Sedimentary Rocks

Sediment

any loose, solid particles that originate from either weathering and erosion of rock, or the chemical precipitation of rock from solution.

Gravel: particles >2mm

Boulder: basketball sized and up

Cobble: between a tennis ball and a basketball

Pebble: between a tennis ball and the thickness of a nickel

Smaller ParticlesSand: grains between 1/16 mm and 2 mm

(between the thickness of a nickel and the thickness of a human hair)

Silt: grains between 1/16 and 1/256 mm (too fine to feel gritty to the touch, but still feels gritty to the teeth)

Clay: grains smaller than 1/256 mm (too fine to feel gritty to the teeth)

vocabulary

• Weathering- destructive processes that change the chemical and physical character of rock near the Earth’s surface.

• Erosion- is the picking up or physical removal of rock particles by water, wind, ice, etc.

mechanical vs. chemical weathering

Weathering

Soil ErosionMadison County, AR 2004 Southwestern Arkansas 2005

Transportation of from one place to another can alter the shape and size of sediment particles.

• Rounding: The grinding away of sharp edges caused by the grinding and scraping of sediment against each other.

• Sorting: Sediment grains can be sorted according to size, shape, or specific gravity by water or wind.

Sorting

Deposition- as stream velocity slows, it can’t hold the larger grains in the water column anymore. At each drop in velocity, progressively smaller particles fall out.

note: Not all sediments are preserved where they are first deposited. For example, a river can deposit sand along its banks, but a large flood can later remove that sand before it is preserved as a rock.

Lithification: The process that compacts, cements, and/or crystallizes loose

sediment together to form a rock.

The processes of Lithification

Compaction: The rearranging of sediment so there is much less open space between the grains.

Cementation: The precipitation of a mineral between the grains that holds the grains together to make a solid rock. Calcite and silica (quartz) are common cements.

Sedimentary rocks are split into two groups: clastic and non-clastic.

• Clastic: rocks formed from the weathered fragments of existing rock.

• Fragments can be large (pebbles) or tiny grains of a mineral (quartz sand).

non-clastic sedimentary rocks

rocks formed from:

1. the chemical precipitation of minerals from solution (rock salt)

2. the formation of rock by living organisms (coral reef)

3. the accumulated and compressed remains of soft bodied organisms (coal).

clastic sedimentary rocks

marine carbonate deposition

non-clastic sedimentary rocks

fossiliferous limestone

evaporites (rock salt)

Some features found in sedimentary rock that are formed during or just after deposition, but before lithification.

Bedding: Individual layers of (mostly) parallel sedimentary rock.

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Some beds are easy to see because of the color differences, others are a distinguished by a change in texture or weathering patterns.

Cross-Bedding: The preserved tracks of dunes as they migrated through air or water.

Photo: A. Marshall

Graded Bedding: A single bed that has grain sizes sorted vertically from large to small.

Ripple Marks: Ripples that have been preserved as a rock.

Photo: A. Marshall

Sedimentary Conglomerate

Identified by the presence of rounded pebbles in a fine grained matrix.

Formation: Forms from the lithification of rounded, but not sorted rock fragments.

Depositional environment(s): Rock that falls into pounding surf or a fast moving river could be rounded but not sorted. Outcrops are usually the small scale you would expect from this kind of depositional history.

Sedimentary Breccia

Identified by large angular fragments in a fine grained matrix.

Formation: formed from the cementation of rock fragments that have not been sorted or rounded, so they probably formed near their source rock.

Depositional environment(s): Steep slopes beneath weathering outcrops could accumulate angular fragments, or a landslide could collect fragments like this. This rock is not very common.

Quartz Sandstone

Identified by sandy texture and lack of any other minerals but quartz.

Formation: formed from the cementation of quartz sand, color depends on the cementing mineral, but can be buff shades of brown, red, yellowish, even green!

Depositional environment(s): river beds and deltas, sand dunes, beaches

Siltstone

Identified by the very fine grain size without the plated look of shale.

Formation: forms from the cementation of very fine quartz grains. Color is usually shades of brown or gray, sometimes yellowish.

Depositional environment(s): transitional zones between moving waters such as rivers, and still, quiet waters such as lakes, and river flood plains.

Shale

Identified by grains too small to be seen with the eye and fissle property.

Formation: forms from very fine grain sediment being compacted and cemented. Color is usually shades of gray and black, can also be brown, yellowish or green.

Depositional environment(s): very still, quiet waters such as deep ocean basins, river flood plains, and lakes.

Fossiliferous Limestone

Identified by the abundance of fossils, and strong reaction with HCl acid.

Formation: formed by the cementing of the hard parts of organisms by calcite. Colors range from white to tan, brown, and gray, sometimes yellowish.

Depositional environment(s): marine setting, warm, shallow water offshore

Dolostone

Identified by the very fine crystalline texture and the weak reaction to HCl.

Formation: forms from the recrystallization of limestone after lithification

Depositional environment(s): Lithified rock where seawater seeping through can bring in extra magnesium that replaces some of the calcite. This process often destroys whatever fossils were present, making it nearly impossible to determine the original environment the rock formed in.

Chalk

Identified by the powdery texture that comes off on your hands.

Formation: forms from the compaction of microfossils

Depositional environment(s): microorganisms in the water column die and pile up on the ocean floor. Eventually they are compacted together to form a rock.

Chert

Identified by very fine grain texture and hardness (it is silica, like quartz).

Formation: 2 ways to get chert: 1. replacement of original rock with silica, 2. accumulation and cementation of very tiny microfossils

Depositional environment(s): 1. groundwater moving through lithified rock replaces some of the original rock structure with silica. Usually seen as lumps, nodules, and veins in other types of rock, or 2. tiny marine fossils build up in the same way chalk is formed, but are more cemented and usually later replaced by fine grain silica, destroying the fossils. Geologist are still debating which idea is the best, if either.

Coal

Identified by black color, light weight and softness (hardness of 1.5-2.5).

Formation: formed by the compaction of organic material that has not completely decayed. Color is black. Anthracite coal is more shiny due to extra heat and pressure after formation, and bituminous is more dull.

Depositional environment(s): an environment with vigorous organic production and water with a low oxygen content are needed. So the best environment to form coal is in a bog or swamp.