Sequence stratigraphy

Last updated

Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety of scales. The essence of the method is mapping of strata based on identification of surfaces which are assumed to represent time lines (e.g. subaerial unconformities, maximum flooding surfaces), thereby placing stratigraphy in chronostratigraphic framework allowing understanding of the evolution of the Earth's surface in a particular region through time. Sequence stratigraphy is a useful alternative to a purely lithostratigraphic approach, which emphasizes solely based on the compositional similarity of the lithology of rock units rather than time significance. Unconformities are particularly important in understanding geologic history because they represent erosional surfaces where there is a clear gap in the record. Conversely within a sequence the geologic record should be relatively continuous and complete record that is genetically related. [1] [2] [3] [4]

Contents

Stratigraphers explain sequence boundaries and stratigraphic units primarily in terms of changes in relative sea level (the combination of global changes in eustatic sea level and regional subsidence caused by tectonic subsidence, thermal subsidence and load-induced subsidence as the weight of accumulated sediment and water cause isostatic subsidence as a sedimentary basin is filled). The net changes resulting from these vertical forces increases or reduces accommodation space for sediments to accumulate in a sedimentary basin. A secondary influence is the rate of sediment supply to the basin which determines the rate at which that space is filled. [5]

Historical development

The origin of sequence stratigraphy can be traced back to the work of L.L. Sloss on interregional unconformities of the North American craton. Sloss recognized six craton-wide sequences representing hundreds of millions of years of earth history. [6] [7] [8] In the late 1960s Sloss had several students, notably Peter Vail, Robert Mitchum, and John Sangree, who completed dissertations studying the Pennsylvanian sedimentary rocks of the North American craton and became aware that global changes in sea level could have been responsible for the numerous widespread unconformities in those rocks. During their subsequent careers as research scientists at Exxon's research division Vail, Mitchum and others pioneered the practice of seismic stratigraphy, the stratigraphic interpretation of seismic reflection profiles to understand the layering and packaging of sedimentary rocks in the subsurface using acoustic imaging. [9] [10] The advent of seismic stratigraphy made it possible to identify sequences representing shorter period of time ranging in duration from tens of thousands to a few million years; and to compare the sequence stratigraphic history around the globe. This in turn led to sequence stratigraphy becoming systematized and understood to have widespread application to stratigraphic study of rock outcrops on the earth's surface as well. During the 1980s this ushered in a revolution in stratigraphy based on the delineation of regional physical surfaces that separate the sedimentary rock into packages representing discrete and sequential periods of time and predictable patterns of sediment depositional history. [11] [12] [4] [3]

Significant surfaces

Sequence boundaries

Sequence boundaries are deemed the most significant surfaces. [13] Sequence boundaries are defined as unconformities or their correlative conformities. Sequence boundaries are formed due to the sea level fall. For example, multi-story fluvial sandstone packages often infill incised valleys formed by the sea level drop associated with sequence boundaries. The incised valleys of sequence boundaries correlate laterally with interfluves, palaeosols formed on the margins of incised valleys. The valley infills are not genetically related to underlying depositional systems as previous interpretations thought. There are four criteria distinguishing incised valley fills from other types of multi-story sandstone deposits: a widespread correlation with a regional, high relief erosional surface that is more widespread than the erosional bases of individual channels within the valley; facies associations reflect a basinward shift in facies when compared with underlying units; erosional base of the valley removes preceding systems tracts and marine bands producing a time gap, the removed units will be preserved beneath the interfluves; increasing channel fill and fine grained units upwards or changes in the character of the fluvial systems reflecting increasing accommodation space. Sandstone bodies associated with incised valleys can be good hydrocarbon reservoirs. There have been problems in the correlation and distribution of these bodies. Sequence stratigraphic principles and identification of significant surfaces have resolved some issues.

Parasequence boundaries

Lesser importance is attached to parasequence boundaries, however, there is a suggestion that flooding surfaces representing parasequence boundaries may be more laterally extensive leaving more evidence than sequence boundaries because the coastal plain has a lower gradient than the inner continental shelf. [14] Parasequence boundaries may be distinguished by differences in physical and chemical properties across the surface such as; formation water salinity, hydrocarbon properties, porosity, compressional velocities and mineralogy. Parasequence boundaries may not form a barrier to hydrocarbon accumulation but may inhibit vertical reservoir communication. After production begins the parasequences act as separate drainage units with the flooding surfaces, which are overlain by shales or carbonate-cemented horizons, forming a barrier to vertical reservoir communications. Sequence stratigraphic principles have optimized production potential once reservoir scale architecture is identified and separate drainage units identified.

Systems tracts

The concept of systems tracts evolved to link the contemporaneous depositional systems. Systems tracts form subdivision in a sequence. Different kinds of systems tracts are assigned on the basis of stratal stacking pattern, position in a sequence, and in the sea level curve and types of bounding surfaces. [15]

Parasequences and stacking patterns

A parasequence is a relatively conformable, genetically related succession of beds and bedsets bounded by marine flooding surfaces and their correlative surfaces. The flooding surfaces bounding parasequences are not of the same scale as the regional transgressive surface that is associated with a sequence boundary.

The parasequences are separated into stacking patterns:

Each stacking pattern will give different information on the behaviour of accommodation space, a major control of which is relative level. So a rapidly progradational pattern will be indicative of falling sea level, rapidly retrogradational is evidence for rapidly transgressing sea level and aggradational will be indicative of gently rising sea level.

Sea level through geologic time

Comparison of two sea level reconstructions during the last 500 Myr. The black bar shows the magnitude of sea level change during the Quaternary glaciations; this is for the past few million years, but the bar is offset further in the past for readability. Phanerozoic Sea Level.png
Comparison of two sea level reconstructions during the last 500 Myr. The black bar shows the magnitude of sea level change during the Quaternary glaciations; this is for the past few million years, but the bar is offset further in the past for readability.

Sea level changes over geologic time. The graph on the right illustrates two recent interpretations of sea level changes during the Phanerozoic. The modern age is depicted on the left side, labeled N for Neogene. The blue spikes near date zero represent the sea level changes associated with the most recent glacial period, which reached its maximum extent about 20,000 years Before Present (BP). During this glaciation event, the world's sea level was about 320 feet (98 meters) lower than today, due to the large amount of sea water that had evaporated and been deposited as snow and ice in Northern Hemisphere glaciers. When the world's sea level was at this "low stand", former sea bed sediments were subjected to subaerial weathering (erosion by rain, frost, rivers, etc.) and a new shoreline was established at the new level, sometimes miles basinward of the former shoreline if the sea floor was shallowly inclined.

Today, sea level is at a relative "high stand" within the Quaternary glacial cycles because of rapid end-Pleistocene and early-Holocene deglaciation. The ancient shoreline of the last glacial period is now under approximately 390 feet (120 meters) of water. Although there is debate among earth scientists whether we are currently experiencing a "high stand" it is generally accepted that the eustatic sea level is rising.

In the distant past, sea level has been significantly higher than today. During the Cretaceous (labeled K on the graph), sea level was so high that a seaway extended across the center of North America from Texas to the Arctic Ocean.

These alternating high and low sea level stands repeat at several time scales. The smallest of these cycles is approximately 20,000 years, and corresponds to the rate of precession of the Earth's rotational axis (see Milankovitch cycles) and are commonly referred to as '5th order' cycles. The next larger cycle ('4th order') is about 40,000 years and approximately matches the rate at which the Earth's inclination to the Sun varies (again explained by Milankovitch). The next larger cycle ('3rd order') is about 110,000 years and corresponds to the rate at which the Earth's orbit oscillates from elliptical to circular. Lower order cycles are recognized, which seem to result from plate tectonic events like the opening of new ocean basins by splitting continental masses.

Hundreds of similar glacial cycles have occurred throughout the Earth's history. The earth scientists who study the positions of coastal sediment deposits through time ("sequence stratigraphers") have noted dozens of similar basinward shifts of shorelines associated with a later recovery. The largest of these sedimentary cycles can in some cases be correlated around the world with great confidence.

The three controls on stratigraphic architecture and sedimentary cycle development are:

Eustatic sea level is the sea level with reference to a fixed point, the centre of the Earth. Relative sea level is measured with reference to the base level, above which erosion can occur and below which deposition can occur. Both eustatic sea level changes and subsidence rates tend to be longer cycles. Sediment supply is largely thought to be controlled by local climatic conditions and can vary rapidly. These variations in local sediment supply affect the local and relative sea level which causes local sedimentary cycles.

Smaller and localised sedimentary cycles are not related to worldwide (eustatic) sea level changes but more to the supply of sediment to the adjacent basins where these sediments are being supplied. For example, when the basinward (oceanward) shift with progradation of shorelines was occurring in the Book Cliffs area of Utah the shorelines were receding or transgressing northwards in Wyoming. These sedimentary cycles are representative of the amount of supply of sediment to the basin. In a transgression, less sediment is being supplied than the rate of increase in the depth of water, and thus the shoreline migrates landward. In a regression, if the water depth is decreasing, the shoreline migrates seaward (basinward) and the previous shoreline is eroded. A regression of the shoreline also occurs if more sediment is being supplied than the shoreline can erode, causing the shoreline to migrate seaward. The latter is called progradation. The cycle of strata deposited during repeated transgressions and regressions creates a depositional sequence.

Economic significance

Sequence stratigraphy is and essential tool in the application of geology to the exploration for oil and gas, as a part of the field of Petroleum geology. Much of the development of this scientific discipline has occurred within or been funded by energy corporations and their geological research labs. [3] [4]

Sequence boundaries have economic significance because these changes in sea level cause large lateral shifts in the depositional patterns of seafloor sediments. These lateral shifts in deposition create alternating layers of good reservoir quality rock (porous and permeable sands) and poorer-quality mudstones (capable of providing a reservoir "seal" to prevent the leakage of any accumulated hydrocarbons that may have migrated into the sandstones). Hydrocarbon prospectors look for places in the world where porous and permeable sands are overlain by low permeability rocks, and where conditions are right for hydrocarbons to be generated and migrate into these "traps".

See also

Related Research Articles

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

<span class="mw-page-title-main">Sedimentary basin</span> Regions of long-term subsidence creating space for infilling by sediments

Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence creates a regional depression that provides accommodation space for accumulation of sediments. Over millions or tens or hundreds of millions of years the deposition of sediment, primarily gravity-driven transportation of water-borne eroded material, acts to fill the depression. As the sediments are buried, they are subject to increasing pressure and begin the processes of compaction and lithification that transform them into sedimentary rock.

<span class="mw-page-title-main">Los Angeles Basin</span> Sedimentary basin located along the coast of southern California

The Los Angeles Basin is a sedimentary basin located in Southern California, in a region known as the Peninsular Ranges. The basin is also connected to an anomalous group of east-west trending chains of mountains collectively known as the Transverse Ranges. The present basin is a coastal lowland area, whose floor is marked by elongate low ridges and groups of hills that is located on the edge of the Pacific Plate. The Los Angeles Basin, along with the Santa Barbara Channel, the Ventura Basin, the San Fernando Valley, and the San Gabriel Basin, lies within the greater Southern California region. The majority of the jurisdictional land area of the city of Los Angeles physically lies within this basin.

The eustatic sea level is the distance from the center of the Earth to the sea surface. An increase of the eustatic sea level can be generated by decreasing glaciation, increasing spreading rates of the mid-ocean ridges or increasing the number of mid-oceanic ridges. Conversely, increasing glaciation, decreasing spreading rates or fewer mid-ocean ridges can lead to a fall in the eustatic sea level.

Sedimentary basin analysis is a geologic method by which the formation and evolution history of a sedimentary basin is revealed, by analyzing the sediment fill and subsidence. Subsidence of sedimentary basins generates the spatial distribution of accommodation infilling sediments. Aspects of the sediment, namely its composition, primary structures, and internal architecture, can be synthesized into a history of the basin fill. Such a synthesis can reveal how the basin formed, how the sediment fill was transported or precipitated, and reveal sources of the sediment fill. From such syntheses models can be developed to explain broad basin formation mechanisms. Examples of such basin classifications include intracratonic, rift, passive margin, strike-slip, forearc, backarc-marginal sea, fold and thrust belt, and foreland basins.

<span class="mw-page-title-main">Great Unconformity</span> Gap in geological strata

Of the many unconformities (gaps) observed in geological strata, the term Great Unconformity is frequently applied to either the unconformity observed by James Hutton in 1787 at Siccar Point in Scotland, or that observed by John Wesley Powell in the Grand Canyon in 1869. Both instances are exceptional examples of where the contacts between sedimentary strata and either sedimentary or crystalline strata of greatly different ages, origins, and structure represent periods of geologic time sufficiently long to raise great mountains and then erode them away.

<span class="mw-page-title-main">Cratonic sequence</span> Very large-scale lithostratographic sequence

A cratonic sequence in geology is a very large-scale lithostratigraphic sequence in the rock record that represents a complete cycle of marine transgression and regression on a craton over geologic time. They are geologic evidence of relative sea level rising and then falling, thereby depositing varying layers of sediment onto the craton, now expressed as sedimentary rock. Places such as the Grand Canyon are a good visual example of this process, demonstrating the changes between layers deposited over time as the ancient environment changed.

Cyclic sediments are sequences of sedimentary rocks that are characterised by repetitive patterns of different rock types (strata) or facies within the sequence. Processes that generate sedimentary cyclicity can be either autocyclic or allocyclic, and can result in piles of sedimentary cycles hundreds or even thousands of metres thick. The study of sequence stratigraphy was developed from controversies over the causes of cyclic sedimentation.

Stratigraphic cycles refer to the transgressive and regressive sequences bounded by unconformities in the stratigraphic record on the cratons. These cycles represent a large scale eustasy cycle since the Cambrian period with further sub-divisions of those units.

The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of the character of a half-graben as its deepest extent is along its eastern and southeastern margins, where it is well defined by a series of sub-parallel faults, most important of which is the Red Rock Fault. These faults divide the basin from the older Carboniferous rocks of the Peak District and the North Staffordshire Coalfield.

The Messinian Erosional Crisis is a phase in the Messinian evolution of the central Mediterranean basin resulting from major drawdown of the Mediterranean seawater.

<span class="mw-page-title-main">Growth fault</span>

Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward the basin and has long-term continuous displacement. Figure one shows a growth fault with a concave upward fault plane that has high updip angle and flattened at its base into zone of detachment or décollement. This angle is continuously changing from nearly vertical in the updip area to nearly horizontal in the downdip area.

<span class="mw-page-title-main">Nias Basin</span>

The Nias Basin is a forearc basin located off the western coast of Sumatra, Indonesia, in the Indian Ocean. The name is derived from the island that bounds its western edge, the island of Nias. The Nias Basin, the island of Nias, and the offshore, submarine accretionary complex, together form a Forearc region on the Sunda Plate/Indo-Australian Plate collisional/subduction boundary. The Forearc region is the area between an oceanic trench and its associated volcanic arc. The oceanic trench associated with the Nias Basin is the Sunda Trench, and the associated volcanic arc is the Sunda Arc.

<span class="mw-page-title-main">Northern North Sea basin</span>

The North Sea is part of the Atlantic Ocean in northern Europe. It is located between Norway and Denmark in the east, Scotland and England in the west, Germany, the Netherlands, Belgium and France in the south.

The Exmouth Plateau is an elongate northeast striking extensional passive margin located in the Indian Ocean roughly 3,000 meters offshore from western and northwestern Western Australia.

One of the major depositional strata in the Himalaya is the Lesser Himalayan Strata from the Paleozoic to Mesozoic eras. It had a quite different marine succession during the Paleozoic, as most parts of it are sparsely fossiliferous or even devoid of any well-defined fossils. Moreover, it consists of many varied lithofacies, making correlation work more difficult. This article describes the major formations of the Paleozoic – Mesozoic Lesser Himalayan Strata, including the Tal Formation, Gondwana Strata, Singtali Formation and Subathu Formation.

<span class="mw-page-title-main">Parnaíba Basin</span>

The Parnaíba Basin is a large cratonic sedimentary basin located in the North and Northeast portion of Brazil. About 50% of its areal distribution occurs in the state of Maranhão, and the other 50% occurring in the state of Pará, Piauí, Tocantins, and Ceará. It is one of the largest Paleozoic basins in the South American Platform. The basin has a roughly ellipsoidal shape, occupies over 600,000 km2, and is composed of ~3.4 km of mainly Paleozoic sedimentary rock that overlies localized rifts.

Seismic stratigraphy is a method for studying sedimentary rock in the deep subsurface based on seismic data acquisition.

In sequence stratigraphy, condensed sections are strata that are thin, yet span a large time interval. They are associated with the maximum flooding surfaces, represented by sedimentary intervals deposited during the maximum marine transgression.

Incised valleys are mountain-valley-like features that commonly result from river down cutting into coastal plains and continental shelves in response to marine regression. They are the key evidence to identify sequence boundary on seismic profiles and outcrops. The magnitudes of their down cutting have been used to estimate the global sea-level variations.

References

  1. ↑ Catuneanu, Octavian (2022). Principles of Sequence Stratigraphy (2nd ed.). San Diego: Elsevier. ISBN   978-0-08-088513-1.
  2. ↑ Emery, Dominic; Myers, Keith (1996). Sequence stratigraphy. Oxford: Blackwell Science. ISBN   978-0-632-03706-3.
  3. 1 2 3 Van Wagoner, J.C.; Posamentier, H.W.; Mitchum, R.M. Jr.; Vail, P.R.; Sarg, J.F.; Loutit, T.S.; Hardenbol, J. (1988). "An Overview of the Fundamentals of Sequence Stratigraphy and Key Definitions". Society of Economic Paleontologists and Mineralogists Special Publication. 42: 39–45.
  4. 1 2 3 Van Wagoner, J.C.; Mitchum, R.M. Jr.; Posamentier, H.W.; Vail, P.R. (1987). Seismic Stratigraphy Interpretation Using Sequence Stratigraphy: Part 2: Key Definitions of Sequence Stratigraphy. American Association of Petroleum Geologists. pp. 11–14.
  5. ↑ "An Online Guide to Sequence Stratigraphy". strata.uga.edu. Archived from the original on 2022-04-01. Retrieved 2018-08-05.
  6. ↑ Sloss, L. L.; Krumbein, W. C.; Dapples, E. C. (1949). "Integrated Facies Analysis". Geological Society of America Memoirs. 39: 91–124. doi:10.1130/MEM39-p91.
  7. ↑ Sloss, L.L. (1950). "Paleozoic stratigraphy in the Montana area". American Association of Petroleum Geologists Bulletin. 34: 425–451.
  8. ↑ Sloss, L. L. (1963). "Sequences in the Cratonic Interior of North America". Geological Society of America Bulletin. 74 (2): 93. doi:10.1130/0016-7606(1963)74[93:SITCIO]2.0.CO;2.
  9. ↑ Vail, P. R.; Mitchum, R. M.; Thompson, S. (1977). "Seismic Stratigraphy and Global Changes of Sea Level, Part 3Relative Changes of Sea Level from Coastal Onlap1". Seismic Stratigraphy — Applications to Hydrocarbon Exploration. doi:10.1306/M26490C5.
  10. ↑ Vail, P.R.; Hardenbol, J.; Todd, R.G. (1984). "Jurassic Unconformities, Chronostratigraphy and Sea-Level Changes from Seismic Stratigraphy and Biostratigraphy". The Jurassic of the Gulf Rim. doi:10.5724/gcs.84.03.0347. ISBN   978-1-944966-02-7.
  11. ↑ Berg, Orville Roger; Woolverton, Donald G. (1985). Seismic Stratigraphy II: An Integrated Approach to Hydrocarbon Exploration. doi:10.1306/M39449. ISBN   0-89181-316-0.
  12. ↑ Cross, T A; Lessenger, M A (May 1988). "Seismic Stratigraphy". Annual Review of Earth and Planetary Sciences. 16 (1): 319–354. Bibcode:1988AREPS..16..319C. doi:10.1146/annurev.ea.16.050188.001535.
  13. ↑ Hampson, G.J., Davies, S. J., Elliott, T., Flint, S. S. & Stollhofen, H. 1999. Incised valley fill sandstone bodies in Upper Carboniferous fluvio-deltaic strata: recognition and reservoir characterisation Southern North Sea analogues. In: Petroleum Geology of NW Europe: Proceedings of the 5th Conference. (Edited by Fleet, A.J. & Boldy, S.A.R.). The Geological Society, London. 771–788.
  14. ↑ Bryant, I.D. 1996. The Application of Physical Measurements to Constrain Reservoir-Scale Sequence Stratigraphic Models. In: Howell, J.A. & Aitken, J.F. (eds). High Resolution Sequence Stratigraphy: Innovations and Applications. Geology Society Special Publication 104. 51–64
  15. ↑ Catuneanu, Octavian (2003). Sequence stratigraphy of clastic systems. St. John's Nfld.: Geological Association of Canada. ISBN   0-919216-90-0.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.