A depositional sequence is defined as a relatively conformable succession of genetically related strata bounded by subaerial unconformities or their correlative conformities. The critical part of this definition is that sequences are bounded above and below by subaerial unconformities, or by correlative conformities, that is, surfaces that correlate updip to a subaerial unconformity. A subaerial unconformity is a surface formed through subaerial exposure and erosion, and includes features formed by downcutting rivers, soil processes, and karst processes.
In addition, every depositional sequence is the record of one cycle of relative sea level. As a result, depositional sequences have a predictable internal structure of surfaces and systems tracts (suites of coexisting depositional systems, such as coastal plains, continental shelves, and submarine fans). There are several models of systems tracts within depositional sequences, and we'll cover what is called the four systems tract model. In this model, all depositional sequences contain the following systems tract in this order: lowstand systems tract, transgressive systems tract, highstand systems tract, and falling-stage systems tract. In this view, a sequence begins with the slow rise following a fall in sea level, and continues through the next fall in sea level. These systems tracts are bounded by important named surfaces. The lowstand and transgressive systems tracts are separated by the transgressive surface. The transgressive and highstand systems tracts are separated by the maximum flooding surface. The highstand and falling-stage systems tract are separated by the basal surface of forced regression.
The systems tracts and changes in accommodation
To understand the formation of a sequence during changes in accommodation, it's easiest to start in the highstand systems tract (HST). During this time, relative sea level is undergoing a slow rise from the combined effects of subsidence and eustasy. During such a slow rise in relative sea level, sediment supply is sufficient to outpace the rise, driving a seaward building of the coast known as progradational stacking. Where a depositional sequence is composed of parasequences, the HST will contain an increasingly progradational set of parasequences. In the late highstand systems tract, eustatic sea level begins to fall, but is outpaced by subsidence, such that relative sea-level is rising, albeit at ever-slower rates.
Important: Sequence stratigraphy is concerned with the relative rates of change in sea level and sedimentation, not the position of sea level per se. Consequently, the plots below indicate the rate of change in sea level, not the position of sea level. The horizontal gray line indicates a zero rate of change, and when the blue relative sea-level line lies above this gray line, sea level is rising. As the blue curve climbs upward, sea level is rising more quickly, and as the blue curve descends towards the gray line, sea level is still rising but at ever slower rates of rise. The opposite is true when the blue curve lies below the gray line: as the blue curve falls below the gray line, sea level is falling at progressively faster rates, and as the blue curve rises toward the gray line, sea level is falling, but at ever slower rates. The brown line indicates sediment supply, which is maintained at always-positive rates, indicating that topography and climate are sufficient to supply sediment continuously. Although sedimentation supply is shown as a constant here, this is not a requirement; it just makes explaining the concepts of sequence stratigraphy easier. Remember, these curves show the rate of change in sea level, not the position of sea level.
Eventually, as the rate of eustatic fall increases, it eventually exceeds the rate of subsidence, leading to a relative fall in sea level. This creates a condition of forced regression, in which the coast is forced to build seaward. On most wave-dominated coasts, the fall in sea level leads to erosion on the shelf as storms erode into the seafloor. This erosion surface that marks the beginning of this falling-stage systems tract (FSST) is known as the basal surface of forced regression. The falling-stage systems tract may contain multiple surfaces of forced regression, and generally does not contain parasequences (cycles bounded by flooding surfaces). During the falling-stage systems tract, rivers may begin to incise on what was formerly a marine shelf, forming what are called incised valleys (not shown on the cross-section below). These incised valleys will tend to widen and grow landwards during the falling-stage systems tract. They will also tend to extend seaward as sea-level continues to fall. Although not shown on these cross-sections, the FSST often marks the beginning of active sand deposition on submarine fans.
Original formulations of sequence stratigraphy (e.g., Van Wagoner et al. 1990) did not recognize a separate systems tract called the FSST and included these strata as part of the late HST. These earlier models are often called the three systems tract model (LST, TST, HST), in contrast to the current four systems model presented here and recognized by most current sequence stratigraphers.
As the rate of eustatic fall slows, it eventually equals the rate of subsidence and is then exceeded by the rate of subsidence, leading to a slow relative rise in sea-level. Just as in the highstand systems tract, this slow rise is outpaced by sedimentation rate, leading to a progradational set of parasequences called the lowstand systems tract (LST). The base of the lowstand systems tract is known as the sequence boundary and marks the greatest extent of subaerial exposure and erosion. During the lowstand systems tract, incised valleys begin to flood and become the site of estuaries. Such estuaries will advance landwards up through the incised valleys as sea level continues to rise, even into subsequent systems tracts. These estuaries act as sediment traps and prevent substantial dispersal of sediment onto the shelf.
As the rate of eustatic rise increases, the rate of relative sea-level rise also increases and eventually outpaces the supply of sediment, leading to retrogradational parasequence stacking. Such retrogradational stacking is called the transgressive systems tract (TST). Retrogradational stacking is marked by well-developed flooding surfaces, that is, flooding surfaces with pronounced deepening. As a result, flooding surfaces within the transgressive systems tract are much more prominent than anywhere else in a depositional sequence. The first of these large flooding surfaces is known as the transgressive surface and separates the underlying lowstand systems tract from the overlying transgressive systems tract. Estuaries are commonly well-developed during the transgressive systems tract as valleys cut during the FSST and LST are flooded. Trapping of sediment within these estuaries starves the shelf of sediment, further enhancing retrogradational stacking.
Eventually the rate of eustatic rise will slow and be outpaced by the rate of sedimentation, leading to progradational stacking in the highstand systems tract. The turnaround from retrogradational stacking in the transgressive systems tract to progradational stacking in the highstand systems tracts generally corresponds to the deepest water depths in a sequence and is called the maximum flooding surface. As estuaries become filled with sediment, rivers build deltas out onto shelves, and this sediment is dispersed by tides and waves to nearby regions. This elevated supply of sediment to the shelves favors the development of progradational stacking.
The end of the depositional sequence is marked by the return of a fall in sea-level and the formation of falling-stage systems tract.
A complete sequence begins at one sequence boundary and ends at the next sequence boundary. A complete sequence consists of four systems tracts, from bottom to top: lowstand systems tract, transgressive systems tract, highstand systems tract, falling-stage systems tract. Although all four systems tracts will be present in the sedimentary basin, not all will be present at any given spot. In particular, falling-stage and lowstand systems tracts are commonly absent in depositionally updip areas. Transgressive and highstand systems tracts may be thin, absent, or difficult to distinguish in depositionally downdip areas.
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