geological sequence strat lecture

8/13/2019 Geological Sequence Strat Lecture

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12/11/13 Geol. 135 - Sequence Stratigraphy

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Geol 135 Sedimentation

J Bret Bennington

Updated 9/99

Sequence Stratigraphy

Sequence stratigraphy is methodology for the analysis of genetically related packages of sedimentary stratathat was initially developed in the 1970s by researchers at Exxon. The goal of sequence stratigraphy is torelate the deposition of strata across a developing tectonic basin to three primary variables:

1. Eustacy change in sea surface elevation2. Subsidence rate of creation of accomodation space by depression of the basin floor.3. Sediment supply amount of sediment being deposited in a basin

Accomadation space can be created by subsidence, by transgression, or by both occuring together.

Subsidence

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

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 occursin clastic settings because transgression raises base level and creates coastal estuaries. These trap sedimentand prevent it from being transported to the shelf. This results in the formation of a condensed intervalduring 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, resultingin a condensed interval.

2. During highstand, when eustatic rise is slow, clastic sediments begin to prograde basinward from theshoreline, creating a clastic wedge . Carbonate sediments have a tendency to aggrade , creating a carbonateplatform .

Relative vs Eustatic Sea Level

Water depth determines what types of depositional environments are possible and therefore, what thecharacteristics 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 absoluteelevation of the ocean surface, which rises and falls over time. Relative sea level is water depth as seen by acrab on the sea floor .


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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 exceedsthe 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 therate 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 (buildoutward 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 toeustatic lowering of sea level or tectonic uplift of the basin floor called a forced regression . A forcedregression 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 shiftlandward (retrogradation).

The sequence stratigraphic model

Depositional Sequence : a sequence is basically a package of strata deposited during a single cycle of sealevel rise and fall. Sequences are bounded above and below by unconformities or their correlativeconformities, 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 anderosion 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 widespreadunconformity surface will develop a sequence boundary. Basinward of the shelf edge the unconformity willcorrelate 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 conceivablydefine 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 formationduring particular phases of rising and falling relative sea level. Systems tracts have also been called faciestracts because they contain strata from related depositional environments.


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Patterns of eustatic sea level change

3. Rates and direction of sea level change vary continuously, resulting incurves of accelerating and decelerating rate. Rates are essentially zero athighstand 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 bysubaerial exposure and significant subaerial erosion associated with stream rejuvenation (increase in streamgradient and rate of flow caused by a lowering of base level). Type 1 boundaries are also marked by anabrupt basinward shift in facies so that nonmarine or shallow marine facies above the boundary overlie deepmarine 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 thecoastal plain and shelf and submarine canyons are eroded into the slope. Sediment bypasses the shelf andslope and is deposited as turbidity currents in submarine fans on the basin floor. Sediment fans can also bedeposited on the slope. As relative sea level stops falling sediments may begin to fill the valleys carved on theshelf, creating a lowstand wedge . Together, the lowstand wedge, slope fan, and basin-floor fan depositsform the LST.

Transgressive Systems Tract (TST) : As sea level starts to rise, base level increases and fluvial depositsform in incised valleys. The shelf becomes flooded again, creating a transgressive surface . The TST ischaracterized 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 willonlap 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 themaximum 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 progradationof 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 earlysea level fall. The upper boundary of the HST is a sequence boundary, formed as sea level fall accelerates


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geological sequence strat lecture

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