Bed (geology)

In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' (5th ed.). Alexandria, Virginia; American Geological Institute. p 61. Specifically in sedimentology, a bed can be defined in one of two major ways.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. First, Campbell and Reineck and SinghReineck, H.E., and Singh, I.B., 1980. ''Depositional Sedimentary Environments'', (2nd ed.) Berlin, Germany: Springer-Verlag, 504 pp. use the term ''bed'' to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are ''small beds'' that constitute the smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed.

Alternatively, a bed can be defined by thickness where a bed is a coherent layer of sedimentary rock, sediment, or pyroclastic material greater than 1 cm thick and a lamina is a coherent layer of sedimentary rock, sediment, or pyroclastic material less than 1 cm thick. This method of defining bed versus lamina is frequently used in textbooks, e.g., Collinson & MountneyCollinson, J., and Mountney, N., 2019. ''Sedimentary Structures'', (4th ed.) Edinburgh, Scotland, Dunedin Academic Press, 320 pp. or Miall.Miall, A.D., 2016. ''Stratigraphy: A Modern Synthesis.'' Dordrecht, Netherlands: Springer. 454 pp. Both definitions have merit and the choice of which one to use will depend on the focus of the specific study on a case by case basis.

In geology, a bedding surface is either a planar, nearly planar, to wavy or curved 3-dimensional surface that visibly separates each successive bed (of the same or different lithology) from the preceding or following bed. Where bedding surfaces occur as cross-sections, e.g., in a 2-dimensional vertical cliff face of horizontal strata, are often referred to as ''bedding contacts''. Within conformable successions, each bedding surface acted as the depositional surface for the accumulation of younger sediment.

Typically, but not always, bedding surfaces record changes in either the rate or type of accumulating sediment that created the underlying bed. Typically, they represent either a period of nondeposition, erosional truncation, shift in flow or sediment regime, abrupt change in composition, or combination of these as a result of changes in environmental conditions. As a result, a bed is typically, but not always, interpreted to represent a single period of time when sediments or pyroclastic material accumulated during uniform and steady paleoenvironmental conditions. However, some bedding surfaces may be postdepositional features either formed or enhanced by diagenetic processes or weathering.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, pp. 679–682.

The relationship between bedding surfaces controls the gross geometry of a bed. Most commonly, the bottom and top surfaces of beds are subparallel to parallel to each other. However, some bedding surfaces of a bed are nonparallel, e.g., wavy, or curved. Differing combinations of nonparallel bedding surfaces results in beds of widely varying geometric shapes such as uniform-tabular, tabular-lenticular, curved-tabular, wedge-shaped, and irregular beds.

Types of beds include cross-beds and graded beds. Cross-beds, or "sets," are not layered horizontally and are formed by a combination of local deposition on the inclined surfaces of ripples or dunes, and local erosion. Graded beds show a gradual change in grain or clast sizes from one side of the bed to the other. A normal grading occurs where there are larger grain sizes on the older side, while an inverse grading occurs where there are smaller grain sizes on the older side.

Bed thickness

Bed thickness is a basic and important characteristic of beds. Besides mapping stratigraphic units and interpreting sedimentary facies, the analysis of bed thickness can be used to recognize breaks in sedimentation, cyclic sedimentation patterns, and gradual environmental changes.Flügel, E. and Munnecke, A., 2010. ''Microfacies of carbonate rocks: analysis, interpretation and application.'' Berlin, Germany, Springer-Verlag, 2004 pp. Such sedimentological studies are typically based on the hypothesis that the thicknesses of stratigraphic units follows a lognormal distribution.Lumsden, D.N., 1971. ''Facies and bed thickness distributions of limestones''. '' Journal of Sedimentary Research'', 41(2), pp.593-598. Differing nomenclatures for the bed and laminae thickness have been proposed by various authors, including McKee and Weir, Ingram,Ingram, R.L., 1954. ''Terminology for the thickness of stratification and parting units in sedimentary rocks''. ''Geological Society of America Bulletin'', 65(9), pp. 937-938. and Reineck and Singh. However, none of them have been universally accepted by Earth scientists.Kelley, V.C., 1956. ''Thickness of strata''. ''Journal of Sedimentary Research'', 26(4), pp.289-300. In the practice of engineering geology, a standardized nomenclature is used for describing bed thickness in Australia,Australian Standards, 1993. ''Geotechnical site investigations. AS1726 – 1993''. Sydney, Australia: Standards Association of Australia, 40 pp. European Union,International Organization for Standardization, 2017. ''14689:2017 Geotechnical investigation and testing — Identification, description and classification of rock''. Geneva, Switzerland: International Organization for Standardization. 122 pp. and United Kingdom.British Standards Institution, 2015. ''BS 5930:2015 Code of practice for ground investigations''. London, England: British Standards Institution. 317 pp.

Examples of widely used bed thickness classifications include Tucker (1982)Tucker, Maurice, E. 1982. ''The Field Description of Sedimentary Rocks''. '' Geological Society of London Handbook'', Open University Press, Milton Keynes, UK, and John Wiley & Sons, NY. Table 5.2, p. 48. and McKee and Weir (1953).

Bed in lithostratigraphy

According to both the North American Stratigraphic Code and International Stratigraphic Guide, a bed is the smallest formal lithostratigraphic unit that can be used for sedimentary rocks. A bed, a stratum, is the smallest formal unit in the hierarchy of sedimentary lithostratigraphic units and is lithologically distinguishable from other layers above and below. Customarily, only distinctive beds, i.e. key beds, marker beds, that are particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units.Murphy, M.A., and Salvador, A., 1999
''International stratigraphic guide—an abridged version''.
''Episodes''. 22(4), pp.255-272.North American Commission on Stratigraphic Nomenclature, 2021
''North American Stratigraphic Code''.
''Stratigraphy''. 18(3), pp.153–204.

In case of volcanic rocks, the lithostratigraphic unit equivalent to a bed is a ''flow''. A flow is “...a discrete, extrusive, volcanic rock body distinguishable by texture, composition, order of superposition, paleomagnetism, or other objective criteria.” A flow is a part of a member as a bed of sedimentary rock is a part of a member.

Engineering considerations

In geotechnical engineering a bedding surface often forms a discontinuity that may have a large influence on the mechanical behaviour (strength, deformation, etc.) of soil and rock masses in tunnel, foundation, or slope construction.

Geologic principles

These are the principles which apply to all geologic features, and can be used to describe the order of events in a feature's geologic history.

* The Law of Superposition states that younger rocks are deposited above older rocks, and remain that way as long as the beds have not been overturned through tectonic activities. This is used to date the stratigraphy and their relative ages.
* The Law of Original Horizontality states that beds are deposited horizontally due to gravity. If the beds are not horizontal, then that is an indication that they have been tilted or warped by geologic processes.
* The Law of Lateral Continuity states that the bed deposits extend laterally in all directions. This implies that two places separated by erosional features with similar rocks may have originally been continuous.
* The law of Cross-Cutting Relationships states that any feature which cuts through another is the younger of the two. This can include faults or igneous dikes cutting through sedimentary bedding.

See also

* Fold (geology)
* Geological formation
* Geological unit
* Lamination (geology)
* Stratigraphy
* Stratum

References

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Stratigraphy