sequence stratigraphy

sequence stratigraphy Sequence stratigraphy is the subject that is concerned with the large-scale, three-dimensional arrangement of sedimentary strata and with the major factors that influence their geometrical relationship. These factors are sea-level change, contemporaneous fault movements, and supply of sediment. The basic conceptual unit, a ‘depositional sequence’, is a package of sedimentary strata bounded above and below by major surfaces. These surfaces are erosion surfaces (‘unconformities’) along some of their length, and are technically defined as the ‘sequence boundaries’. The geometrical relationships within and between depositional sequences are observable from geophysical data, particularly seismic reflection profiles, which represent an acoustic image through a rock sequence. Because seismic reflection profiles are an important tool in hydrocarbon exploration, sequence stratigraphy as a discipline was first developed extensively within the oil industry. Depositional sequence geometries may also be observed in mountainside exposures of relatively undeformed strata, or by piecing together evidence from exposures on a much smaller scale.

A simple depositional sequence for particulate (detrital) sediment is shown in Fig. 1. Sediments laid down in different depositional environments are distinguished by different tints, and the sequence boundaries are also indicated. The geometries have been related to changing sea level because sea level exerts a strong influence on the heights to which sediment may accumulate. At the sequence boundary, sediment accumulated only in the most distal locations, and hence it is inferred that sea level was falling or at a low stand. At the opposite extreme, in the middle portion of the depositional sequence, deposition occurred near the source, and it is inferred that sea level was rising or at high stand; at the same time more distal locations may have become completely starved of sediment, and the surface generated as a result has been called a ‘maximum flooding surface’.

The proximal region of the sequence boundary will be characterized by erosion of the underlying strata and may include features such as river valleys cut into previously deposited marine strata below. Conversely, the distal region of the sequence boundary may be represented by an increased abundance of sedimentary debris eroded from more landward sites. The maximum flooding surface has a different character. The more distal portion of the maximum flooding surface represents, like the sequence boundary, a depositional break, 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 characterized by slowly accumulating marine deposits containing an abundant pelagic fauna and well-developed mineralization of the sea floor.

One school of thought prevalent in the early development of sequence stratigraphy was that the sea-level changes responsible for generating depositional sequences were global in extent. Thus, by recognizing the key surfaces of depositional sequences in well-dated sedimentary successions, composite global sea-level curves could be constructed for Phanerozoic time. There are, however, many soundly based objections to the notion of global sea-level change as the dominant control on depositional sequence geometries.

Sea-level change taking place in a specific place on the Earth is also the result of localized subsidence and uplift. The rates of subsidence or uplift in many geological settings are likely to have been comparable to, or in excess of, global sea-level change. This is particularly the case during times in Earth history when there were no large, land-based polar ice caps, as during the Mesozoic. Additionally, changes in the rate at which sediment was supplied to a basin would alter significantly the time of formation of a sequence boundary or maximum flooding surface. For example, where there was an abundant supply of sediment, the shoreline would have begun to move seawards much earlier than it would when sediment was scarce. Furthermore, movements along major faults would have affected not just subsidence and uplift but also the local supply of sediment. Thus, the extent to which global sea-level changes may have influenced sequence stratigraphy will have depended both on time and place.

Carbonate systems (limestones and dolomites) may be expected to have responded differently to sea-level changes because the sedimentary grains were produced mainly in situ rather than having been transported from a hinterland. Many carbonate-producing processes require warmth and light in a shallow-water setting, and are particularly sensitive to changes in nutrient supply. As a result, the large-scale geometrical relationships of carbonate systems are very different from those of detrital systems. During relative sea-level rise carbonate production would generally have been extremely effective; large volumes of shallow-water carbonate grains would accumulate, and enough would be produced for export of carbonate grains to deep-water settings. During falls in sea level, large regions of potentially productive shelf would be exposed and sediment production effectively shut down, so that little sediment would have been able to accumulate in either shallow-water or deep-water settings. It is clear also that in the past some sea-level rises have been associated with increases in nutrient supply that have led to the demise of the carbonate sedimentary system, to form a ‘drowning unconformity’. The lower depositional angles of overlying detrital sediments may create stratal geometries that mimic those expected with extreme sea-level fall in purely detrital systems.

Stephen Hesselbo