Thegeologic record instratigraphy,paleontology and othernatural sciences refers to the entirety of the layers of rockstrata. That is, deposits laid down byvolcanism or bydeposition of sediment derived fromweathering detritus (clays,sands etc.). This includes all itsfossil content and the information it yields about the history of the Earth: its pastclimate, geography, geology and theevolution of life on its surface. According to thelaw of superposition,sedimentary andvolcanic rock layers are deposited on top of each other. They harden over time to become a solidified (competent) rock column, that may be intruded byigneous rocks and disrupted bytectonic events.
At a certain locality on the Earth's surface, the rock column provides across section of thenatural history in the area during the time covered by the age of the rocks. This is sometimes called therock 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 majorgeologic periods—for the particulargeographic region or regions. The geologic record is in no one place entirely complete[1] for where geologic forces oneage provide a low-lying region accumulatingdeposits much like a layer cake, in the next may have uplifted the region, and the same area is instead one that isweathering 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 newlandforms and features. Sediment core data at the mouths of large riverinedrainage 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 thegeologic time scale using the law of superposition, for wheretectonic forces have uplifted one ridge newly subject toerosion andweathering infolding andfaulting the strata, they have also created a nearby trough orstructural 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.
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 calledbiostratigraphy and utilizes amassedgeobiology andpaleobiological knowledge. Fossils can be used to recognize rock layers ofthe same or differentgeologic ages, thereby coordinating locally occurringgeologic stages to the overallgeologic timeline.
The pictures of the fossils of monocellular algae in thisUSGS figure were taken with a scanning electron microscope and have been magnified 250 times.
In the U.S. state ofSouth Carolina three marker species of fossil algae are found in a core of rock whereas inVirginia only two of the three species are found in theEoceneSeries of rock layers spanning threestages and the geologic ages from 37.2–55.8MA.
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 localrock 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 tocorrelate 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.
| Segments of rock (strata) inchronostratigraphy | Time spans ingeochronology | 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 | 38 defined, tens of millions of years |
| Stage | Age | 101 defined, millions of years |
| Chronozone | Chron | subdivision of an age, 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 spanstage to the largest thickest strataeonothem and time spanseon. 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.
{{citation}}: CS1 maint: multiple names: authors list (link)