sedimentary

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Terms: clastic, detrital, siliciclastic, terrigenous, bedding, chemical, biochemical, organic, sedimentary structure, fossil, shale, sandstone, limestone (carbonate), evaporite, diagenesis, lithification, cementation, recrystallization
Sedimentary rocks are formed on the earth's surface by the deposition and lithification of sediment. They comprise the weathered rind of the lithosphere. Because sediment is deposited incrementally over a surface, sedimentary rocks are layered and, unless deformed by some tectonic event, horizontal.  Layers of sedimentary rocks are call strata, and each stratum contains information about the climate, sedimentary processes, and organisms present at the time it was deposited. Grand Canyon
Figure 1.  Over 5,000 feet of sedimentary rocks are exposed in the Grand Canyon in Arizona.  They record over 500,000,000 years of earth history.

What is sediment?

Sediment is any material deposited on the earth's surface that is derived from rocks by weathering. The products of weathering include rock fragments (detritus), new minerals, such as kaolinite and hematite, and dissolved ions, which may later precipitate from water. All of these components, along with organic debris, comprise the material that make up sedimentary rocks. 

Terms: clastic and detrital both refer to fragmental material. Detrital refers to rock-derived particles only, whereas clastic can refer to any particle regardless of composition or origin (e.g. volcanic glass, shell fragments, and detrital grains).  Sediment composed of silicate minerals originating from the weathering of silica-rich metamorphic and igneous rocks is more specifically called siliciclastic.

Why are sedimentary rocks important?

The formation of sedimentary rocks

Once created sediment is eroded, transported, deposited, and eventually lithified.  The process may take a few thousand years to several million years depending on the climate, topography, and the prevalence of geologic agents.

Transportational history of sediment
Detrital sediments are carried by gravity, glaciers, rivers, waves and wind to where they are eventually deposited. Some sediment may be picked up and deposited again and again, experiencing several cycles of erosion and deposition before ever being incorporated into a rock. The longer a detrital sediment has been exposed to weathering and transportation the more mature it becomes. Maturity is recognized by the degree of sorting, roundness of the grains, and the absence of unstable rock fragments such as ferromagnesium minerals and plagioclase. A highly mature sediment containing well-rounded grains of quartz, has experienced may cycles of erosion and deposition and may be far traveled from its original source. Quartz sand deposited along the Gulf of Mexico may have originated from rocks exposed on the eastern flanks of the Appalachian Mountains or Western margin of the Rockies.  On the other hand, sediment deposited in an alluvial fan at the base of a mountain front in a fault-bound basin will contain a wide range of coarse mineral and rock fragments, often angular or subrounded, and poorly sorted.
Deposition

Physical Agents of Deposition: The agents that carry sediment, such as water, wind and ice, also deposit it--mostly in some sort of depositional basin.   It may be deposited by waves breaking on the shores of an epicontinental sea, by streams flowing into a rift basin, or by turbidity currents flowing into the deep ocean from the continental slope. Both the texture and the composition of a sedimentary rock reflect a sediment's source, transportational history, and depositional environment.  Sedimentary structures called bedforms (figs.2-5) disclose the agent of deposition, and also provide clues as to the climate, water depth, and other environmental factors. Because sediments are laid down incrementally on the surface, sedimentary rocks are typically stratified--composed of layers, which are separated by horizontal discontinuities called bedding planes.  A pulse of sand from a stream in flood may be followed by mud deposited later from quieter waters--each layer above preserving the features of the bed below.  A climatic change may dry up the river and bury its sediments in dune deposits.

Dunes ripples
Figure 2.  Navajo Sandstone, Utah.  The inclined strata are the foreset beds  formed on the down-wind side of giant dunes that existed over 200 million years old. Figure 3. Linear wave ripples in a thinly-bedded sandstone (Moenkopi Fm, Utah) records a coastal setting.
kiabab limestone mudcracks
Figure 4.  Calcite precipitated by organisms forms shells and reefs that are preserved as limestone. (Kiabab limestone on the rim of the Grand Canyon.) Figure 5.  Modern mudcracks, similar to those preserved on the surfaces of shales deposited on a tidal flat or playa.


Deposition by precipitation: Dissolved mineral material carried by water will eventually crystallize from solution forming a chemical sedimentary rock, such as limestone, rock salt, and gypsum. Chemical sedimentary rocks precipitated directly from water typically have a crystalline texture, which is composed of interlocking crystals. However, sometimes precipitation is aided by organisms. For example, shellfish, mammals, marine micro-organisms, and corals, induce mineral precipitation to form hard parts (either endoskeletons, exoskeletons) or living quarters. Sedimentary rocks produced in this manner are called biochemical. The texture of these rocks typically reflects the organic structure of the organisms that created it. Over time these rocks will recrystallize, developing a more interlocking crystalline texture (fig. 3).  The original organic structure may be partially or entirely destroyed.

How does sediment become rock?

Some chemical and biochemical materials, such as rock salt and coral, are deposited as rock. However, most sediment goes through some process of lithification to become a solid rock. The first step in preserving a sediment in the rock record is to bury it beneath more sediment.  This separates it from geologic agents that would normally erode and remove it.  Once buried physical and chemical changes (diagenesis) occur in the sediment. For example the pore spaces between grains is decreased (compaction), mineral material carried by migrating fluids is precipitated--binding the grains together, and some soluble minerals may recrystallized The upper limit of diagenesis, although arbitrary and debatable,  is somewhere 150-300 °C and a few kilometers in depth.  Beyond these limits minerals start to alter and metamorphism begins.  Lithification, turning a sediment into a rock, is accomplished during diagenesis. The processes of lithification are summarized:

Classification of Sedimentary Rocks

Sedimentary rocks and be divided into three categories:

1. Detrital sedimentary rocks: These rocks are composed of mineral grains and/or rock fragments transported and deposited by some geologic agent. Detrital rocks are typically composed of mineral grains derived from silicate rocks. Because these rocks are composed of particles they have a clastic texture, and are classified first by grain size and second by composition (table 1).
Table 1. Classification of detrital sedimentary rocks.
Clast size
Sediment
Rock Name
Variations
>2 mm

(head of a hat pin or larger)

rounded gravel
conglomerate
quartz pebble --, lithic--, chert--, etc.
angular gravel
breccia
1/16 - 2 mm
sand
sandstone
quartz sandstone (quartz)
arkose (feldspar, qtz., mica, lithic1 fragments, etc.)
<1/16 mm

(not visible)

silt and/or

clay(<1./256 mm)

mudstone (massive2)
silty shale, shaly mudstone
shale (fissile3)
Contains all clast sizes
matrix supported
paraconglomerate

Terms
1 lithic: fragment of rock such as basalt, chert, quartizite, etc.2 massive: uniform homogeneous appearance with no preferred parting or breakage.
3Fissile: Breaks into thin parallel layers. Property of fine-grained sedimentary rocks with abundant clay or micaceous minerals.

2. Chemical and biochemical sedimentary rocks: These rocks (table 2) are chemically precipitated and reflect climate where they form. They have a wide range of textures depending on mode of precipitation and age. Older rocks typically are recrystallized and have denser crystalline textures.


3. Organic sedimentary rocks:
Organic rocks(table 4) are composed of the remains or altered remains of plants and organisms. (Organic remains = hydrocarbon compounds that are the sole product of life processes. I distinguish these from the inorganic remains of organisms, such as carbonate material derived from shells.)

Table 2. Classification of organic sedimentary rocks

Composition
Texture
Name

partially decomposed organic matter
carbon content <65%

bioclastic
(brown, loose or friable)
peat
completely altered organic matter: macerals1
carbon content >75%
amorphous
(black, vitreous to dull luster often in visible layers, blocky to conchoidal fractured)
coal

1 maceral: naturally occurring, homogeneous, amorphous, organic compound having a standard but variable chemical composition. A particular maceral is derived from the alteration of certain types of organic tissues (e.g. vitrinite=woody tissue, sporonite=spores and pollen, cutinite=waxy cuticules of leaves etc.


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