Embry sepm

15122nd Annual Gulf Coast Section SEPM Foundation Bob F. Perkins Research Conference—2002

Transgressive-Regressive (T-R) Sequence Stratigraphy

Embry, Ashton F. Geological Survey of Canada

Calgary, Alberta, Canada, T2L 2A7

e-mail: [email protected]

Abstract

A sequence, as originally defined by Sloss and colleagues, was a stratigraphic unitbounded by subaerial unconformities. Such a stratigraphic unit proved to be of limited valuebecause, in most instances, sequences could be recognized only on the margins of a basinwhere subaerial unconformities were present. Vail and colleagues greatly expanded the utilityof sequences for basin analysis when they redefined the term as a unit bounded by unconfor-mities or correlative conformities. The addition of correlative conformities allowed a sequenceto potentially be recognized over an entire basin.

This revised definition has led to the formulation of four different types of sequences,each having a different set of bounding surfaces. Vail and colleagues have defined two types: atype 1 depositional sequence and a type 2 depositional sequence. A type 1 depositionalsequence utilizes a subaerial unconformity as the unconformable portion of the boundary anda time line equivalent to the start of base level fall for the correlative conformity. Because thesubaerial unconformity migrates basinward during base level fall, much of it is thereforeincluded within such a sequence rather than being on the boundary. Also it is impossible toobjectively recognize a time line that corresponds to the start of base level fall. For these rea-sons a type 1 depositional sequence has little practical value.

A type 2 depositional sequence also uses the subaerial unconformity as the unconform-able portion of the boundary but uses a time line equivalent to the end, rather than the start, ofbase level fall for the correlative conformity. This resolves the problem of including a portionof the unconformity inside the sequence. However, it is essentially impossible to objectivelyrecognize a time line that corresponds with the end of base level fall (start of base level rise)and thus this type of sequence also has no practical value. Galloway proposed the use of maxi-mum flooding surfaces as sequence boundaries and named such a unit a genetic stratigraphicsequence. This alleviated the problem of major subjectivity in boundary recognition becausemaximum flooding surfaces can be determined by objective scientific analysis. However, thissequence type founders on the problem that the subaerial unconformity occurs within thesequence and thus it lacks genetic coherency on the basin margins.

To overcome these major deficiencies in sequence definition, Embry and Johannessenhave defined a fourth type of sequence that they term a T-R sequence. This sequence uses thesubaerial unconformity as the unconformable portion of the boundary and the maximum regres-sive surface as the correlative conformity. This methodology keeps the subaerial unconformityon the boundary and also provides for a correlative conformity that can be objectively deter-mined. It thus avoids the fatal flaws of previously defined types. A T-R sequence can be dividedinto a transgressive systems tract below and a regressive systems tract above by using the maxi-mum flooding surface as a mutual boundary. T-R sequence stratigraphy, unlike the otherproposed methodologies, has maximum practical utility with a minimum of stultifying jargon.

Reprinted from Proceedings of the 22nd Annual Bob F. Perkins Research Conference, © 2002 with permission from The Gulf Coast Section-SEPM Foundation.

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Introduction

Sloss et al. (1949) first used the term sequence for a stratigraphic unit bounded byregional subaerial unconformities and in doing so they initiated the discipline of sequencestratigraphy. Mitchum et al. (1977), as part of a watershed collection of papers on sequencestratigraphy, revised the definition of a sequence to “a stratigraphic unit composed of geneti-cally related strata bounded at the top and bottom by unconformities or their correlativeconformities.” With the inclusion of the concept that sequence boundaries can be extendedbasinward along “correlative conformities,” this definition provides for a sequence that poten-tially can be delineated over an entire basin and not just on the basin margins where mostunconformities occur.

There seems to be widespread agreement that the unconformable portion of a sequenceboundary should coincide with a subaerial unconformity and/or a shoreface ravinement sur-face that has eroded through a subaerial unconformity. Such an unconformable stratigraphicsurface occurs mainly on the basin margin and most often disappears towards the center of abasin. The extension of a sequence boundary into the conformable succession of the more cen-tral regions of a basin, that is the delineation of the correlative conformity, has been the subjectof both confusion and debate.

Over the past 25 years, a variety of different combinations of stratigraphic surfaces havebeen proposed as sequence boundaries. Each combination can be regarded as a specific type ofsequence and its usage as a specific type of sequence stratigraphy. However, before each pro-posed sequence type can be described and discussed, it is important to examine what sequencestratigraphy is and to describe the various surfaces of sequence stratigraphy which potentiallycan be employed as part or all of a sequence boundary. After the various surfaces aredescribed, the possible sequence types are discussed. Following this, systems tracts, which arethe component units of sequences, are reviewed.

This paper is basically a review of a sequence stratigraphic methodology that has been indevelopment since 1972. It is based on the widespread and spectacular exposures of Devonianand Mesozoic clastic strata in the Canadian Arctic Archipelago as well as subsurface data (wells,seismic) for these strata (Embry, 1991a, 1991b). Notably, it has been formulated independent ofand in parallel with Exxon sequence stratigraphy (Embry and Klovan, 1974; Embry 1983, 1986,1988, 1990, 1993, 1995; Embry and Johannessen, 1992) and it provides stratigraphers with asomewhat different perspective on, and an alternative approach to, sequence analysis.

Sequence Stratigraphy

To understand what makes a given type of stratigraphy “tick” and how to best apply it, itis critical to understand what basic rock property change is used to delineate stratigraphic con-tacts for that discipline. Furthermore, it is also important to know what phenomena areresponsible for generating these changes in the rock record in the first place. For example,when one considers biostratigraphy, it is clear that changes in fossil content are used to delin-eate the boundaries of biostratigraphic units. We also know that such changes in fossil contentare mainly driven by a combination of evolution and shifting environments. With this knowl-edge, we can go a long way in understanding the basic tenets of biostratigraphy.

In sequence stratigraphy, we utilize various changes in depositional trends as bound-aries. Examples of such changes in trend are the change from sedimentation to subaerialerosion and the change from transgressive (deepening-upward) trend to a regressive (shallow-ing-upward) trend. These changes, which are recognized as specific types of surfaces (e.g.,subaerial unconformity for the change from sedimentation to subaerial erosion, maximumflooding surface for the change from transgressive to regressive), are used as the boundaries of



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units of sequence stratigraphy (sequence, systems tract). As was first discussed by Barrell(1917) almost a century ago, we know such changes in depositional trend and the associatedrecognized surfaces are exclusively or commonly generated by changes in base level.

These concepts give us a basic understanding of sequence stratigraphy from which aworking definition of sequence stratigraphy can be derived: “Sequence stratigraphy consistsof the recognition and correlation of changes in depositional trends in the rock record. Suchchanges, which were generated by the interplay of sedimentation and shifting base level, arenow recognized by sedimentological criteria and geometrical relationships.” I would note thatthis definition is somewhat different from other definitions of the discipline that have tended tobe somewhat circular, incomplete. or far off base. For example, Van Wagoner et al. (1990)define sequence stratigraphy as “the study of genetically related facies within a framework ofchronostratigraphically significant surfaces.” This has little to do with sequence stratigraphyand more closely defines facies analysis. Furthermore, the term “chronostratigraphically sig-nificant” adds nothing to the definition because stratigraphic surfaces from all the varioustypes of stratigraphic analysis have chronostratigraphic significance. Emery and Myers (1996)define sequence stratigraphy as “the subdivision of sedimentary basin fills into genetic pack-ages bounded by unconformities and their correlative conformities”. This is certainly a markedimprovement over the Van Wagoner et al. (1990) effort but still leaves much to be desired.This definition would be equivalent to saying lithostratigraphy consists of subdividing the sed-imentary rock record into lithostratigraphic units. Hopefully the offered definition provides abetter characterization of the discipline.

Base Level Changes and Changes in Depositional Trend

Introduction

As mentioned above, the natural phenomenon of oscillating base level is exploited forthe development of sequence stratigraphy just as the phenomenon of changing magnetic polar-ity is exploited in magnetostratigraphy. In this section I briefly discuss what is base level, whatcauses base level to oscillate, and the changes in depositional trends that result during onecycle of base level rise and fall.

Base Level Change

There has been a lot of confusion about what base level means in a stratigraphic senseand this is in part because the term has also been used somewhat differently in a geomorphicsense. Harry Wheeler (1964) has succinctly reviewed the history of the use of the term baselevel in stratigraphy. Recently Tim Cross (Cross, 1991; Cross and Lessenger, 1998) has clearlyshown how the concept of base level has direct application to sequence stratigraphy.

Base level, in a stratigraphic sense, is not a real, physical surface but rather is an abstractsurface that represents a surface of equilibrium between erosion and deposition. It can bethought of as a ceiling for sedimentation, and thus in any area where base level lies below theEarth’s surface no sediment accumulation is possible and erosion will occur. Where it liesabove the Earth’s surface, deposition can and usually does occur in the available space. Thespace between the Earth’s surface and base level, which is available for sedimentation, hasbeen called accommodation space (Jervey, 1988). Of course, the places where base level inter-sects the earth’s surface are equilibrium points between areas of erosion and areas ofdeposition. Such points define the edges of a depositional basin. As Cross and Lessenger(1998) explain “ Stratigraphic base level is a descriptor of the interactions between processes


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that create and remove accommodation space and surficial processes that bring sediment orthat remove sediment from that space.”

Because of the dynamic nature of the Earth, base level rarely remains static in any givenarea and is generally moving upwards or downwards relative to a datum below the surface ofthe Earth. Thus, base level changes represent changes in the distance between base level andthe datum. A datum is used rather than the Earth’s surface itself to ensure the concept of baselevel change is independent of sedimentation.

There are two main drivers of base level change. The first one is tectonics that results inupward or downward movements of the datum in relation to the center of the Earth. In this sit-uation the datum, and not base level, is moving. Downward movement of the datum is referredto as subsidence and, in a relative sense, results in rising base level and increased accommoda-tion space. Conversely, upward movement of the datum (uplift) results in base level fall as thetwo reference horizons approach each other and accommodation space is reduced.

The second driver of base level movement is eustatic sea level change that records themovements of the surface of the ocean in relation to the center of the earth. In this case, the datumremains stationary and base level moves. Thus, rising eustatic sea level equals rising base leveland increased accommodation space and falling eustatic sea level equates to falling base level anddecreased accommodation space. Furthermore, any reduction or increase of volume in the sedi-mentary column due to such phenomena, such as compaction, salt solution, and salt intrusion alsowill cause changes in base level and the amount of accommodation space available.

Although we know that a variety of factors control the movement of base level relativeto the datum, it is critical to understand that these factors act at the same time within a sedi-mentary basin and it is impossible to determine the effect of each factor separately (Burton etal., 1987). Their combined, net effects can be determined and this is expressed as changes inbase level. Thus, use of the term base level avoids irresolvable arguments of whether tectonicsor eustasy is responsible for additions and reductions in accommodation space and the accom-panying changes in depositional trends. Some authors have a priori decided that eithertectonics or eustasy is the key variable (e.g., Posamentier et al., 1988, use eustasy) but such adogmatic approach is not helpful and should be avoided. The term relative sea level change issometimes used (Van Wagoner et al., 1990) in the same context as base level change but I pre-fer the term base level change which has priority (Barrell, 1917) and which does not result in

any confusion in regards to moving sea levels. Given that base level is continually changing, the key point for sequence stratigraphy is

that such oscillations in base level result in a number of recognizable changes of depositionaltrends within a sedimentary succession that was deposited during the oscillation. Such changesin depositional trend are due to the interaction between changing rates of the addition or reduc-tion of accommodation space and the rate of sedimentation. For example, when allaccommodation space is eliminated in a given area, there is a major change from sedimenta-tion to erosion. Such a change in depositional trend results in the occurrence of various typesof unconformities that can then be used for correlation and the delineation of various types ofsequence stratigraphic units.

Changes in Depositional Trends

There are two main types of change in depositional trend that result from base levelmovements. These are (1) a change from sedimentation and accumulation to erosion and vice-versa and (2) the change from a shallowing-upward trend (regression) to a deepening-upward(transgressive) one and vice-versa. During a cycle of base level rise and fall, six importantchanges of depositional trend, which represent variations of the two main types, occur. Four



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occur during base level rise and two during fall. These changes occur either over a short orlong time interval when compared to the duration of the complete cycle. These six changes indepositional trend are:

Base level rise:

1. Expansion of deposition and accumulation of nonmarine strata in a landward directionacross a subaerial erosion surface.

2. Change from a regressive trend to a transgressive trend in a marine succession.3. Cessation of sedimentation along the shoreline and the start of net erosion along the

shoreline.4. Change from a transgressive trend to a regressive one in marine strata.

Base level fall:

5. Cessation of sedimentation on the basin edge and gradual basinward expansion of sub-aerial erosion.

6. Development of sea floor erosion on the inner shelf and expansion of this submarineerosion surface basinward.

The reader might wonder why cessations of sedimentation on the marine slope (subma-rine channeling, slumps, etc.) are not included in the above list. The main reasons for this arebecause, as emphasized by Galloway (1998), such cessations of sedimentation can occur atalmost any time during a base level cycle and they are often localized.

The six significant changes in depositional trend listed above produce a number of dis-tinctive, recognizable horizons in the sedimentary record and these horizons are the ones thatare used for sequence stratigraphic analysis (Fig. 1). Their development is elaborated uponbelow and criteria for the recognition of each specific horizon are discussed in the next section.

When base level starts to rise, new accommodation space begins to be created in areasformerly undergoing erosion. This results in landward expansion of the basin margin and pro-gressive onlap of the underlying erosion surface by nonmarine strata throughout the entiretime of base level rise. With rising base level, less sediment is transported to the marine por-tion of a basin because of reduced fluvial gradients and increased sediment storage in thenonmarine area along the expanding basin margin. During the initial stage of rise, enough sed-iment still reaches the marine area to allow the shoreline to continue to advance basinward(regression) as it had during the previous time of sea level fall. However, such advancementoccurs at a declining rate until finally the rate of base level rise at the shoreline exceeds the rate

Figure 1. During a single cycle of base level fall andrise, six specific stratigraphic surfaces that representregional changes in depositional trend are gener-ated. During base level fall, a subaerial unconfor-mity and regressive surface of marine erosionmigrate basinward. Between the start of base levelrise and the start of transgression, the maximumregressive surface is formed. During the entire inter-val of transgression a shoreface ravinement migrateslandward and a shoreface ravinement-unconform-able and/or shoreface ravinement-normal areformed. Following the start of regression, a maxi-mum flooding surface forms.


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of sediment supply and the shoreline ceases its seaward movement and begins to shift land-

ward (transgression). This change from regression to transgression results in two major

changes in depositional trend. Along the shoreline, net erosion occurs and this zone of shore-

face erosion moves landward during the transgression. The erosion surface is known as a

shoreface ravinement and it develops during the entire time transgression occurs (Fig. 1). This

erosional surface may or may not cut down through the underlying subaerial unconformity

(shoreface ravinement-unconformable or shoreface ravinement-normal). Also, with the initia-

tion of transgression, less sediment is deposited at any given shelf locality due to the

increasing distance from the sediment source as well as the overall reduced supply to the

marine area. This results in a significant change from a shallowing-upward trend that charac-

terized the preceding regression to a deepening-upward one. The horizon that marks this

significant change is herein known as the maximum regressive surface (Fig. 1).

Eventually the rate of base level rise slows and sedimentation at the shoreline once again

exceeds the rate of removal by waves. The development of the shoreface ravinement stops and

the shoreline reverses direction and begins to move seaward (regression). This results in

increased sedimentation to the marine basin and coarser sediment begins to prograde across the

shelf. This produces a change from a deepening-upward trend to a shallowing-upward one, and

the horizon that marks this change in trend is known herein as a maximum flooding surface

(Fig. 1). Thus, four sedimentary horizons that represent changes in depositional trend are pro-

duced during base level rise. These are maximum regressive surface, shoreface ravinement-

unconformable, shoreface ravinement-normal and maximum flooding surface. Also, during this

time the subaerial unconformity that develops during the preceding base level fall is progres-

sively covered with sediment completing its development as a distinctive stratigraphic horizon.

With the start of base level fall, sediment accommodation space is reduced and sedimen-

tation ceases on the basin margin. Subaerial erosion advances basinward during the entire time

of fall and this produces a subaerial unconformity that reaches its maximum basinward extent

at the end of base level fall (Fig.1). The seaward movement of the shoreline, which began in

the waning stages of base level rise, continues throughout base level fall but at a faster pace.

Also when base level starts to fall, the inner part of the marine shelf begins to be eroded as

described by Plint (1988). This is due to the regrading of the shelf as it attempts to equilibrate

with falling base level. This inner shelf erosion surface moves seaward during the entire inter-

val of base level fall and is progressively covered by prograding shoreface deposits. This

results in a widespread horizon known as the regressive surface of marine erosion (Fig.1).

In summary, four surfaces are formed during base level rise: the shoreface ravinement-

normal, the shoreface ravinement- unconformable, the maximum regressive surface and the

maximum flooding surface and two surfaces are formed during base level fall: the subaerial

unconformity and the regressive surface of marine erosion. These six surfaces are the heart of

sequence stratigraphy and are discussed in more detail below. Most importantly, because these

surfaces form at specific times during a base level cycle, they have a specific and predictable

arrangement to each other in time and space. This arrangement can be regarded as a sequence

stratigraphic model and one version of it is presented in Figure 2.



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Figure 2. A schematic cross section which shows the spatial relationships of the six surfaces of sequencestratigraphy: subaerial unconformity, regressive surface of marine erosion, shoreface ravinement-unconform-able, shoreface ravinement-normal, maximum regressive surface, and maximum flooding surface. Becausethese surfaces are generated during specific times of a base level transit cycle, they always have a similar rela-tionship to one another, and this arrangement of surfaces constitutes a model for sequence stratigraphy.

Surfaces of Sequence Stratigraphy

Introduction

Each type of stratigraphy has one or more types of surfaces that are recognized and usedfor correlation and unit delineation. For example in lithostratigraphy, there is really only onetype of surface recognized and that is the one that marks a significant change in lithology. Thiskeeps lithostratigraphy relatively uncomplicated. Things become more complex when morethan one type of surface is recognized within a given type of stratigraphy. In biostratigraphy, anumber of different types of surfaces have been defined and include a boundary that marks thefirst appearance of a species, a boundary that marks the last appearance of a species, and aboundary that marks a significant change in the fossil assemblage. The identification of morethan one type of surface can result in numerous types of units being defined.

Above, it has been shown that a number of changes in depositional trend occur during acycle of base level rise and fall. These changes in trend result in six distinctive, stratigraphicsurfaces that can potentially be used for correlation and to define units in sequence stratigra-phy. The six surfaces are: subaerial unconformity, regressive surface of marine erosion,shoreface ravinement-unconformable, shoreface ravinement-normal, maximum regressive sur-face and maximum flooding surface. The first two are generated during base level fall and thelast four during base level rise. As will be discussed, some of these surfaces are very useful forcorrelation and for delineating sequence stratigraphic units whereas others are not.


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Below, each of these six surfaces is described with emphasis on the criteria that allow itsobjective recognition and its differentiation from the other surfaces. Also, the relationship ofeach surface to time lines and its consequent usefulness in sequence stratigraphy are assessed.I have also included a discussion of what I call a “within-trend facies change,” which repre-sents a significant change in facies but not a change in depositional trend. It is thus not asurface of sequence stratigraphy, but I include it here because it is a very important boundaryfor facies analysis and it sometimes is mistaken for a surface of sequence stratigraphy.

Subaerial Unconformity (SU)

The stratigraphic surface most often associated with sequence stratigraphy is a subaerialunconformity. This surface forms when base level falls and the surface of the earth is exposed tosubaerial erosion processes such as fluvial and wind action. Throughout the time of base levelfall, it expands seaward as the basin edge is progressively exposed. During the subsequent baselevel rise, the subaerial unconformity is onlapped by nonmarine sediments and is preserved as adiscrete surface. A subaerial unconformity marks a cessation in sedimentation and is thus char-acterized by a sharp erosive contact in many cases. Underlying strata can be highly variable andsometimes are marked by the diagenetic effects of soil development. A key characteristic of asubaerial unconformity is that nonmarine strata (i.e., strata deposited landward of the shoreline)overlie it. Thus, the defining attributes of a subaerial unconformity are an erosive surface or soilhorizon that is overlain by nonmarine strata and truncates underlying strata.

A subaerial unconformity has an important relationship to time lines. It develops overthe entire time of base level fall and therefore can be considered to be diachronous. However,time lines do not pass through the surface as they do for many diachronous surfaces. The rea-son for this is that in most cases all strata below a subaerial unconformity are entirely olderthan all strata above the unconformity. The subaerial unconformity truncates time lines that arebelow it, and the time lines above it display an onlap relationship. Thus, a subaerial unconfor-mity can be regarded as a time line barrier, and this feature makes a subaerial unconformity animportant surface for establishing a quasi-chronostratigraphic framework and for using as aunit boundary.

Regressive Surface of Marine Erosion (RSME)

This type of sequence stratigraphic surface was first defined and discussed by Plint(1988). It is an erosional surface that develops on the inner shelf during a base level fall. Whenbase level starts to fall, the slope of the inner shelf is no longer in equilibrium with the currentsand it becomes an area of net erosion. Currents slowly remove sediment in order to re-establishan equilibrium profile and this erosional area migrates basinward during the entire interval ofbase level fall. At the same time, sediment is deposited in the shoreface and this sedimentdownlaps onto the erosion surface as the shoreface sediments prograde seaward. These sedi-ments eventually become capped by a subaerial unconformity.

Given the above, the characteristics of an RSME are an erosion surface which overliesshallowing-upward, marine shelf strata and is overlain by shallowing-upward, shoreface strata.These characteristics are unique compared with the other erosional surfaces described hereinand allow the RSME erosion to be identified with confidence. The RSME develops during theentire time of base level fall and thus it is very diachronous. Time lines pass through the sur-face at a high angle and with some offset. Thus, it is not a time line barrier like a subaerialunconformity. The highly diachronous nature of the surface makes it unsuitable for being partof a stratigraphic framework and for bounding a sequence stratigraphic unit.



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Shoreface Ravinement-Unconformable (SR-U)

Another prominent unconformity surface that is generated during a base level cycle is ashoreface ravinement. It commonly is confused with a subaerial unconformity despite its verydifferent origin. Whereas the subaerial unconformity forms during base level fall and regres-sion, a shoreface ravinement forms during the interval of base level rise when transgressionoccurs. When the transgression begins and the shoreline starts to move landward, the shore-face ravinement is cut by shoreface wave action that removes sediment and transports itmainly seaward. This occurs primarily because of the landward translation of the shelf equilib-rium profile. Throughout the entire interval of transgression, this erosive action moves steadilylandward and removes previously deposited shoreline and nonmarine sediments. This resultsin a widespread erosion surface that separates underlying sediments from overlying lowershoreface to offshore sediment.

The two most important characteristics of a shoreface ravinement are a sharp, erosivecontact and the occurrence of directly overlying marine strata that display a deepening-upwardtrend (transgressive).

It is crucial to determine if the shoreface ravinement in question has eroded through anunderlying subaerial unconformity or not. If this has happened, then the shoreface ravinementinherits the time line barrier property of a subaerial unconformity and all strata below are olderthan all those above. In this case, the ravinement surface is referred to as a shoreface ravine-ment-unconformable. One way of determining if a shoreface ravinement is an unconformabletype is to examine the nature of the underlying strata. If they are marine, then it is very likelythat an interval of nonmarine strata and a subaerial unconformity have been eroded and theshoreface ravinement is a time line barrier.

Shoreface Ravinement-Normal (SR-N)

If the shoreface ravinement in question has not eroded through the underlying subaerialunconformity, then it is classified as a shoreface ravinement–normal. It has many characteris-tics in common with the shoreface ravinement–unconformable in that it is a sharp, scouredsurface overlain by deepening-upward marine strata. However, the distinguishing characteris-tic of a shoreface ravinement-normal is that underlying strata overlie a subaerial unconformityand consist of sediments deposited landward of the shoreline. The shoreface ravinement-nor-mal is commonly a highly diachronous surface and time lines pass through it, offset and at ahigh angle. Because of this high diachroniety, such a surface has limited value in sequencestratigraphy but is important in facies analysis.

Maximum Regressive Surface (MRS)

Soon after base level starts to rise, the rate of rise begins to exceed the rate of sedimenta-tion at the shoreline and the shoreline begins to move landward. This marks the start oftransgression and at this time sediment supply to the adjacent marine shelf decreases, and thewater depth at any nearshore locality begins to increase. This results in a change in the shelfsuccession from a shallowing-upward trend that developed during the previous regression to adeepening-upward one that reflects the ensuing transgression. The surface that marks this sig-nificant and distinctive change in depositional trend is herein referred to as the maximumregressive surface. This surface has been called a variety of names including transgressive sur-face, conformable transgressive surface, maximum progradation surface, or by the moregeneral term, flooding surface. Because there is considerable confusion associated with theabove names, it seems best to use the more descriptive and less ambiguous term, maximumregressive surface.


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For practical purposes, this surface is confined to marine strata and is characterized bythe change from a shallowing-upward trend to a deepening-upward one. Clearly the recogni-tion of this surface depends on the availability of data from which general water depths inwhich the strata were deposited can be interpreted (i.e., facies analysis). The actual surfacemay occur within a gradational interval of facies change or it can be rather abrupt with minorscouring marking it.

In summary, a maximum regressive surface is characterized by the change from shal-lowing-upward marine strata below and deepening-upward marine strata above with noevidence of substantial erosion. It is not recognized in nonmarine strata because in most casesits place is taken by either the subaerial unconformity or a shoreface ravinement-unconform-able. In some cases it is theoretically possible that the change from regression to transgressionis recorded in nonmarine strata that directly overlie a subaerial unconformity. However, it isimpossible to objectively identify such a boundary in nonmarine strata and, given its likelyrare occurrence, the practical solution is to interpret all nonmarine strata overlying a subaerialunconformity as having been deposited during transgression. In the vast majority of situationsthis will likely be entirely correct. In these cases, the subaerial unconformity marks the changein trend from regressive sedimentation below and transgressive sedimentation above.

The change from a shallowing-upward trend to a deepening-upward one will not beginat the same time everywhere in a marine area because rates of sediment supply and base levelchange vary throughout the marine area. In general, it begins to form in basinward localities atthe start of base level rise and ends at the start of landward movement of the shoreline. Thisresults in a maximum regressive surface being somewhat diachronous over its extent but frommy experience it appears that such regional diachroniety is low and that time lines cross aMRS at a very low angle. This low diachroniety makes the MRS a very useful surface for cor-relation and for helping to establish a regional stratigraphic framework, one of the main goalsof sequence stratigraphy.

Maximum Flooding Surface (MFS)

The maximum flooding surface is basically the opposite of the maximum regressive sur-face. It is generated at the time when a shallowing-upward trend replaces a deepening-upwardone. This change begins at the shoreline when transgression ends and regression begins andtakes place in the waning phases of base level rise when the rate of sediment supply begins toexceed the rate of base level rise. At this time the shoreline begins to move basinward (regres-sion) and consequently marine areas receive a higher supply of sediment. This results in achange from a deepening-upward trend in the marine strata that developed during transgres-sion to a shallowing-upward trend that reflects regression.

The MFS is most readily recognizable in marine clastic strata where it marks the bound-ary between a deepening-upward succession and an overlying shallowing-upward one. Such aboundary can occur within a gradational succession and thus be completely conformable or itcan be a scoured surface on which anywhere from a little to a lot of erosion has occurred. Sucherosion would be due to marine currents that were able to effect a net removal of sediment dueto low sediment supply to offshore areas at the height of transgression. In situations where thecontact is conformable, it often occurs within “condensed” deposits that represent very lowsedimentation rates. In these circumstances, its exact placement can be difficult, and I suggestplacing it at the base of the first obvious coarser interval associated with, or directly overlying,the condensed interval.

Unlike the maximum regressive surface, the maximum flooding surface can sometimesbe recognized in nonmarine strata. In this case it occurs at the boundary between nonmarinestrata that display an interpreted trend of a decreasing distance from the shoreline (transgres-



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sive) and overlying nonmarine strata that represent an interpreted trend of increasing distancefrom the shoreline (regressive). Because it is more difficult to recognize trends in distancefrom shoreline in nonmarine strata than it is to identify water depth trends in marine strata, theidentification of the maximum flooding surface is much more tenuous in nonmarine strata.

Like the MRS, the MFS is also somewhat time transgressive and is generated later inoffshore areas. However such diachroniety tends to be low, making the MFS a very useful sur-face in sequence stratigraphic analysis.

Within-Trend Facies Contact

This is not a surface of sequence stratigraphy but is one that sometimes can be mistakenfor one of the surfaces of sequence stratigraphy. It is simply a notable facies change that occurswithin a succession of regressive or transgressive strata and it does not mark any change indepositional trend. Because such a facies boundary can be a scour surface, it can be misinter-preted as regressive surface of marine erosion if it occurs within a regressive succession andseparates relatively high-energy deposits (e.g., shoreface sandstone) from lower energy ones(e.g., offshore shale). In this case, the only way to distinguish the two would be to determine ifa subaerial unconformity (or shoreface ravinement-unconformable) is present above and/orlandward of the horizon. Within-trend facies contacts are the key boundaries of facies analysisthat is done once a sequence stratigraphic framework has been established.

Types of Sequences

Introduction

The above-described surfaces of sequence stratigraphy can be used simply for correla-tion without delineating any specific types of units. However, sequence stratigraphy alsoallows units to be delineated with the surfaces of sequence stratigraphy acting as boundaries ofthe units. The two different types of units that have been defined so far are the sequence andthe systems tract. A sequence is primarily defined by its bounding unconformities and thishonors the original definition by Sloss et al. (1949). The Mitchum et al. (1977) definition of asequence, while still emphasizing unconformable boundaries, includes the added provisionthat sequence boundaries also can be recognized in the conformable succession of the centralportion of a basin where the bounding unconformities are no longer present. This is accom-plished by allowing a sequence to be bounded by unconformities “or their correlativeconformities.”

The extended definition of what constitutes a sequence boundary has led to four differ-ent types of sequence boundaries, each with a distinctive combination of unconformable andconformable portions, having been defined. This has resulted in there being four differenttypes of sequences available for use in sequence analysis. Very often authors do not make itclear what specific type of sequence they are using in their sequence stratigraphic analysis andthis can result in confusion and misunderstanding. It is important to understand how each typeof sequence boundary is defined and delineated and to be able to recognize the specific typethat is being used in a given study.

In this section, the four types of sequence boundaries are described. Also, each type isevaluated as to its utility in sequence analysis using the following criteria. The boundaries of asequence must be stratigraphic surfaces that can be recognized by objective, scientificallysound observations and interpretations or else sequence boundaries can be drawn willy-nilly atthe whim of the interpreter. Any boundaries that do not meet this criterion would seem to havelittle value in scientific analysis or for petroleum exploration. Furthermore, the unconformable


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and conformable portions of the boundary should form a single through-going surface in mostcases. This means that the landward end of the conformable surface that is used for the bound-ary in the more central portions of a basin must be co-terminus with the basinward end of theunconformity that forms the boundary on the basin flanks. This constraint seems self-evidentbecause a boundary must be a continuous surface to be a true boundary.

Another constraint is that the bounding surfaces should be developed in most deposi-tional settings. This is required because if a type of stratigraphic surface used for sequencedelineation is not commonly developed in most basins then that type of sequence would havevery limited applicability. Finally, to have the most utility, the unconformable portion of theboundary should be a time line barrier and the conformable portion should have relatively lowdiachroniety. This last constraint allows sequence boundaries to be part of an effective, quasi-chronostratigraphic framework for subsequent facies analysis.

Currently, there are four different types of sequences that have proposed for sequenceanalysis. These are type 1 depositional sequence (Posamentier et al, 1988), type 2 depositionalsequence (Posamentier et al., 1988), genetic stratigraphic sequence (Galloway, 1989) and T-Rsequence (Embry and Johannessen, 1992). Each type is defined by a specific combination ofstratigraphic surfaces for the unconformable and conformable portion of the sequence bound-ary and each is illustrated in Figure 3.

Type 1 Depositional Sequence

Posamentier et al. (1988) define this type of sequence, the boundary of which is charac-terized by a subaerial unconformity on the basin margin, and a time line approximately

equivalent to the start of base level fall farther basinward (Fig.3). In some areas the base ofsubmarine fan deposits is used as a proxy for such a time line. Recently Posamentier and Allen(1999) and Posamentier and Morris (2000) have greatly elaborated on this type of sequence.The most problematic aspect of this type of sequence is that the time line equivalent to the start

of base level fall has no distinguishing characteristics and cannot be identified with any sem-blance of scientific objectivity (Embry, 1995). Posamentier and Morris (2000) defend the useof a cryptic time line as the correlative conformity although they admit that such a “surfacemay have little objective expression” (Posamentier and Morris, 2000, p. 38). Furthermore, use

of the base of submarine fan deposits as a proxy for the time line is not suitable because such aboundary is commonly a highly diachronous within-trend facies contact on a regional basis.Similarly, in ramp settings, authors sometimes use the RSME as a type 1 sequence boundary.

This is most inappropriate given the highly diachronous nature of such a surface and its patchydistribution.

Another drawback of this type of sequence is that the basinward portion of the unconfor-mity is placed within the sequence and not on the sequence boundary. The reason for this is

that the portion of the subaerial unconformity that develops during base level fall must liewithin the sequence by definition (see Posamentier and Morris, 2000, Figure 22). Given theseserious flaws, a type 1 depositional sequence is not a practical unit for sequence analysis, and I

would discourage its use.



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Figure 3. A schematic cross section illustrating the surfaces of sequence stratigraphy and the boundaries of thefour different types of sequences that have been defined. The sequence types are type 1 depositional sequence(T1DS); type 2 depositional sequence (T2DS); genetic stratigraphic sequence (GSS); and T-R sequence (T-RS).Note that both types of depositional sequences use a time line for a boundary and thus have no practical utility.Both the T-R and genetic stratigraphic sequences have objectively recognizable boundaries, but the GSS is nota useful type because it includes the unconformity within the sequence. Only the T-R sequence has recogniz-able boundaries and keeps the unconformity on the boundary. Thus, only a T-R sequence has practical utility.

Type 2 Depositional Sequence

Posamentier et al. (1988) also defined this type of sequence which has a boundary dis-tinguished by a subaerial unconformity on the basin margin and the time line equivalent to thestart of base level rise farther basinward where the unconformity is no longer present (Fig.3).This type of sequence seems to be more widely accepted than the type 1 depositionalsequence. The main difference is the correlative conformity of the type 2 is the time line at thestart of base level rise, whereas for the type 1 it is at the time line at the start of base level fall.Van Wagoner et al. (1990), Hunt and Tucker (1992), Helland-Hansen and Gjelberg (1994), andPlint and Nummedal (2000) all favor the type 2 depositional sequence over the type 1 mainlyto avoid having a portion of the unconformity within the sequence. In a type 1 depositionalsequence, all the strata deposited during base level fall are placed immediately above thesequence boundary whereas in a type 2 such strata are placed directly below the sequence


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boundary. Kolla et al. (1995) and Posamentier and Morris (2000) discuss the reasons why theconformable portion of the sequence boundary is best equated with the start of base level fall(type 1) whereas Hunt and Tucker (1992, 1995) and Plint and Nummedal (2000) argue for itsplacement coincident with the start of base level rise (type 2).

One significant problem associated with a type 2 depositional sequence boundary is thelack of objective criteria for the recognition of the time line which coincides with the start ofbase level rise. As emphasized by Embry (1995), there is no significant shift in sedimentarypatterns or supply rates so that a recognizable, regional stratigraphic boundary would be cre-ated at the change from base level fall to base level rise. Over much of the marine area ashallowing-upward trend in sedimentation simply continues as base level fall changes to baselevel rise. Notably, no objective, scientific criteria for recognizing the conformable portion of atype 2 depositional sequence boundary have ever been described in the literature. A type 2depositional sequence is very impractical unit because the conformable portion of the bound-ary cannot be objectively determined.

Genetic Stratigraphic Sequence

Galloway (1989) defined a genetic stratigraphic sequence following the groundbreakingwork of Frazier (1974). It is sometimes known as a regressive-transgressive (R-T) sequence.This sequence is bound by only one type of surface, a maximum flooding surface (MFS)(Fig.3). A MFS generally consists of both unconformable and conformable portions, and thusthis sequence type is seemingly compatible with the Mitchum et al. (1977) definition of asequence. Furthermore, Vail et al. (1977) recognize the MFS on seismic sections, where it wastermed a downlap surface, and they consider it to be a sequence boundary. In later publica-tions, Vail and associates cease the practice of designating a MFS as a sequence boundary.

Because only one surface type is used, there is no possibility of a discontinuous bound-ary. Furthermore, the MFS can be objectively recognized by scientific analysis, and theboundary has a low diachroniety. Thus, it would seem that a MFS might have a lot of utility asa sequence boundary. However, a serious drawback to this sequence type is the fact that itsusage results in the subaerial unconformity lying within the sequence rather than on its bound-aries. Given that, on the basin flanks, a subaerial unconformity can separate two very different,structurally discordant units, a genetic stratigraphic sequence that includes such an unconfor-mity would consist of two genetically unrelated units. This does not fit the original Mitchum etal. (1977) definition and runs counter to the one of the goals of sequence stratigraphy which isto delineate separate genetic units. There can be no doubt that, when Sloss et al. (1949) origi-nally defined a sequence, they were thinking of major subaerial unconformities, across whichthere is potentially major loss of section, as the bounding surfaces. The use of maximumflooding surfaces as sequence boundaries seems to be stretching the definition of a sequencetoo far. However, it must be mentioned that the MFS is an excellent surface for correlatingstrata, and their great utility in this regard should not be confused with their inappropriatenessas a sequence boundary, an entirely separate function.

Transgressive-Regressive (T-R) Sequence

Embry and Johannessen (1992) have defined this type of sequence and Embry (1993, 1995)has discussed it further. It is similar to the type 1 and type 2 depositional sequences describedabove in that the unconformable portion of the sequence boundary consists of a subaerial uncon-formity or shoreface ravinement-unconformable. However, basinward of the termination of theunconformity the boundaries of these three different sequence types diverge (Fig.4). As shown in



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Figure 4 the maximum regressive surface (MRS), which can be considered a conformable sur-face, is used as the correlative conformity portion of the T-R sequence boundary.

I strongly advocate the use of this type of sequence for sequence analysis because it isthe only type that meets all the criteria for practicality and usefulness. A T-R sequence isbound by objectively recognizable stratigraphic surfaces. The unconformity is a time barrierand the MRS has low diachroniety as previously discussed. Finally, in most cases, the MRS isco-terminus with the unconformity having a short span of shoreface ravinement linking theMRS with the subaerial unconformity. In many cases, a shoreface ravinement-unconformableforms most, or even, the entire unconformable portion of the boundary. Embry (1995) notesthat in rare cases such a continuous relationship may not be present and that the basinward endof the subaerial unconformity may lie stratigraphically below the MRS. Such a relationshiphas also been illustrated by Helland-Hansen and Gjelberg (1994) using a theoretical model. Itappears that such a discontinuous relationship, although theoretically possible, is very rare innature and that in almost all documented cases the unconformable and conformable portionsof a T-R sequence boundary form a single through-going boundary. It should be noted that itwould be extremely difficult to document a discontinuous relationship, and thus such a theo-retical possibility is of little practical interest.

Summary

Although four types of sequences have been advocated since Mitchum et al. (1977) firstproposed their revised definition of a sequence, only one type results in a genetically consis-tent unit that can be delineated in a practical and scientific manner. That type is a T-Rsequence that employs a subaerial unconformity or shoreface ravinement-unconformable forthe unconformable portion of the boundary and a maximum regressive surface for the con-formable portion. Other proposed types of sequences are not suitable because they include allor a portion of the subaerial unconformity within the sequence and /or have conformableboundaries which cannot be recognized by objective scientific analysis.

Systems Tracts

Introduction

The sequence is the primary unit of sequence stratigraphy, and it is best defined bybounding unconformities, such as a subaerial unconformity and/or a shoreface ravinement-unconformable and conformities consisting of maximum regressive surfaces. A sequence can

Figure 4. A comparison of systems tract schemes fora type 1 depositional sequence, a type 2 depositionalsequence, and a T-R sequence. Only the T-Rsequence has systems tracts that have objectivelyrecognizable boundaries. LST: lowstand systemtract; TST: transgressive systems tract; HST: high-stand systems tract; FRST: forced regressive sys-tems tract; FSST: falling sea level systems tract;RST: regressive systems tract; and SB: sequenceboundary.


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be subdivided into distinctive units that are called systems tracts. Like a sequence, a given sys-tems tract must be bound by specific, recognizable sequence stratigraphic surfaces if it is tohave utility. Below, I review previously proposed methods for subdividing a sequence into sys-tems tracts. Following this, I present my preferred method for sequence subdivision thatcontrasts somewhat with the methods of others.

Previous Work

The term systems tract was first introduced to define a linkage of contemporaneous dep-ositional environments, forming the subdivision of a seismic-stratigraphic unit (Brown andFisher, 1977). The term was expropriated by Exxon stratigraphers (Van Wagoner et al., 1988)to define various units within a depositional sequence, each unit supposedly being distin-guished by stratal stacking patterns, position within the sequence, and types of boundingsurfaces. The Exxon workers (Posamentier and Vail, 1988) also propose that each system tractcorresponds to a certain interval on a eustatic sea level curve and this unfortunate postulate hasbeen the source of unending confusion and problems.

Posamentier and Vail (1988) divide a sequence into three systems tracts: lowstand,transgressive and highstand (Fig.4). The lowstand systems tract (LST) is the basal subdivisionof a sequence, and it is envisioned as having been deposited during base level fall and duringthe early part of the subsequent base level rise that precedes the start of transgression. Thebasal boundary of the LST corresponds with the sequence boundary and in most cases is atime line (correlative conformity) coincident with the start of base level fall (Fig.4).

The insurmountable problems associated with the objective recognition of time linesplague the delineation of the lower boundary of a LST and for the most part a lowstand systemstract cannot be objectively recognized. In an attempt to circumvent this problem, Posamentierand Vail (1988) suggest that the base of submarine fan deposits be used as the sequence bound-ary and the base of the LST. In actuality, such a contact on a regional basis is a highlydiachronous within-trend facies contact and is not suitable for use as a unit boundary insequence stratigraphy. The top of the LST in a marine setting is designated as the transgressivesurface which is equivalent to the maximum regressive surface in the present terminology(Fig.4). This upper boundary, in contrast to the lower one, is a practical, recognizable boundary.

An LST is also sometimes recognized as overlying the unconformable portion of thesequence boundary (e.g., strata in an incised valley). These strata are those that are depositedduring the initial stage of base level rise when regression is still occurring. Such strata consistof nonmarine strata which onlap the subaerial unconformity. The major problem associatedwith the delineation of a LST in this case is not the determination of a lower boundary but isthe objective determination of an upper boundary. The lower boundary is the subaerial uncon-formity and the upper boundary is the horizon that is equivalent to the start of transgression.This theoretical upper boundary cannot be objectively determined in nonmarine strata. Variousattempts have been made to delineate a LST on top of an unconformity. For example, VanWagoner et al. (1990) designate the entire section of nonmarine strata between the subaerialunconformity below and the shoreface ravinement-normal above as LST. (See their Fig. 28.)Such a methodology has no merit because clearly most, and in many cases all, of the nonma-rine strata in a given section are transgressive. Furthermore the shoreface ravinement-normalin this case is a highly diachronous surface and is not suitable for a systems tract boundary.

Some authors have drawn the upper boundary of the LST at a within trend facies changefrom a fluvial facies to a brackish water facies. This is also not an acceptable method becausesuch a facies boundary is highly diachronous and is entirely unsuitable for a systems tractboundary. Given that it is very likely that most nonmarine strata initially deposited on anunconformity were deposited during transgression, and that LST nonmarine sediments are



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commonly eroded by the overlying ravinement, all nonmarine strata above the unconformityare best placed in the transgressive systems tract which is described below. I emphasize thateven if some of the nonmarine strata overlying an unconformity have been deposited beforetransgression commenced and escaped erosion by the SR, it is basically impossible to objec-tively differentiate such nonmarine strata from overlying nonmarine strata that have beendeposited during transgression. Thus, any attempt to recognize an LST overlying a subaerialunconformity is an incredibly subjective exercise and is doomed to failure because of the lackof any objective criteria for differentiating LST nonmarine strata from TST nonmarine strata.

In stratigraphic situations where submarine fans deposits are not present, Posamentierand Vail (1988) proposed a second sequence model (Type 2) in which the initial systems tractoverlying the sequence boundary was termed a shelf margin system tract (SMST). This unitwas envisioned as having been deposited between the start of base level rise and the start oftransgression. The lower boundary of the SMST was defined as an unrecognizable time lineequivalent to the start of base level rise and the upper boundary was designated as the maxi-mum regressive surface (transgressive surface in their terminology). Because of the completelack of objective criteria for recognizing the lower boundary, the SMST has not been used insequence stratigraphic studies and has no practical value.

The next systems tract in the Exxon model is the transgressive system tract (TST). Thisunit is defined by the maximum regressive surface (transgressive surface of Exxon) at the baseand the maximum flooding surface above (Fig.4). It is envisioned as having been depositedduring base level rise when the rate of rise exceeded the rate of sediment supply over themarine area and the shoreline transgressed landward. As previously discussed, both the lowerand upper bounding surfaces can be objectively recognized with scientific criteria and this sys-tems tract has much utility.

The third and uppermost systems tract in the Exxon model is the highstand systems tract(HST) and it is defined by the maximum flooding surface below and the sequence boundaryabove (Fig.4). It is envisioned as having been deposited during the waning stage of base levelrise. Where the sequence boundary is an unconformity, the HST has scientifically recogniz-able boundaries. Unfortunately, where the boundary is a time line (correlative conformity), theboundary between the HST and overlying LST (or SMST) of the next sequence cannot beobjectively determined, and in these instances the HST loses its identity and usefulness.

Hunt and Tucker (1992) initiated the next phase in the evolution of systems tract termi-nology. It was taken as a given that the base of a sequence must coincide with the base of theLST which in the Exxon primary model was placed at a hypothetical time line or at the base ofsubmarine fan strata. Hunt and Tucker (1992) recognized that most of the submarine fan stratawere deposited during base level fall and were consequently time equivalent to strata whichunderlie the unconformable portion of the sequence boundary farther up slope. Thus, substan-tial strata below the sequence boundary on the shelf were time equivalent to strata above thesequence boundary in the basin. To them, this violated a fundamental tenet of sequence stratig-raphy which decreed that all the strata above the sequence boundary should be younger thanall those below it.

To correct this fundamental flaw in the Exxon model, they advocate use of a type 2 dep-ositional sequence and added a fourth systems tract that they named the forced regressivewedge systems tract (FRST). It is designated as the highest system tract in a sequence, lyingdirectly below the sequence boundary. The boundaries of this new systems tract are either atime line at the start of base level fall or the base of submarine fan strata at the base of the sys-tems tract and a time line equivalent to the start of base level rise at the top (Fig.4). To them,the FRST represents all strata deposited during base level fall although in reality this is some-times not the case because the base of submarine fan strata is highly diachronous and in manysituations is generated well after base level has begun to fall.


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With this revision, a sequence could now theoretically be subdivided into four systemstracts that are defined by changes in either the direction of base level movement or changes inthe direction of shoreline movement. The four systems tracts and their defined basal bound-aries are:

1. Lowstand (LST) with a time line at the start of base level rise at the base.2. Transgressive (TST) with a stratigraphic horizon equivalent to the start of landward

movement of the shoreline (MRS) at the base.3. Highstand (HST) with a horizon equivalent to the start of seaward movement of the

shoreline (MFS) at the base.4. Forced regressive (FRST) with the time line equivalent to the start of base level fall at

the base.

It is essential to understand that only the TST has boundaries that can be recognized byobjective scientific analysis. The LST, HST and FRST each have one or two boundaries that can-not be objectively determined, and thus these systems tracts have essentially no practical value.

At this same time as Hunt and Tucker (1992) were proposing the FRST, Nummedal etal. (1992) proposed a fourth system tract for strata deposited during base level fall. They pro-posed the name falling sea level system tract (FSST). They were working with shallow waterclastic strata and chose the regressive surface of marine erosion as the basal boundary andeither the subaerial unconformity or the time line equivalent to the start of base level rise as theupper boundary. These choices for both the lower and upper boundaries of a FSST, which isessentially equivalent to a FRST, have no practicality, being either unrecognizable or highlydiachronous.

Currently, systems tract definition and nomenclature is in a very sorry state, and thereare no clear scientific criteria for recognizing most of the proposed systems tracts. Only thetransgressive systems tract has well defined, recognizable boundaries. Major problems areassociated with the objective recognition of the highstand, lowstand, shelf margin, forcedregressive and falling sea level systems tracts.

Practical Systems Tracts

A practical solution to the problem of highly subjective systems tracts is to insist thatany proposed systems tract have well defined, specific boundaries that can be recognized byobjective, scientific criteria. In short, each boundary must be one of the surfaces of sequencestratigraphy rather than an invisible time line. Furthermore, each boundary must have reason-ably low diachroniety or be a time line barrier. Note that this is the same philosophy that isadvocated for defining the boundaries of a sequence. The only sequence stratigraphic surfacesthat meet these criteria are subaerial unconformity (time barrier), shoreface ravinement-uncon-formable (time barrier), maximum regressive surface (low diachroniety), and maximumflooding surface (low diachroniety). Notably, the regressive surface of marine erosion and theshoreface ravinement-normal are not suitable because they are both highly diachronous sur-faces similar to within-trend facies changes. As discussed in the previous chapter the SU, SR-U and MRS are used as the boundaries of a T-R sequence. This leaves only the maximumflooding surface (MFS) for subdividing a sequence into practical systems tracts.

This results in two systems tracts: the transgressive systems tract below and the regres-sive systems tract above (Figs.5, 6) (Embry and Johannessen, 1992; Embry, 1993). Thetransgressive system tract as defined in this manner is very similar to the TST of the Exxonmodel. The only difference is that in the T-R sequence all strata above the subaerial unconfor-mity are placed in the TST. As discussed above, the Exxon model often refers to fluvial stratadirectly overlying the unconformity as LST and places the lower boundary of the TST at the



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base of brackish water or marine strata. Such contacts are highly diachronous and are notappropriate for a systems tract boundary.

The regressive systems tract (RST) encompasses all strata between the MFS below andeither the unconformity (SU or SR-U) or maximum regressive surface (MRS) (i.e., sequenceboundary) above. Thus, it can be objectively delineated. It encompasses the HST, LST,SMST, and FRST (FSST) of other authors (Figs.5 and 6) and avoids the intractable problemsassociated with the objective recognition of each of these units.

Summary

Systems tracts are stratigraphic subdivisions of sequences. Most previously defined sys-tems tracts have been based on theoretical considerations rather than clear definitions thatemphasize objective criteria for recognizing the boundaries of a given designated unit. Thisresults in great confusion because of wide variability in how systems tract boundaries aredrawn and in what constitutes a given system tract. Terms such as lowstand, highstand, shelfmargin, falling sea level, and forced regressive systems tract, systems tract depend on the rec-ognition of a highly subjective time surfaces and consequently have no practical usage. Inmost instances the best one can do is to subdivide a sequence into two systems tracts, trans-gressive below and regressive above, the mutual boundary being a maximum flooding surface(Figs. 5 and 6).

Figure 5. A schematic cross section illustrating the boundaries of the various types of systems tracts that havebeen defined. Only the transgressive system tract (TST) and the regressive systems tract (RST) have bound-aries that can be determined by objective scientific analysis. The other types have one or more unrecognizableboundaries (hypothetical time lines) and have no practical use. See Figures 2 and 4 for stratigraphic surfaceand systems tract acronyms.


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SummarySequence stratigraphy is best seen as the delineation and correlation of changes in depo-

sitional trends that are generated during a base level cycle. The surfaces that represent thesechanges in depositional trends can be used for correlation or the delineation of sequences andtheir component systems tracts as long as they are not highly diachronous. The surfaces havingthe greatest utility include a subaerial unconformity, a shoreface ravinement-unconformable, amaximum regressive surface, and a maximum flooding surface. The first three surfaces are usedto define the sequence boundary and such a sequence is termed a T-R sequence. The MFSallows a T-R sequence to be subdivided into a transgressive systems tract below and a regres-sive systems tract above. In contrast to all other proposed sequence and systems tract types, theT-R sequence and its two component systems tract are the only sequence stratigraphic units forwhich all boundaries can be recognized by objective scientific analysis. This makes T-Rsequence analysis the only practical methodology currently available for sequence stratigraphy.

Figure 6. Gamma ray and sonic logs of a successionof shallow marine (offshore to shoreface) stratashowing a T-R sequence (MRS boundaries) and theTST and RST of the sequence. Any attempt to delin-eate time lines equivalent to the start and end of baselevel fall has no scientific basis, and thus any attemptto subdivide the interval of the RST into a HST,FRST and LST (all of which are theoreticallypresent) is futile.



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