In geology and related fields, a stratum (: strata) is a layer of rock or sediment characterized by certain Lithology properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as either bedding surfaces or bedding planes.[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. .] Prior to the publication of the International Stratigraphic Guide,[Neuendorf, K.K.E., Mehl, Jr., J.P., and Jackson, J.A. , eds., 2005. Glossary of Geology 5th ed. Alexandria, Virginia, American Geological Institute. 779 pp. .] Related terms are substrate and substratum (pl. substrata), a stratum underlying another stratum.
Characteristics
Typically, a stratum is generally one of a number of parallel layers that lie one upon another to form enormous thicknesses of strata.
The bedding surfaces (bedding planes) that separate strata represent episodic breaks in deposition associated either with periodic
erosion, cessation of deposition, or some combination of the two.
Stacked together with other strata, individual stratum can form composite
stratigraphy that can extend over hundreds of thousands of square kilometers of the
Earth's surface. Individual strata can cover similarly large areas. Strata are typically seen as bands of different colored or differently structured material exposed in
, road cuts,
quarry, and
river banks. Individual bands may vary in thickness from a few millimeters to several meters or more. A band may represent a specific mode of deposition: river
silt, beach
sand, coal
swamp,
sand dune,
lava bed, and others.
Types
In the study of rock and sediment strata, geologists have recognized a number of different types of strata, including
bed,
flow,
band, and
key bed.
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 geological 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 a
marker 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.
Gallery
File:Strata-french-alps.jpg|Strata on a mountain face in the French Alps
File:Rockstrata3435.JPG|Interstate road cut through limestone and shale strata in East Tennessee
File:Rock_Strata.jpg|Rock strata at Depot Beach, New South Wales
File:Rainbow_Basin.JPG|Rainbow Basin Syncline in the Barstow Formation near Barstow, California. Folded strata.
File:OrdOutcropTN.JPG|Outcrop of Upper Ordovician limestone and minor shale, central Tennessee
File:Geology_of_Cyprus-Chalk.jpg|Chalk Layers in Cyprus showing classic layered structure
File:HeavyMineralsBeachSand.jpg|Heavy minerals (dark) as thin strata in a quartz beach sand (Chennai, India)
File:Stratified Island near La Paz, Baja California Sur, Mexico.jpg|Stratified Island near La Paz, Baja California Sur, Mexico
See also
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Archaeological horizon
-
Bed (geology)
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Geological formation
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Geologic unit
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Lamination (geology)
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Law of superposition
External links
