Tag Archives: correlative conformity of Hunt and Tucker

Which sequence stratigraphic model is that?

The evolution of sequence stratigraphic models

Any scientific model of worth will be tested – and tested again. Models frequently undergo modification to account for any deficiencies or are replaced by new models. Sequence stratigraphy is no different; the original model (Vail and others) has gone through several iterations, including two proposals for alternative models. Most of the changes are based on different interpretations of the timing of events that produce key stratigraphic surfaces and systems tracts in relation to fluctuating baselevels, as well as the preservation potential for surfaces like the basal surface of forced regression. One problem of note is the contentious issue of identification and timing of correlative surfaces, or correlative conformities. For example, a subaerial unconformity develops during baselevel fall and the ensuing regression; this surface forms down depositional dip as the shoreline shifts basinward; the hiatus is a minimum at the youngest shoreline. At any time during this basinward shift there will be a correlative conformity in the marine parts of the basin – shelf, slope, and beyond. So, which correlative conformity does one choose – the one at the beginning of regression, the one at the end of regression, or somewhere in between? Identification of a correlative conformity in the stratigraphic record poses an additional problem; tracking seismic reflections across a basin is reasonably successful but putting one’s finger on a particular bedding plane in outcrop, core, or wireline logs is far less so, particularly in deposits that accumulate basinward of storm wave-base.

The diagram below (modified from Catuneanu et al, 2010) shows some of the important steps in this iterative process.

Summary of the 6 sequence stratigraphic models, their systems tracts, sequence boundaries, and relationship between their bounding surfaces and stages of baselevel change. Modified from Catuneanu et al. 2010, Figure 2. Systems tracts abbreviations are: HST = highstand, LST = lowstand, TST = transgressive, FSST = Falling stage, and RST = regressive, mfs = maximum flooding surface, mrs = maximum regressive surface. On the sea level curve, NR = normal regression, TR = transgression

Summary of the 6 sequence stratigraphic models, their systems tracts, sequence boundaries, and the relationship between their bounding surfaces and stages of baselevel change. Modified from Catuneanu et al. 2010, Figure 2. Systems tracts abbreviations are: HST = highstand, LST = lowstand, TST = transgressive, FSST = Falling stage, and RST = regressive, mfs = maximum flooding surface, mrs = maximum regressive surface. On the sea level curve, NR = normal regression, TR = transgression

Depositional sequence I is the original model proposed by Vail, Mitchum and others in AAPG Memoir 26 (1977), based on seismic stratigraphy. The sequence boundary is a subaerial unconformity and its correlative conformity, that begin to form at the onset of baselevel fall.

Model II (e.g. Posamentier et al. 1988) maintains the sequence boundary of Model I and is divided into four systems tracts; the HST, LST, TST, and a shelf margin systems tract. There was also an attempt to clarify issues with the correlative conformity at the base of the lowstand fan. At this juncture, the Exxon group proposed two types of sequence based on the degree of erosion beneath a subaerial unconformity and its down-dip extent: Type 1 sequences are bound by a subaerial unconformity that extends to the edge of the shelf, the associated facies also transiting seawards. The subaerial unconformity bounding a Type 2 sequence is confined to the basin margin. Type 2 sequences also contain the shelf margin systems tract (SMST) which overlies the sequence boundary and is overlain by a transgressive surface. Thus, the SMST accumulates during baselevel fall. Amidst the confusion that followed, several problems quickly became apparent with this scheme (Embry, 2002; Catuneanu, 2006):

Distinguishing between Type 1 and Type 2 unconformities was quite subjective.
The shelf margin and lowstand systems tracts are basically the same thing; they both form from sediment shed off the basin margin and shelf during baselevel fall. One problem here is that if a Type 2 sequence was identified, then a shelf margin systems tract must follow. In other words, the model was determining whether a shelf margin or lowstand systems tract was present, rather than the nature of the rocks themselves.
The correlative conformity basinward of the subaerial unconformity occurs within the lowstand deposits, and
In many parts of a marine basin, the correlative conformity has “little objective expression” (quoted in Embry, 2002); in other words, there may be little or no stratigraphic and sedimentologic criteria upon which one could identify such a boundary.

Depositional model III attempts to rectify these problems by combining sequence Types 1 and 2 and moving the sequence boundary to the end of baselevel fall (i.e. the beginning of baselevel rise) (e.g. Van Wagoner et al, 1988). The shelf margin systems tract was also discarded. In this version, the former lowstand fan deposits become part of the late highstand systems tract. However, the beginning of baselevel rise does not coincide with the end of regression – regression continues for some time because there is plenty of sediment available in shallow marine environments. Thus, there may be no specific stratigraphic record of any surface that indicates the start of sea level rise.

Comparison of the Exxon systematics corresponding to Model III, and the changes wrought by Hunt and Tucker (1992) in Model IV. Note the different placements of the correlative conformity (sequence boundary), the inclusion of a forced regressive systems tract that includes the down-stepping shoreface wedges and the correlative fan deposits, and the lowstand prograding wedge in the normal regressive portion of early baselevel rise. Modified from Hunt and Tucker, 1992.

Depositional model IV is, perhaps, the first significant departure from the original Exxon story. It was based on the recognition that forced regression occurs when sediment supply is less than the rate of baselevel fall (progradation and aggradation require sediment supply to exceed baselevel fall). To account for this dynamic in the sequence stratigraphic model a falling stage systems tract (FSST) was added by Plint (1988) and Hunt and Tucker (1992). The lower boundary is the basal surface of marine erosion. A subaerial unconformity occurs at the top of the FSST, stepping downward in concert with the shoreline trajectory; the correlative conformity is placed at the base of the redefined lowstand systems tract. Here, the LST begins with the final stages of baselevel fall and continues during normal regression in the early stage of baselevel rise; it overlies the basinward part of the FSST. Of all the depositional sequence model iterations, the Hunt and Tucker scheme is the probably the most accepted.

Two alternative models:

Genetic sequences (Galloway, 1989) are bound by maximum flooding surfaces (MFS). Galloway’s model relies on Frazier’s (1974) definition of depositional episodes, that represent time-bound units of progradation, aggradation and retrogradation, and form in response to transgression and regression. A Genetic sequence is the stratigraphic record of a depositional episode. Although a Genetic sequence records changes in baselevel, it is not tied to eustatic cycles – in fact, Galloway’s model was partly an attempt to extricate sequence stratigraphy from the idea that eustasy was the driving force. It was also a kind of plea for stratigraphy to return to objective lithofacies analysis, rather than being model driven.

The MFS was chosen as the Genetic sequence boundary because:

It is the surface that signals the end of transgression and landward shoreline excursion, and the beginning of regression (note that the end of transgression does not coincide with the end of baselevel rise).
It has low diachroneity.
It is an easily recognizable and mappable surface. Maximum flooding surfaces commonly overlie condensed sections or omission surfaces; they signal an abrupt lithological change from coarse-grained or cemented lithologies, to fine-grained beds that indicate the beginning of regression.
The MFS can be traced into coeval non-marine deposits where they interfinger with marine strata; Galloway maintains that the surface can also be traced to deeper parts of the basin.
The MFS is the only surface that can, in principle, be used across all parts of a basin.

A major criticism of Galloway’s scheme is that subaerial unconformities occur within a sequence, which means that it can no longer be considered as a relatively conformable stratigraphic unit. And unless the correlative conformity coincides with a condensed section or omission surface, its identification in deep water strata suffers from the same kinds of problems that the other models contend with.

Transgressive-regressive (T-R) sequences

Sequence stratigraphic models according to Embry (2002), including Genetic sequences (GS) and T-R sequences. The boundaries of depositional sequence (DS) models II and III are also indicated. SU-R = subaerial unconformity – ravinement surface, RSME = regressive surface of marine erosion at the base of each forced regressive wedge.

The general plan of attack for T-R sequences was formulated by Embry and Johannessen (1993). T-R sequences are bound by subaerial unconformities and their marine equivalents, maximum regressive surfaces (MRS); the subaerial unconformity may be partly or wholly replaced by a ravinement surface. This model avoids the complications of correlative conformities in shallow marine strata and maintains the general mantra of relatively conformable successions (unlike Genetic Sequences). The relative ease of identification of the bounding surfaces in outcrop and seismic profiles gives this model some advantage over its predecessors. T-R sequences contain only regressive and transgressive systems tracts; the boundary at the top of the TST is the maximum flooding surface.

However, like depositional sequences, there is a common problem of identifying the correlative conformity in deep water deposits including the deep shelf seaward of storm wave-base, and slope-submarine fan deposits that are dominated by sediment gravity flows and mass transport deposits (slumps, slides etc.). An additional problem is the claim that the MRS has low diachroneity which, as Catuneanu (2006) and others have noted, may be true along depositional dip, but not along depositional strike because of significant variations in sedimentation rates from one part of the basin margin to another.

The regressive component of T-R sequences assembles the highstand, falling stage, and lowstand systems tracts of depositional and Genetic sequences into a single stratigraphic entity (RSTs). This has the advantage of simplicity. But there is also an advantage in knowing which components derived from normal and forced regression – this information helps us understand basin dynamics. This can still be done by identifying the relevant depositional systems within the RST, but there is no explicit inclusion of the corresponding systems tracts within the T-R model.

Which model is best?

If the geological literature is anything to go by, one might assume that the Exxon-type depositional sequence schemas are the only models worth considering; Genetic and T-R sequence models barely get a mention. And yet there are as many advantages and disadvantages to depositional sequence schemes as there are for the two alternatives. Perhaps it’s a function of shouting loudest. To compound the issue, it is not always clear in many publications which depositional sequence version is being used.

A group of stratigraphers has grappled with these problems, publishing their deliberations and recommendations in several papers (e.g. Catuneanu et al. 2010, and 2011). The group acknowledges the historical significance of the depositional sequence models introduced by Vail and his colleagues, but not their primacy. Several conclusions stand out:

All the models have value, they are all sensible and logical; each has advantages and disadvantages.
No single model solves all stratigraphic problems.
The fundamental basis for stratigraphic interpretation is sound lithofacies analysis.
Choose the model that best suits the geological environment and the available data: interpretation of the stratigraphy should be based on geology and not driven by a model.
Identify this model in your reporting.

This is sound advice!

This post is part of the How To…series on Stratigraphy and Sequence Stratigraphy

Sequence stratigraphic surfaces

1 Reply

A brief look at the key stratigraphic surfaces that help us construct sequence stratigraphic models.

Look at any pile of sedimentary or volcanic rock and you will see a multitude of stratigraphic surfaces: bedding planes, lava and ash-flow tops, unconformities, and erosional discordances. Each is important in its own way and has a story to tell.

Genetic stratigraphic models, like sequence stratigraphy, use specific surfaces to define (and in some respects be defined by) abrupt changes in baselevel (regression, transgression) and sediment accommodation, changes in the direction of shoreline migration, the boundaries of stratigraphic trends (e.g. coarsening or fining upwards trends), and the stacking of parasequences and other high-order cycles. The main stratigraphic surfaces are (Catuneanu et al. 2011):

Subaerial unconformity
Correlative conformity
Regressive surface of marine erosion (RSME)
Maximum flooding surface (MSF)
Maximum regressive surface (MRS)
Two kinds of ravinement surface (tide- and wave-formed)

All have been referred to by different names and synonyms over the decades since sequence stratigraphy was first formalized (Payton, 1977). Details on this terminological jungle are documented by Catuneanu (2006).

There are four conditions under which these surfaces form – conditions that are related to the different stages of baselevel rise and fall, shoreline trajectory, sediment accommodation, and depositional environments. For each stage, refer to the relative sea level curve below:

Stages of regression and transgression in relation to relative sea level, in a sequence stratigraphy context

Beginning of baselevel fall and forced regression: Forced regression takes place when relative sea level fall outpaces sediment supply, forcing the shoreline and associated shelf facies seaward. Deposition associated with forced regression produces a succession of seaward-stacked shoreface wedges, where each wedge in succession is stranded as the shoreline moves seaward. In profile, the shoreline trajectory defining these wedges, downsteps in concert with falling sea level. Landward, a correlative surface will develop via fluvial incision.
The end of forced regression and the beginning of normal regression: In this case normal regression produces a typical prograding (the shoreline trajectory is approximately horizontal) and aggrading profile (the shoreline trajectory has a vertical component) and usually signifies that the supply of sediment is keeping pace with or exceeding the changes to baselevel and accommodation. Landward, fluvial incision will slow or cease as drainage adjusts to the lowstand baselevel.

End of regression and the beginning of transgression: Transgression occurs when baselevel rises and accommodation increases, such that the shoreline and associated shelf facies move landward. The back-stepping sediment packages are referred to as retrogradational. Landward, rivers will begin to aggrade, or change their sinuosity to accommodate the rising baselevel.
End of transgression and the beginning of the next phase of normal regression: Here, the shoreline trajectory reverses such that deposition changes from retrogradational to progradational – aggradational.

Schematic dip-section showing shoreline trajectories and depositional trends resulting from normal and forced regression, and transgression. across a siliciclastic shelf.

Subaerial unconformity

As relative sea level falls, the shoreline migrates seawards and the siliciclastic shelf or carbonate platform is progressively exposed. Exposure of sediment and bedrock results in various kinds of weathering – wind and water erosion, soil development, and diagenesis. Exposed marine carbonates are particularly prone to meteoric diagenesis because a fresh watertable will move seaward in concert with the shoreline. Rivers draining across the shelf will erode to depths controlled primarily by local baselevel (during sea level fall this baselevel will also fall); much of the sediment produced by these lowstand rivers will be delivered to the slope and deep basin.

Subaerial unconformities are sequence boundaries. The hiatus at a subaerial unconformity is commonly variable because the depth and breadth of erosion changes across the exposed shelf. They are generally considered to represent chronostratigraphic surfaces; in reality, it takes time for shorelines to move seaward or retreat landward, but the hiatus is considered to be short relative to the duration of an entire sequence – hence its chronostratigraphic rank.

Subaerial unconformities develop during regression and sea level lowstand, and for at least some of the subsequent transgression until the shelf is completely flooded.

Karstic and erosional relief on a subaerial unconformity developed on Ordovician carbonates, onlapped by Paleocene estuarine and shallow shelf deposits. Ellesmere Island.

Outcrop and schematic reconstruction of onlapping Paleocene estuarine – sandspits & bars deposits attached to Ordovician bedrock headlands, Ellesmere I. Detail of the unconformity and onlap surface is shown in the previous image.

Successive vadose pisolite layers indicate variable meteoric watertables indicate prolonged subaerial exposure of platform carbonates, Paleoproterozoic, Belcher Islands.

Successive vadose pisolite layers indicate variable meteoric watertables and prolonged subaerial exposure of platform carbonates, Paleoproterozoic, Belcher Islands.

Valley (outlined) erosded into Pleistocene coastal dunes during sea level lowstand. The vallety was later filled by a younger dune complex as sea level rose. Kariotahi, New Zealand

Valley (outlined) eroded into Pleistocene coastal dunes during sea level lowstand. The valley was later filled by a younger dune complex as sea level rose. Kariotahi, New Zealand

Same location as image above. The dipping surface is the valley margin, cut into Pleistocene dunes. The weathered profile in the lower dune sands contains abundant limonite-goethite hard-pans that developed from groundwater seepage.

Correlative conformity

A subaerial unconformity encroaches upon the shelf or platform in concert with the shoreline, until the end of regression. As sea level falls, sediment is delivered to the slope and basin floor, accumulating as a relatively conformable, deep-water, succession – commonly submarine fan and related deposits. Although the subaerial unconformity (the sequence boundary) ends at the lowest shoreline, there will be an equivalent surface, marking the end of sea level fall (regression) at the top of the lowstand deposits. This surface is referred to as a correlative conformity. It is the surface corresponding to the end of relative sea level fall and, according to Hunt and Tucker (1992) the end of forced regression. As such, it is the marine extension of a sequence boundary. The Hunt and Tucker systematics also indicate a period of normal regression at the beginning of sea level (baselevel) rise, when the rate of rise is relatively low compared with the rate of sediment supply (they call this the Lowstand Prograding Wedge). Thus, the correlative conformity, and therefore the sequence boundary, lie between the deposits of forced regression and normal regression.

Diagrammatic view (dip-section) of a subaerial unconformity across a shelf and forced regressive wedges, and the correlative conformity over basin floor submarine fans. The overlying lowstand prograding wedge is deposited during normal regression at the beginning of the subsequent sea level rise.

Regressive surface of marine erosion

This surface is manifested as an abrupt contact beneath the regressive shoreface wedges (commonly resistant sandstone), that form during forced regression and are subsequently abandoned as baselevel falls. The RSME commonly develops where wave orbitals impact the sea floor, at and above fairweather wave-base. It forms in concert with the basinward down-stepping shoreline trajectory as sea level falls (Plint, 1988). The RSME will erode mudrocks deposited on the mid to outer shelf. Sandstones deposited above the RSME are commonly crossbedded, reflecting the high-energy shoreface conditions. The RSME overlies highstand deposits that accumulated during the previous stage of normal regression.

Successive sharp-based shoreface sandstones deposited during forced regression. The sandstones overlie highstand deposits, and are in turn overlain by transgressive mudstones. Paleocene, Axel Heiberg Island. The subaerial unconformity probably lies atop the highest sharp-based wedge (right), but is covered by scree.

Maximum regressive surface (transgressive surface)

This surface represents the sea floor at the time when regression ends and transgression begins, and the shoreline trajectory reverses from seaward to landward. Its preservation potential is relatively high because erosion by continued regression has ceased, and burial by deeper water sediment begins as shoreline migration reverses (landward). However, in some circumstances shoreface ravinement during transgression may erode the MRS (and the subaerial unconformity).

Fluvial channels lie between two shelf parasequences. Lowstand fluvial incision – the landward extension of the max reg surface that represents the position of the sea floor at the maximum basinward extent of the sea level lowstand. Jurassic, Bowser Basin.

Channelized, crossbedded, fluvial sandstones lie between two shelf parasequences. Fluvial incision took place during sea level lowstand – it is interpreted as the landward extension of the maximum regressive surface that represents the position of the sea floor at the maximum basinward extent of the sea level lowstand. Jurassic, Bowser Basin.

Maximum flooding surface

During the late stages of transgression, the rate of sediment supply is usually very low, resulting in thin, condensed stratigraphy that commonly includes organic mudstone (black shale), calcareous mudstone or limestone, glauconite, or phosphatized fossils and nodules. Gamma tool signatures frequently record high levels of natural radioactivity through this condensed interval. The MFS is usually located at the top of this condensed section.

The MFS is the paleo sea floor at the end of transgression, marking the change to normal regression where the shoreline trajectory reverses from landward to seaward. The initiation of progradation during the subsequent normal regression means that the MFS will be a downlap surface for prograding clinoforms.

The (transgressive) condensed section overlies coarsening-upward, highstand deposits from the previous regression. The MFS is in turn overlain by muddy lithologies of the succeeding normal regression. The contrast between the condensed strata and the overlying muddy facies is expressed in outcrops as an abrupt change from resistant to recessive exposure. In outcrop, this produces a stair-like appearance in successive cycles (a good example is shown below).

Two surfaces are present in this top part of a Jurassic shelf parasequence; the maximum regressive surface or transgressive surface, fossiliferous pebbly mudstones depsited during transgression, and a condensed, cemented mudstone, the top of which is the maximum flood surface. The MFS is overlain by normal shelf deosits of the next parasequence

Two surfaces are present in the top part of a Jurassic shelf parasequence; the maximum regressive surface or transgressive surface (MRS-TS), fossiliferous pebbly mudstones deposited during transgression, and a condensed, cemented mudstone, the top of which is the maximum flood surface. The MFS is overlain by normal shelf deposits of the next parasequence. Bowser Basin.

Typical stair-like exposure of stacked parasequences; the resistant beds are normal and forced regressive sandstones, capped by resistant, transgressive deposits. The prominent bench corresponds to the maximum flood surface. Jurassic, Bowser Basin.

Ravinement surface

During transgression the shoreline, beach, and shoreface migrate landward, the attendant waves and tidal currents scouring as they go. The distinction between wave- and tide-dominated ravinement is often made, although depending on coastal dynamics, both can occur along the same stretch of coast. The depth of erosion of the underlying regressive deposits and their stratigraphic surfaces, such as the subaerial unconformity, RSME and MRS, depends on the wave climate and the efficiency of tidal currents to redistribute the eroded sediment. Thus, it is possible for the ravinement surface to overlie highstand deposits directly, where beach and other associated shoreline deposits have been removed. In other situations, scouring may be quite shallow, such that lagoon and estuarine deposits are preserved below the ravinement surface.

As the shoreline moves landward, the ravinement surface is overlain by deeper shoreface deposits. The contrast in facies through a ravinement surface is one of the more recognisable characteristics in outcrop and core. For example, deeper shoreface deposits containing interbedded mudstone and sandstone, or typical storm wave-base structures like hummocky crossbeds, will onlap the ravinement surface that, in turn, overlies lagoonal muds. Trace fossils, benthic faunas, and sedimentary structures will help your identification of the erosional surface separating these disparate facies. Facies transitions across a ravinement surface will vary depending on whether delta lobes, sand barriers, tidal channels, lagoons or estuaries are transgressed.

Wave erosion of Late Quaternary tidal flat and salt marsh – mangrove deposits at two locations: above – an estuary in Auckland Harbour (New Zealand), and the Galveston coast (below). The ravinement surface in both examples is at the level of the modern beach-tidal flat. The resulting stratigraphy will present in ascending order as tidal flat or beach sands, overlain by a thin remnant of salt marsh mud, the scoured (wave) ravinement surface, overlain by lower intertidal or subtidal deposits.

Ravinement at coal contact has removed any beach and shoreface deposits, and is instead overlain by thin silty sandstones. The top of these sandstones is inferred to be a MFS. Above the MFS is a thick unit of normal regressive mudstone (HST). Eocene, Ellesmere Island.

This is part of the How To…series on Stratigraphy and Sequence Stratigraphy

Geological Digressions

Earth science resources, images, glossaries, and SCICOMM for students and non-specialists

Tag Archives: correlative conformity of Hunt and Tucker

Which sequence stratigraphic model is that?