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Strata layers laid down in historic times in New Orleans, before becoming compressed by chemistry and geological forces. The relationship between such soft deposits and eventual sedimentary rocks is obvious. This cut was an attempt to find bedrock near a residential street near the lower breach of the London Avenue Canal after restoring the levees which has been plowed/excavated clear by the Army Corp of Engineers, showing a nascent stratigraphy in the large deposits of silt deposited by flooding during various times in recent earth history. |
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The Geologic record in stratigraphy, Chronostratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — depositions laid down in volcanism or by weathering detritus (clays, sands etc.) including 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 (first proposed in the mid-seventeenth century by the Danish naturalist Nicolas Steno) sedimentary and volcanic rocklayers are deposited on top of each other. They harden over time to become a competent rock column, that may occasionally be intruded by igneous rocks— over a particular length of time.
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| “ | Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top. | ” |
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 species of the species are found in the Eocene series (geology) of rock layers spanning three stages and the geologic ages from 37.2–55.8 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 earth scientists and 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. |
At a certain locality on the Earth's surface, the rock column provides a cross section of the natural history of 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 complete1 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. These discontinuities in any given rock column have been seized upon historically by some groups and members of fundamentalist religions to argue against the system, but natural scientists are confident the overall schema is correct and fundamentally sound. 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.
However using broadly occurring deposited layers trapped within differently located rock columns, geologist 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 the land, they have also created a nearby trough (valley) region that lies at a relative lower elevation that can accumulate additional deposits.(see this Grand Canyon region cross-section) By comparing overall formations, geotopology and local strata, calibrated by those layers which are widespread, a nearly complete geologic record has been constructed since the 1600s.
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| Segments of rock (strata) in chronostratigraphy | Periods of time in geochronology | Notes |
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4 total, half a billion years or more |
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12 total, several hundred million years |
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tens of millions of years |
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millions of years |
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smaller than an age/stage, not used by the ICS timescale |
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 (geology) to the largest thickest strata eonothem and time spans eon (geology). 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 (above ).
Notes
- ^ a b Richard Burky, 1990. "An Overview of the Geologic Record". Retrieved on 2008-06-21.
- ^ International Commission on Stratigraphy. "International Stratigraphic Chart". Retrieved on 2008-06-17.
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Wikipedia content modification information:
- This page was last modified on 9 January 2009, at 20:34.
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