform systems, to form a drowning unconformity (Schlager 1989). The lower depositional angles of the siliciclastic sediments may create stratal geometries that mimic those expected when sea-level falls below the shelf-slope break of purely siliciclastic systems.

It has been claimed by some that global sea-level change is the dominant influence on the formation of all depositional sequences (e.g. Haq et al. 1988), but this is not accepted as fact by the majority of workers in the field.  The extent to which global sea-level change may influence the sequence stratigraphical record will depend on both time and location.  In many depositional settings (for example, active plate margins) more localised relative sea-level changes commonly dominate the stratigraphical record.  On the other hand high magnitude, high rate sea-level fluctuations have undoubtedly occurred during periods of glaciation and deglaciation (glacio-eustatic sea-level changes).

Sequence stratigraphy is now widely used as a means of subdividing, correlating and dating sediments (e.g. Hesselbo &  Parkinson 1996), especially within the hydrocarbons industry. However, it has yet to be applied to Quaternary successions to any great extent.  Although it has most commonly adopted in marine coastal settings, attempts have also been made, with mixed success, to apply the principles in fluvial, glaciomarine and lacustrine situations.

It should also be noted that sequence stratigraphy is often applied uncritically; in particular, age assignment of strata based solely on correlation with a supposedly global sea-level curve has not proved to be a robust method.

 

Further details on sequence stratigraphy

Below are listed two recent publications that provide detailed examination and explanation of the Sequence Stratigraphical approach to stratigraphical classification.  For further detail on this subject, the reader should refer to the list of references provided in each document.

Embry, A.F. 2009. Practical sequence stratigraphy. Canadian Society of Petroleum Geologists, 81 p.

Catuneanu, O. et al. 2011. Sequence stratigraphy; methodology and nomenclature. Newsletters on stratigraphy 44, 173-245.

 

*This guide is based on that produced by Rawson et al. (2002) for the Stratigraphy Commission of the Geological Society of London.