Sequence stratigraphy
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]
Stratigraphers explain sequence boundaries and
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.
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]
- A lowstand systems tract (LST) forms when the rate of sedimentation outpaces the rate of sea level rise during the early stage of the sea level curve. It is bounded by a subaerial unconformity or its correlative conformity at the base and maximum regressive surface at the top.
- A transgressive systems tract (TST) is bounded by maximum regressive surface at the base and maximum flooding surface at the top. This systems tracts forms when the rate of sedimentation is outpaced by the rate of sea level rise in the sea level curves.
- A highstand systems tract (HST) occurs during the late stage of base level rise when the rate of sea level rise drops below the sedimentation rate. In this period of sea level highstand is formed. It is bounded by maximum flooding surface at the base and composite surface at the top.
- Regressive systems tract forms in the marine part of the basin during the base level fall. Subaerial unconformities form in the landward side of the basin at the same time.
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
Sea level changes over
Today, sea level is at a relative "high stand" within the
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
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 changes
- Subsidence rate of the basin
- Sediment supply.
Smaller and localised sedimentary cycles are not related to worldwide (eustatic) sea level changes but more to the supply of sediment to the adjacent
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
See also
- Cratonic sequence – Very large-scale lithostratographic sequence
- Cyclostratigraphy – Study of astronomically forced climate cycles within sedimentary successions
- Relative sea level
References
- ISBN 978-0-08-088513-1.
- ISBN 978-0-632-03706-3.
- ^ a b c 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.
- ^ a b c 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.
- ^ "An Online Guide to Sequence Stratigraphy". strata.uga.edu. Archived from the original on 2022-04-01. Retrieved 2018-08-05.
- .
- ^ Sloss, L.L. (1950). "Paleozoic stratigraphy in the Montana area". American Association of Petroleum Geologists Bulletin. 34: 425–451.
- .
- doi:10.1306/M26490C5.
- ISBN 978-1-944966-02-7.
- ISBN 0-89181-316-0.
- .
- ^ 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.
- ^ 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
- ISBN 0-919216-90-0.
Further reading
- Coe, Angela L. (2002). The Sedimentary Record of Sea-Level Change. Cambridge, New York: Cambridge University Press. pp. 57–98. ISBN 0-521-53842-4.
External links
- Sequence Stratigraphy authoritative online encyclopedia from SEPM, a scientific society whose publications have been central to defining sequence stratigraphy.
- An Online Guide to Sequence Stratigraphy by the University of Georgia's Stratigraphy Lab.
- USC's Sequence Stratigraphy Web a fairly extensive online education resource
- A chart of sea level for the past 140,000 years (The different orders of cyclicity can be seen as higher frequency chatter on an overall asymmetric cycle. Today's date is on the right side of this chart.)