Sequence stratigraphy was developed from seismic stratigraphy in the 1970s, by workers in the Exxon research facility. It was founded on the same principle as used in seismic stratigraphy, that seismic reflectors are time surfaces and that unconformites are bounding surfaces that separate strata into time-coherent packages. Moreover, it was recognized from the circum-Atlantic passive margins that the stratal pattern in one area could be correlated with others that were far distant. The stratal patterns were as distinctive as the biostratigraphic correlations. The two also matched. It was clear that the stratal patterns in these areas of low tectonic activity were the signatures of sea-level rise and fall. The implications of sequence stratigraphy are profound. An explanation of strata in terms of relative sea-level fluctuations and a combination of eustatic sea-level change and tectonic subsidence allows an understanding of why sediment packages develop where they do. It can therefore provide a predictive tool for determining the likely presence of source rocks, and the distribution of reservoirs and seals.
A brief history about stratigraphy
Stratigraphy in one form or another has been around since the 1600s. In 1669, Nicholas Steno, a Darnish geologist working in Italy recognized that strata are formed as heavy particles settle out of a fluid. He also recognized that some strata contain remnants of other strata, and so must be younger. He thus developed three principles that form the basis of all stratigraphy-younger layers lie on top of older layers, layers are initially horizontal, and layers continue until they run into a barrier. For over 300 years after Steno, stratigraphers worked at unraveling the history of the earth, correlating fossils from one continent to another, assigning names, ages, and eventually physical mechanisms to the creation of rock layers. By 1950, most of the major geologic time units had been named. By 1900, most layers had relative ages, and rock types had been associated with certain positions of the shoreline, which was known to move with time. At the turn of the century, shoreline movement was attributed to tectonic activity—the rising and falling of continents. This view was challenged in 1906 when Eduard Suess hypothesized that changes in shoreline position were related to sea level changes and occurred on a global scale; he called the phenomenon eustasy. However, Suess was not able to refute evidence presented by opponents of this theory—in many locations there were discrepancies between rock types found and types predicted by sea level variation. In 1961, Rhodes W. Fairbridge summarized the main mechanisms of sea level change: tectono-eustasy, controlled by deformation of the ocean basin; sedimento-eustasy, controlled by addition of sediments to basins, causing sea level rise; glacio-eustasy, controlled by climate, lowering sea level during glaciation and raising it during deglaciation. He recognized that all these causes may be partially applicable, and are not mutually incompatible. He believed that while eustatic hypotheses apply worldwide, tectonic hypotheses do not and vary from region to region.
Stratigraphy is the science of describing the vertical and lateral relationships of rocks. It is the study of the spatial relationships between bodies of sedimentary rocks. These relationships may be based on rock type, called lithostratigraphy, on age, as in chronostratigraphy, on fossil content, label biostratigraphy, or on magnetic properties, named magnetostratigraphy.
The basic unit in sequence stratigraphy is, of course, the sequence. This has been defined as "a relatively conformable, genetically related succession of strata bounded by unconformities or their correlative conformities". Sequence boundaries form when the water depth decreases. A sequence is made up of parasequences and parasequence sets. A parasequence is defined as a relatively conformable, genetically related succession of beds or bed sets, bounded by marine-flooding surfaces or their correlative unconformities. The parasequence set is a group of genetically linked parasequences that form a distinctive stacking pattern. Such parasequence sets are typically bounded by major marine-flooding surfaces or their correlative surfaces. Smaller units of subdivision are beds and laminae, both sometimes grouped into sets. The key property of the bounding surfaces to sequences, parasequence sets, parasequences, and beds is that they are chronostratigraphically significant surfaces. Thus each surface is a physical boundary that separates all the rocks above from those below.
Components of the lowstand systems tract on a shelf/slope break margin. The coastal onlap and shelf/slope lines are marked.
Stratal geometries in a Type 1 sequence on a shelf/slope break margin. Five separate sedimentary packages are shown, traditionally assigned to three systems tracts: lowstand, transgressive, and highstand.
It is important to point out that the terminology adopted above is derived from the "Vail" school of thought. An alternative sequence scheme has been developed by Galloway (1989), in which the maximum flooding surfaces are used to bound what he terms "genetic sequences." In the Galloway genetic sequences, the unconformity is in the middle. This has led to widespread confusion. There are merits to both schemes. On seismic data the unconformities tend to be most easily identified, while on wireline log data maximum flooding surfaces are commonly more obvious.
Sediments accumulate where accommodation space exists and where a sediment supply exists. Accommodation space is created and destroyed by the interplay of tectonic subsidence or uplift and sea level (in the marine environment). Changes in absolute sea level (eustasy) and basement can be considered as changes in relative sea level. For example, basement subsidence equates to relative sea-level rise, and basement uplift to relative sea-level fall. The effect of relative sea-level rise will be a landward shift of facies belts, while the opposite will occur during sea-level fall. Because the angle of slope from land to shelf to continental slope to basin is not constant, the effects of sea-level fall and rise change dramatically depending upon the magnitude of the sea-level change. The critical points on the shelf slope-basin profile are the shelf/slope break and the point of coastal onlap.
The shelf/slope break is the boundary between the less steeply dipping shelf (0.1°) and the more steeply dipping slope (2-7°). The point at which sedimentation begins on the coastal plain is the point of coastal onlap. Above this point is an area of nondeposition and/or erosion.
During cycles of relative sea-level fall, two types of sequence can develop depending on whether or not the fall in the sea level is sufficient to expose the shelf/slope break to erosion. When the sea level falls sufficiently to expose the shelf/slope break, the sequence is termed Type 1. The large drop in sea level will cause erosion of the shelf area and the coastal plain beyond. Incised valleys and canyons may form up slope from the shelf/slope break, while deposition of coarse elastics occurs in the basin (the basin-floor fan). As sedimentation responds to the sea-level drop, the slope fan and slope wedge succeed the basin-floor fan. Collectively, these three elements are termed the "lowstand systems tract" (LST). Moreover, much of the LST could contain reservoir lithologies dominated by sediment gravity flows, including turbidites. The LST will in turn be buried beneath the "transgressive systems tract" (TST), which is formed as the sea level rises. This will culminate in a "maximum flooding surface" (MFS). Typically, during periods of maximum flooding, large areas of shelf lie beneath shallow water. In consequence, circulation of the water column can be poor. Such conditions are often ideal environments for source-rock development. A high sea level also reduces physical erosion, resulting in deposition of fine-grained lithologies, which are potential seals. The "highstand systems tract" (HST) succeeds the transgressive systems tract. The HST builds over and downlaps onto the TST. Initially, the sediment pile aggrades until accommodation space is exhausted and then progradation takes over. As the sea level falls again, the next sequence boundary is created.
In situations where the sea level does not fall below the shelf/slope break, extensive coarse-grade sedimentation does not occur in the basin but, instead, occurs at the shelf margin, the so-called "shelf margin systems tract" (SMST). The complete shelf margin systems tract, with the transgressive systems tract and the highstand systems tract, collectively form a Type 2 sequence.
A carbonate highstand systems tract (HST) of an escarpment margin, showing continued aggradation but a progressive thinning of topsets. In the basin and at the basin slope, carbonate shed from the platform top during highstand onlaps the talus apron of the transgressive systems tract. Some talus also may be shed from the carbonate margin during highstand. MFS = maximum flooding surface; TST = transgressive systems tract.
Acarbonate lowstand systems tract (LST) on a humid escarpment-margin system. Here the sea level has fallen significantly below the margin, and the exposed platform top is karstified and may become incised by fluvial channels. Siliciclastic sediment may be deposited in the basin, onlapping the carbonate slope, and failure of the margin may also result in the deposition of talus cones at base of slope. In situ carbonate production is likely to be minor on very steep margins. HST = highstand systems tract; TST = transgressive systems tract; SB = sequence boundary.
The situation in carbonate systems is quite different from that in clastics. Because carbonates predominantly develop in situ in shallow shelf settings, large sea-level falls can shut down the carbonate factory.
Exposure of the shelf often promotes chemical weathering, the formation of karst above the water table, and meteoric diagenesis (cementation) below the water table. Thus sea-level fall may generate no sediment into the basin. At times of high sea level, the opposite can be true. Productivity is high when the shelf is flooded. Large areas of shelf may overproduce carbonate material that may be shed from the shelf into deep water during sea-level highstand.
Application of sequence stratigraphy
As an exploration tool, sequence stratigraphy is used to locate reservoir sands. In deep water basins with high sedimentation rates, sands are commonly first laid down as submarine fans on the basin floor and later as deposits on the continental slope or shelf. However, as sea level starts slowly rising onto the continental shelf, sands are deposited a great lateral distance from earlier slope and basin deposits.
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