Stratum

Ingeologyand related fields, astratum(pl.:strata) is a layer ofrockorsedimentcharacterized by certainlithologicproperties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as eitherbedding surfacesorbedding planes.^[1]Prior to the publication of the International Stratigraphic Guide,^[1]older publications have defined a stratum as being either equivalent to a singlebedor composed of a number of beds; as a layer greater than 1 cm in thickness and constituting a part of a bed; or a general term that includes bothbedandlamina.^[2]Related terms aresubstrateandsubstratum(pl.substrata), a stratum underlying another stratum.

Goldenville strata in quarry inBedford,Canada. These areMiddle Cambrianmarine sediments. This formation covers over half ofNova Scotiaand is recorded as being 8,800 m (29,000 ft) thick in some areas.

Characteristics

ThePermianthroughJurassicstrata in theColorado Plateauarea of southeasternUtahdemonstrate the principles ofstratigraphy.These strata make up much of the famous prominent rock formations in widely spaced protected areas such asCapitol Reef National ParkandCanyonlands National Park.From top to bottom: Rounded tan domes of theNavajo Sandstone,layered redKayenta Formation,cliff-forming, vertically jointed, redWingate Sandstone,slope-forming, purplishChinle Formation,layered, lighter-redMoenkopi Formation,and white, layeredCutler Formationsandstone. Picture fromGlen Canyon National Recreation Area,Utah.

Typically, a stratum is generally one of a number of parallel layers that lie one upon another to form enormousthicknessesof strata.^[1]The bedding surfaces (bedding planes) that separate strata represent episodic breaks in deposition associated either with periodicerosion,cessation of deposition, or some combination of the two.^[3]^[4]Stacked together with other strata, individual stratum can form compositestratigraphic unitsthat can extend over hundreds of thousands of square kilometers of theEarth's surface. Individual stratum can cover similarly large areas. Strata are typically seen as bands of different colored or differently structured material exposed incliffs,road cuts,quarries,andriverbanks. Individual bands may vary in thickness from a few millimeters to several meters or more. A band may represent a specific mode ofdeposition:riversilt,beachsand,coalswamp,sand dune,lavabed, etc.

Types of stratum

In the study of rock and sediment strata, geologists have recognized a number of different types of strata, includingbed,flow,band,andkey bed.^[1]^[5]A bed is a single stratum that is lithologically distinguishable from other layers above and below it. In the classification hierarchy of sedimentary lithostratigraphic units, a bed is the smallest formal unit. However, only beds that are distinctive enough to be useful for stratigraphic correlation and geologic mapping are customarily given formal names and considered formal lithostratigraphic units. The volcanic equivalent of a bed, a flow, is a discrete extrusive volcanic stratum or body distinguishable by texture, composition, or other objective criteria. As in case of a bed, a flow should only be designated and named as a formal lithostratigraphic unit when it is distinctive, widespread, and useful for stratigraphic correlation. A band is a thin stratum that is distinguishable by a distinctive lithology or color and is useful in correlating strata. Finally, a key bed, also called amarker bed,is a well-defined, easily identifiable stratum or body of strata that has sufficiently distinctive characteristics, such as lithology or fossil content, to be recognized and correlated during geologic field or subsurface mapping.^[1]^[5]

Gallery

Strata on a mountain face in the French Alps
Interstate road cut throughlimestoneandshalestrata in EastTennessee
Rock strata atDepot Beach,New South Wales
Rainbow Basin Syncline in theBarstow FormationnearBarstow, California.Folded strata.
Outcrop of Upper Ordovician limestone and minor shale, central Tennessee
Chalk Layers inCyprusshowing classic layered structure
Heavy minerals (dark) as thin strata in a quartz beach sand (Chennai,India)
Stratified Island nearLa Paz, Baja California Sur,Mexico

References

^^a ^b ^c ^d ^eSalvador, A. ed., 1994.International stratigraphic guide: a guide to stratigraphic classification, terminology, and procedure. 2nd ed.Boulder, Colorado, The Geological Society of America, Inc., 215 pp.ISBN 978-0-8137-5216-7.
^Neuendorf, K.K.E., Mehl, Jr., J.P., and Jackson, J.A., eds., 2005.Glossary of Geology5th ed. Alexandria, Virginia, American Geological Institute. 779 pp.ISBN 0-922152-76-4.
^Davies, N.S., and Shillito, A.P. 2021,True substrates: the exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces.Sedimentology.published online 22 May 2021, doi: 10.1111/sed.12900.
^Davies, N.S., and Shillito, A.P. 2018,Incomplete but intricately detailed: the inevitable preservation of true substrates in a time-deficient stratigraphic record.Geology,46, 679–682.
^^a ^bMurphy, MA., and Salvador, A., 1999.International stratigraphic guide—an abridged version.Episodes,22(4), pp.255-272.

External links

GeoWhen Database

[Salvador1994a-1] Salvador, A. ed., 1994.International stratigraphic guide: a guide to stratigraphic classification, terminology, and procedure. 2nd ed.Boulder, Colorado, The Geological Society of America, Inc., 215 pp.ISBN 978-0-8137-5216-7.

[NeuendorfOthers2005a-2] Neuendorf, K.K.E., Mehl, Jr., J.P., and Jackson, J.A., eds., 2005.Glossary of Geology5th ed. Alexandria, Virginia, American Geological Institute. 779 pp.ISBN 0-922152-76-4.

[DaviesOthers2021a-3] Davies, N.S., and Shillito, A.P. 2021,True substrates: the exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces.Sedimentology.published online 22 May 2021, doi: 10.1111/sed.12900.

[DaviesOthers2018a-4] Davies, N.S., and Shillito, A.P. 2018,Incomplete but intricately detailed: the inevitable preservation of true substrates in a time-deficient stratigraphic record.Geology,46, 679–682.

[MurphyOthers1999a-5] Murphy, MA., and Salvador, A., 1999.International stratigraphic guide—an abridged version.Episodes,22(4), pp.255-272.

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