Geologic record

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The layer cake like appearance of concordant strata laid down over hundreds of thousands of years (in northern Spain) Concordant.jpg
The layer cake like appearance of concordant strata laid down over hundreds of thousands of years (in northern Spain)
Strata turned sideways by orogenic forces near San Sebastian, Spain End of cobbles.jpg
Strata turned sideways by orogenic forces near San Sebastián, Spain

The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sands etc.). This includes all its fossil content and the information it yields about the history of the Earth: its past climate, geography, geology and the evolution of life on its surface. According to the law of superposition, sedimentary and volcanic rock layers are deposited on top of each other. They harden over time to become a solidified (competent) rock column, that may be intruded by igneous rocks and disrupted by tectonic events.

Contents

Correlating the rock record

Horseshoe Canyon Formations exposed in Horseshoe Canyon near Drumheller, Alberta Horseshoe Canyon Alberta Nov 1988.jpg
Horseshoe Canyon Formations exposed in Horseshoe Canyon near Drumheller, Alberta

At a certain locality on the Earth's surface, the rock column provides a cross section of the natural history in the area during the time covered by the age of the rocks. This is sometimes called the rock history and gives a window into the natural history of the location that spans many geological time units such as ages, epochs, or in some cases even multiple major geologic periods for the particular geographic region or regions. The geologic record is in no one place entirely complete [1] for where geologic forces one age provide a low-lying region accumulating deposits much like a layer cake, in the next may have uplifted the region, and the same area is instead one that is weathering and being torn down by chemistry, wind, temperature, and water. This is to say that in a given location, the geologic record can be and is quite often interrupted as the ancient local environment was converted by geological forces into new landforms and features. Sediment core data at the mouths of large riverine drainage basins, some of which go 7 miles (11 km) deep thoroughly support the law of superposition.[ clarification needed ]

However using broadly occurring deposited layers trapped within differently located rock columns, geologists have pieced together a system of units covering most of the geologic time scale using the law of superposition, for where tectonic forces have uplifted one ridge newly subject to erosion and weathering in folding and faulting the strata, they have also created a nearby trough or structural basin region that lies at a relative lower elevation that can accumulate additional deposits. By comparing overall formations, geologic structures and local strata, calibrated by those layers which are widespread, a nearly complete geologic record has been constructed since the 17th century.

Discordant strata example

USGS correlation diagram Biostratigraphie usgs.png
USGS correlation diagram

Correcting for discordancies can be done in a number of ways and utilizing a number of technologies or field research results from studies in other disciplines.

In this example, the study of layered rocks and the fossils they contain is called biostratigraphy and utilizes amassed geobiology and paleobiological knowledge. Fossils can be used to recognize rock layers of the same or different geologic ages , thereby coordinating locally occurring geologic stages to the overall geologic timeline.

The pictures of the fossils of monocellular algae in this USGS figure were taken with a scanning electron microscope and have been magnified 250 times.

In the U.S. state of South Carolina three marker species of fossil algae are found in a core of rock whereas in Virginia only two of the three species are found in the Eocene Series of rock layers spanning three stages and the geologic ages from 37.255.8 MA.

Comparing the record about the discordance in the record to the full rock column shows the non-occurrence of the missing species and that portion of the local rock record, from the early part of the middle Eocene is missing there. This is one form of discordancy and the means geologists use to compensate for local variations in the rock record. With the two remaining marker species it is possible to correlate rock layers of the same age (early Eocene and latter part of the middle Eocene) in both South Carolina and Virginia, and thereby "calibrate" the local rock column into its proper place in the overall geologic record.

Units in geochronology and stratigraphy [2]
Segments of rock (strata) in chronostratigraphy Time spans in geochronology Notes to
geochronological units
Eonothem Eon 4 total, half a billion years or more
Erathem Era 10 defined, several hundred million years
System Period 22 defined, tens to ~one hundred million years
Series Epoch 34 defined, tens of millions of years
Stage Age 99 defined, millions of years
Chronozone Chron subdivision of an age, not used by the ICS timescale

Lithology vs paleontology

Consequently, as the picture of the overall rock record emerged, and discontinuities and similarities in one place were cross-correlated to those in others, it became useful to subdivide the overall geologic record into a series of component sub-sections representing different sized groups of layers within known geologic time, from the shortest time span stage to the largest thickest strata eonothem and time spans eon. Concurrent work in other natural science fields required a time continuum be defined, and earth scientists decided to coordinate the system of rock layers and their identification criteria with that of the geologic time scale. This gives the pairing between the physical layers of the left column and the time units of the center column in the table at right.

Related Research Articles

<span class="mw-page-title-main">Geology</span> Scientific study of the composition, structure, and history of Earth

Geology is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth sciences, including hydrology. It is integrated with Earth system science and planetary science.

<span class="mw-page-title-main">Geologic time scale</span> System that relates geologic strata to time

The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy and geochronology. It is used primarily by Earth scientists to describe the timing and relationships of events in geologic history. The time scale has been developed through the study of rock layers and the observation of their relationships and identifying features such as lithologies, paleomagnetic properties, and fossils. The definition of standardised international units of geologic time is the responsibility of the International Commission on Stratigraphy (ICS), a constituent body of the International Union of Geological Sciences (IUGS), whose primary objective is to precisely define global chronostratigraphic units of the International Chronostratigraphic Chart (ICC) that are used to define divisions of geologic time. The chronostratigraphic divisions are in turn used to define geochronologic units.

<span class="mw-page-title-main">Sedimentary rock</span> Rock formed by the deposition and cementation of particles

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.

<span class="mw-page-title-main">Stratigraphy</span> Study of rock layers and their formation

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostratigraphy, biostratigraphy, and chronostratigraphy.

<span class="mw-page-title-main">Geochronology</span> Science of determining the age of rocks, sediments and fossils

Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as paleomagnetism and stable isotope ratios. By combining multiple geochronological indicators the precision of the recovered age can be improved.

<span class="mw-page-title-main">Historical geology</span> Study of the geological history of Earth

Historical geology or palaeogeology is a discipline that uses the principles and methods of geology to reconstruct the geological history of Earth. Historical geology examines the vastness of geologic time, measured in billions of years, and investigates changes in the Earth, gradual and sudden, over this deep time. It focuses on geological processes, such as plate tectonics, that have changed the Earth's surface and subsurface over time and the use of methods including stratigraphy, structural geology, paleontology, and sedimentology to tell the sequence of these events. It also focuses on the evolution of life during different time periods in the geologic time scale.

<span class="mw-page-title-main">Unconformity</span> Rock surface indicating a gap in the geological record

An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record. The significance of angular unconformity was shown by James Hutton, who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland.

<span class="mw-page-title-main">Law of superposition</span> In undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence

The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time. This is paramount to stratigraphic dating, which requires a set of assumptions, including that the law of superposition holds true and that an object cannot be older than the materials of which it is composed. To illustrate the practical applications of superposition in scientific inquiry, sedimentary rock that has not been deformed by more than 90° will exhibit the oldest layers on the bottom, thus enabling paleontologists and paleobotanists to identify the relative ages of any fossils found within the strata, with the remains of the most archaic lifeforms confined to the lowest. These findings can inform the community on the fossil record covering the relevant strata, to determine which species coexisted temporally and which species existed successively in perhaps an evolutionarily or phylogenetically relevant way.

<span class="mw-page-title-main">Biostratigraphy</span> Stratigraphy which assigns ages of rock strata by using fossils

Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. The primary objective of biostratigraphy is correlation, demonstrating that a particular horizon in one geological section represents the same period of time as another horizon at a different section. Fossils within these strata are useful because sediments of the same age can look completely different, due to local variations in the sedimentary environment. For example, one section might have been made up of clays and marls, while another has more chalky limestones. However, if the fossil species recorded are similar, the two sediments are likely to have been laid down around the same time. Ideally these fossils are used to help identify biozones, as they make up the basic biostratigraphy units, and define geological time periods based upon the fossil species found within each section.

<span class="mw-page-title-main">Geological formation</span> Fundamental unit of lithostratigraphy

A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region. It is the fundamental unit of lithostratigraphy, the study of strata or rock layers.

<span class="mw-page-title-main">Lithostratigraphy</span> Sub-discipline of stratigraphy

Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology.

<span class="mw-page-title-main">Relative dating</span> Determining the relative order of past events

Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.

Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time.

<span class="mw-page-title-main">Bed (geology)</span> Layer of sediment, sedimentary rock, or pyroclastic material

In geology, a bed is a layer of sediment, sedimentary rock, or volcanic rock "bounded above and below by more or less well-defined bedding surfaces". Specifically in sedimentology, a bed can be defined in one of two major ways. First, Campbell and Reineck and Singh use the term bed to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are small beds that constitute the smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

<span class="mw-page-title-main">Eonothem</span> Totality of rock strata laid down during a certain eon of the geologic timescale

In stratigraphy and geology, an eonothem is the totality of rock strata laid down in the stratigraphic record deposited during a certain eon of the continuous geologic timescale. The eonothem is not to be confused with the eon itself, which is a corresponding division of geologic time spanning a specific number of years, during which rocks were formed that are classified within the eonothem. Eonothems have the same names as their corresponding eons, which means during the history of the Earth only four eonothems were formed. Oldest to newest these are the Hadean, Archean, Proterozoic, and Phanerozoic. A rock stratum, fossil or feature present in the "upper Phanerozoic" eonothem would therefore have originated within the "later Phanerozoic" eon. In practice, the rock column is discontinuous:

Technically, a complete geologic record doesn't occur anywhere. For such a record to develop would require the area to have been receiving sedimentary deposits continually ever since the origin of the earth. Nowhere is such a situation known to exist. If it did exist, we could not effectively look at the strata because they would still be buried, and modern strata would continue to be deposited on top of them. The earth's surface has been far too dynamic to allow that to occur anywhere. No area has been in such a static condition throughout the earth's long history. Areas that have had sediment deposited on them at one time are later uplifted and eroded. In some places this has occurred many times. There is ample evidence to prove such a sequence of events.

In stratigraphy, paleontology, geology, and geobiology, an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their characteristic remanent magnetization (ChRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. This is possible because volcanic flows acquire a thermoremanent magnetization and sediments acquire a depositional remanent magnetization, both of which reflect the direction of the Earth's field at the time of formation. This technique is typically used to date sequences that generally lack fossils or interbedded igneous rock. It is particularly useful in high-resolution correlation of deep marine stratigraphy where it allowed the validation of the Vine–Matthews–Morley hypothesis related to the theory of plate tectonics.

<span class="mw-page-title-main">Stratigraphic column</span>

A stratigraphic column is a representation used in geology and its subfield of stratigraphy to describe the vertical location of rock units in a particular area. A typical stratigraphic column shows a sequence of sedimentary rocks, with the oldest rocks on the bottom and the youngest on top.

References

  1. Committee on the Geologic Record of Biosphere Dynamics, Board on Earth Sciences and Resources, Board on Life Sciences, Division on Earth and Life Studies, National Research Council (2005), The Geological Record of Ecological Dynamics: Understanding the Biotic Effects of Future Environmental Change, National Academies Press, p. 14, ISBN   9780309095808 {{citation}}: CS1 maint: multiple names: authors list (link)
  2. Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy.