Geol 135 Sedimentation
J Bret Bennington
Sequence stratigraphy is methodology for the analysis of genetically related packages of sedimentary strata that was initially developed in the 1970s by researchers at Exxon. The goal of sequence stratigraphy is to relate the deposition of strata across a developing tectonic basin to three primary variables:
Accomadation space can be created by subsidence, by transgression, or by both occuring together.
Subsidence is usually modeled as continuously producing accomodation space at a uniform rate. This is because changes in subsidence rate occur over longer periods of time than most sea level changes.
Sediment supply fills accomodation space. Sediment supply is episodic at short time scales but more predictable at longer time scales. Clastic sedimentation can also behave differently than carbonate sedimentation due to the different sources of sediment.
1. In general, sediment deposition is inhibited during transgressions, particularly if they are rapid. This occurs in clastic settings because transgression raises base level and creates coastal estuaries. These trap sediment and prevent it from being transported to the shelf. This results in the formation of a condensed interval during transgression. In carbonate settings a rapid rise of sea level can temporarily outpace the ability of reef-building organisms to grow upward. This causes the carbonate factory to be temporarily depressed, resulting in a condensed interval.
2. During highstand, when eustatic rise is slow, clastic sediments begin to prograde basinward from the shoreline, creating a clastic wedge. Carbonate sediments have a tendency to aggrade, creating a carbonate platform.
Relative vs Eustatic Sea Level
Water depth determines what types of depositional environments are possible and therefore, what the characteristics will be of the sediments deposited. Water depth varies depending on the relative rates of subsidence and sediment supply, as well as eustacy.
Keep in mind the difference between relative and eustatic sea level. Eustatic sea level is the absolute elevation of the ocean surface, which rises and falls over time. Relative sea level is water depth as seen by a crab on the sea floor.
A decrease in relative water depth = regression seaward shift of facies belts
An increase in relative water depth = transgression landward shift of facies belts
It is possible for a regression to occur during a eustatic sea level rise, if the rate of sediment supply exceeds the rate at which subsidence and eustacy create accomodation space.
It is possible for transgression to occur during a eustatic sea level fall, if the rate of subsidence exceeds the rate of sediment supply and eustatic fall.
Relationships between rates of the three variables
Sediment supply = accomodation space - Deposits will aggrade vertically and there will be no shift in the position of the shoreline and related facies belts. Relative sea level remains unchanged.
Sediment supply > accomodation space - Deposits will aggrade vertically and then prograde (build outward from the shoreline) creating a pattern of regression. Facies belts shift seaward and shallow water deposits overlie deeper water deposits.
A distinction is made between regression due to progradational filling of the basin and regression due to eustatic lowering of sea level or tectonic uplift of the basin floor called a forced regression. A forced regression can also be thought of as occuring due to a negative rate of production of accomodation space. This causes facies to shift basinward while shallow water deposits become exposed and are eroded.
Sediment supply < accomodation space Relative sea level rises during a transgression and facies shift landward (retrogradation).
The sequence stratigraphic model
Depositional Sequence: a sequence is basically a package of strata deposited during a single cycle of sea level rise and fall. Sequences are bounded above and below by unconformities or their correlative conformities, meaning that they are deposited between episodes of significant sea level fall.
Unconformities and sequence boundary
Sea level fall of several tens of meters will cause subaerial exposure of the coastal plain and downcutting and erosion by rivers draining out to the receeding shoreline. On the shelf, deep water sediments will be overlain by shallow water sediments. If sea level fall is extensive enough to expose the entire shelf, then a widespread unconformity surface will develop a sequence boundary. Basinward of the shelf edge the unconformity will correlate to a stratigraphic horizon in the more continuous stratigraphic sequence of deep water sediments. This horizon is the correlative conformity.
There is no cycle order (length or time scale) implicit in the definition of a sequence - one could conceivably define a sequence for any order of cycle. In practice, however, sequences are reserved for packages of strata bounded by regionally significant unconformities marked by significant erosion on the shelf and coastal plain.
Systems tracts: systems tracts are packages of strata within a sequence that can be attributed to formation during particular phases of rising and falling relative sea level. Systems tracts have also been called facies tracts because they contain strata from related depositional environments.
Patterns of eustatic sea level change
3. Rates and direction of sea level change vary continuously, resulting in curves of accelerating and decelerating rate. Rates are essentially zero at highstand and lowstand.
Parasequences and parasequence sets: these are packages of strata within systems tracts that consist of sediment cycles formed by lower order sea level cycles. Parasequences and parasequence sets are defined by marine-flooding surfaces identified by abrupt transitions to deeper water depth.
Parasequence sets can be progradational, retrogradational, or aggradational, depending on the higher order trend in relative sea level.
There are two main idealized models for deposition during cycles of sea level rise and fall:
Type 1 sequences
A type 1 sequence is bounded below by a Type 1 sequence boundary, which is an unconformity marked by subaerial exposure and significant subaerial erosion associated with stream rejuvenation (increase in stream gradient and rate of flow caused by a lowering of base level). Type 1 boundaries are also marked by an abrupt basinward shift in facies so that nonmarine or shallow marine facies above the boundary overlie deep marine rocks below the boundary. Basically, a type 1 sequence begins with relative sea level at its lowest point.
Stage 1: Lowstand systems tract (LST): During a relative fall in sea level, valleys are eroded into the coastal plain and shelf and submarine canyons are eroded into the slope. Sediment bypasses the shelf and slope and is deposited as turbidity currents in submarine fans on the basin floor. Sediment fans can also be deposited on the slope. As relative sea level stops falling sediments may begin to fill the valleys carved on the shelf, creating a lowstand wedge. Together, the lowstand wedge, slope fan, and basin-floor fan deposits form the LST.
Transgressive Systems Tract (TST): As sea level starts to rise, base level increases and fluvial deposits form in incised valleys. The shelf becomes flooded again, creating a transgressive surface. The TST is characterized by one or more retrogradational parasequence sets, meaning that each subsequent cycle of lower order sea level change results in the shoreline stepping landward. Landward of the initial shoreline, where there are no deposits from the lowstand wedge or the transgressive surface, these parasequences will onlap directly onto the sequence boundary.
Seaward of the initial shoreline the TST deposits are often thin due to sediment starvation as clastic sediments become trapped in estuaries. This causes the deposition of a condensed section, which includes the maximum flooding surface stratigraphic level of maximum relative sea level. Above the MFS parasequences change from retrogradational to aggradational.
Highstand Systems Tract (HST): This systems tract is characterized by aggradation and then progradation of parasequences as the rate of sea level rise slows, stops, and then reverses. Often this is the thickest part of the sequence because clastics stored in estuaries during sea level rise a flushed out onto the shelf during early sea level fall. The upper boundary of the HST is a sequence boundary, formed as sea level fall accelerates and begins to expose the coastal plain and shelf to erosion once again.
Note that during early HST relative sea level may still be rising on the shelf, but at a slow rate compared to sediment supply. At the same time, transgression may stop at the shoreline if sediment supply begins to exceed the creation of accomodation space.
Depositional Sequences in Carbonate Environments
The same sequences and sequence boundaries described above can be recognized in carbonate depositional basins. The main difference is in the mechanisms generating sediment supply. Carbonate sediments are generated by the "carbonate factory" which is dependent on the following:
Transgressive and Highstand Tracts
During transgression and highstand, shelves are flooded and shallow water habitat is at a maximum. Because of the ability of carbonate producing organisms to grow quickly under these conditions, sediment production can keep pace with sea level rise and accomodation space and thick, aggradational sequences of shallow water carbonates are produced.
The rate of carbonate sedimentation is much less basinward of the shelf edge because of the deeper water and relative lack of carbonate producing organisms. Most of the sediment influx here is from shallow water carbonates being transported offshore by storms and by slumping.
If sea level rise is great enough to outpace carbonate sediment production (this can occur during glacio-eustacy when melting glacial ice quickly raises sea level) then water will deepen over the shelf, shutting down the carbonate factory until a forced regression causes shallowing again. This drowned shelf condition is easy to recognize in a carbonate succession by the presence of a condensed section of deep water deposits overlying shallow water carbonates.
During relative sea level fall the carbonate shelf becomes subaerially exposed, resulting in karstification of the carbonates in a humid climate or the development of an evaporitic sabkha in an arid climate. The zone of carbonate production moves basinward and occupies a narrow zone along the shelf margin. Lowstand wedges of transported carbonates can form in deeper water, but they are usually less well developed than their clastic counterparts.