Bed (geology)

In geology, a bed is a layer of sediment, sedimentary rock, or volcanic rock "bounded above and below by more or less well-defined bedding surfaces".^[1] A bedding surface is three-dimensional surface, planar or curved, 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.^[1][2]

Beds of sedimentary rock at Parque Geológico do Varvito, Itu, São Paulo, Brazil

Originally horizontal beds of sedimentary rock were tilted by the Alpine orogeny, at Angles, Alpes-de-Haute-Provence, France

Definitions

Specifically in sedimentology, a bed can be defined in one of two major ways.^[2] First, Campbell^[3] and Reineck and Singh^[4] 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.^[2]

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.^[5] This method of defining bed versus lamina is frequently used in textbooks, e.g., Collinson & Mountney^[6] or Miall.^[7] 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.^[2]

Interpretation

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.^[2][8]

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.^[9]

Types

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.^[4][6][9]

Bed thickness

Beds of lava flows exposed on the island of La Gomera

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.^[10] Such sedimentological studies are typically based on the hypothesis that the thicknesses of stratigraphic units follows a log-normal distribution.^[10][11] Differing nomenclatures for the bed and laminae thickness have been proposed by various authors, including McKee and Weir,^[5] Ingram,^[12] and Reineck and Singh.^[4] However, none of them have been universally accepted by Earth scientists.^[10][13] In the practice of engineering geology, a standardized nomenclature is used for describing bed thickness in Australia,^[14] the European Union,^[15] and the United Kingdom.^[16]

Examples of widely used bed thickness classifications include Tucker (1982)^[17] and McKee and Weir^[5] (1953).

Classification of Thickness of Stratification

Bedding class	Tucker (1982)[17]	McKee and Weir[5] (1953)
Very thick	> 1 m	> 120 cm
Thick	30 cm – 1 m	60–120 cm
Medium	10 – 30 cm
Thin	3 – 10 cm	5–60 cm
Very thin	1 – 3 cm	1–5 cm
Thickly laminated	3 – 10 mm	2 mm - 1 cm
Thinly laminated	< 3 mm	< 2 mm

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.^[18][19]

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.^[18][19]

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

Vertical cross-sections of bed sequences to illustrate (from top to bottom) the Law of Superposition, the Law of Original Horizontality, the Law of Lateral Continuity, and Cross-Cutting Relationship

Main articles: Relative dating and Outline of geology § Principles of geology

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.^[20]
• 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.^[21]
• 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.^[20]
• 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
• Interbedding
• Lamination (geology)
• Stratigraphy

References

1. 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. ISBN 0-922152764
2. 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.
3. Campbell, Charles V. (February 1967). "Lamina, Laminaset, Bed and Bedset". Sedimentology. 8 (1): 7–26. Bibcode:1967Sedim...8....7C. doi:10.1111/j.1365-3091.1967.tb01301.x – via Wiley Online Library.
4. Reineck, H.E., and Singh, I.B., 1980. Depositional Sedimentary Environments, (2nd ed.) Berlin, Germany: Springer-Verlag, 504 pp. ISBN 978-3642962912
5. McKee, Edwin D.; Weir, Gordon W. (1953). "Terminology for Stratification and Cross-Stratification in Sedimentary Rock". Bulletin of the Geological Society of America. 64 (4). Geological Society of America: 381–390. Bibcode:1953GSAB...64..381M. doi:10.1130/0016-7606(1953)64[381:TFSACI]2.0.CO;2 – via GeoScienceWorld.
6. Collinson, J., and Mountney, N., 2019. Sedimentary Structures, (4th ed.) Edinburgh, Scotland, Dunedin Academic Press, 320 pp. ISBN 978-1903544198
7. Miall, A.D., 2016. Stratigraphy: A Modern Synthesis. Dordrecht, Netherlands: Springer. 454 pp. ISBN 978-3319243023
8. 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.
9. Boggs, Jr., Sam (2006). Principles of Sedimentology and Stratigraphy (PDF) (4th ed.). Upper Saddle River, NJ: Prentice Hall. ISBN 0-13-154728-3. Archived from the original (PDF) on 2022-03-05. Retrieved 2021-05-09.
10. Flügel, E. and Munnecke, A., 2010. Microfacies of carbonate rocks: analysis, interpretation and application. Berlin, Germany, Springer-Verlag, 2004 pp. ISBN 978-3662499610
11. Lumsden, D.N., 1971. Facies and bed thickness distributions of limestones. Journal of Sedimentary Research, 41(2), pp.593-598.
12. 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.
13. Kelley, V.C., 1956. Thickness of strata. Journal of Sedimentary Research, 26(4), pp.289-300.
14. Australian Standards, 1993. Geotechnical site investigations. AS1726 – 1993. Sydney, Australia: Standards Association of Australia, 40 pp.
15. 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.
16. British Standards Institution, 2015. BS 5930:2015 Code of practice for ground investigations. London, England: British Standards Institution. 317 pp. ISBN 978-0580800627
17. 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.
18. Murphy, M.A., and Salvador, A., 1999. International stratigraphic guide—an abridged version. Episodes. 22(4), pp.255-272.
19. North American Commission on Stratigraphic Nomenclature, 2021. North American Stratigraphic Code. Stratigraphy. 18(3), pp.153–204.
20. Steno, Nicolaus (1671). The Prodromus to a Dissertation Concerning Solids Naturally Contained within Solids: Laying a Foundation for the Rendering a Rational Attempt both of the Frame and the several Changes of the Masse of the Earth, as also of the various Productions in the same. Translated by Oldenburg, Henry (2nd ed.). London: F. Winter – via Biodiversity Heritage Library.
21. Levin, Harold L. (2009). The Earth Through Time. John Wiley & Sons, Inc. p. 15. ISBN 978-0-470-38774-0.