Depositional Sequences And Systems Tracts


The terminology used predates the advent of sequence stratigraphy: terms such as transgression and regression to describe sea-level changes have been in use for a long time and the idea that strata are sometimes arranged into repeating cycles of lithologies was established in the 19th century. The development of the concepts of sequence stratigraphy has resulted in the introduction of a large number of new terms into sedimentary geology and although at first this new terminology seems to be quite daunting, it is generally quite logical and easy to relate to the concepts. To illustrate and introduce the application of sequence stratigraphic terminology, and to show how facies distributions can relate to changes in relative sea level, two types of continental margin are considered, each with certain conditions regarding rates of sea-level change and sediment supply. These two case studies are the ones most often illustrated in texts on sequence stratigraphy, but variations on these two are not only possible, but common, depending on the rates of sea-level rise and fall, changes in sediment supply, and the shape of the bathymetry of the shelf. 
  1. A continental margin with a distinct shelf break: sediment supply is assumed to be constant, and the sea level falls to below the edge of the shelf so that there is erosion on the shelf during sea-level fall. Beyond the edge of the shelf lies a slope and a deeper basin area that receives sediment during certain stages of the sea-level cycle.
  2. A continental shelf that is a sloping ramp with no distinct change in slope: the sediment supply is again considered to be constant and is relatively high such that deposition occurs throughout the cycle. A deeper basin area may exist, but is not strongly influenced by sea-level fluctuations on the ramp.
Shelf-break margin depositional sequence


Highstand

The highstand is the period of high sea level during the cycle and the beds deposited during this period are called the highstand systems tract (HST). (A ‘systems tract’ is the term used in sequence stratigraphy for strata deposited during a stage of a depositional sequence.) The beds show either an aggradational or a progradational pattern as the shoreline shifts seawards across the shelf. Sediment is supplied by rivers from the hinterland and most of the accumulation occurs on the shelf with little sediment reaching the deeper basin.

Sequence boundary

During sea-level fall erosion of the shelf occurs as rivers erode into the sediment deposited during the previous cycle: where erosion is localised the rivers cut incised valleys. This erosion creates an unconformity, which in this context is also called a sequence boundary (SB). It marks the end of the previous depositional sequence and the start of a new one: depositional sequences are defined as the packages of beds that lie between successive sequence boundaries. If the sea level falls to the level of the shelf edge then both the detritus from the hinterland being carried by the rivers and the material eroded from the shelf are carried beyond the edge of the shelf. This sediment forms a succession of turbidites on the basin floor that are deposited during the period of falling sea level, forming a basin floor submarine fan. There is no unconformity within the basin floor succession to mark the sequence boundary, so the start of the next sequence is marked by a correlative conformity, a surface that is laterally equivalent to the unconformity that forms the sequence boundary on the shelf.

Lowstand

The interval of low sea level is called a lowstand and the deposits of this period are called the lowstand systems tract (LST). The relative sea level is rising slowly but the rate of sediment supply is relatively high. The rivers cease to erode on the shelf but the shelf continues to be by-passed: sedimentation continues to occur on the basin-floor fan as turbidites (also referred to as lowstand fan deposits). Sediments also start to build up above the fan at the base of the slope to form a lowstand wedge. The pattern of beds in these deposits is initially progradational, becoming aggradational in the lowstand wedge as the rate of sea level increases.

Transgressive surface

The point at which the rate of creation of accommodation due to relative sea-level rise exceeds the rate of sediment supply to fill the space is called the transgressive surface (TS). It marks the start of retrogradational patterns within the sedimentary succession as accommodation outpaces sediment supply. If sediment supply is relatively low the transgressive surface may be erosional: surfaces of erosion formed during transgression are called ravinement surfaces and they form because high wave energy in the shallow water that floods over the land surface can result in erosion.

Transgressive systems tract

Deposits on the shelf formed during a period of relative sea level rising faster than the rate of sediment supply are referred to as the transgressive systems tract (TST). They show a retrogradational pattern within the beds as the shoreline moves landwards. Sediment is no longer supplied to the basin floor because there is now sufficient accommodation on the shelf. The relative sea-level rise results in the formation of estuaries as the incised valleys are flooded with seawater: estuarine sedimentation is characteristic of the transgressive systems tract. The rise in base level further upstream creates accommodation for the accumulation of fluvial deposits within the valleys.

Maximum flooding surface

As the rate of sea-level rise slows down the depositional system reaches the point where the accommodation is balanced by sediment supply: when this happens transgression ceases and the shoreline initially remains static and then starts to move seawards. This point of furthest landward extent of the shoreline is called the maximum flooding surface (MFS): it should be noted that it does not represent the highest sea level in the cycle, which occurs later in the highstand systems tract. As the point of the maximum flooding surface is approached the outer part of the shelf is starved of sediment because there is abundant accommodation near the shoreline: very low sedimentation rates on the shelf can be recognised by a number of features including concentrations of authigenic glauconite and phosphorites, condensed beds rich in fossils and evidence of sea-floor cementation from hardgrounds and firmgrounds.

Highstand

A return to aggradational and progradational patterns of shelf sedimentation marks the onset of the highstand systems tract above the maximum flooding surface. Continued relative sea-level rise creates accommodation within the continental realm: fluvial deposition is no longer confined within incised valleys (cf. transgressive systems tract) resulting in deposition in rivers and on overbank areas over wide areas of the coastal plain.

Ramp margin depositional sequence 

Highstand

Highstand deposition on a ramp margin is essentially the same as for the shelf-break example, with an aggradational to progradational pattern of deposition on the inner part of the margin.

Sequence boundary

The sequence boundary in the ramp succession is placed on the surface on which there is the first evidence of erosion caused by sea-level fall. Erosion starts at the landward end, but further seaward there will be continued sedimentation, with the geometry of the strata changing from building up to stepping down (i.e. from geometry IV to geometry VI). This change in stratal geometry may not always be easy to recognise in practice. It is worth noting that in the original schemes for shelfbreak margins, the highstand was considered to continue into the initial stages of sea-level fall and the sequence boundary placed at the point when erosion was widespread across the shelf: if this approach is applied to a ramp margin it would be placed at some point within the succession deposited during sea-level fall. Alternative scheme in which the sequence boundary is placed above all the strata deposited during the period of falling sea level, but this creates the situation where the sequence boundary lies within the package of strata deposited in the basin in the shelf-break setting.

Falling stage systems tract

Sediments deposited during the period from the onset of the relative fall in sea level until the point where it stops falling are considered to form the falling stage systems tract (FSST). The sediment is supplied by rivers from the hinterland and also as a result of erosion of the landward part of the ramp. Erosion by rivers may create incised valleys between the landward part of the ramp and the shoreline. The shoreline moves seawards and steps down as the relative sea level falls and hence the geometry of the strata is progradational and down-stepping. Note that under the alternative scheme, these deposits would be considered to be below the sequence boundary and they refer to them as the forced regressive wedge systems tract (FRWST).

Lowstand

The lowstand systems tract is deposited during the early stages of sea-level rise. The inner part of the ramp is no longer erosional and the water level starts to rise in the incised valleys. The geometry of the strata in the outer part of the ramp is progradational, becoming aggradational as the rate of sea-level rise increases and the shoreline stops moving seawards and becomes stationary.
Transgressive surface, transgressive systems tract, maximum flooding surface, highstand
The processes and patterns of sedimentation during these stages of rising sea level are essentially the same as for the shelf-break margin depositional sequence.
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