Stratigraphy, a branch of geology, studies rock layers and layering (stratification). Geology (from Greek γη gê, "earth" and λόγος Logos, "speech" lit In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes It is primarily used in the study of sedimentary and layered volcanic rocks. Sedimentary rock is one of the three main rock types (the others being igneous and Metamorphic rock) Volcanic rock is an Igneous rock of volcanic origin Texture Volcanic rocks are usually fine-grained or Aphanitic to glassy in Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy. Lithostratigraphy is a sub-dicipline of Stratigraphy, the geological Science associated with the study of strata or rock layers Biostratigraphy is the branch of Stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the Fossil assemblages contained

Historical development

Engraving from William Smith's monograph on identifying strata based on fossils

The theoretical basis for the subject was established by Nicholas Steno who 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. Nicolas Steno ( Danish: Niels Stensen; Latinized to Nicolaus Stenonis) ( January 10, 1638 - November 25, The law of superposition (or the principle of superposition) is a key axiom based on observations of Natural history that is a foundational principle of sedimentary The Principle of Original Horizontality was proposed by the Danish geological pioneer Nicholas Steno (1638-1686 The principle of lateral continuity states that layers of Sediment initially extend laterally in all directions in other words they are laterally continuous

The first practical large scale application of stratigraphy was by William Smith in the 1790s and early 1800s. William Smith ( March 23 1769 &ndash August 28 1839) was an English Geologist, credited with creating the first nationwide 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. A geologic map or geological map is a special-purpose Map made to show Geological features In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes Another influential application of stratigraphy in the early 1800s was a study by Georges Cuvier and Alexandre Brongniart of the geology of the region around Paris. Baron Georges Léopold Chrétien Frédéric Dagobert Cuvier ( August 23 1769 &ndash May 13, 1832) was a French naturalist Alexandre Brongniart (1770 &ndash 1847 was a French Chemist, Mineralogist, and Zoologist, who collaborated with Georges Cuvier on

Lithologic stratigraphy

Main article: Lithostratigraphy

Lithostratigraphy, or lithologic stratigraphy, is the most obvious. Lithostratigraphy is a sub-dicipline of Stratigraphy, the geological Science associated with the study of strata or rock layers It deals with the physical lithologic or rock type change both vertically in layering or bedding of varying rock type and laterally reflecting changing environments of deposition, known as facies change. In Geology, petrology (from Greek πέτρα petra, rock and λόγος logos, knowledge is the study of rocks and the conditions on which The term " Facies " can also refer to distinctive Facial expressions associated with conditions such as Williams syndrome. Key elements of stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries mean in terms of depositional environment. One of stratigraphy's basic concepts is codified in the Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. The law of superposition (or the principle of superposition) is a key axiom based on observations of Natural history that is a foundational principle of sedimentary

Chalk Layers in Cyprus - showing classic layered structure

Chemostratigraphy is based on the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides Carbon and oxygen isotope ratios vary with time and are used to map subtle changes in the paleoenvironment This has led to the specialized field of isotopic stratigraphy. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals, particularly carbonates, and fossil diversity with time, related to changes in palaeoclimates. A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition a highly ordered atomic structure and specific In Chemistry, a carbonate is a salt or Ester of Carbonic acid. Paleoclimatology (also Palaeoclimatology) is the study of Climate change taken on the scale of the entire History of Earth.

Biostratigraphy

Main article: Biostratigraphy

Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Biostratigraphy is the branch of Stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the Fossil assemblages contained Palaeontology redirects here For the Scientific journal, see Palaeontology (journal. FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. 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. The principle of faunal succession is based on the observation that sedimentary rock strata contain Fossilised flora and fauna, and that these eVolution is the third Album by eLDee, it was due to be released in 2008 It provides strong evidence for formation (speciation) of and the extinction of species. Speciation is the Evolutionary process by which new biological Species arise In Biology and Ecology, extinction is the cessation of existence of a Species or group of taxa. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. The geologic time scale was developed during the 1800s based on the evidence of biologic stratigraphy and faunal succession. The geologic time scale is a chronologic schema (or idealized Model) relating Stratigraphy to time that is used by Geologists and other 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. Radiometric dating (often called radioactive dating) is a technique used to date materials usually based on a comparison between the observed abundance of a naturally occurring

One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from world-wide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals and traps in petroleum geology. In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon. Petroleum geology refers to the specific set of geological disciplines that are applied to the search for Hydrocarbons ( Oil exploration)

Chronostratigraphy

Main article: Chronostratigraphy

Chronostratigraphy is the branch of stratigraphy that studies the absolute age of rock strata. Chronostratigraphy is the branch of Stratigraphy that studies the age of rock strata in relation to Time. In Geology and related fields a stratum (plural strata) is a layer of rock or Soil with internally consistent characteristics that distinguishes

Chronostratigraphy is based upon deriving geochronological data for rock units, both directly and by inference, so that a sequence of time relative events of rocks within a region can be derived. In the Natural sciences under the umbrella of Natural history, Geochronology is the Science of determining the absolute age of rocks, Fossils 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.

Magnetostratigraphy

When measurable magnetic properties of rocks vary stratigraphically they may be the bases for related but different kinds of stratigraphic units known collectively as "magnetostratigraphic units" ("magnetozones"). The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. Such reversals of the polarity have taken place many times during geologic history. They are recorded in the rocks because the rocks may record the direction of the Earth's magnetic field at or near the time of rock formation (see paleomagnetism). Paleomagnetism is the study of the record of the Earth's magnetic field preserved in various Magnetic Minerals through time The direction of the remnant magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called "magnetostratigraphic polarity units".

Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic stratigraphic sections. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their Detrital Remanent Magnetization (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. This is possible because when very fine-grained magnetic minerals (< 17 micrometres) fall through the water column, they orient themselves with Earth's magnetic field. Upon burial, that orientation is preserved. The minerals, in effect, behave like tiny compasses.

If the ancient magnetic field was oriented similar to today's field (North Magnetic Pole near the North Rotational Pole) the strata retain a Normal Polarity. If the data indicate that the North Magnetic Pole was near the South Rotational Pole, the strata exhibit Reversed Polarity.

Sampling procedures

Oriented paleomagnetic core samples are collected in the field using a Pomeroy Drill. A minimum of three samples is taken from each sample site for statistical purposes. Spacing of the sample sites within a stratigraphic section depends on: 1) the type of depositional environment: The farther away from the orogenic front, the closer the sample spacing due to generally lower rates of deposition; and 2) the suitability of the rocks for paleomagnetic analysis. 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. It is more likely that these samples will deliver a reliable paleomagnetic signal.

Analytical procedures

Samples are first analyzed in their natural state to obtain their Natural Remanent Magnetization (NRM). Natural Remanent Magnetization (Abbreviated NRM is the permanent magnetism of a rock The NRM is then stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component. The stable component is usually interpreted to be the DRM.

DRM orientations of all samples from a site are then compared and their magnetic polarity is determined with Fisher statistics. Using Watson's criteria, the statistical significance of each sample site is evaluated. The latitudes of the Virtual Geomagnetic Poles from those sites determined to be statistically significant are plotted against the stratigraphic level at which they were collected. These data are then abstracted to the standard black and white magnetostratigraphic columns in which black indicates Normal polarity and white is Reversed polarity.

Correlation and ages

Because the polarity of a stratum can only be Normal or Reversed, variations in the rate at which the sediment accumulated can cause the thickness of a given polarity zone to vary from one area to another. This presents the problem of how to differentiate different zones of like polarities between different stratigraphic sections. To overcome the possibility of confusion at least one isotopic age (or at least an age based on paleontological data) needs to be collected from each section. These are usually obtained from intercalated airfall volcanic material. With the aid of the independent isotopic age or ages, the local magnetostratigraphic column is correlated with the Global Magnetic Polarity Time Scale (GMPTS).

Because the age of each reversal shown on the GMPTS is relatively well known, the correlation establishes numerous time lines through the stratigraphic section. These ages provide relatively precise dates for features in the rocks such as fossils, changes in sedimentary rock composition, changes in depositional environment, etc. They also constrain the ages of cross-cutting features such as faults, dikes, and unconformities.

Sediment accumulation rates

Perhaps the most powerful application of these data is to determine the rate at which the sediment accumulated. This is accomplished by plotting the age of each reversal (in millions of years ago) vs. the stratigraphic level at which the reversal is found (in meters). This provides the rate in meters per million years which is usually rewritten in terms of millimeters per year (which is the same as kilometers per million years).

These data are also used to model basin subsidence rates. Knowing the depth of a hydrocarbon source rock beneath the basin-filling strata allows calculation of the age at which the source rock passed through the generation window and hydrocarbon migration began. Because the ages of cross-cutting trapping structures can usually be determined from magnetostratigraphic data, a comparison of these ages will assist reservoir geologists in their determination of whether or not a play is likely in a given trap.

Another application of these results derives from the fact that they illustrate when sediment accumulation rates changed. Such changes require explanation. The answer is often related to either climatic factors or to tectonic developments in nearby or distant mountain ranges. Evidence to strengthen this interpretation can often be found by looking for subtle changes in the composition of the rocks in the section. Changes in sandstone composition are often used for this type of interpretation.

Archaeological stratigraphy

Main article: Archaeological stratigraphy

In the field of archaeology, soil stratigraphy is used to better understand the processes that form and protect archaeological sites. In Archaeology, especially in the course of Excavation, stratification is a paramount and base concept Archaeology, archeology, or archæology (from Greek grc ἀρχαιολογία archaiologia – grc ἀρχαῖος archaīos An archaeological site is a place (or group of physical sites in which evidence of past activity is preserved (either Prehistoric or historic or contemporary and The law of superposition holds true, and this can help date finds or features from each context, as they can be placed in sequence and the dates interpolated. Archaeology, archeology, or archæology (from Greek grc ἀρχαιολογία archaiologia – grc ἀρχαῖος archaīos Phases of activity can also often be seen through stratigraphy, especially when a trench or feature is viewed in section (profile). In Archaeology a section is a view in part of the archaeological sequence showing it in the vertical plane as a cross section, and thereby illustrating As 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, as they are found, for example, in the contexts of urban archaeology. The Harris matrix or Winchester seriation diagram is a tool used to depict the temporal succession of Archaeological contexts and thus the sequence of deposition on Urban archaeology is a sub discipline of archaeology specialising in the material past of Towns and Cities where long-term human habitation has often left a rich record

See also

• Important publications in stratigraphy
• Key bed
• Sequence stratigraphy
• Sedimentary basin analysis
• Harris matrix

External links

• University of South Carolina Sequence Stratigraphy Web
• Front Range stratigraphy
• International Commission on Stratigraphy
• University of Georgia (USA) Stratigraphy Lab