STRATIGRAPHY SS2001- Notes on LECTURE 3

Sequence stratigraphy

1. Introduction

Over the past several decades, carbonate facies models of ramps (Ahr, 1973; Read, 1985),

shelves (Wilson, 1975; Read, 1985) and craton settings (Irwin, 1965; Shaw, 1964) have been

routinely used for describing and interpreting lateral facies relationships in ancient carbonate

platforms . They offer a static representation of a carbonate platform by depicting an idealized

distribution pattern of facies and paleoenvironments, usually during an instant in time and in the

absence of realtive sea-level changes. However, during the history of a carbonate platform or a

siliciclastic environment appear, migrate, disappear, and reappear to a large extent in response to

depositional and erosional processes associated with marine transgressions and regressions

imposed by relative changes in sea-level. Thus the predictive capacity of the facies models is

limited by their static view of time and relative sea level changes.

Sequence stratigraphy integrates time and relative sea-level changes to track the migration of

facies. Sequence stratigraphy is rooted mainly in seismic stratigraphic sequence analysis, and its

strength lies in its potential to predict facies within a chronostratigraphically constrained framework

of unconformity-bound depositional sequences.

Sequence stratigraphy is done using outcrops, well logs or cores, and interpretations may depend

on rather different sets of data. However, the basic geometrical criteria remain the same. Using

the methodology developed for seismic sequences by Vail et al. (1977), interpreters analyze

seismic reflections to describe stratal geometry and delineate the systematic patterns of lap-out

and truncation of strata against chronostratigraphically constrained surfaces. In this manner, they

establish the presence of unconformity-bound depositional sequences, deduce relative sea-level

changes, and describe the depositional and erosional history of an area.

• Difference between lithostratigraphic units and sequence stratigraphy, which has a geological time

significance

• Significance to industry: lithostratigraphy does not predict changes in lithologies, whereas with

ss one can predict subsurface lithological patterns and changes in permeability

2. Historical perspective of seismic stratigraphy and major developments

The concepts and techniques of seismic stratigraphy were first introduced in a number of papers

published in in AAPG Mem. 26 (1977), but the ideas behind this new method can be traced further

back. For the part of seismic stratigraphy dealing with stratigraphic interpretation, an historical

perspective cannot be separated from sequence stratigraphy.

• See Dott's summary (1992) of century-old controversies over the origin of cyclic sedimentation

and eustatic versus tectonic controls on sea-level

• Sloss (1963) published his major sequences correlatable across the North American craton,

the Indian Tribal names still appearing as super-sequences on the Haq et al. (1987) chart.

• AAPG memoir 1977. Development of digitally recorded and processed multichannel seismic

data made available large-scale 2D images throughout basins in the world. Industry had the

lead over academics.

• AAPG mem. 33 (1984), ideas wereexpanded

• 1987, publication of Haq et al. Chart

• SEPM sp. Publ. 42, 1988. New concepts introduced, such as parasequences and

accomodation space.

• 90's Many publications questioning certain aspects of sequence stratigraphy, or validity of

interbasinal correlations, or alternative models for the development of sequences

• High-resolution, subseismic scale sequence stratigraphy both in siliciclastic(e.g, Weimer et al.,

1990) and carbonates (Hardie et al., 1986; Goldhammer et al., 1991). Milankovitch theory of

orbital forcing was revived to explain the origin of high-frequency subsequence scale cycles

• Computer modeling packages developed to replicate and analyze the sedimentary fill of

sedimentary basins (e.g., SEDPAK, Mr Sediment,….)

• Inverse seismic modellng based on physical properties (e.g., Biddle et al., 1994;…)

3. Definition of sequences

We will review the major definitions and get familiar with the concepts, starting with geometrical

criteria of seismic stratigraphy. The seismic stratigraphic approach allows us to break up a basin's

stratigraphy into genetically related packages termed depositional sequences.

Geometric analysis of a depositional sequence- Unconformities

Definition of unconformity- An unconformity is a surface of erosion or non-deposition that

separates younger strata from older rocks and represents a significant hiatus.

Unconoformities are classified on the basis of the structural relationships between the underlying

and overlying rocks. They represent breaks in the stratihraphic sequence, that is, they record

periods of time that are not represented in the stratigraphic column. Unconformities also record a

fundamental change the environement (from deposition to non deposition and/or erosion) which

generally represents an important tectonic event.

The recognition and mapping of unconformities are the first steps in understanding the geological

history of a basin or a geological province- whether recognized in seiemic lines, outcrops or well

data- and are used as boundaries of stratigraphic units.

• Types of uncoformities (nonconformity, angular unconformities, disconformity, and

paraconformity)

Geometric analysis of a depositional sequence- relationships of strata to sequenceboundaries

A seismic sequence is a depositional sequence identified on a seismic section. It is a relatively

conformable succesion of relections interpreted as genetically related strata. It is bounded at its

top and base by surfaces of discontinuity marked by reflection terminations interpreted as

unconformities or the correlative conformities. A seismic sequence consists of genetically related

strata.

Because it is determined by a single objective criterion-the physical relationships of the strata

themselves- the depositional sequence is useful in establishing a framework for stratigraphic

analysis.

The concept of sequence stratigraphy was initially developed at Exxon by Vail and collegues, and

diffused with publication of AAPG Memoir 27 (1977).

• The original definition of a depositional sequence is by Vail et al., 1977 and states that

a depositional sequence is a stratigraphic unit composed of genetically related strata and

bounded at its top and base by unconformities or their correlative surfaces

A depositional sequence is chronostratigraphically significant because it was deposited during

a given interval of geologic time limited by the ages of the sequence boundaries where they are

conformities, although the age range of the strata within the sequence may differ from place to

place where the boundaries are unconformities .

• Relation of strata to sequence boundaries have been assigned different names, and are based

on the parallelism, or lack of between the strata and the boundary itself. It is important to be

familiar with these terms as they have been used commonly to define sequences in seismic

sections, and also in outcrop (see later the discussion regarding the problem of imaging real

geometries with seismics). Baselap is lapout at the lower boundary of a depositional sequence.

Two types are recognized: (1) Onlap is baselap in which a stratum (horizonthal or inclined) laps

out against an originally inclined surface of greater inclination. (2) Downlap is baselap in which an

initially inclined stratum terminates downdip against an initially horizontal or inclined surface.

Toplap is lapout at the upper boundary of a depositional sequence. Erosional truncation is the

lateral termination of a stratum by erosion.

• Chronostratigraphic significance: A depositional sequence is chronostratigraphically

significant because it was deposited during a given interval of geologic time limited by the

ages of the sequence boundaries where they are conformities, although the ages range of the

strata within the sequence may differ from place to place where the boundaries are

unconformities.

Two types of chronostratigraphic surfaces are related to sequences: (1) unconformities and

their correlative conformities forming sequence boundaries, and (2) stratal (bedding) surfaces

withi sequences.

The definition of depositional sequence was modified by Vail et al. (1984; 1987), Posamentier and

Vail (1988), to include systems tracts. A system tract is associated with a segment of the eustatic

curve and its timing in any given basin will depend on local subsidence and sediment supply.

A sequence is now defined as "a relatively conformable succession of genetically related strata

bounded at its top and base by unconformities and their correlative conformities. It is composed of

a succession of systems tracts and it is interpreted to be deposited between eustatic fall inflection

points".

They also distinguish between sequences of type 1 and 2 according to the type of sequence

boundaries bounding the sequences (Type 1: subaerial exposure of shelf margin, Type 2:

subaerial exposure limited to shelf area). Sequence bounding unconformities are initiated at times

when the rate of sea level fall exceeds the rate of subsidence. As subsidence rates increase

seaward on most platforms, the unconformities pass downdip into correlative conformities.

• Show several cases of stratal terminations from seismics

4. Controls over sequence development

Sequence stratigraphic units result from the interaction of: (1) rates of subsidence, (2) rate of

eustatic sea level change, and (3) sedimentation rate. The combination of the first two points, or

relative sea level changes, has been considered by many workers the primary control. To look at a

different view, Schlager (1993) has pointed out with the use of modelling that changes in sediment

supply can result in the same patterns generated by relative sea-level fluctuations. Caveat!

Sequence stratigraphic analysis, integrated with other stratigraphic techniques, biostratigraphy,

magnetostratigraphy, and radiometric data was used by Haq et al. (1988) to build sea-level cycle

charts. It is important to look how these charts are constructed, on the basis of which data and

assumptions, in order to mantain a critical view. Although eustatic sea level fluctuations are

important in formation of depositional sequences, tectonics and varying sediment supply also are

important and may be difficult to separate without high resolution biostratigraphic data on a global

scale. Consequently, relative sea level curves which are the sum of tectonic subsidence and

eustatic sea level change can be employed where global eustatic curves are poorly documented.

Sarg (1988) was the first to address specifically the issue of sequence stratigraphy in carbonate

systems . He intepreted changes in carbonate productivity, as well as platform or bank growth and

the resultant facies distribution, as the result of short-term eustatic fluctuations superimposed on

longer term changes. He pointed out that carbonate platforms associated with sea-level

highstands are characterized by relatively thick aggradational to progradational geometries,

bounded below by the top of a transgressive unit and above by a sequence boundary. Two types

of high stand platfoms, keep up and catch up, are distinguished. A keep-up carbonate highstand

platform is interpreted to represent a relatively rapid rate of accumulation that is able to keep pace

with periodic rises in relative sea level. Keep up margins are usually grain-rich and tend to form

mounded/oblique stratal configuration at the platform/bank margin. A catch up cabonate

highstand is intepreted to represent a relatively slow rate of accumulation that is characterized by

micrite-rich parasequences, and generally displays a sigmoidal depositional profile at the

platformbank margin. This classification never has been taken much popularity.

Sarg tried also to integrate diagenetic processes and products in the characterization of sequence

boundaries. During type 1 sequence boundaries, when sea level falls at a rate high enough to

drop below the preceding platform/bank margin, he expects to observe slope front erosion and

seaward movement of a freshwater lens. Diagenetic effects and relative proportion of marine vs.

meteroic processes would depend on many variables, such as extent of sea level fall, duration of

exposure, climate. During type 2 sequence boundaries, sea level is interpreted to fall to a position

at or just below the bank margin, and the inner-platfiorm area is exposed. In general, the dominant

meteoric effect will be in the inner platform. In synthesis, Sarg interprets sea-level changes to be

the major control, in analogy with siliciclastics. In this regard, see his interpretation of Triassic

depositional sequences .

Handford and Loucks (1994) more recently have addressed in great detail sequence stratigraphy

in carbonate settings, also stressing very much the role of sea-level changes as a control on

geometries and stratal patterns. However, they take into account fundamental principles of

carbonate deposition and geologic-based observations, and construct depositional sequence and

systems tract models for a variety of rimmed shelves and ramps. They take into account the fact

that different anmounts of carbonate sediments are produced and can accumulate in any portion of

a carbonate system. Depositional sequences from different settings comprise depositional systems

deposited during lowstand, transgressive and highstand conditions. Lowstand: carbonate

sediment production is reduced on rimmed shelves because a relatively small shallow water area

is available for sediment production. Transgression: carbonate sedimentation initiates in restricted

environments and later as more open conditions develop, open marine facies including patch reefs

may locally develop atop flooded platforms and ramps. Retrogradational parasequences form and

subsequentely drown, and shelf edges tend to aggrade, backstep, and drown if the rate of sea-

level rise is high. Highstand: Sea-wards progradation may partially infill inner to outer shelf seas

under the influence of high rates of sediment production. Slope and basinal environments receive

excess shelf and shelf-edge derived material.

Handford and Loucks also consider different expressions of sequence boundaries in shelf, margin

and toe-of-slope setting depending on different types of climate.

5. An alternative view in sequence stratigraphy

Schlager (1992, 1993) added another perspective to the sequence stratigraphic models in

carbonate settings, pointing out many previously underevaluated aspects of the carbonate

environments. In particular, he has shown how eustacy alone is inadequate in explaing observed

patterns and other controls must be considered. Differences in depositional systems, called

depositional bias, as well as environmental changes strongly influence sequence patterns.

These include high-stand shedding of carbonates, drowning unconformities, effects of slope on

areas of sediment production, patterns of sediment dispersal...

Schlager (1992) proposes that another definition for sequences and sequence boundaries, more

process-oriented, is needed, and he suggests a definition which is broad enough to allow an

objective assessment of the respective impact of sea-level and environmnetal change on

sequence patterns. He proposes "a sequence can be viewed as a relatively conformable

succession of strata deposited under the same regime of sediment input and dispersal". A

"sequence boundary represent a geometrically manifest change in the pattern of sediment input

and dispersal". E.g., isopachs above and below the mid-Cretaceous unconformity in the Gulf of

Mexico; this unconformity represents a fundamental change in the sediment input pattern of the

Gulf, or drowning unconformities

Schlager emphasizes that seismic unconformities and outcrop unconformities may not match. Also

be aware of the problem of imaging with seismic real geometries: example of Picco di Valandro,

Tiassic, by Biddle et al. (1992). This causes complications and problems when comparing

seismics and outcrops. In fact, seismic image according to the frequency used might show

unconformities that correspond to transitional boundaries in outcrop (Biddle et al., 1992).

Some differences of carbonate and siliciclastic systems are important and affect the sequence

stratigraphic development of the two different systems. In summary these are: (1) carbonate

systems tend to build elevated margins that build to sea level at the shelf break, (2) some

carbonate systems tend to export most of their sediment offshore during high-stands of sea level

(high-stand shedding), (3) carbonate systems are reliable records of sea level that can be read

both in changes of their biotic associations and in the diagenetic processes at unconformities, and

(4) carbonate platforms can drown particularly when they are isolated, whereas siliciclastics can be

shut off and build again to sea level simply as a function of sediment input.

5. Systems tracts and carbonate cycles.

Major depositional sequences (2nd order) are 10-to 50 m.y. duration and commonly contain minor

depositional sequences (3rd order), 0.5 to 5 m.y. duration. Depositional sequences are made up

by systems tracts, which consist of all the facies deposited during either low stand, transgression

or highstand. These are termed lowstand (LST), transgressive (TST) and highstand (HST)

systems tracts. The transgressive surface (ts) or flooding surface separates the LST from the

TST. The maximum flooding surface (mfs) separates the TST from the HST. Most systems

tracts are themselves composed of small scale shallowing upward cycles from a meter to 10 m or

more, often bounded by flooding surfaces. These units have been termed parasequences, and

are commonly partly related to Milankovitch cyclicity and associated sea level changes.

References

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Coast Association of Geological Societies, v. 23, p. 221-225.

Biddle, K.T., Schlager, W., Rudolph, K.W., and Bush, T.L., 1992, Seismic model of a

progradational carbonate platform, Picco di Vallandro, the Dolomites, northern Italy: American

Association of Petroleum Geologists Bulletin, v. 76, p. 14-30.

Handford, C.R. and Loucks, R.G., 1994, Carbonate depositional sequences and systems tracts-

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