Bed (geology)

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Beds of sedimentary rock at Parque Geologico do Varvito, Itu, Sao Paulo, Brazil Webysther 20211009102044 - Parque Geologico do Varvito.jpg
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 StratotypeBarremienA.JPG
Originally horizontal beds of sedimentary rock were tilted by the Alpine orogeny, at Angles, Alpes-de-Haute-Provence, France

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] 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]

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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]

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. [1] [2]

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 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 Volcanic Landscapes of La Gomera 17.jpg
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 classTucker (1982) [17] McKee and Weir [5] (1953)
Very thick> 1 m> 120 cm
Thick30 cm – 1 m60–120 cm
Medium10 – 30 cm
Thin3 – 10 cm5–60 cm
Very thin1 – 3 cm1–5 cm
Thickly laminated3 – 10 mm2 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 Geologic Principles.jpg
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

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.

See also

Related Research Articles

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Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Sedimentology encompasses the study of modern sediments such as sand, silt, and clay, and the processes that result in their formation, transport, deposition and diagenesis. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.

<span class="mw-page-title-main">Stratigraphy</span> Study of rock layers and their formation

Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostratigraphy, biostratigraphy, and chronostratigraphy.

<span class="mw-page-title-main">Lithology</span> Description of the physical characteristics of a rock unit

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

<span class="mw-page-title-main">Unconformity</span> Rock surface indicating a gap in the geological record

An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger layer, but the term is used to describe any break in the sedimentary geologic record. The significance of angular unconformity was shown by James Hutton, who found examples of Hutton's Unconformity at Jedburgh in 1787 and at Siccar Point in Berwickshire in 1788, both in Scotland.

<span class="mw-page-title-main">Stratum</span> Layer of sediment, rock or soil with internally consistent characteristics

In geology and related fields, a stratum is a layer 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. Prior to the publication of the International Stratigraphic Guide, 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. Related terms are substrate and substratum (pl.substrata), a stratum underlying another stratum.

<span class="mw-page-title-main">Geological formation</span> Fundamental unit of lithostratigraphy

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<span class="mw-page-title-main">Conglomerate (geology)</span> Sedimentary rock composed of smaller rock fragments

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<span class="mw-page-title-main">Lithostratigraphy</span> Sub-discipline of stratigraphy

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<span class="mw-page-title-main">Relative dating</span> Determining the relative order of past events

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The Roxbury Conglomerate, also informally known as Roxbury puddingstone, is a name for a rock formation that forms the bedrock underlying most of Roxbury, Massachusetts, now part of the city of Boston. The bedrock formation extends well beyond the limits of Roxbury, underlying part or all of Quincy, Canton, Milton, Dorchester, Dedham, Jamaica Plain, Brighton, Brookline, Newton, Needham, and Dover. It is named for exposures in Roxbury, in the Boston area. It is the Rock of the Commonwealth in Massachusetts.

<span class="mw-page-title-main">Cross-bedding</span> Sedimentary rock strata at differing angles

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<span class="mw-page-title-main">Graded bedding</span> Type of layering in sediment or sedimentary rock

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References

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