Stratigraphy is a branch of geology which studies rock layers and layering. It is primarily
used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two
related subfields: lithologic stratigraphy or lithostratigraphy, and biologic stratigraphy
or biostratigraphy.
Historical development
Nicholas Steno established the theoretical basis for stratigraphy when he introduced
the law of superposition, the principle of original horizontality and the principle of
lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment.
The first practical large-scale application of stratigraphy was by William Smith in the
1790s and early 19th century. Smith, known as the "father of English geology," created
the first geologic map of England and first recognized the significance of strata or rock
layering and the importance of fossil markers for correlating strata. Another influential
application of stratigraphy in the early 19th century was a study by Georges Cuvier and
Alexandre Brongniart of the geology of the region around Paris.
Lithologic stratigraphy
Lithostratigraphy, or lithologic stratigraphy, provides the most obvious visible layering.
It deals with the physical contrasts in lithology, or rock type. Such layers can occur both vertically
- in layering or bedding of varying rock type - and laterally - reflecting changing environments
of deposition, known as facies change. Key elements of stratigraphy involve understanding
how certain geometric relationships between rock layers arise and what these geometries
mean in terms of the depositional environment. Stratigraphers have codified a basic concept
of their discipline in the Law of Superposition, which simply states that, in an undeformed
stratigraphic sequence, the oldest strata occur at the base of the sequence.
Chemostratigraphy studies the changes in the relative proportions of trace elements and
isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with
time, and researchers can use them to map subtle changes in the paleoenvironment. This
has led to the specialized field of isotopic stratigraphy.
Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals,
particularly carbonates and of fossil diversity with time, related to changes in palaeoclimates.
Biostratigraphy
Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers.
Strata from widespread locations containing the same fossil fauna and flora are correlatable
in time. Biologic stratigraphy was based on William Smith's principle of faunal succession,
which predated, and was one of the first and most powerful lines of evidence for, biological
evolution. It provides strong evidence for formation of and the extinction of species.
The geologic time scale was developed during the 19th century, based on the evidence of
biologic stratigraphy and faunal succession. This timescale remained a relative scale until
the development of radiometric dating, which gave it and the stratigraphy it was based
on an absolute time framework, leading to the development of chronostratigraphy.
One important development is the Vail curve, which attempts to define a global historical
sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy
is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir
rocks, seals, and traps in petroleum geology. Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the absolute, not relative, age
of rock strata. The branch is based upon deriving geochronological data for rock units, both
directly and inferentially, so that a sequence of time-relative events of rocks within a
region can be derived. In essence, chronostratigraphy seeks to understand the geologic history of
rocks and regions. The ultimate aim of chronostratigraphy is
to arrange the sequence of deposition and the time of deposition of all rocks within
a geological region and, eventually, the entire geologic record of the Earth.
A gap or lacuna in the geological record may be caused by erosion, in which case it is
called a vacuity, or by non-deposition, where is known as a stratigraphic hiatus. It is
called a hiatus because deposition was on hold for a period of time. A physical lacuna
may represent both a period of non-deposition and a period of erosion. A fault may cause
the appearance of a hiatus. Magnetostratigraphy
Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic
sequences. The method works by collecting oriented samples at measured intervals throughout
a section. The samples are analyzed to determine their detrital remanent magnetism, that is,
the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary
rocks, this is possible because, when very fine-grained magnetic minerals fall through
the water column, they orient themselves with Earth's magnetic field. Upon burial, that
orientation is preserved. The minerals behave like tiny compasses. For volcanic rocks, magnetic
minerals, which form as the melt cools, orient with the ambient magnetic field.
Oriented paleomagnetic core samples are collected in the field; mudstones, siltstones, and very
fine-grained sandstones are the preferred lithologies because the magnetic grains are
finer and more likely to orient with the ambient field during deposition. If the ancient magnetic
field were oriented similar to today's field, the strata would retain a normal polarity.
If the data indicate that the North Magnetic Pole were near the South Rotational Pole,
the strata would exhibit reversed polarity. Results of the individual samples are analyzed
by removing the natural remanent magnetization to reveal the DRM. Following statistical analysis,
the results are used to generate a local magnetostratigraphic column that can then be compared against the
Global Magnetic Polarity Time Scale. This technique is used to date sequences that
generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling
means that it is also a powerful technique for the estimation of sediment-accumulation
rates. Archaeological stratigraphy
In the field of archaeology, soil stratigraphy is used to better understand the processes
that form and protect archaeological sites. The law of superposition holds true, and this
can help date finds or features from each context because they can be placed in sequence
and the dates interpolated. Phases of activity can also often be seen through stratigraphy,
especially when a trench or feature is viewed in section. Because pits and other features
can be dug down into earlier levels, not all material at the same absolute depth is necessarily
of the same age, but close attention has to be paid to the archeological layers. The Harris-matrix
is a tool to depict complex stratigraphic relations when they are found, for example,
in the contexts of urban archaeology. See also
Assise Erygmascope
Harris matrix Important publications in stratigraphy
International Commission on Stratigraphy Key bed
Sedimentary basin analysis Sequence stratigraphy
Sadler effect References
Christopherson, R. W., 2008. Geosystems: An Introduction to Physical Geography, 7th ed.,
New York: Pearson Prentice-Hall. ISBN 978-0-13-600598-8 External links
ICS Subcommission for Stratigraphic Information University of South Carolina Sequence Stratigraphy
Web Front Range stratigraphy
International Commission on Stratigraphy University of Georgia Stratigraphy Lab
Stratigraphy.net A stratigraphic data provider. Agenames.org A global index of stratigraphic
terms