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 Bedding planes Horizontal Beds of Sedimentary Rock

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 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.

Bedding planes 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.

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. 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.