Page 178 - Laboratory Manual in Physical Geology

Layering of Sediment its texture. The texture of a sediment or sedimentary rock
The result of deposition is a deposit of sediment. So is a description of its parts and their sizes, shapes, and
erosion, transportation, and deposition are a sequence arrangement ( FIGURE 6.3 ).
of related events. The events are also episodic (happen

infrequently, not continuously). Erosion happens when Grain Size. The particles that make up sedimentary rocks
it rains, transportation happens when it floods, and are called grains . Size of the grains is commonly expressed
deposition happens when flood waters accumulate in these Wentworth classes, named after C. K. Wentworth,
in a lake or ocean and stop moving (and sediment an American geologist who devised the scale in 1922:
settles out or precipitates out of the water). The net ■ Gravel includes grains larger than 2 mm in diameter
result is, therefore, a layered deposit. Each time a new (granules, pebbles, cobbles, and boulders).
episode of flood water washes into the lake or ocean,
a new layer of sediment is deposited on top of the last ■ Sand includes grains from 1/16 mm to 2 mm in
(older) one. In between the depositional events, there diameter (in decimal form, 0.0625 mm to 2.000 mm).
is nondeposition (a time during which no deposition This is the size range of grains in a sandbox. The
occurs). The times of nondeposition become surfaces, grains are visible and feel very gritty when rubbed
called bedding planes , between the layers of sediment between your fingers.
(called beds , bedding , or strata ).
■ Silt includes grains from 1/256 mm to 1/16 mm
in diameter (in decimal form, 0.0039 mm to
Lithification of Sediment 0.0625 mm). Grains of silt are usually too small
Lithification is the process of changing loose particles to see, but you can still feel them as very tiny
of sediment (unconsolidated sediment) to solid rock gritty grains when you rub them between your
(consolidated sediment). This happens most often when fingers or teeth.
sediment is compacted (squeezed together) or cemented
(glued together by tiny crystals or chemical residues). ■ Clay includes grains less than 1/256 mm diameter
(in decimal form, 0.0039 mm). Clay-sized grains are
too small to see, and they feel smooth (like chalk dust)
Composition and Textures of when rubbed between your fingers or teeth. Note that
Sediments and Sedimentary Rocks the word clay is used not only to denote a grain size,
but also a clay mineral. However, clay mineral crystals
Sediment and sedimentary rocks are described, classified, are usually clay-sized.
named, and interpreted on the basis of their composition
and textures.
Rounding of Sediment. All sediment has a source

Composition of Sediment and (place of origin; FIGURE 6.1 ). Sediments deposited quickly
Sedimentary Rocks at or near their source tend to lack abrasion. Sediments
The composition of a sediment or sedimentary rock is that have been moved about locally (as in waves on a
a description of the kinds and abundances of grains that beach) or transported away from their source are abraded
compose it ( FIGURE 6.2 ). Sediments and sedimentary (worn). Roundness is a description of the degree to
rocks are classified as biochemical (bioclastic), chemical, which the sharp corners and points of a fragmented grain
or detrital (siliciclastic) based on their composition. have been worn away and its profile has become round

Biochemical sediments and rocks consist of whole and ( FIGURE 6.3 ). A newly formed clast is very angular . As it
broken (bioclastic) parts of organisms, such as shells and is transported and worn it will become subangular , then
plant fragments. Chemical sediments and rocks consist subround , and then well rounded . A freshly broken rock
of chemical residues and intergrown mineral crystals fragment, mineral grain, or seashell has sharp edges and is

precipitated from aqueous solutions. The precipitated described as angular. The more rounded a grain becomes,
minerals commonly include gypsum, halite, hematite, the smaller it generally becomes. Gravel gets broken
limonite, calcite, dolomite, and chert (microcrystalline and abraded down into sand, and sand gets broken and
variety of quartz). Detrital sediments and rocks consist of abraded into silt and clay-sized grains. When combined,
siliciclastic grains (rock fragments, quartz, feldspar, clay the silt plus clay mixture is called mud .
minerals) that are also detrital grains—rock fragments and
mineral grains that were worn and transported away from Sorting of Sediment. Different velocities of wind and

the landscape. water currents are capable of transporting and naturally
separating different densities and sizes of sediments from
Textures of Sediment and Sedimentary one another. Sorting is a description of the degree to which
Rocks one size class of sediment has been separated from the oth-
ers ( FIGURE 6.3 ). Poorly sorted sediments consist of a mixture
Processes of weathering, transportation, precipitation, of many different sizes of grains. Well-sorted sediments
and deposition that contribute to the formation of a
consist of grains that are of similar size and/or density.
sediment or sedimentary rock also contribute to forming

Sedimentary Processes, Rocks, and Environments ■ 157