DEFINITION OF BEDDING

Also known as stratification, bedding is a primary

depositional layering in sedimentary rocks that is defined by

variations in grain size of clastic grains, mineralogy, rock

type, or other distinguishing features. It forms during the

movement, sorting, and deposition of sediments by depositing

currents, or it may reflect changes in the character of the

environment during deposition of different beds. In a sedimentary

rock, beds may be stacked in a series of layers of

similar or variable thickness, and the overall character and

changes within a sedimentary sequence may be described by

noting the changes between and within beds at various

heights in the sedimentary or stratigraphic pile. Stratification

or bedding may form at many different scales, and understanding

the character and mechanisms of formation of the

strata can lead to a deep understanding of the depositional

history of the rock unit or sedimentary basin in which the

rocks formed. Major changes in the character of strata or

beds can be used to define different formations in a rock

sequence and form the basic subdivision for geologic mapping

of an area.

Several styles of bedding are more common than others.

Uniform beds are those that do not change in character from

base to top. Graded bedding refers to the phenomena whereby

the largest sized grains are deposited at the base of the

bed, and finer grained particles are deposited progressively

upward toward the top of the bed. Graded bedding is produced

by a flow regime where the depositing current is losing

velocity during deposition, dropping first the heavy coarsegrained

particles, and then the finer grained particles as the

current loses its strength. Cross bedding refers to the characteristic

where prominent layers are oriented obliquely, typically

at angle of 15–23 degrees, from the main originally

horizontal bedding planes. These are produced by air or

water currents that deposit sand or other particles in ripples

or dunes, with the sedimentary particles forming layers as

they slide down and accumulate on the slip surfaces of the

dunes and ripples.

Bedding in rock sequences defines patterns that reflect

major and minor changes in the sedimentary depositional

environments. For instance, when sea levels rise relative to a

coastline, a marine transgression results, and typically strata

will change from sandstone to shale to limestone in a vertical

sequence. Recognizing that this sequence of rock types in a

bedded stratigraphic unit represents a deepening upward

cycle would lead the geologist to understand the past series of

events that led to the formation of these layers.

With the exception of cross-bedding, most beds are

deposited horizontally, providing a reference frame for understanding

folding and tilting of rocks in orogenic and deformation

belts. The amount of tilting of a rock sequence can be

estimated by measuring the inclination (called dip) of the

bedding planes relative to the horizontal plane.

See also CROSS-BEDDING; SEDIMENTARY ROCKS;

STRATIGRAPHY.

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