STRATIGRAPHY SS2001- Notes on LECTURE 3 Sequence stratigraphy1. IntroductionOver 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 beenroutinely used for describing and interpreting lateral facies relationships in ancient carbonateplatforms . They offer a static representation of a carbonate platform by depicting an idealizeddistribution pattern of facies and paleoenvironments, usually during an instant in time and in theabsence of realtive sea-level changes. However, during the history of a carbonate platform or asiliciclastic environment appear, migrate, disappear, and reappear to a large extent in response todepositional and erosional processes associated with marine transgressions and regressionsimposed by relative changes in sea-level. Thus the predictive capacity of the facies models islimited by their static view of time and relative sea level changes.Sequence stratigraphy integrates time and relative sea-level changes to track the migration offacies. Sequence stratigraphy is rooted mainly in seismic stratigraphic sequence analysis, and itsstrength lies in its potential to predict facies within a chronostratigraphically constrained frameworkof unconformity-bound depositional sequences.Sequence stratigraphy is done using outcrops, well logs or cores, and interpretations may dependon rather different sets of data. However, the basic geometrical criteria remain the same. Usingthe methodology developed for seismic sequences by Vail et al. (1977), interpreters analyzeseismic reflections to describe stratal geometry and delineate the systematic patterns of lap-outand truncation of strata against chronostratigraphically constrained surfaces. In this manner, theyestablish the presence of unconformity-bound depositional sequences, deduce relative sea-levelchanges, 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 permeability2. Historical perspective of seismic stratigraphy and major developmentsThe concepts and techniques of seismic stratigraphy were first introduced in a number of paperspublished in in AAPG Mem. 26 (1977), but the ideas behind this new method can be traced furtherback. For the part of seismic stratigraphy dealing with stratigraphic interpretation, an historicalperspective cannot be separated from sequence stratigraphy.
• See Dotts 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.• 90s 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 sequencesWe will review the major definitions and get familiar with the concepts, starting with geometricalcriteria of seismic stratigraphy. The seismic stratigraphic approach allows us to break up a basinsstratigraphy into genetically related packages termed depositional sequences. Geometric analysis of a depositional sequence- UnconformitiesDefinition of unconformity- An unconformity is a surface of erosion or non-deposition thatseparates younger strata from older rocks and represents a significant hiatus.Unconoformities are classified on the basis of the structural relationships between the underlyingand overlying rocks. They represent breaks in the stratihraphic sequence, that is, they recordperiods of time that are not represented in the stratigraphic column. Unconformities also record afundamental change the environement (from deposition to non deposition and/or erosion) whichgenerally represents an important tectonic event.The recognition and mapping of unconformities are the first steps in understanding the geologicalhistory of a basin or a geological province- whether recognized in seiemic lines, outcrops or welldata- 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 sequence boundariesA seismic sequence is a depositional sequence identified on a seismic section. It is a relativelyconformable succesion of relections interpreted as genetically related strata. It is bounded at itstop and base by surfaces of discontinuity marked by reflection terminations interpreted asunconformities or the correlative conformities. A seismic sequence consists of genetically relatedstrata.Because it is determined by a single objective criterion-the physical relationships of the stratathemselves- the depositional sequence is useful in establishing a framework for stratigraphicanalysis.The concept of sequence stratigraphy was initially developed at Exxon by Vail and collegues, anddiffused 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 andVail (1988), to include systems tracts. A system tract is associated with a segment of the eustaticcurve 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 stratabounded at its top and base by unconformities and their correlative conformities. It is composed ofa succession of systems tracts and it is interpreted to be deposited between eustatic fall inflectionpoints".They also distinguish between sequences of type 1 and 2 according to the type of sequenceboundaries bounding the sequences (Type 1: subaerial exposure of shelf margin, Type 2:subaerial exposure limited to shelf area). Sequence bounding unconformities are initiated at timeswhen the rate of sea level fall exceeds the rate of subsidence. As subsidence rates increaseseaward on most platforms, the unconformities pass downdip into correlative conformities.• Show several cases of stratal terminations from seismics4. Controls over sequence developmentSequence stratigraphic units result from the interaction of: (1) rates of subsidence, (2) rate ofeustatic sea level change, and (3) sedimentation rate. The combination of the first two points, orrelative sea level changes, has been considered by many workers the primary control. To look at adifferent view, Schlager (1993) has pointed out with the use of modelling that changes in sedimentsupply 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 cyclecharts. It is important to look how these charts are constructed, on the basis of which data andassumptions, in order to mantain a critical view. Although eustatic sea level fluctuations areimportant in formation of depositional sequences, tectonics and varying sediment supply also areimportant and may be difficult to separate without high resolution biostratigraphic data on a globalscale. Consequently, relative sea level curves which are the sum of tectonic subsidence andeustatic 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 carbonatesystems . He intepreted changes in carbonate productivity, as well as platform or bank growth andthe 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-levelhighstands 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 typesof high stand platfoms, keep up and catch up, are distinguished. A keep-up carbonate highstandplatform is interpreted to represent a relatively rapid rate of accumulation that is able to keep pacewith periodic rises in relative sea level. Keep up margins are usually grain-rich and tend to formmounded/oblique stratal configuration at the platform/bank margin. A catch up cabonatehighstand is intepreted to represent a relatively slow rate of accumulation that is characterized bymicrite-rich parasequences, and generally displays a sigmoidal depositional profile at theplatformbank margin. This classification never has been taken much popularity.Sarg tried also to integrate diagenetic processes and products in the characterization of sequenceboundaries. During type 1 sequence boundaries, when sea level falls at a rate high enough todrop below the preceding platform/bank margin, he expects to observe slope front erosion andseaward 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 ofexposure, climate. During type 2 sequence boundaries, sea level is interpreted to fall to a positionat or just below the bank margin, and the inner-platfiorm area is exposed. In general, the dominantmeteoric effect will be in the inner platform. In synthesis, Sarg interprets sea-level changes to bethe major control, in analogy with siliciclastics. In this regard, see his interpretation of Triassicdepositional sequences .Handford and Loucks (1994) more recently have addressed in great detail sequence stratigraphyin carbonate settings, also stressing very much the role of sea-level changes as a control ongeometries and stratal patterns. However, they take into account fundamental principles ofcarbonate deposition and geologic-based observations, and construct depositional sequence andsystems tract models for a variety of rimmed shelves and ramps. They take into account the factthat different anmounts of carbonate sediments are produced and can accumulate in any portion ofa carbonate system. Depositional sequences from different settings comprise depositional systemsdeposited during lowstand, transgressive and highstand conditions. Lowstand: carbonatesediment production is reduced on rimmed shelves because a relatively small shallow water areais available for sediment production. Transgression: carbonate sedimentation initiates in restrictedenvironments and later as more open conditions develop, open marine facies including patch reefsmay locally develop atop flooded platforms and ramps. Retrogradational parasequences form andsubsequentely 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 seasunder the influence of high rates of sediment production. Slope and basinal environments receiveexcess shelf and shelf-edge derived material.Handford and Loucks also consider different expressions of sequence boundaries in shelf, marginand toe-of-slope setting depending on different types of climate.
5. An alternative view in sequence stratigraphySchlager (1992, 1993) added another perspective to the sequence stratigraphic models incarbonate settings, pointing out many previously underevaluated aspects of the carbonateenvironments. In particular, he has shown how eustacy alone is inadequate in explaing observedpatterns and other controls must be considered. Differences in depositional systems, calleddepositional bias, as well as environmental changes strongly influence sequence patterns.These include high-stand shedding of carbonates, drowning unconformities, effects of slope onareas of sediment production, patterns of sediment dispersal...Schlager (1992) proposes that another definition for sequences and sequence boundaries, moreprocess-oriented, is needed, and he suggests a definition which is broad enough to allow anobjective assessment of the respective impact of sea-level and environmnetal change onsequence patterns. He proposes "a sequence can be viewed as a relatively conformablesuccession 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 inputand dispersal". E.g., isopachs above and below the mid-Cretaceous unconformity in the Gulf ofMexico; this unconformity represents a fundamental change in the sediment input pattern of theGulf, or drowning unconformitiesSchlager emphasizes that seismic unconformities and outcrop unconformities may not match. Alsobe 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 comparingseismics and outcrops. In fact, seismic image according to the frequency used might showunconformities that correspond to transitional boundaries in outcrop (Biddle et al., 1992).Some differences of carbonate and siliciclastic systems are important and affect the sequencestratigraphic development of the two different systems. In summary these are: (1) carbonatesystems tend to build elevated margins that build to sea level at the shelf break, (2) somecarbonate 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 readboth 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 beshut 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 minordepositional 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, transgressionor highstand. These are termed lowstand (LST), transgressive (TST) and highstand (HST)systems tracts. The transgressive surface (ts) or flooding surface separates the LST from theTST. The maximum flooding surface (mfs) separates the TST from the HST. Most systemstracts are themselves composed of small scale shallowing upward cycles from a meter to 10 m ormore, often bounded by flooding surfaces. These units have been termed parasequences, andare commonly partly related to Milankovitch cyclicity and associated sea level changes.ReferencesAhr, W.M., 1973, The carbonate ramp: an alternative to the shelf model: Transactions of the Gulf 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- responses of carbonate platforms to relative sea-level changes, in Loucks, R.G. and Sarg, J.F. (Rick). eds., Carbonate Sequence Stratigraphy, AAPG Memoir 57, p. 3-41.Haq, B.U., Hardenbol, J., and Vail, P.R., 1987, Chronology of fluctuating sea levels since the Triassic: Science, v. 235, p. 1156-1167.Irwin, M.L., 1965, General theory of epeiric clear water sedimentation: American Association of Petroleum Geologists Bulletin, v. 49, p. 445-459.Posamentier, H.W., and Vail, P.R., 1988, Eustatic control on clastic deposition II-sequence and systems tracts models, in Wilgus, C.K., et al., eds., Sea level changes: an integrated approach: SEPM Special Publication 42, p. 125-154.Read, J.F., 1985, Carbonate platform facies models: American Association of Petroleum Geologists Bulletin, v. 69, p. 1-21.Sarg, J.F., 1988, Carbonate sequence stratigraphy, in Wilgus, C.K., Hastings, B.S., Kendall, C.G.S.C., Posamentier, H.W., Ross, C.A., and Van Wagoner, J.C., eds., Sea-Level Changes: An Integrated Approach: Tulsa, OK, SEPM Special Publication No. 42, p. 155-182.Schlager, W., 1991, Depositional bias and environmental change — important factors in sequence stratigraphy: Sedimentary Geology, v. 70, p. 109-130.Schlager, W., 1992, Sedimentology and sequence stratigraphy of reefs and carbonate platforms: Tulsa, OK, American Association of Petroleum Geologists Continuing Education Course Note Series n. 34, 71 p.Schlager, W., 1991, Accomodation and supply- a dual control on stratigraphic sequences: Sedimentary Geology, v. 86, p. 111-136.Shaw, A.B., 1964, Time in stratigraphy: New York, McGraw-Hill, 353 p.Vail, P.R., Hardenbol, J., and Todd, R.G., 1984, Jurassic unconformities, chronostratigraphy, and sea-level changes from seismic stratigraphy and biostratigraphy, in Schlee, J.S., ed.,
Interregional Unconformities and Hydrocarbon Accumulation: Tulsa, OK, American Association of Petroleum Geologists Memoir 36, p. 129-144.Vail, P.R., Todd, R.G., and Sangree, J.B., 1977, Seismic stratigraphy and global changes of sea level, Part five: chronostratigraphic significance of seismic reflections, in Payton, C.E., ed., Seismic Stratigraphy — Applications to Hydrocarbon Exploration: Tulsa, OK, American Association of Petroleum Geologists Memoir 26, p. 99-116.Van Wagoner , J.C., Posamentier, H.W., Mitchum, R.M., et al., , 1988, An overview of the fundamentals of sequence stratigraphy and key definitions, in Wilgus, C.K., et al., eds., Sea lelevl changes: an integrated approach: SEPM Special Publication 42, p. 39-45.Wilson, J.L., 1975, Carbonate Facies in Geologic History: New York, Springer Verlag, 471 p.