Sediments and Diagenesis The Rock Cycle is a group of changes. It is an ongoing process. Igneous...

Sediments and DiagenesisThe Rock Cycle is a group

of changes. It is an ongoing process.

Igneous rock can change into sedimentary rock or into metamorphic rock.

Sedimentary rock can change into metamorphic rock or into igneous rock.

Metamorphic rock can change into igneous or sedimentary rock.

Sediments and DiagenesisSediment & Diagenesis

75% of all the rocks exposed at the Earth’s land surface (only 5% of Earth’s outer 10 miles - so mostly at the surface)

I. Sediment

Rock fragments created by weathering and erosion

Accumulates everywhere—oceans, beaches, lakes, rivers, deserts, swamps, caves, base of glaciers

A. Classified based on its origin.

1. Detrital: composed of transported solid fragments (detritus) of preexisting rocks

2. Chemical: forms when previously dissolved minerals either precipitate from solution or are extracted from water by living organisms and converted to shells/skeletons

Sediments and Diagenesis

B. Sediment Transport and Deposition

1. DetritalGenerally move from high

ground to low ground by the pull of gravity (assisted by water, wind, or glacial ice)

Deposited when the carrying material loses it’s capacity to carry the sediment

2. ChemicalIons remain in solution until

there’s a change in the water’s temperature, pressure, or chemical composition and then the ions precipitate

Sediments

C. Sediment Texture: Detrital sediment and chemical sediment are classified based on composition.

1. Grain sizeGrain composition - some

minerals are stronger than others.

a. Distance - smaller grains travel longer distances.

b. Energy of the transportation medium - high energy environment moves larger grains.

Sediments

Sediments

2. Grain shapea. Distance transported - over far

distances, grained become more rounded.

b. Transportation medium

High energy river, wind produce rounded grains

Middle of glacier, mudflow, debris flow preserve angular grains – not enough distance to round grains

Sediments

C. Sediment Texture: detrital sediment and chemical sediment are classified based on composition.

3. SortingRelated to the carrying

capacity of the transport medium

Well sorted = windMedium - well sorting =

wavesMedium - poor sorting =

streamsPoorly sorted = glaciers,

debris flows

Sediments

Sediments

II. Turning sediments into rock

Eventually accumulated sediment turns into rock

A. Diagenesis: All the chemical, physical, and biological changes that take place after sediments are deposited.

Burial

Alteration by groundwater Recrystallization: regrowth of less

stable minerals into more stable ones.

Lithification: occurs within the upper few kilometers of the crust at temperatures < 200C (400F)



B. Lithification: the process by which unconsolidated sediments are transformed into solid sedimentary rocks (part of diagensis)

1. Compaction: pressure (from overlying sediment) reduces the volume of sediment—

Compaction forces out air and water and packs grains together.



B. Lithification

2. Cementation

Cements grains together - ions dissolved in water by chemical weathering may be deposited by groundwater circulating through the sediment.

Calcium carbonate = CaCO3

Calcium ions plus CO2 and H2O

Silica = SiO2 is produced by chemical weathering of silicate igneous rocks

Iron oxide = Fe2O3


III. Types of Sedimentary rocks

A. Detrital Sedimentary rocks: made of sediment that is transported as solid particles

Particle size is the primary basis for distinguishing various detrital sedimentary rocks.

Particle size indicates environment of deposition

Water and air currents sort by size.

Stronger current = larger grain sizes



A. Detrital Sedimentary rocks: made of sediment that is transported as solid particles

1. Shale (mudstone, siltstone)

>50% of all sedimentary rocks:

Silt and clay Small grain size = settling from

quiet, non-turbulent water

Silt and class form in lakes, lagoons, swamps, deep ocean.

Need quiet water depositional setting



A. Detrital Sedimentary rocks

2. Sandstone: sand sized particles (1/16 – 2 millimeters)

~25% of all sedimentary rocks

Windblown dunes, some rivers, and beaches

Shape and sorting important for determining depositional environment.

Sorting: well sorted = wind & wavespoorly sorted = streams

Shape: well rounded = water or wind transported over long distances

Angular = glacier or debris flow

Transported over short distances



A.Detrital Sedimentary rocks

2. Sandstone: sand sized particles (1/16 – 2 millimeters) (




Sorting: well sorted = wind & wavespoorly sorted = streams

Shape: well rounded = water or wind transported over long distances

Angular = glacier

Transported over short distances




2. Sandstone: sand sized particles (1/16 – 2 millimeters) (




Sorting: well sorted = wind & waves; poorly sorted = streams

Shape: well rounded = transported over long distances (water)

Angular = transported over short distances (glacier)



A.Detrital Sedimentary rocks

3. Conglomerate and Breccia—

Composed of gravels (pea to large boulders, >2 mm)

Conglomerate: composed of rounded grains of difference sizes.

Formed in energetic mountain streams or coasts (storm deposits)

Breccia: composed of angular pieces.

Did not travel far: glaciers, landslides




3. Conglomerate and Breccia

Composed of gravels (pea to large boulders, >2 mm)

Conglomerate: composed of rounded grains of difference sizes.

Formed in energetic mountain streams or coasts (storm deposits)

Breccia: composed of angular pieces.

Did not travel far: glaciers, landslides, (close to source)


Sediments and Diagenesis The Rock Cycle is a group of changes. It is an ongoing process. Igneous...

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