1. 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.

2. • 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)

3. Geometric analysis of a depositional sequence- relationships of strata to sequence
boundaries
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.

4. 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

5. 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.

6. 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

7. 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.
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