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.
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