A discipline involving the description and interpretation of layered sediments and rocks, and especially their correlation and dating. Correlation is a procedure for determining the relative age of one deposit with respect to another. The term “dating” refers to any technique employed to obtain a numerical age, for example, by making use of the decay of radioactive isotopes found in some minerals in sedimentary rocks or, more commonly, in associated igneous rocks. To a large extent, layered rocks are ones that accumulated through sedimentary processes beneath the sea, within lakes, or by the action of rivers, the wind, or glaciers; but in places such deposits contain significant amounts of volcanic material emplaced as lava flows or as ash ejected from volcanoes during explosive eruptions. See also: Dating methods; Igneous rocks; Rock age determination; Sedimentary rocks
Sedimentary successions are locally many thousands of meters thick owing to subsidence of the Earth’s crust over millions of years. Sedimentary basins therefore provide the best available record of Earth history over nearly 4 billion years. That record includes information about surficial processes and the varying environment at the Earth’s surface, and about climate, changing sea level, the history of life, variations in ocean chemistry, and reversals of the Earth’s magnetic field. Sediments also provide a record of crustal deformation (folding and faulting) and of large-scale horizontal motions of the Earth’s lithospheric plates (continental drift). Stratigraphy applies not only to strata that have remained flat-lying and little altered since their time of deposition, but also to rocks that may have been strongly deformed or recrystallized (metamorphosed) at great depths within the Earth’s crust, and subsequently exposed at the Earth’s surface as a result of uplift and erosion. As long as original depositional layers can be identified, some form of stratigraphy can be undertaken. See also: Basin; Continental drift; Fault and fault structures; Sedimentology
An important idea first articulated by the Danish naturalist Nicolaus Steno in 1669 is that in any succession of strata the oldest layer must have accumulated at the bottom, and successively younger layers above. It is now understood that it is not necessary to rely on the present orientation of layers to determine their relative ages because most sediments and sedimentary rocks contain numerous features, such as current-deposited ripples, minor erosion surfaces, or fossils of organisms in growth position, that have a well-defined polarity with respect to the up direction at the time of deposition (so-called geopetal indicators). This principle of superposition therefore applies equally well to tilted and even overturned strata. Only where a succession is cut by a fault is a simple interpretation of stratigraphic relations not necessarily possible, and in some cases older rocks may overlie younger rocks structurally. See also: Depositional systems and environments
The very existence of layers with well-defined boundaries implies that the sedimentary record is fundamentally discontinuous. This is because the processes that control the production, transport, and accumulation of sediments are themselves markedly episodic. Infrequent events such as landslides, floods, and hurricanes play a disproportionate role at time scales of years to thousands of years. At longer time scales, patterns of sediment accumulation in sedimentary basins are influenced by variations in subsidence and by changes in sea level and climate. Discontinuities are present in the stratigraphic record at a broad range of scales, from that of a single layer or bed to physical surfaces that can be traced laterally for many hundreds of kilometers. Large-scale surfaces of erosion or nondeposition are known as unconformities, and they can be identified on the basis of both physical and paleontological criteria. For example, as first recognized by James Hutton on the Scottish Isle of Arran in 1787, sediments beneath an unconformity may have been tilted and truncated prior to the deposition of overlying sediments, or superposed strata may have accumulated in different and unrelated environments. Alternatively, certain elements in the expected succession of fossils may be missing, indicating the existence of a hiatus or time gap. See also: Paleontology; Unconformity
Most stratal discontinuities possess time-stratigraphic significance because strata below a discontinuity tend to be everywhere older than strata above. To the extent that unconformities can be recognized and traced widely within a sedimentary basin, it is possible to analyze sedimentary rocks in a genetic framework, that is, with reference to the way they accumulated. This is the basis for the modern discipline of sequence stratigraphy, so named because intervals bounded by unconformities have come to be called sequences.
It has long been recognized that the character of sedimentary layers tends to change laterally according to the varying environment in which the sediment accumulated. An important attribute of physical surfaces in sedimentary successions is that they pass laterally through these changes in character or facies. Variations in the location of lateral facies transitions through time lead to the vertical stacking of different facies. In contrast to physical surfaces, vertical facies transitions tend to vary markedly in age from one place to another. For example, at the time of the last glacial maximum some 20,000 years ago, the level of the sea stood approximately 120 m (400 ft) lower than today, and the Atlantic shoreline of New York was located near the present edge of the shelf about 100 km (60 mi) from the modern shoreline. Sea-level rise associated with retreat of the Laurentide ice sheet led to gradual drowning of the then-broader coastal plain, with the exact time of marine inundation, and hence vertical facies transition, varying according to location. See also: Facies (geology)
Traditional stratigraphic analysis has focused on variations in the intrinsic character or properties of sediments and rocks—properties such as composition, texture, and included fossils (lithostratigraphy and biostratigraphy)—and on the lateral tracing of distinctive marker beds such as those composed of ash from a single volcanic eruption (tephrostratigraphy). The techniques of magnetostratigraphy and chemostratigraphy are also based on intrinsic characteristics, although these techniques require sophisticated laboratory analysis. Sequence stratigraphy attempts to integrate these approaches in the context of stratal geometry, thereby providing a unifying framework in which to investigate the time relations between sediment and rock bodies as well as to measure their numerical ages (chronostratigraphy and geochronology). Seismic stratigraphy is a variant of the technique of sequence stratigraphy in which unconformities are identified and traced in seismic reflection profiles on the basis of reflection geometry. See also: Geochronometry
This type of analysis describes sedimentary successions in terms of variations in lithic character. Lithostratigraphic units are the ones that appear on geological maps, and their boundaries are usually defined on the basis of attributes that can be easily and consistently recognized in the field. These boundaries are in general diachronous, that is, of varying age; except in the broadest sense, they are not necessarily helpful in establishing time correlation.
This technique makes use of an observation, initially made by William Smith in England in the late eighteenth and early nineteenth centuries, that strata of different ages contain different fossils. Paleontologists have determined that, to a large extent, variations in fossil faunas and floras are due to evolution. However, the distribution of specific fossils is influenced also by provinciality, environmental factors, and migration; and the first (or last) occurrences of a given fossil in different successions cannot necessarily be interpreted as indicating a single time horizon. Most useful for time correlation are planktonic microfossils that are abundant, rapidly evolving, rapidly dispersed, of wide geographic range, and relatively insensitive to environmental factors. Biostratigraphy is also most useful in the correlation of sediments and sedimentary rocks of Phanerozoic and latest Proterozoic age (younger than about 575 million years). In older rocks, fossils consist predominantly of primitive bacteria, Eukarya, and Archaea lacking shells or skeletons, and biostratigraphic correlation is possible only with very coarse resolution. See also: Fossil; Organic evolution; Proterozoic
This is a method for correlation that makes use of the presence of ash layers in some sedimentary successions. Correlation may be achieved by a combination of pattern matching (observing the varying distribution of ash beds in a vertical succession); analysis of mineralogy, chemistry, and textural characteristics; determination of magnetic polarity; and direct dating by means of isotopic techniques. Tephrostratigraphy is an especially useful approach in areas downwind of volcanoes because volcanic eruptions can be regarded as instantaneous; because ash layers may be distributed widely in nonmarine as well as marine settings, and in pre-Phanerozoic strata in which biostratigraphy is undertaken with difficulty; and because, unlike most sediments, the layers may be amenable to high-resolution geochronology. See also: Volcano
This technique involves the measurement of the remanent magnetization of sediments and sedimentary rocks. This is acquired initially at the time of deposition through the preferential alignment of magnetic grains in the Earth’s field. The orientation of the magnetization depends on the paleolatitude of the sample site, which tends to change on time scales of tens of millions of years as a result of continental drift. At shorter time scales, the polarity of the Earth’s magnetic field reverses, and it is the patterns of reversal in magnetization in a sedimentary succession that are most useful for correlation purposes. See also: Magnetic reversals; Paleomagnetism
One difficulty with magnetostratigraphy is that one or more secondary magnetizations of different orientation may also be acquired during burial and diagenesis. Standard procedure includes an attempt to remove such overprints in a stepwise manner by means of heating to progressively higher temperatures or subjecting the sample to an alternating field of progressively increasing strength. Tests are also available to check the age of individual components of magnetization, for example, according to whether measurements from samples on different limbs of a fold cluster better before or after the effects of folding are removed. In the case of the latter, the magnetization predates folding, the age of which can be determined geologically. Unfortunately, not all sediments are amenable to magnetostratigraphy, even with the most sensitive magnetometers. In that the sedimentary record is itself discontinuous, magnetostratigraphic data are commonly ambiguous, and for the purpose of correlation need to be integrated with other stratigraphic observations. Magnetostratigraphy has proven useful in the dating of cores of deep marine sediments as old as several tens of millions of years, sediments in which high-resolution biostratigraphy and chemostratigraphy are possible. The technique has also been applied in nonmarine sedimentary rocks and in strata as old as the Proterozoic. In the case of such old deposits, the precise age of the observed components of magnetization is commonly difficult to establish with confidence. See also: Deep-marine sediments; Rock magnetism
This involves the measurement of ratios of certain isotopes or the analysis of rare elements and noble metals present in exceedingly small concentrations in sediments and sedimentary rocks. In sediments deposited within the past few tens of millions of years, for example, the concentrations of the isotope oxygen-18 relative to oxygen-16 vary as a function of ocean temperature and the volume of continental ice; such ratios offer a way of achieving very high resolution correlation. On longer time scales, secular changes have also been established in the concentrations of isotopes of strontium, carbon, and sulfur. These have been increasingly useful in improving the resolution of correlation in rocks of both Phanerozoic and Proterozoic age. See also: Isotope
Sequence and seismic stratigraphy
This technique is concerned with the interpretation of sediments and sedimentary rocks in a geometrical context. The gross geometry of unconformities and other stratal surfaces can be determined in outcrop, by reference to geophysical logs in closely spaced boreholes or wells, and in seismic reflection profiles (seismic stratigraphy). The key to seismic stratigraphy is that depositional surfaces are commonly associated with contrasts in acoustic impedance, the product of sediment or rock density, and the velocity of seismic waves traveling through the Earth between a source and receiver at the Earth’s surface. It is this property that determines the extent to which seismic energy is reflected from a given interface. Most reflection events seen on a seismic section are composites of reflections from individual interfaces, that is, they are not strictly reflectors; but in many cases, the configuration of reflections mimics the stratal geometry at the resolution of the seismic data, typically on the order of a few tens of meters. Unconformities are recognized in seismic sections on the basis of the oblique termination of seismic events, indicative of onlap and offlap (the progressive updip termination of strata against an underlying and overlying surface, respectively). By interpreting grids of seismic sections and tracing unconformities to correlative conformities where the hiatus represented is effectively absent, and by calibration against appropriately positioned boreholes or wells, a conventional age may be obtained for each unconformity. See also: Acoustic impedance; Seismology; Well logging
Although the existence of unconformity-bounded depositional units had been recognized previously, it was the development of seismic stratigraphy in the petroleum industry, beginning in the late 1950s, that refocused attention on the pervasive nature of large-scale stratigraphic discontinuities in sediments and sedimentary rocks. A comparatively recent innovation has been to acquire seismic reflection data along exceedingly closely spaced lines. These three-dimensional seismic data permit the mapping of reflection geometry at very high resolution, and in horizontal planes at any depth selected as well as in vertical profiles. See also: Petroleum geology; Seismic stratigraphy
Conventional stratigraphy currently recognizes two kinds of stratigraphic unit: material units, distinguished on the basis of some specified property or properties or physical limits; and temporal or time-related units. A common example of a material unit is the formation, a lithostratigraphic unit defined on the basis of lithic characteristics and position within a stratigraphic succession. Each formation is referred to a section or locality where it is well developed (a type section), and assigned an appropriate geographic name combined with the word formation or a descriptive lithic term such as limestone, sandstone, or shale (for example, Tapeats Sandstone). Some formations are divisible into two or more smaller-scale units called members and beds. In other cases, formations of similar lithic character or related genesis are combined into composite units called groups and supergroups.
Other examples of material units are the biozone, the basic unit of biostratigraphy, defined or characterized by its fossil content (for example, Discoaster multiradiatus Zone); the lithodeme, a body of intrusive, highly deformed or metamorphosed rock that does not conform to the principle of superposition (for example, Manhattan Schist); the polarity zone, a body of sediment or rock unified by specified remanent-magnetic properties (for example, Deer Park Reversed Polarity Zone); and the geosol, a body of sediment or rock composed of one or more soil horizons. The alloformation is a mappable stratiform body of sedimentary rock defined and identified on the basis of its bounding discontinuities. See also: Metamorphism
Temporal stratigraphic units are of two types. The first type, a chronostratigraphic unit, is a body of rock established as the material reference for all rocks formed during the same span of time (for example, Devonian System). Boundaries are synchronous, by definition. Systems are divisible into series, stages, and chronozones, and they are combined into erathems and eonothems (for example, Paleozoic and Phanerozoic, respectively). The second type, a geochronologic unit, is a division of time traditionally distinguished on the basis of the rock record as expressed by chronostratigraphic units (for example, Devonian Period). Periods are divisible into epochs, ages, and chrons, and they are combined into eras and eons, exactly matching the terms for chronostratigraphic units (see table). See also: Geologic time scale
Much of the terminology for temporal stratigraphic units dates back to the nineteenth century. In many cases, the locations originally selected to specify units have proven inferior in terms of completeness, structural simplicity, or degree of exposure to regions elsewhere on Earth where systematic geological studies were not undertaken until more recently. Modern international protocol is to focus on the definition of bases of systems at Global Stratotype Sections and Points (GSSPs or “golden spikes”). For example, the concept of a Cambrian System was established in northern Wales in 1835 by Adam Sedgwick, and named for the Celtic word “Cymru,” meaning Welsh people. By international agreement, the base of the Cambrian is now defined at the first appearance of the trace fossil Treptichnus pedum in an immensely thick section in southeastern Newfoundland. Effort is under way to define a GSSP for the terminal Proterozoic, the first new system to be established and named in more than 100 years.
Sequence stratigraphic terminology
Sequence stratigraphy differs from conventional stratigraphy in two important respects. The first is that basic units (sequences) are defined on the basis of bounding unconformities and correlative conformities rather than material characteristics or age. The second is that sequence stratigraphy is fundamentally not a system for stratigraphic classification (compare alloformation), but a procedure for determining how sediments accumulate. Beginning in the early 1980s, it became clear that the surfaces being recognized as sequence boundaries are associated at least locally with subaerial erosion or nondeposition or with evidence for abrupt shallowing of the marine environment. Indeed, it was the emergence of such evidence that led to the still-popular idea that sequence development is due primarily to fluctuations in sea level. That particular idea is not necessarily correct, particularly for long spans of Earth history lacking evidence for significant continental glaciation, but it has left a lasting legacy in sequence stratigraphic terminology. Also, while sequences came to be regarded as representing cycles of sea-level change, and hence spans of time of global extent, they are specifically not chronostratigraphic units for two reasons. The first is that they are not defined as a reference for geological time. The second is that, as dated at correlative conformities, sequence boundaries are not necessarily globally synchronous or even regionally synchronous. Some unconformity surfaces are demonstrably diachronous, with sediments overlying a surface in some cases older than sediments that elsewhere underlie the same surface. Such relations do not pose a serious problem for time correlation under most circumstances, and they are of considerable interest for the insights they provide about how such surfaces develop. However, the mere existence of such examples means that sequences cannot be regarded in general as chronostratigraphic. For these reasons, sequence stratigraphy currently falls outside the realm of conventional stratigraphic nomenclature.
Every example is different in detail, but commonalities exist in the manner in which sediments are assembled within sequences. An important one is the tendency for seaward and landward shifts of the shoreline (regression and transgression, respectively) to be out of phase with respect to the development of sequence boundaries. This makes it possible to divide many sequences into three stratigraphic elements called systems tracts (see illus.). Three systems tracts are recognized in older literature: highstand, lowstand/shelf margin, and transgressive. The transgressive systems tract is associated with overall transgression of the shoreline, and its top is marked by an interval of sediment starvation in relatively deep water and by maximum flooding of the adjacent basin margin. Highstand and lowstand/shelf margin units are both associated with regression of the shoreline, and they are separated by a sequence boundary. The terminology is derived from the assumption of a sea-level or eustatic origin. In practice, the concept of a systems tract is applicable on purely observational grounds, and specifically does not imply sedimentation at any particular stand of sea level. In at least one example, where eustasy is known to have been important, highstand sedimentation continued very nearly to the eustatic minimum. Lowstand and shelf-margin systems tracts are conventionally distinguished according to whether they overlie type 1 or type 2 sequence boundaries. In practice, a continuum exists between prominent (type 1) and less prominent (type 2) unconformities, and such distinctions are not necessarily very useful. Similarly, doubt exists about the universal applicability or significance of subdivisions of the lowstand systems tract. Many examples are now known in which no lowstand unit is present.
Bilal U. Hag
Conceptual cross sections in relation to (a) depth and (b) geological time showing stratal geometry, the distribution of silliciclastic facies, and standard nomenclature for sequences associated with a shelf-slope break. Systems tracts: SMST, shelf margin; HST, highstand; TST, transgressive; LST, lowstand. Sequence boundaries: sb2, type 2; sb1, type 1. Other abbreviations: iss, interval of sediment starvation; ts, transgressive surface; iv, fluvially incised valley; lsw, lowstand prograding wedge; sf, slope fan; bff, basin floor fan. (After N. Christie-Blick and N. W. Driscoll, Sequence stratigraphy, Annu. Rev. Earth Planet. Sci., 23:451–478, 1995)
N. Christie-Blick and N. W. Driscoll, Sequence stratigraphy, Annu. Rev. Earth Planet. Sci., 23:451–478, 1995
North American Commission on Stratigraphic Nomenclature, North American Stratigraphic Code, Amer. Ass. Petrol. Geologists Bull., 67:841–875, 1987
A. Salvador (ed.), International Stratigraphic Guide, 2d ed., Geological Society ofAmerica, Boulder, 1994
C. J. Allègre, G. Manhès, and C. Göpel, The age of the Earth, Geochim. Cosmochim. Acta, 59:1445–1456, 1995
W. A. Berggren et al., A revised Cenozoic geochronology and chronostratigraphy, in W. A. Berggren et al. (eds.), Geochronology, Time Scales and Global Stratigraphic Correlation, SEPM (Soc. Sediment. Geol.) Spec. Publ., 54:129–212, 1995
J. W. Cowie, W. Ziegler, and J. Remane, Stratigraphic Commission accelerates progress, 1984 to 1989, Episodes, 12:79–83, 1989
K. Davidek et al., New uppermost Cambrian: U-Pb date from Avalonian Wales and the age of the Cambrian-Ordovician boundary, Geol. Mag., 135:303–309, 1998
F. M. Gradstein et al., A Triassic, Jurassic and Cretaceous time scale, in W. A. Berggren et al. (eds.), Geochronology, Time Scales and Global Stratigraphic Correlation, SEPM (Soc. Sediment. Geol.) Spec. Publ., 54:95–126, 1995
J. P. Grotzinger et al., Biostratigraphic and geochronologic constraints on early animal evolution, Science, 270:598–604, 1995
W. B. Harland et al., A Geologic Time Scale 1989, Cambridge University Press, 1990
P. E. Olsen et al., High-resolution stratigraphy of the Newark rift basin (early Mesozoic, eastern North America), Geol. Soc. Amer. Bull., 108:40–77, 1996
E. T. Rasbury et al., U-Pb dates of paleosols: Constraints on late Paleozoic cycle durations and boundary ages, Geology, 26:403–406, 1998
R. D. Tucker et al., New U-Pb zircon ages and the duration and division of Devonian time, Earth Planet. Sci. Lett., 158:175–186, 1998
An Online Guide to Sequence Stratigraphy
Geologic Timechart (British Geological Survey)
Geologic Time Scale (USGS)
Geologic Story at Grand Canyon (National Park Service)
Sedimentary Rocks (University of British Columbia)