Stratum
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8,800 m (29,000 ft) thick in some areas.
In geology and related fields, a stratum (plural: strata) is a layer, generally tabular body, of rock or sediment characterized by certain lithologic 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.[1] Prior to the publication of International Stratigraphic Guide,[1] older publications have defined a stratum as either being either equivalent to a single bed or 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 both bed and lamina.[2]
Characteristics
Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural processes. They may extend over hundreds of thousands of square kilometers of the Earth's surface. Strata are typically seen as bands of different colored or differently structured material exposed in cliffs, road cuts, quarries, and river banks. Individual bands may vary in thickness from a few millimeters to a kilometer or more. A band may represent a specific mode of deposition: river silt, beach sand, coal swamp, sand dune, lava bed, etc.
Types of Stratum
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.[1][3] 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 customnarily 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 units when it is distinctive, widespread, and useful for stratigraphic correlation. A band is a thin stratum that is distingushiable 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 recognize and correlate during geologic field mapping or subsurface work.[1][3]
Gallery
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Strata on a mountain face in the French Alps -
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Rock strata at Depot Beach, New South Wales -
Rainbow Basin Syncline in the Barstow Formation near Barstow, California. Folded strata. -
Outcrop of Upper Ordovician limestone and minor shale, central Tennessee. -
Chalk Layers in Cyprus - showing classic layered structure. -
Heavy minerals (dark) as thin strata in a quartz beach sand (Chennai, India). -
Stratified Island near La Paz, Baja California Sur, Mexico.
See also
- Archaeological horizon
- Geologic formation
- Geologic map
- Geologic unit
- Law of superposition
- Bed (geology)
References
- ^ a b c d 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
- ^ 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. ISBN 0-922152-76-4
- ^ a b Murphy, MA., and Salvador, A., 1999. International stratigraphic guide—an abridged version. Episodes, 22(4), pp.255-272.
External links
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