DEFINITION OF SEDIMENTARY ROCKS

Those rocks that have consolidated

from accumulations of loose sediment, in turn produced by

physical, chemical, or biological processes. Common mechanical

processes involved in the formation of sediments include

the breaking, transportation of fragments, and accumulation

of older rocks; chemical processes include the precipitation

of minerals by chemical processes or evaporation of water;

common biological processes include the accumulation of

organic remains.

Soils and other products of weathering of rocks are continuously

being removed from their sources and deposited

elsewhere as sediments. This process can be observed as gravel

in streambeds, on alluvial fans, and in windblown deposits.

When these sediments are cemented together, commonly by

minerals deposited from water percolating through the

ground, they become sedimentary rocks. Other types of sedimentary

rocks are purely chemical in origin and were formed

by the precipitation of minerals from an aqueous solution.

Clastic sediments (also detritus) are the accumulated particles

of broken rocks, some with the remains of dead organisms.

The word clastic is from the Greek word klastos,

meaning broken. Most clastic particles have undergone various

amounts of chemical change and may have a continuous

gradation in size from huge boulders to submicroscopic particles.

Size is the main basis for classifying clastic sediments

and sedimentary rocks. The texture of the sedimentary rock

or individual sedimentary particles forms as additional criteria

for classifying sedimentary rocks.

Clastic sediments may be transported by wind, water,

ice, or gravity, and each method of transport leaves specific

clues as to how it was transported and deposited. For

instance, if sediments are transported by gravity in a landslide,

the resulting deposit consists of a poorly sorted mixture

including everything that was in its path, whereas if the

sediment was transported by wind, it has a very uniform

grain size and typically forms large dunes. Clastic sediments

are deposited when the transporting agent can no longer

carry them. For instance, if the wind stops, the dust and

sand will fall out, whereas sediments transported by streams

are deposited when the river velocity slows down. This happens

either where the stream enters a lake or the ocean or

when a flood stage lowers and the stream returns to a normal

velocity and clears up. Geologists can look at old rocks

and tell how fast the water was flowing during deposition

and can also use clues such as the types of fossils, or the way

the individual particles are arranged, as clues to decipher the

ancient environment.

Chemical sediment is sediment formed from the precipitation

of minerals from solution in water. They may form

from biochemical reactions from activities of plants and animals

that live in the water, or they may form from inorganic

reactions in the water, induced by things such as hot springs,

or simply the evaporation of seawater. This produces a variety

of salts, including ordinary table salt. Chemical sedimentary

rocks are classified according to their main chemical

component, with common types including limestone (made of

predominantly calcite), dolostone (consisting of more than 50

percent dolomite), rock salt (composed of NaCl), and chert

(whose major component is SiO2).

Most sedimentary rocks display a variety of internal and

surface marking known as sedimentary structures that can be

used to interpret the conditions of formation. Stratification

results from a layered arrangement of particles in a sediment

or sedimentary rock that accumulated at the surface of the

earth. The layers are visible and different from adjacent layers

because of differences (such as size, shape, or composition)

in the particles between different layers, and because of

differences in the way the particles are arranged between different

layers. Bedding is the layered arrangement of strata in

a body of rock. Parallel strata refers to sedimentary layers in

which individual layers are parallel. The presence of parallel

strata usually means that the sediments were deposited

underwater, such as in lakes or in the deep sea. Some sediments

with parallel layers have a regular alternation between

two or more types of layers, indicating a cycle in the depositional

environment. These can be daily, yearly, or some other

rhythm influenced by solar cycles. One unusual type of layered

rock is a varve, which is a lake sediment that forms a

repeating cycle of coarse-grained sediments with spring tides,

and fine-clay with winter conditions, when the suspended

sediments gradually settle out of the water column. Cross

strata are layers that are inclined with respect to larger layers

that they occur in. Most cross-laminated deposits are sandy

or coarser, and they form as ripples that move along the surface.

The direction of inclination of the cross strata is the

direction that the water used to flow.

Sorting is a sedimentary structure that refers to the distribution

of grain sizes within a sediment or sedimentary rock.

Sediments deposited by wind are typically well-sorted, but

those deposited by water may show a range of sorting. If the

grains have the same size throughout a bed it is called a uniform

layer. A gradual transition from coarse to fine-grained,

or fine to coarse-grained, is known as a graded bed. Graded

beds typically reflect a change in current velocity during

deposition. Non-sorted layers represent a mixture of different

grain sizes, without order. These are common in rock falls,

avalanche deposits, landslides, and from some glaciers.

Rounding is a textural term that describes the relative shape

or roundness of grains. When sediments first break off from

their source area, they tend to be angular, and reflect the

shape of joints, or internal mineral forms. With progressive

transportation by wind or water, abrasion tends to smooth

the grains and make them rounded. The greater the transport

distance, in general, the greater the rounding.

Surface features on sedimentary layers also yield clues

about the depositional environment. Like ripple marks or

footprints on the beach, many features preserved on the surface

of strata offer clues about the origin of sedimentary

rocks and the environments in which they formed. Ripple

marks show the direction of ancient currents, whereas tool

marks record places where an object was dragged by a current

across a surface. Flute marks are grooves produced by

turbulent eddies in the current scouring out small pockets on

the paleosurface. Mud cracks reveal that the surface was

wet, then desiccated by subaerial exposure. Other types of

surface marks may include footprints and animal tracks in

shallow water environments, and raindrop impressions in

subaerial settings.

Fossils are remains of animals and plants preserved in

the rock, that can also tell clues about past environments. For

instance, deep marine fossils are not found in lake environments,

and dinosaur footprints are not found in deep marine

environments.

See also CARBONATE; CLASTIC ROCKS; EVAPORITE.

sedimentation See SEDIMENTARY ROCKS.

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