چینه شناسی-Stratigraphy
Stratigraphy
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
Basic
principles
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
Lithostratigraphy
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.
Biostratigraphy
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
Tephrostratigraphy
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
Magnetostratigraphy
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
Chemostratigraphy
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
Stratigraphic
nomenclature
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.
Nicholas Christie-Blick
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)
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An Online Guide to
Sequence Stratigraphy
Geologic Timechart
(British Geological Survey)
Geologic Time Scale (USGS)
Geologic Story at
Sedimentary Rocks
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