The rock cycle is a theoretical model of the constant recycling of rocks as they form, are destroyed, and then reform. We began our discussion of the rock cycle with igneous rock, and we now discuss sedimentary rocks.

You see in the lesson of weathering how weathering produces sediment. In this lesson, we explain more about sediment origin, as well as the erosion, transportation, sorting, deposition, and eventual lithification of sediments to form sedimentary rock. Because they have such diverse origins, sedimentary rocks are difficult to classify. We divide them into clastic, chemical, and organic sedimentary rocks, but this classification is not entirely satisfactory. Furthermore, despite their great variety, only three sedimentary rocks are very common - shale, sandstone, and limestone.

Sedimentary rocks contain numerous clues to their origin and the environment in which they were deposited. Geologists determine this information from the shape and sequence of rock layers and from the sediment grains and the sedimentary structures such as a fossils, cross-beds, ripple marks, and mud cracks that are preserved in the rock.

Sedimentary rocks are important because they are widespread and because many of them, such as coal and limestone, are economically important. About three-fourths of the surface of the continents is blanketed with a relatively thin skin of sedimentary rocks. Concentrated in sedimentary rocks are important natural resources such as crude oil, natural gas, ground water, salt, gypsum, uranium, and iron ore.

Questions

What is lithification?

What is a sedimentary rock?

What are different processes that form sedimentary rock?

What are two common processes associated with lithification?

What mainly causes agents of transport, like wind or water, to deposit sediment?

What is a clastic sedimentary rock?

Where do sediments come from?

What are roles of water in a sedimentary rock formation?

What are processes that change sediment into sedimentary rock?

What do chemical and detrital sedimentary rocks have in common?

What properties that are used to determin rock names of clastic/chemical sedimentary rocks?

What are different types of clastic sedimentary rock?

Which sedimentary rock is made of silt and/or clay and splits easily into layers?

Name some of different chemical sedimentary rocks?

What makes a shale different than a mudstone?

What is rounding?

What is sorting?

What arethe relations between sorting and rounding to the formation process of sedimentary rock?

What is the most likely cause of a detrital sediment with highly rounded grains?

What is the different between breccia and conglomerate?

Three types of sandstone? Composition of each type?

What are the range of sizes of particles, i.e., gravel, sand, silt, clay?

What is a biochemical sedimentary rock?

What is a bed of sedimentary rock?

What is lamina?

What is bedding plane?

What is graded bedding? What is cross-bedded?

What is mud crack? What is ripple?

What is the most important thing that all sedimentary rocks can tell you, specifically regarding depositional environment?

Extra readings

1. Sedimentary rocks have highly diverse origins and are difficult to classify, but only three are very common: shale, sandstone and limestone. Seventy five percent of continental surfaces are covered by sedimentary rocks.

2. Sediment is unconsolidated particles of either preexisting rocks or chemical precipitates. It is classified by size: gravel> 2 mm< sand> 1/16 mm<"mud", without regard to composition, although most grains of"mud" size are clay minerals.

3. Rounding (grinding away sharp edges) and sorting (separation by size) occur during transportation, usually by streams. Size decreases downstream in a river. Deposition occurs when agents of transportation lose their energy. Preservation of sediments requires their burial and is favored in subsiding basins.

4. Lithification converts loose sediment to sedimentary rock, usually by compaction (reduces pore space) and cementation (fills remaining pore space). These rocks have a clastic texture.

5. Not all sedimentary rocks form from sediment. Some form through crystallization of minerals from solution (example, calcite). These rocks have a crystalline texture, but that texture can also result from recrystallization that has destroyed an originally clastic texture.

6. A section on types of sedimentary rocks expands the discussion of clastic versus crystalline textures, and includes organic rocks as well. Clastic sedimentary rocks are classified by grain size and composition. Breccia (angular) and conglomerate (rounded) (the term "till" is deleted in 9th edition) have a gravel fraction. Sandstones contain sand-size grains, distinguished as quartz sandstone (>90% quartz), arkose (>25% feldspar), and graywacke (>15% matrix = silt and clay). Graywackes result from deposition by turbidity currents. Lithified silt forms siltstone, while combinations of silt and clay form shale. Predominately clay-size particles form claystone and mudstone.

7. Limestone is composed mostly of calcite through the action of organisms or as an inorganic precipitate. Varieties include coquina (cemented shells), bioclastic limestone (coarse-grained fossils), chalk (very fine-grained bioclastic limestone), oolitic limestone (small spheres of calcite), tufa and travertine (crystalline precipitates) and recrystallized limestone (original texture lost). Dolomite is a mineral CaMg(CO3)2 and a rock (sometimes called dolostone) that occurs as a replacement of limestone and destroys its original texture.

8. Chert is a fine-grained, sedimentary rock composed almost entirely of silica. It

can be a replacement, inorganic precipitate, or bioclastic, and may be recrystallized. Evaporites are sedimentary rocks formed from evaporation of seawater. They have crystalline textures and include rock gypsum and rock salt. Coal forms from consolidation of plant material, originally as peat. Compaction transforms peat to coal and several varieties are recognized. Organic material preserved in marine muds change to oil and natural gas through increased heat and pressure provided by burial.

9. Sedimentary structures form before lithification. Horizontal bedding planes are the most common feature of sedimentary rocks and reflect original horizontality and superposition. Cross bedding is inclined and most common in sandstones as a reflection of wind or water currents. Graded bedding exhibits a vertical change in grain size and characterizes turbidity current deposition. Mud cracks require air-drying of very fine-grained sediments. Ripple marks are either symmetric (waves) or asymmetric (currents) and form in any clastic rock. Fossils are traces of plants or animals buried by sediment and preserved as unaltered original material, replacements, molds or carbon films. Fossils may occur in any sedimentary rock type, but are most common in limestones.

10. Formations are bodies of rock recognized as a convenient means to map, describe and interpret the geology of a region. The first name is a geographic location where it is well exposed, and the second name is its rock type. The bounding surfaces are called contacts.

11. Source area of the sedimentary rock is determined by the composition of its grains (for example feldspar, quartz and mica indicate a granitic source). Sedimentary deposits thin away from their source, and sedimentary structures may help determine direction of current flow.

12. Continental environments include glacial environments, alluvial fans, river channels, that usually have a gravel component. Flood plains and lakes usually develop shales. Dunes have high angle cross-bedding. Shallow marine environments include deltas (usually with thick siltstone and shale, cut by sandstone channels), beaches and barrier islands (well sorted, quartz sandstone), dunes (high angle cross bedding), lagoons (shales), shallow marine shelves (widespread sandstone, siltstone and shale), and reefs (massive limestone cores). Deep marine environments receive deposition from turbidity currents.

13. The distribution of sedimentary rocks may be controlled by plate tectonics. Convergent boundaries accumulate thick clastic deposits in sedimentary basins. Turbidity currents dominate forearc basins, while sediments derived from rising mountains fill backarc basins. These deposits may now be found in mountains marking those plate boundaries. Transform boundaries may have organic rich deposits, while diverging boundaries form rift valleys with gravels, lake deposits and evaporites.

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Technical terms

bedding

An arrangement of layers or beds of rock.

bedding plane

A nearly flat surface separating two beds of sedimentary rock.

cement

The solid material that precipitates in the pore space of sediments, binding the grains together to form solid rock.

cementation

The chemical precipitation of material in the spaces between sediment grains, binding the grains together into a hard rock.

chemical sedimentary rock

A rock composed of material precipitated directly from solution.

chert

A hard, compact, fine-grained sedimentary rock formed almost entirely of silica.

clastic sedimentary rock

A sedimentary rock composed of fragments of preexisting rock.

clastic texture

An arrangement of rock fragments bound into a rigid network by cement.

clay

Sediment composed of particles with diameter less than 1/256 mm.

coal

A sedimentary rock formed from the consolidation of plant material. It is rich in carbon, usually black, and burns readily.

compaction

A loss in overall volume and pore space of a rock as the particles are packed closer together by the weight of overlying material.

conglomerate

A coarse-grained sedimentary rock (grains coarser than 2 mm) formed by the cementation of rounded gravel.

contact

Boundary surface between two different rock types or ages of rocks.

cross-bedding

An arrangement of relatively thin layers of rock inclined at an angle to the more nearly horizontal bedding planes of the larger rock unit.

crystalline texture

An arrangement of interlocking crystals.

crystallization

Crystal development and growth.

deposition

The settling or coming to rest of transported material.

dolomite

A sedimentary rock composed mostly of the mineral dolomite.

environment of deposition

The location in which deposition occurs, usually marked by characteristic physical, chemical, or biological conditions.

evaporite

Rock that forms from crystals precipitating during evaporation of water.

formation

A body of rock of considerable thickness that has a recognizable unity or similarity making it distinguishable from adjacent rock units. Usually composed of one bed or several beds of sedimentary rock, although the term is also applied to units of metamorphic and igneous rock. A convenient unit for mapping, describing, or interpreting the geology of a region.

fossil

Traces of plants or animals preserved in rock.

graded bed

A stream that exhibits a delicate balance between its transporting capacity and the sediment load available to it.

gravel

Rounded particles coarser than 2 mm in diameter.

limestone

A sedimentary rock composed mostly of calcite.

lithification

The consolidation of sediment into sedimentary rock.

matrix

Fine-grained material found in the pore space between larger sediment grains.

mud crack

Polygonal crack formed in very fine grained sediment as it dries.

organic sedimentary rock

Rock composed mostly of the remains of plants and animals.

original horizontality

The deposition of most water-laid sediment in horizontal or near-horizontal layers that are essentially parallel to Earth’s surface.

pore space

The total amount of space taken up by openings between sediment grains.

recrystallization

The development of new crystals in a rock, often of the same composition as the original grains.

ripple mark

Any of the small ridges formed on sediment surfaces exposed to moving wind or water. The ridges form perpendicularly to the motion.

rounding

The grinding away of sharp edges and corners of rock fragments during transportation.

sand

Sediment composed of particles with a diameter between 1/16 mm and 2 mm.

sandstone

A medium-grained sedimentary rock (grains between 1/16 mm and 2 mm) formed by the cementation of sand grains.

sediment

Loose, solid particles that can originate by (1) weathering and erosion of preexisting rocks, (2) chemical precipitation from solution, usually in water, and (3) secretion by organisms.

sedimentary breccia

A coarse-grained sedimentary rock (grains coarser than 2 mm) formed by the cementation of angular rubble.

sedimentary rock

Rock that has formed from (1) lithification of any type of sediment, (2) precipitation from solution, or (3) consolidation of the remains of plants or animals.

sedimentary structure

A feature found within sedimentary rocks, usually formed during or shortly after deposition of the sediment and before lithification.

shale

A fine-grained sedimentary rock (grains finer than 1/16 mm in diameter) formed by the cementation of silt and clay (mud). Shale has thin layers (laminations) and an ability to split (fissility) into small chips.

silt

A tabular intrusive structure concordant with the country rock.