|University of Georgia||Geology Department||Stratigraphy Lab||Steven Holland|
A depositional sequence is defined as a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities. Like the definition of a parasequence, this definition obscures many of the significant features of a depositional sequence. What the definition does emphasize is that every sequence is bounded above and below by unconformities, or by correlative conformities, surfaces that correlate updip to an unconformity. An unconformity is somewhat narrowly defined here as a surface formed through subaerial exposure and erosion. Furthermore, every depositional sequence is the record of one cycle of relative sea level. Because of this, depositional sequences have a predictable internal structure consisting of major stratal surfaces and systems tracts, which are suites of coexisting depositional systems, such as coastal plains, continental shelves, and submarine fans. In vertical succession, all depositional sequences are composed of the following elements in this order: sequence boundary, lowstand systems tract, transgressive surface, transgressive systems tract, maximum flooding surface, highstand systems tract, and the following sequence boundary.
figure adapted from Van Wagoner et al. (1990)
Lowstand Systems Tract
The lowstand systems tract is the set of depositional systems active during the time of relatively low sea level following the formation of the sequence boundary. If a distinct shelf-slope break exists and relative sea level has fallen sufficiently, the lowstand systems tract may include two distinct parts, the lowstand fan and the lowstand wedge.
The lowstand fan consists of a basin-floor submarine fan. This fan may contain a series of feeder channels as well as distinct fan lobes. The lowstand fan typically displays aggradational stacking and is overlain by the lowstand wedge. During the time of lowest relative sea levels on siliciclastic margins, rivers begin to incise into the exposed shelf and this sediment is shunted directly off the shelf edge to feed submarine fans.
The lowstand wedge consists of a progradational set of parasequences building out from the pre-existing continental slope. In siliciclastic systems, the lowstand wedge may be characterized by shelf-edge deltas and shorelines. In systems lacking a distinct shelf-slope break or in cases where relative sea level does not fall sufficiently, only a lowstand wedge may form, with no lowstand fan. During the late lowstand, relative sea level begins to rise slowly, allowing the incised valleys to flood and form estuaries. River sediment is trapped in these estuaries and is prevented from reaching the shelf; this trapping becomes even more effective during the transgressive systems tract.
Following the relative fall in sea level that produces the sequence boundary, relative sea-level begins to bottom out and and eventually begins to rise slowly, but at a very slow rate. This slow rate of accommodation coupled with relatively high supply of sediment results in the progradational stacking typical of the lowstand wedge.
Transgressive Systems Tract
The transgressive systems tract consists of a retrogradational set of parasequences. It is underlain by the transgressive surface and overlain by the maximum flooding surface. As in any retrogradational set of parasequences, flooding surfaces within the transgressive systems tract are unusually prominent and display strong facies contrasts and pronounced deepening. These flooding surfaces may display variable but commonly strong degrees of sediment starvation, discussed in more detail below. Because the parasequences backstep, the transgressive systems tract displays an overall deepening-upward succession, although each component parasequence is shallowing-upward. In siliciclastic systems, much sediment is trapped in estuaries, so the continental shelf is relatively starved of sediment during major transgressions. A relatively minor amount of sand is reworked along the shoreline and little sediment is transported to the outer continental shelf. Consequently, individual parasequences of the TST are relatively thin nearshore sands with thinner offshore deposits and the TST as a whole is therefore commonly quite thin relative to other systems tracts.
As relative sealevel continues to rise, accommodation space is produced at a faster rate than it can fill with sediments, and a retrogradational set of parasequences forms. At each flooding surface in the transgressive systems tract, the short term relative rise in sea level adds to the long term rise in relative sea level to produce an unusually rapid rise and a highly pronounced flooding surface.
Highstand Systems Tract
The highstand systems tract consists of an aggradational to progradational set of parasequences that overlies the maximum flooding surface and that is overlain by the next sequence boundary. As the parasequences pass from aggradational to progradational stacking, the flooding surfaces are increasingly subdued at the expense of overall shallowing
In siliciclastic systems, estuaries have either been filled with sediment by the beginning of the highstand systems tract or are finally filled in the earliest phases of the highstand systems tract. Once sediment is no longer trapped in estuaries, rivers are free to build seaward and form deltas. In portions of coastlines between deltas, sandy wave-dominated shoreline deposits may form.
During the highstand systems tract, the rate of relative sea level rise begins to slow and relative sea level eventually begins to fall prior to the next sequence boundary. Throughout the highstand systems tract however, accommodation space is created or destroyed at a relatively slow rate. Coupled with the increased supply of sediment to the shelf as estuaries are filled, progradational stacking is increasingly favored over aggradational stacking. As relative sealevel begins to fall, a new sequence boundary begins to form; this sequence boundary will begin to erode into the underlying highstand systems tract. Although the highstand systems tract is most prone to erosional removal during sequence boundary formation, even lower systems tracts or entire sequences may be removed during extremely low or long relative sea-level lowstands.
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