In
geology, a diachronism (
Greek ''dia'', "through" + ''chronos'', "time" + ''-ism''), or diachronous deposit, is a
sedimentary rock formation in which the material, although of a similar nature, varies in age with the place where it was deposited.
[Whitten F.G.A & Brooks J.R.V., (1972),''A Dictionary of Geology'', Penguin, ]
Typically this occurs as a result of a
marine transgression
A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...
or regression, or the progressive development of a
delta
Delta commonly refers to:
* Delta (letter) (Δ or δ), a letter of the Greek alphabet
* River delta, at a river mouth
* D ( NATO phonetic alphabet: "Delta")
* Delta Air Lines, US
* Delta variant of SARS-CoV-2 that causes COVID-19
Delta may also ...
. As the shoreline advances or retreats, a succession of continuous deposits representing different environments (for example beach, shallow water, deeper water) may be left behind. Although each type of deposit (
facies) may be continuous over a wide area, its age varies according to the position of the shoreline through time.
An example is the sandy beds near the end of the lower
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
of the west of England (the ''Drybrook sandstone'' of the
Forest of Dean). Deposition of this began much later in the
Bristol
Bristol () is a city, ceremonial county and unitary authority in England. Situated on the River Avon, it is bordered by the ceremonial counties of Gloucestershire to the north and Somerset to the south. Bristol is the most populous city in ...
area than further north.
The detection of diachronous beds can be quite problematic since fossil assemblages tend to migrate geographically with their environment of formation. They are generally revealed by the presence of marker species, fossils which can be dated reliably from other beds.
Other usages
The term may also be applied to other features that vary in age, such as
erosion surfaces, areas of uplift, etc. It is also sometimes applied to fossils which appear sporadically at different times in different places due to migration, though such usage is regarded by some authors as incorrect.
In academic librarianship, the adjectival form, diachronous, is used in the context of "diachronous obsolescence" to describe the reduction of usefulness of a book or journal volume over several years.
[Line, Maurice B. "Changes in the use of literature with time - obsolescence revisted". Library Trends 41(4), Spring 1993.]
References
Sedimentary rocks
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