Transgressive-Regressive (T-R) Sequence Stratigraphy

Transcription

1 Transgressive-Regressive (T-R) Sequence Stratigraphy Embry, Ashton F. Geological Survey of Canada Calgary, Alberta, Canada, T2L 2A7 Abstract A sequence, as originally defined by Sloss and colleagues, was a stratigraphic unit bounded by subaerial unconformities. Such a stratigraphic unit proved to be of limited value because, in most instances, sequences could be recognized only on the margins of a basin where subaerial unconformities were present. Vail and colleagues greatly expanded the utility of sequences for basin analysis when they redefined the term as a unit bounded by unconformities or correlative conformities. The addition of correlative conformities allowed a sequence to 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: a type 1 depositional sequence and a type 2 depositional sequence. A type 1 depositional sequence utilizes a subaerial unconformity as the unconformable portion of the boundary and a time line equivalent to the start of base level fall for the correlative conformity. Because the subaerial unconformity migrates basinward during base level fall, much of it is therefore included within such a sequence rather than being on the boundary. Also it is impossible to objectively recognize a time line that corresponds to the start of base level fall. For these reasons a type 1 depositional sequence has little practical value. A type 2 depositional sequence also uses the subaerial unconformity as the unconformable portion of the boundary but uses a time line equivalent to the end, rather than the start, of base level fall for the correlative conformity. This resolves the problem of including a portion of the unconformity inside the sequence. However, it is essentially impossible to objectively recognize 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 maximum flooding surfaces as sequence boundaries and named such a unit a genetic stratigraphic sequence. This alleviated the problem of major subjectivity in boundary recognition because maximum flooding surfaces can be determined by objective scientific analysis. However, this sequence type founders on the problem that the subaerial unconformity occurs within the sequence and thus it lacks genetic coherency on the basin margins. To overcome these major deficiencies in sequence definition, Embry and Johannessen have defined a fourth type of sequence that they term a T-R sequence. This sequence uses the subaerial unconformity as the unconformable portion of the boundary and the maximum regressive surface as the correlative conformity. This methodology keeps the subaerial unconformity on the boundary and also provides for a correlative conformity that can be objectively determined. It thus avoids the fatal flaws of previously defined types. A T-R sequence can be divided into a transgressive systems tract below and a regressive systems tract above by using the maximum flooding surface as a mutual boundary. T-R sequence stratigraphy, unlike the other proposed methodologies, has maximum practical utility with a minimum of stultifying jargon. 22nd Annual Gulf Coast Section SEPM Foundation Bob F. Perkins Research Conference

2 Introduction Embry Sloss et al. (1949) first used the term sequence for a stratigraphic unit bounded by regional subaerial unconformities and in doing so they initiated the discipline of sequence stratigraphy. Mitchum et al. (1977), as part of a watershed collection of papers on sequence stratigraphy, revised the definition of a sequence to a stratigraphic unit composed of genetically related strata bounded at the top and bottom by unconformities or their correlative conformities. With the inclusion of the concept that sequence boundaries can be extended basinward along correlative conformities, this definition provides for a sequence that potentially can be delineated over an entire basin and not just on the basin margins where most unconformities occur. There seems to be widespread agreement that the unconformable portion of a sequence boundary should coincide with a subaerial unconformity and/or a shoreface ravinement surface that has eroded through a subaerial unconformity. Such an unconformable stratigraphic surface occurs mainly on the basin margin and most often disappears towards the center of a basin. The extension of a sequence boundary into the conformable succession of the more central regions of a basin, that is the delineation of the correlative conformity, has been the subject of both confusion and debate. Over the past 25 years, a variety of different combinations of stratigraphic surfaces have been proposed as sequence boundaries. Each combination can be regarded as a specific type of sequence and its usage as a specific type of sequence stratigraphy. However, before each proposed sequence type can be described and discussed, it is important to examine what sequence stratigraphy is and to describe the various surfaces of sequence stratigraphy which potentially can be employed as part or all of a sequence boundary. After the various surfaces are described, the possible sequence types are discussed. Following this, systems tracts, which are the component units of sequences, are reviewed. This paper is basically a review of a sequence stratigraphic methodology that has been in development since It is based on the widespread and spectacular exposures of Devonian and 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 of and 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 a somewhat 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, it is critical to understand what basic rock property change is used to delineate stratigraphic contacts for that discipline. Furthermore, it is also important to know what phenomena are responsible 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 delineate the boundaries of biostratigraphic units. We also know that such changes in fossil content are mainly driven by a combination of evolution and shifting environments. With this knowledge, we can go a long way in understanding the basic tenets of biostratigraphy. In sequence stratigraphy, we utilize various changes in depositional trends as boundaries. Examples of such changes in trend are the change from sedimentation to subaerial erosion and the change from transgressive (deepening-upward) trend to a regressive (shallowing-upward) trend. These changes, which are recognized as specific types of surfaces (e.g., subaerial unconformity for the change from sedimentation to subaerial erosion, maximum flooding surface for the change from transgressive to regressive), are used as the boundaries of 152

3 Transgressive-Regressive (T-R) Sequence Stratigraphy 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 associated recognized surfaces are exclusively or commonly generated by changes in base level. These concepts give us a basic understanding of sequence stratigraphy from which a working definition of sequence stratigraphy can be derived: Sequence stratigraphy consists of the recognition and correlation of changes in depositional trends in the rock record. Such changes, which were generated by the interplay of sedimentation and shifting base level, are now recognized by sedimentological criteria and geometrical relationships. I would note that this definition is somewhat different from other definitions of the discipline that have tended to be 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 of chronostratigraphically significant surfaces. This has little to do with sequence stratigraphy and more closely defines facies analysis. Furthermore, the term chronostratigraphically significant adds nothing to the definition because stratigraphic surfaces from all the various types of stratigraphic analysis have chronostratigraphic significance. Emery and Myers (1996) define sequence stratigraphy as the subdivision of sedimentary basin fills into genetic packages bounded by unconformities and their correlative conformities. This is certainly a marked improvement 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 sedimentary rock record into lithostratigraphic units. Hopefully the offered definition provides a better 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 for the development of sequence stratigraphy just as the phenomenon of changing magnetic polarity is exploited in magnetostratigraphy. In this section I briefly discuss what is base level, what causes base level to oscillate, and the changes in depositional trends that result during one cycle of base level rise and fall. Base Level Change There has been a lot of confusion about what base level means in a stratigraphic sense and this is in part because the term has also been used somewhat differently in a geomorphic sense. Harry Wheeler (1964) has succinctly reviewed the history of the use of the term base level in stratigraphy. Recently Tim Cross (Cross, 1991; Cross and Lessenger, 1998) has clearly shown 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 abstract surface that represents a surface of equilibrium between erosion and deposition. It can be thought of as a ceiling for sedimentation, and thus in any area where base level lies below the Earth s surface no sediment accumulation is possible and erosion will occur. Where it lies above the Earth s surface, deposition can and usually does occur in the available space. The space between the Earth s surface and base level, which is available for sedimentation, has been called accommodation space (Jervey, 1988). Of course, the places where base level intersects the earth s surface are equilibrium points between areas of erosion and areas of deposition. 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 153

4 Embry that create and remove accommodation space and surficial processes that bring sediment or that remove sediment from that space. Because of the dynamic nature of the Earth, base level rarely remains static in any given area and is generally moving upwards or downwards relative to a datum below the surface of the Earth. Thus, base level changes represent changes in the distance between base level and the datum. A datum is used rather than the Earth s surface itself to ensure the concept of base level change is independent of sedimentation. There are two main drivers of base level change. The first one is tectonics that results in upward or downward movements of the datum in relation to the center of the Earth. In this situation the datum, and not base level, is moving. Downward movement of the datum is referred to as subsidence and, in a relative sense, results in rising base level and increased accommodation space. Conversely, upward movement of the datum (uplift) results in base level fall as the two reference horizons approach each other and accommodation space is reduced. The second driver of base level movement is eustatic sea level change that records the movements of the surface of the ocean in relation to the center of the earth. In this case, the datum remains stationary and base level moves. Thus, rising eustatic sea level equals rising base level and increased accommodation space and falling eustatic sea level equates to falling base level and decreased accommodation space. Furthermore, any reduction or increase of volume in the sedimentary column due to such phenomena, such as compaction, salt solution, and salt intrusion also will 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 relative to the datum, it is critical to understand that these factors act at the same time within a sedimentary basin and it is impossible to determine the effect of each factor separately (Burton et al., 1987). Their combined, net effects can be determined and this is expressed as changes in base level. Thus, use of the term base level avoids irresolvable arguments of whether tectonics or eustasy is responsible for additions and reductions in accommodation space and the accompanying changes in depositional trends. Some authors have a priori decided that either tectonics or eustasy is the key variable (e.g., Posamentier et al., 1988, use eustasy) but such a dogmatic approach is not helpful and should be avoided. The term relative sea level change is sometimes used (Van Wagoner et al., 1990) in the same context as base level change but I prefer 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 depositional trends within a sedimentary succession that was deposited during the oscillation. Such changes in depositional trend are due to the interaction between changing rates of the addition or reduction of accommodation space and the rate of sedimentation. For example, when all accommodation space is eliminated in a given area, there is a major change from sedimentation to erosion. Such a change in depositional trend results in the occurrence of various types of unconformities that can then be used for correlation and the delineation of various types of sequence stratigraphic units. Changes in Depositional Trends There are two main types of change in depositional trend that result from base level movements. These are (1) a change from sedimentation and accumulation to erosion and viceversa 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 important changes of depositional trend, which represent variations of the two main types, occur. Four 154

5 Transgressive-Regressive (T-R) Sequence Stratigraphy occur during base level rise and two during fall. These changes occur either over a short or long time interval when compared to the duration of the complete cycle. These six changes in depositional trend are: Base level rise: 1. Expansion of deposition and accumulation of nonmarine strata in a landward direction across 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 subaerial erosion. 6. Development of sea floor erosion on the inner shelf and expansion of this submarine erosion surface basinward. The reader might wonder why cessations of sedimentation on the marine slope (submarine channeling, slumps, etc.) are not included in the above list. The main reasons for this are because, as emphasized by Galloway (1998), such cessations of sedimentation can occur at almost 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 distinctive, recognizable horizons in the sedimentary record and these horizons are the ones that are used for sequence stratigraphic analysis (Fig. 1). Their development is elaborated upon below and criteria for the recognition of each specific horizon are discussed in the next section. Figure 1. During a single cycle of base level fall and rise, six specific stratigraphic surfaces that represent regional changes in depositional trend are generated. During base level fall, a subaerial unconformity and regressive surface of marine erosion migrate basinward. Between the start of base level rise and the start of transgression, the maximum regressive surface is formed. During the entire interval of transgression a shoreface ravinement migrates landward and a shoreface ravinement-unconformable and/or shoreface ravinement-normal are formed. Following the start of regression, a maximum flooding surface forms. When base level starts to rise, new accommodation space begins to be created in areas formerly undergoing erosion. This results in landward expansion of the basin margin and progressive onlap of the underlying erosion surface by nonmarine strata throughout the entire time of base level rise. With rising base level, less sediment is transported to the marine portion of a basin because of reduced fluvial gradients and increased sediment storage in the nonmarine area along the expanding basin margin. During the initial stage of rise, enough sediment 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 advancement occurs at a declining rate until finally the rate of base level rise at the shoreline exceeds the rate 155

6 Embry of sediment supply and the shoreline ceases its seaward movement and begins to shift landward (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 shoreface 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 initiation 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 characterized 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 produced during base level rise. These are maximum regressive surface, shoreface ravinementunconformable, shoreface ravinement-normal and maximum flooding surface. Also, during this time the subaerial unconformity that develops during the preceding base level fall is progressively covered with sediment completing its development as a distinctive stratigraphic horizon. With the start of base level fall, sediment accommodation space is reduced and sedimentation 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 interval 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 ravinementnormal, 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

7 Transgressive-Regressive (T-R) Sequence Stratigraphy Figure 2. A schematic cross section which shows the spatial relationships of the six surfaces of sequence stratigraphy: subaerial unconformity, regressive surface of marine erosion, shoreface ravinement-unconformable, shoreface ravinement-normal, maximum regressive surface, and maximum flooding surface. Because these surfaces are generated during specific times of a base level transit cycle, they always have a similar relationship 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 used for correlation and unit delineation. For example in lithostratigraphy, there is really only one type of surface recognized and that is the one that marks a significant change in lithology. This keeps lithostratigraphy relatively uncomplicated. Things become more complex when more than one type of surface is recognized within a given type of stratigraphy. In biostratigraphy, a number of different types of surfaces have been defined and include a boundary that marks the first appearance of a species, a boundary that marks the last appearance of a species, and a boundary that marks a significant change in the fossil assemblage. The identification of more than 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 a cycle of base level rise and fall. These changes in trend result in six distinctive, stratigraphic surfaces that can potentially be used for correlation and to define units in sequence stratigraphy. The six surfaces are: subaerial unconformity, regressive surface of marine erosion, shoreface ravinement-unconformable, shoreface ravinement-normal, maximum regressive surface and maximum flooding surface. The first two are generated during base level fall and the last four during base level rise. As will be discussed, some of these surfaces are very useful for correlation and for delineating sequence stratigraphic units whereas others are not. 157

8 Embry Below, each of these six surfaces is described with emphasis on the criteria that allow its objective recognition and its differentiation from the other surfaces. Also, the relationship of each 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 represents a significant change in facies but not a change in depositional trend. It is thus not a surface of sequence stratigraphy, but I include it here because it is a very important boundary for 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 subaerial unconformity. This surface forms when base level falls and the surface of the earth is exposed to subaerial erosion processes such as fluvial and wind action. Throughout the time of base level fall, it expands seaward as the basin edge is progressively exposed. During the subsequent base level rise, the subaerial unconformity is onlapped by nonmarine sediments and is preserved as a discrete surface. A subaerial unconformity marks a cessation in sedimentation and is thus characterized by a sharp erosive contact in many cases. Underlying strata can be highly variable and sometimes are marked by the diagenetic effects of soil development. A key characteristic of a subaerial 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 soil horizon that is overlain by nonmarine strata and truncates underlying strata. A subaerial unconformity has an important relationship to time lines. It develops over the 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 reason for this is that in most cases all strata below a subaerial unconformity are entirely older than all strata above the unconformity. The subaerial unconformity truncates time lines that are below it, and the time lines above it display an onlap relationship. Thus, a subaerial unconformity can be regarded as a time line barrier, and this feature makes a subaerial unconformity an important surface for establishing a quasi-chronostratigraphic framework and for using as a unit 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. When base level starts to fall, the slope of the inner shelf is no longer in equilibrium with the currents and it becomes an area of net erosion. Currents slowly remove sediment in order to re-establish an equilibrium profile and this erosional area migrates basinward during the entire interval of base level fall. At the same time, sediment is deposited in the shoreface and this sediment downlaps onto the erosion surface as the shoreface sediments prograde seaward. These sediments eventually become capped by a subaerial unconformity. Given the above, the characteristics of an RSME are an erosion surface which overlies shallowing-upward, marine shelf strata and is overlain by shallowing-upward, shoreface strata. These characteristics are unique compared with the other erosional surfaces described herein and allow the RSME erosion to be identified with confidence. The RSME develops during the entire time of base level fall and thus it is very diachronous. Time lines pass through the surface at a high angle and with some offset. Thus, it is not a time line barrier like a subaerial unconformity. The highly diachronous nature of the surface makes it unsuitable for being part of a stratigraphic framework and for bounding a sequence stratigraphic unit. 158

9 Transgressive-Regressive (T-R) Sequence Stratigraphy Shoreface Ravinement-Unconformable (SR-U) Another prominent unconformity surface that is generated during a base level cycle is a shoreface ravinement. It commonly is confused with a subaerial unconformity despite its very different origin. Whereas the subaerial unconformity forms during base level fall and regression, a shoreface ravinement forms during the interval of base level rise when transgression occurs. When the transgression begins and the shoreline starts to move landward, the shoreface ravinement is cut by shoreface wave action that removes sediment and transports it mainly seaward. This occurs primarily because of the landward translation of the shelf equilibrium profile. Throughout the entire interval of transgression, this erosive action moves steadily landward and removes previously deposited shoreline and nonmarine sediments. This results in a widespread erosion surface that separates underlying sediments from overlying lower shoreface to offshore sediment. The two most important characteristics of a shoreface ravinement are a sharp, erosive contact and the occurrence of directly overlying marine strata that display a deepening-upward trend (transgressive). It is crucial to determine if the shoreface ravinement in question has eroded through an underlying subaerial unconformity or not. If this has happened, then the shoreface ravinement inherits the time line barrier property of a subaerial unconformity and all strata below are older than all those above. In this case, the ravinement surface is referred to as a shoreface ravinement-unconformable. One way of determining if a shoreface ravinement is an unconformable type is to examine the nature of the underlying strata. If they are marine, then it is very likely that an interval of nonmarine strata and a subaerial unconformity have been eroded and the shoreface ravinement is a time line barrier. Shoreface Ravinement-Normal (SR-N) If the shoreface ravinement in question has not eroded through the underlying subaerial unconformity, then it is classified as a shoreface ravinement normal. It has many characteristics in common with the shoreface ravinement unconformable in that it is a sharp, scoured surface overlain by deepening-upward marine strata. However, the distinguishing characteristic of a shoreface ravinement-normal is that underlying strata overlie a subaerial unconformity and consist of sediments deposited landward of the shoreline. The shoreface ravinement-normal is commonly a highly diachronous surface and time lines pass through it, offset and at a high angle. Because of this high diachroniety, such a surface has limited value in sequence stratigraphy 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 sedimentation at the shoreline and the shoreline begins to move landward. This marks the start of transgression and at this time sediment supply to the adjacent marine shelf decreases, and the water depth at any nearshore locality begins to increase. This results in a change in the shelf succession from a shallowing-upward trend that developed during the previous regression to a deepening-upward one that reflects the ensuing transgression. The surface that marks this significant and distinctive change in depositional trend is herein referred to as the maximum regressive surface. This surface has been called a variety of names including transgressive surface, conformable transgressive surface, maximum progradation surface, or by the more general term, flooding surface. Because there is considerable confusion associated with the above names, it seems best to use the more descriptive and less ambiguous term, maximum regressive surface. 159

10 Embry For practical purposes, this surface is confined to marine strata and is characterized by the change from a shallowing-upward trend to a deepening-upward one. Clearly the recognition of this surface depends on the availability of data from which general water depths in which the strata were deposited can be interpreted (i.e., facies analysis). The actual surface may occur within a gradational interval of facies change or it can be rather abrupt with minor scouring marking it. In summary, a maximum regressive surface is characterized by the change from shallowing-upward marine strata below and deepening-upward marine strata above with no evidence of substantial erosion. It is not recognized in nonmarine strata because in most cases its place is taken by either the subaerial unconformity or a shoreface ravinement-unconformable. In some cases it is theoretically possible that the change from regression to transgression is recorded in nonmarine strata that directly overlie a subaerial unconformity. However, it is impossible to objectively identify such a boundary in nonmarine strata and, given its likely rare occurrence, the practical solution is to interpret all nonmarine strata overlying a subaerial unconformity as having been deposited during transgression. In the vast majority of situations this will likely be entirely correct. In these cases, the subaerial unconformity marks the change in trend from regressive sedimentation below and transgressive sedimentation above. The change from a shallowing-upward trend to a deepening-upward one will not begin at the same time everywhere in a marine area because rates of sediment supply and base level change vary throughout the marine area. In general, it begins to form in basinward localities at the start of base level rise and ends at the start of landward movement of the shoreline. This results in a maximum regressive surface being somewhat diachronous over its extent but from my experience it appears that such regional diachroniety is low and that time lines cross a MRS at a very low angle. This low diachroniety makes the MRS a very useful surface for correlation and for helping to establish a regional stratigraphic framework, one of the main goals of sequence stratigraphy. Maximum Flooding Surface (MFS) The maximum flooding surface is basically the opposite of the maximum regressive surface. It is generated at the time when a shallowing-upward trend replaces a deepening-upward one. This change begins at the shoreline when transgression ends and regression begins and takes place in the waning phases of base level rise when the rate of sediment supply begins to exceed the rate of base level rise. At this time the shoreline begins to move basinward (regression) and consequently marine areas receive a higher supply of sediment. This results in a change from a deepening-upward trend in the marine strata that developed during transgression to a shallowing-upward trend that reflects regression. The MFS is most readily recognizable in marine clastic strata where it marks the boundary between a deepening-upward succession and an overlying shallowing-upward one. Such a boundary can occur within a gradational succession and thus be completely conformable or it can be a scoured surface on which anywhere from a little to a lot of erosion has occurred. Such erosion would be due to marine currents that were able to effect a net removal of sediment due to low sediment supply to offshore areas at the height of transgression. In situations where the contact is conformable, it often occurs within condensed deposits that represent very low sedimentation rates. In these circumstances, its exact placement can be difficult, and I suggest placing 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 sometimes be recognized in nonmarine strata. In this case it occurs at the boundary between nonmarine strata that display an interpreted trend of a decreasing distance from the shoreline (transgres- 160

11 Transgressive-Regressive (T-R) Sequence Stratigraphy sive) and overlying nonmarine strata that represent an interpreted trend of increasing distance from the shoreline (regressive). Because it is more difficult to recognize trends in distance from shoreline in nonmarine strata than it is to identify water depth trends in marine strata, the identification 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 in offshore areas. However such diachroniety tends to be low, making the MFS a very useful surface in sequence stratigraphic analysis. Within-Trend Facies Contact This is not a surface of sequence stratigraphy but is one that sometimes can be mistaken for one of the surfaces of sequence stratigraphy. It is simply a notable facies change that occurs within a succession of regressive or transgressive strata and it does not mark any change in depositional trend. Because such a facies boundary can be a scour surface, it can be misinterpreted as regressive surface of marine erosion if it occurs within a regressive succession and separates 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 if a subaerial unconformity (or shoreface ravinement-unconformable) is present above and/or landward of the horizon. Within-trend facies contacts are the key boundaries of facies analysis that 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 correlation without delineating any specific types of units. However, sequence stratigraphy also allows units to be delineated with the surfaces of sequence stratigraphy acting as boundaries of the units. The two different types of units that have been defined so far are the sequence and the systems tract. A sequence is primarily defined by its bounding unconformities and this honors the original definition by Sloss et al. (1949). The Mitchum et al. (1977) definition of a sequence, while still emphasizing unconformable boundaries, includes the added provision that sequence boundaries also can be recognized in the conformable succession of the central portion of a basin where the bounding unconformities are no longer present. This is accomplished by allowing a sequence to be bounded by unconformities or their correlative conformities. The extended definition of what constitutes a sequence boundary has led to four different types of sequence boundaries, each with a distinctive combination of unconformable and conformable portions, having been defined. This has resulted in there being four different types of sequences available for use in sequence analysis. Very often authors do not make it clear what specific type of sequence they are using in their sequence stratigraphic analysis and this can result in confusion and misunderstanding. It is important to understand how each type of sequence boundary is defined and delineated and to be able to recognize the specific type that is being used in a given study. In this section, the four types of sequence boundaries are described. Also, each type is evaluated as to its utility in sequence analysis using the following criteria. The boundaries of a sequence must be stratigraphic surfaces that can be recognized by objective, scientifically sound observations and interpretations or else sequence boundaries can be drawn willy-nilly at the whim of the interpreter. Any boundaries that do not meet this criterion would seem to have little value in scientific analysis or for petroleum exploration. Furthermore, the unconformable 161

12 Embry and conformable portions of the boundary should form a single through-going surface in most cases. This means that the landward end of the conformable surface that is used for the boundary in the more central portions of a basin must be co-terminus with the basinward end of the unconformity that forms the boundary on the basin flanks. This constraint seems self-evident because a boundary must be a continuous surface to be a true boundary. Another constraint is that the bounding surfaces should be developed in most depositional settings. This is required because if a type of stratigraphic surface used for sequence delineation is not commonly developed in most basins then that type of sequence would have very limited applicability. Finally, to have the most utility, the unconformable portion of the boundary should be a time line barrier and the conformable portion should have relatively low diachroniety. This last constraint allows sequence boundaries to be part of an effective, quasichronostratigraphic framework for subsequent facies analysis. Currently, there are four different types of sequences that have proposed for sequence analysis. These are type 1 depositional sequence (Posamentier et al, 1988), type 2 depositional sequence (Posamentier et al., 1988), genetic stratigraphic sequence (Galloway, 1989) and T-R sequence (Embry and Johannessen, 1992). Each type is defined by a specific combination of stratigraphic surfaces for the unconformable and conformable portion of the sequence boundary 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 characterized 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 of submarine 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 semblance of scientific objectivity (Embry, 1995). Posamentier and Morris (2000) defend the use of a cryptic time line as the correlative conformity although they admit that such a surface may 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 a boundary 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 patchy distribution. Another drawback of this type of sequence is that the basinward portion of the unconformity 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 lie within the sequence by definition (see Posamentier and Morris, 2000, Figure 22). Given these serious flaws, a type 1 depositional sequence is not a practical unit for sequence analysis, and I would discourage its use. 162

13 Transgressive-Regressive (T-R) Sequence Stratigraphy Figure 3. A schematic cross section illustrating the surfaces of sequence stratigraphy and the boundaries of the four 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 not a useful type because it includes the unconformity within the sequence. Only the T-R sequence has recognizable 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 distinguished by a subaerial unconformity on the basin margin and the time line equivalent to the start 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 depositional sequence. The main difference is the correlative conformity of the type 2 is the time line at the start 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), and Plint and Nummedal (2000) all favor the type 2 depositional sequence over the type 1 mainly to avoid having a portion of the unconformity within the sequence. In a type 1 depositional sequence, all the strata deposited during base level fall are placed immediately above the sequence boundary whereas in a type 2 such strata are placed directly below the sequence 163

14 Embry boundary. Kolla et al. (1995) and Posamentier and Morris (2000) discuss the reasons why the conformable 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 its placement coincident with the start of base level rise (type 2). One significant problem associated with a type 2 depositional sequence boundary is the lack of objective criteria for the recognition of the time line which coincides with the start of base level rise. As emphasized by Embry (1995), there is no significant shift in sedimentary patterns or supply rates so that a recognizable, regional stratigraphic boundary would be created at the change from base level fall to base level rise. Over much of the marine area a shallowing-upward trend in sedimentation simply continues as base level fall changes to base level rise. Notably, no objective, scientific criteria for recognizing the conformable portion of a type 2 depositional sequence boundary have ever been described in the literature. A type 2 depositional sequence is very impractical unit because the conformable portion of the boundary cannot be objectively determined. Genetic Stratigraphic Sequence Galloway (1989) defined a genetic stratigraphic sequence following the groundbreaking work 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 thus this sequence type is seemingly compatible with the Mitchum et al. (1977) definition of a sequence. Furthermore, Vail et al. (1977) recognize the MFS on seismic sections, where it was termed a downlap surface, and they consider it to be a sequence boundary. In later publications, 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 boundary. Furthermore, the MFS can be objectively recognized by scientific analysis, and the boundary has a low diachroniety. Thus, it would seem that a MFS might have a lot of utility as a sequence boundary. However, a serious drawback to this sequence type is the fact that its usage results in the subaerial unconformity lying within the sequence rather than on its boundaries. 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 unconformity would consist of two genetically unrelated units. This does not fit the original Mitchum et al. (1977) definition and runs counter to the one of the goals of sequence stratigraphy which is to delineate separate genetic units. There can be no doubt that, when Sloss et al. (1949) originally defined a sequence, they were thinking of major subaerial unconformities, across which there is potentially major loss of section, as the bounding surfaces. The use of maximum flooding surfaces as sequence boundaries seems to be stretching the definition of a sequence too far. However, it must be mentioned that the MFS is an excellent surface for correlating strata, and their great utility in this regard should not be confused with their inappropriateness as 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 described above in that the unconformable portion of the sequence boundary consists of a subaerial unconformity or shoreface ravinement-unconformable. However, basinward of the termination of the unconformity the boundaries of these three different sequence types diverge (Fig.4). As shown in 164

15 Transgressive-Regressive (T-R) Sequence Stratigraphy Figure 4. A comparison of systems tract schemes for a type 1 depositional sequence, a type 2 depositional sequence, and a T-R sequence. Only the T-R sequence has systems tracts that have objectively recognizable boundaries. LST: lowstand system tract; TST: transgressive systems tract; HST: highstand systems tract; FRST: forced regressive systems tract; FSST: falling sea level systems tract; RST: regressive systems tract; and SB: sequence boundary. Figure 4 the maximum regressive surface (MRS), which can be considered a conformable surface, 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 is the only type that meets all the criteria for practicality and usefulness. A T-R sequence is bound by objectively recognizable stratigraphic surfaces. The unconformity is a time barrier and the MRS has low diachroniety as previously discussed. Finally, in most cases, the MRS is co-terminus with the unconformity having a short span of shoreface ravinement linking the MRS with the subaerial unconformity. In many cases, a shoreface ravinement-unconformable forms most, or even, the entire unconformable portion of the boundary. Embry (1995) notes that in rare cases such a continuous relationship may not be present and that the basinward end of the subaerial unconformity may lie stratigraphically below the MRS. Such a relationship has also been illustrated by Helland-Hansen and Gjelberg (1994) using a theoretical model. It appears that such a discontinuous relationship, although theoretically possible, is very rare in nature and that in almost all documented cases the unconformable and conformable portions of a T-R sequence boundary form a single through-going boundary. It should be noted that it would be extremely difficult to document a discontinuous relationship, and thus such a theoretical possibility is of little practical interest. Summary Systems Tracts Although four types of sequences have been advocated since Mitchum et al. (1977) first proposed their revised definition of a sequence, only one type results in a genetically consistent unit that can be delineated in a practical and scientific manner. That type is a T-R sequence that employs a subaerial unconformity or shoreface ravinement-unconformable for the unconformable portion of the boundary and a maximum regressive surface for the conformable portion. Other proposed types of sequences are not suitable because they include all or a portion of the subaerial unconformity within the sequence and /or have conformable boundaries which cannot be recognized by objective scientific analysis. Introduction The sequence is the primary unit of sequence stratigraphy, and it is best defined by bounding unconformities, such as a subaerial unconformity and/or a shoreface ravinementunconformable and conformities consisting of maximum regressive surfaces. A sequence can 165

16 Embry be subdivided into distinctive units that are called systems tracts. Like a sequence, a given systems tract must be bound by specific, recognizable sequence stratigraphic surfaces if it is to have utility. Below, I review previously proposed methods for subdividing a sequence into systems tracts. Following this, I present my preferred method for sequence subdivision that contrasts somewhat with the methods of others. Previous Work The term systems tract was first introduced to define a linkage of contemporaneous depositional environments, forming the subdivision of a seismic-stratigraphic unit (Brown and Fisher, 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 distinguished by stratal stacking patterns, position within the sequence, and types of bounding surfaces. The Exxon workers (Posamentier and Vail, 1988) also propose that each system tract corresponds to a certain interval on a eustatic sea level curve and this unfortunate postulate has been 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 subdivision of a sequence, and it is envisioned as having been deposited during base level fall and during the early part of the subsequent base level rise that precedes the start of transgression. The basal boundary of the LST corresponds with the sequence boundary and in most cases is a time line (correlative conformity) coincident with the start of base level fall (Fig.4). The insurmountable problems associated with the objective recognition of time lines plague the delineation of the lower boundary of a LST and for the most part a lowstand systems tract cannot be objectively recognized. In an attempt to circumvent this problem, Posamentier and Vail (1988) suggest that the base of submarine fan deposits be used as the sequence boundary and the base of the LST. In actuality, such a contact on a regional basis is a highly diachronous within-trend facies contact and is not suitable for use as a unit boundary in sequence stratigraphy. The top of the LST in a marine setting is designated as the transgressive surface 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 the sequence boundary (e.g., strata in an incised valley). These strata are those that are deposited during the initial stage of base level rise when regression is still occurring. Such strata consist of nonmarine strata which onlap the subaerial unconformity. The major problem associated with the delineation of a LST in this case is not the determination of a lower boundary but is the objective determination of an upper boundary. The lower boundary is the subaerial unconformity 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. Various attempts have been made to delineate a LST on top of an unconformity. For example, Van Wagoner et al. (1990) designate the entire section of nonmarine strata between the subaerial unconformity 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 nonmarine strata in a given section are transgressive. Furthermore the shoreface ravinement-normal in 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 change from a fluvial facies to a brackish water facies. This is also not an acceptable method because such a facies boundary is highly diachronous and is entirely unsuitable for a systems tract boundary. Given that it is very likely that most nonmarine strata initially deposited on an unconformity were deposited during transgression, and that LST nonmarine sediments are 166

17 Transgressive-Regressive (T-R) Sequence Stratigraphy commonly eroded by the overlying ravinement, all nonmarine strata above the unconformity are best placed in the transgressive systems tract which is described below. I emphasize that even if some of the nonmarine strata overlying an unconformity have been deposited before transgression commenced and escaped erosion by the SR, it is basically impossible to objectively differentiate such nonmarine strata from overlying nonmarine strata that have been deposited during transgression. Thus, any attempt to recognize an LST overlying a subaerial unconformity is an incredibly subjective exercise and is doomed to failure because of the lack of any objective criteria for differentiating LST nonmarine strata from TST nonmarine strata. In stratigraphic situations where submarine fans deposits are not present, Posamentier and Vail (1988) proposed a second sequence model (Type 2) in which the initial systems tract overlying the sequence boundary was termed a shelf margin system tract (SMST). This unit was envisioned as having been deposited between the start of base level rise and the start of transgression. The lower boundary of the SMST was defined as an unrecognizable time line equivalent to the start of base level rise and the upper boundary was designated as the maximum regressive surface (transgressive surface in their terminology). Because of the complete lack of objective criteria for recognizing the lower boundary, the SMST has not been used in sequence stratigraphic studies and has no practical value. The next systems tract in the Exxon model is the transgressive system tract (TST). This unit is defined by the maximum regressive surface (transgressive surface of Exxon) at the base and the maximum flooding surface above (Fig.4). It is envisioned as having been deposited during base level rise when the rate of rise exceeded the rate of sediment supply over the marine area and the shoreline transgressed landward. As previously discussed, both the lower and upper bounding surfaces can be objectively recognized with scientific criteria and this systems 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 boundary above (Fig.4). It is envisioned as having been deposited during the waning stage of base level rise. Where the sequence boundary is an unconformity, the HST has scientifically recognizable boundaries. Unfortunately, where the boundary is a time line (correlative conformity), the boundary between the HST and overlying LST (or SMST) of the next sequence cannot be objectively 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 terminology. It was taken as a given that the base of a sequence must coincide with the base of the LST which in the Exxon primary model was placed at a hypothetical time line or at the base of submarine fan strata. Hunt and Tucker (1992) recognized that most of the submarine fan strata were deposited during base level fall and were consequently time equivalent to strata which underlie the unconformable portion of the sequence boundary farther up slope. Thus, substantial strata below the sequence boundary on the shelf were time equivalent to strata above the sequence boundary in the basin. To them, this violated a fundamental tenet of sequence stratigraphy which decreed that all the strata above the sequence boundary should be younger than all those below it. To correct this fundamental flaw in the Exxon model, they advocate use of a type 2 depositional sequence and added a fourth systems tract that they named the forced regressive wedge systems tract (FRST). It is designated as the highest system tract in a sequence, lying directly below the sequence boundary. The boundaries of this new systems tract are either a time line at the start of base level fall or the base of submarine fan strata at the base of the systems 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 sometimes not the case because the base of submarine fan strata is highly diachronous and in many situations is generated well after base level has begun to fall. 167

18 Embry With this revision, a sequence could now theoretically be subdivided into four systems tracts that are defined by changes in either the direction of base level movement or changes in the direction of shoreline movement. The four systems tracts and their defined basal boundaries 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 by objective scientific analysis. The LST, HST and FRST each have one or two boundaries that cannot 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 et al. (1992) proposed a fourth system tract for strata deposited during base level fall. They proposed the name falling sea level system tract (FSST). They were working with shallow water clastic strata and chose the regressive surface of marine erosion as the basal boundary and either the subaerial unconformity or the time line equivalent to the start of base level rise as the upper boundary. These choices for both the lower and upper boundaries of a FSST, which is essentially equivalent to a FRST, have no practicality, being either unrecognizable or highly diachronous. Currently, systems tract definition and nomenclature is in a very sorry state, and there are no clear scientific criteria for recognizing most of the proposed systems tracts. Only the transgressive systems tract has well defined, recognizable boundaries. Major problems are associated with the objective recognition of the highstand, lowstand, shelf margin, forced regressive and falling sea level systems tracts. Practical Systems Tracts A practical solution to the problem of highly subjective systems tracts is to insist that any proposed systems tract have well defined, specific boundaries that can be recognized by objective, scientific criteria. In short, each boundary must be one of the surfaces of sequence stratigraphy rather than an invisible time line. Furthermore, each boundary must have reasonably low diachroniety or be a time line barrier. Note that this is the same philosophy that is advocated for defining the boundaries of a sequence. The only sequence stratigraphic surfaces that meet these criteria are subaerial unconformity (time barrier), shoreface ravinement-unconformable (time barrier), maximum regressive surface (low diachroniety), and maximum flooding surface (low diachroniety). Notably, the regressive surface of marine erosion and the shoreface ravinement-normal are not suitable because they are both highly diachronous surfaces 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 maximum flooding surface (MFS) for subdividing a sequence into practical systems tracts. This results in two systems tracts: the transgressive systems tract below and the regressive systems tract above (Figs.5, 6) (Embry and Johannessen, 1992; Embry, 1993). The transgressive system tract as defined in this manner is very similar to the TST of the Exxon model. The only difference is that in the T-R sequence all strata above the subaerial unconformity are placed in the TST. As discussed above, the Exxon model often refers to fluvial strata directly overlying the unconformity as LST and places the lower boundary of the TST at the 168

19 Transgressive-Regressive (T-R) Sequence Stratigraphy base of brackish water or marine strata. Such contacts are highly diachronous and are not appropriate for a systems tract boundary. The regressive systems tract (RST) encompasses all strata between the MFS below and either the unconformity (SU or SR-U) or maximum regressive surface (MRS) (i.e., sequence boundary) 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 problems associated with the objective recognition of each of these units. Summary Systems tracts are stratigraphic subdivisions of sequences. Most previously defined systems tracts have been based on theoretical considerations rather than clear definitions that emphasize objective criteria for recognizing the boundaries of a given designated unit. This results in great confusion because of wide variability in how systems tract boundaries are drawn and in what constitutes a given system tract. Terms such as lowstand, highstand, shelf margin, falling sea level, and forced regressive systems tract, systems tract depend on the recognition of a highly subjective time surfaces and consequently have no practical usage. In most instances the best one can do is to subdivide a sequence into two systems tracts, transgressive 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 have been defined. Only the transgressive system tract (TST) and the regressive systems tract (RST) have boundaries that can be determined by objective scientific analysis. The other types have one or more unrecognizable boundaries (hypothetical time lines) and have no practical use. See Figures 2 and 4 for stratigraphic surface and systems tract acronyms. 169