Sedimentary Rocks

Sedimentary Rocks:

Sediments are loose rock particles, produced by one of three mechanisms:

1) Weathering of preexisting rocks, followed by transportation and deposition (produce clastic sedimentary rocks such as sandstones, breccias, and conglomerates).
2) Chemical precipitation of minerals from water (produce chemical sedimentary rocks such as limestone and evaporites).
3) Accumulation of biological matter such as shells and plant fragments (such as coal).

Sediments that are buried may harden into sedimentary rock through a process know as lithification (a combination of compaction and cementation)

From Sediment to Sedimentary Rock

Prior to lithification, sediment experiences two major events:

1)  Transportation- The movement of sediment away from its source rock by water, wind, or ice

Rounding of particles occurs due to abrasion during transport.  Rounding increases as transport distance increases.

Size sorting occurs by transport agents, especially running water.  Sediment size decreases as transport distance increases.

2)  DepositionThe environment of deposition is the location where deposition occurred.  Examples of environments of deposition are:

River Channel
Lake Bottom
Deep Ocean
Desert Dunes

Classification of Sedimentary Rocks by Origin

1) Clastic (or detrital) sedimentary rocksForm from the cementation of sediment grains that come from pre-existing rocks.  This is the most common sedimentary rock type.  Clastic sedimentary rocks are classified by grain size, and to a lesser extent by chemical composition.

Boulder - >256 mm
Cobble - 64 to 256 mm
Pebble - 2 to 64 mm
Sand - 1/16 to 2 mm
Silt - 1/256 to 1/16 mm
Clay - <1/256 mm

Breccia- A coarse-grained clastic rock composed of angular rock fragments cemented together (poor rounding).

Breccia© Marli Miller, University of Oregon.

Conglomerate- A coarse-grained clastic rock made of rounded gravel cemented together (good rounding).

Conglomerate, © Marli Miller, University of Oregon.

Sandstone- A medium-grained clastic rock.

Sandstone, Public Domain Image, USGS (Minerals in your World Project).

Sandstones are subclassified based on the chemical composition:

Quartz sandstone - Contains >90% quartz grains.
Arkose - Contains mostly feldspar and quartz grains mixed together.
Graywacke - Contains sand grains surrounded by dark, fine-grained matrix (they are "dirty" sandstones).

Siltstone- A fine-grained clastic rock.  Siltstones have a gritty feel, and the individual grains are visible with a hand lens.

Siltstone, Public Domain Image, USGS (Minerals in your World Project).

Shale- A fine-grained clastic rock containing silt and clay-sided grains.  Shale spits into thin layers, a behavior known as fissile.  Shales feel smoother than siltstones.

Shale, Public Domain Image, USGS (Minerals in your World Project).

Mudstone/Claystone- Mudstone is made of silt and clay-sized grains.  They are blocky (non-fissile).

2) Chemical (and biochemical) sedimentary rocksChemical sedimentary rocks form by the precipitation of minerals from water (this process may or may not involve the actions of organisms).  In contrast to the clastic textures of the rocks discussed above, chemical sedimentary rocks have crystalline textures.

LimestoneA chemical sedimentary rock composed mainly of calcite (CaCO3).

Fossiliferous limestone, Public Domain Image, USGS (Minerals in your World Project).

Most limestones are biochemical, but many are inorganic.

They often contain easily recognizable fossils (fossiliferous).

Chemical alteration of limestone in Mg-rich water can produce dolomite, CaMg(CO3)2.

ChertA hard, compact, fine-grained chemical sedimentary rock composed almost entirely of silica (SiO2).

Chert, Public Domain Image, USGS (Minerals in your World Project).

Chert can occur as layers or as lumpy nodules within other sedimentary rocks, especially within limestone.

Some cherts are primary precipitates, but others formed by replacement of pre-existing material by silica.

Evaporates- Evaporites are chemical sedimentary rocks that grow upward from seas and salt-rich lakes due to water evaporation.

Delicate evaporites
© Larry Fellows, Arizona Geological Survey.

Common evaporite deposits are gypsum, CaSO4•2(H2O), and halite (NaCl).

Organic sedimentary rocksFossil fuels are sedimentary rocks with a biological origin.

Coal- Coal is a sedimentary rock formed from the compaction of partially decayed plant material (requires stagnant water and rapid burial).

Oil and natural gas- “Cooking” below Earth's surface can change organic solids into oil and natural gas.  These fossil fuels rise and accumulate in porous overlying rocks.

Sedimentary Structures

Sedimentary structure- A features within a sedimentary rock that provides clues about the environment of deposition.

1) Bedding: Series of horizontal layers within an outcrop of rock.  The most common sedimentary structure.
Shale with interbedded limestone. © Marli Miller, University of Oregon.

2) Ripple marks: Small ridges formed on the surface of sediment layer by moving wind or water.  Symmetrical ripples represent water wave ripples, whereas assymetrical ripples represent water current or wind current ripples.

Sand dunes of Death Valley, © Marli Miller, University of Oregon.
Ripples on sandy beach in southern Alaska, © Marli Miller, University of Oregon.

Ripple marks on sandstone of the Triassic Chinle Formation, © Marli Miller, University of Oregon.

3) Cross-bedding- Series of thin, inclined layers within a horizontal bed of rock that represent the preservation of migrating dunes.  You are looking at the side of a bed rather than the top of the bed.

Cross-bedding structures in the Navajo Sandstone of Zion National Park, © Marli Miller, University of Oregon.

4) Graded bedding- A type of bedding where grain size gets smaller from bottom to top due to sorting of grains under water.  This type of bedding results from underwater landslides (called turbidity currents) initiated by earthquakes.

Graded bedding in matrix supported conglomerate from the Pliocene Copper Canyon formation, © Marli Miller, University of Oregon.

5) Mud Cracks/Dessication Cracks- Polygonal cracks formed in mud that dries upon exposure to air (common in dry lake beds and tidal flats).

Mud cracks in the bed of the Amargosa River in California's Death Valley National Park, © Michael Collier.

Ancient mudcracks (shrinkage cracks) preserved in red-brown mudstone near the base of the Watahomigi Formation, Public Domain Image, United States Geological Survey.

6) Fossils- Any evidence of past life preserved in rock.  Hard parts (shells, bones) are the most easily preserved parts of organisms.
Fossils can give detailed information about the environment of deposition.
Trilobite. © Oklahoma University

Animal footprints in Coconino Sandstone of Aubrey Cliffs, Arizona- Courtesy United States Geological Survey.


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