Sedimentary RocksEvery stone a story, like a rosary.

---Joan Osborne

WeatheringThe sediments that make up sedimentary rocks are produced by:

Mechanical & Chemical Weathering

Sedimentary Rocks and ProcessesSedimentary Rocks and Processes

ClasticClastic Non-ClasticNon-Clastic

Sediments are derived from fragments of pre-existing

rocks (clastsclasts).

All sedimentary rocks not composed of clasts

Categorized primarily by clast size

Categorized primarily by composition

Clastic RocksClastic RocksClastic RocksClastic Rocks

Formed from clasts (bits and pieces of pre-existing rocks). Formed from clasts (bits and pieces of pre-existing rocks). The major processes in clastic rock formation are:The major processes in clastic rock formation are:

• WeatheringWeathering of clasts from pre-existing rock

• TransportTransport of clasts

• DepositionDeposition of clasts

• LithificationLithification of clasts

clast size

WeatheringWeathering processes continue to alter the clasts until depositiondeposition. In general, both physical and chemical weathering tend to make clasts smaller and rounder.

Sorting of clasts by size occurs mostly during transportationtransportation.

e.g., conglomerate, breccia, sandstone, siltstone, shalee.g., conglomerate, breccia, sandstone, siltstone, shale

http://www.glossary.oilfield.slb.com/

Wentworth Scale

Boulders

Cobbles

Gravel (pebbles)

Sand

Silt

Clay

Sedimentary clasts are classified by grain sizegrain size (the size of the individual clasts) using the Wentworth Scale:

Larger clasts require more energy to

transport

Smaller clasts require less energy

to transport

Clast SortingClast Sorting The farther the clasts have traveled from the source, the more well sortedwell sorted they tend to be.

Clast RoundingClast Rounding

As clasts are rolled around during transportationtransportation, sharp corners tend to be knocked off, and the clasts become more roundedrounded the farther they are transported.

Angular grains are usually found only near the source rock.

Deposition

Proximal Depositional Proximal Depositional EnvironmentsEnvironments

Near the source rock, the clasts have not experienced much physical or chemical weathering.

These areas contain poorly rounded and poorly sorted clasts.

Clasts composed of chemically unstable minerals (e.g., feldspar) are most common close to the source rock.

Distal Depositional Distal Depositional EnvironmentsEnvironments

Clasts experience both chemical and physical weathering as they are transported. Thus, the further they have traveled from the source the more altered the clasts are.

Distal areas contain well rounded and well sorted clasts. Large clasts (e.g., boulders) are rare (they have either been left upstream, or have weathered into smaller clasts).

Clasts composed of chemically unstable minerals (e.g., feldspar) are very rare in distal settings. Clay minerals derived from feldspars are common.

http://www.flmnh.ufl.edu/alligatorfund/potm-nov04.html

http://www.ufz.de/index.php?de=5366http://www.ufz.de/index.php?de=5366

Clast size indicates ancient relative current velocity

>=1.8 km/hr (strong currents)

weak moderate

Clastic Sedimentary Rocks

Formed from clasts (bits and pieces of pre-existing rocks) transported, deposited,and lithified

Chemical Sedimentary RocksChemical Sedimentary RocksFormed from precipitation of minerals within the depositional basin. Includes

biominerals, evaporites, coal, chert, etc.

Chemical Sedimentary RocksChemical Sedimentary Rocks

http://csmres.jmu.edu/geollab/Fichter/SedRx/chembiokey.html

EvaporitesEvaporites

minerals precipitated from evaporating water

Siliceous RocksSiliceous Rocks

Silica and opalline silica precipitate either organically or inorganically

Coal and PeatCoal and Peat

Carbon-rich deposits formed from build-up of plant material in swamps

CarbonatesCarbonates

Variety of limestones composed of calcite, aragonite, and dolomite precipitated either organically or inorganically

Corals and SnailsCorals and Snails

Common Minerals:CalciteCalcite CaCO3

AragoniteAragonite CaCO3

DolomiteDolomite (CaMg)CO3

CarbonatesCarbonates - - LimestoneLimestone Biochemical (corals)

Bioclastic (chalk, coquina)

Inorganic – chemical precipitate

travertine

tufa

oolitic limestone

Chalk Limestone formed from the microscopic calcite skeletons of tiny animals.

Siliceous Sedimentary RocksSiliceous Sedimentary RocksMicrocrystalline quartzMicrocrystalline quartz and opalopal precipitated from dissolved ions in sedimentary environments. Can be precipitated organically or inorganically.

Microcrystalline quartz can also precipitate in pore spaces to form cement.

http://www.diskworks.com/octoberopals/opal1.html

Scanning Electron Micrograph (SEM) Scanning Electron Micrograph (SEM) of tiny, siliceous skeletons of radiolarians, which are a source silica chertchert

Ionic Composition of SeawaterIonic Composition of Seawater

Cl- 18,980

(SO4)2- 2,649

(HCO3)- 140

Anions

Na+ 10,556

Mg2+ 1,272

K+ 380

Ca2+ 400

Cations

parts per million

Ions: 34,387 ppmH2O: 965,613 ppm

35 o/oo normal marine salinity

Total

NaCl

Common Mineral Salts

KMgCl3•6H2O - CarnaliteMgCO3 - MagnesiteCaMg(CO3) - DolomiteCaSO4 - Anhydrite

Other Mineral SaltsOther Mineral Salts

- Halite

CaSO4•H2O

KCl - Sylvite

- Gypsum

Fossil example of the past activity of organisms mixing sediment --- an example of fossil BIOTURBATION