Sedimentary Rocks

Sedimentary Rocks Sediments = pieces of solid

material deposited on the Earth’s surface.

Sedimentary Rock Rocks that are composed of the

weathered remains of preexisting rock, or plant and animal remains.

Sedimentary rocks commonly originate from sediments laid down in horizontal strata by water or wind.

Horizontal layers called “Beds” separated by “Bedding planes”, are a common feature in sedimentary rocks.

Horizontal Beds of Sedimentary Rock

BedsBedding planes

Sedimentary Rocks How is a sedimentary rock

formed?

Sediments get compacted and cemented together.

Sedimentary Rocks Detrial – made from fragments of

other rocks, that have been transported, deposited, then compacted and cemented together. Shale, sand, conglomerate, siltstone,

breccia Classified by the size of the fragments in

the rocks Think about it: What environments would

create these types of rock? High energy fast moving water (carrying

fragments), then water slows down (lake or ocean) and deposits fragments.

Cemented Rocks Detrial sedimentary rock – rocks

composed of weathered sediments: Pebbles or gravel – usually quartz Sand – usually quartz Clay and silt – weathered feldspars

and mica : held together by a natural cement

Silica – commonly gray-white in color Calcite – commonly gray-white in

color Iron oxide – commonly reddish to

yellow brown in color, or by compaction of clay and silt.

Conglomerate – cemented sand, silt, and pebble sediments. If large fragments are angular this rock is called a breccia.

Sandstone – cemented

quartz sand grains. Feels

gritty. Unfilled spaces between

grains make most

sandstones porous and

permeable to water.

Shale – clay and silt sized particles lithified by dehydration and compaction. Note the cleavage at bedding planes. Thumps when you tap it with a nail and, moistened, it smells like damp earth.

Bedding planes

Sandstone in the Pinnacle Desert, Australia

Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL

Sedimentary Rocks Chemical – formed when

minerals fall out of solution. From evaporation of salt water or from chemical reactions. Rock salt, rock gypsum, some

limestones Think about it: Where would these

rocks form? Sea, lake, swamps, or underground

waters that contain dissolved minerals

Chemical Sedimentary Rock Rocks formed either as

precipitates or as evaporites of dissolved chemical sediments.

Mineral salts that accumulate in water become concentrated by evaporation until they precipitate from solution, or

Mineral salts are deposited as a result of the total evaporation of the solution.

Rock salt, the mineral halite (NaCl), left as an evaporite as a shallow sea evaporated.

Alabaster, the mineral gypsum (CaSO4), also an evaporite.

Compact (or precipitate) limestone, the mineral calcite (CaCO3), precipitated from sea water as evaporation increased concentration. Many cavern systems are formed in this type of limestone.

Sedimentary Rocks Organic Rocks – formed from the

remains of plants and animals. Shells of marine animals pile up,

compact and cement to create fossiliferous limestone (coquina).

Where would these form? Oceans

Plants pile up and compress over time to form coal.

Where would these form? Swamps – large amount of build up of

organic material.

Organic Sedimentary Rock

Rocks formed from the altered remains of plant and animal material

An exception to the definition of rocks as a mixture of minerals (Remember: Minerals are inorganic substances).

Organic material exists in sequences of sedimentary rock to the extent that it cannot be ignored as a true rock type.

Organic Sedimentary Rock (Cont)

Molds, casts, and other traces of ancient plant and animal life, called fossils, are a common feature of sedimentary rock.

Some layers of rock, coal and limestone in particular, are formed, almost, entirely of the remains of living things.