Horizontal Beds of Sedimentary Rock Beds Bedding planes
How is a sedimentary rock formed?
Sediments get compacted and cemented together.
Deposition Compaction Cementation
Clastic – 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
Clastic 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
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
Chemical – formed when minerals fall out of solution. From evaporation of salt water or from chemical reactions.
Rock salt, rock gypsum, some limestones
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
Rock salt , the mineral halite (NaCl), left as an evaporite as a shallow sea evaporated. Alabaster , the mineral gypsum (CaSO 4 ), also an evaporite.
Compact (or precipitate) limestone , the mineral calcite (CaCO 3 ), precipitated from sea water as evaporation increased concentration. Many cavern systems are formed in this type of limestone.
Organic Rocks – formed from the remains of plants and animals.
Shells of marine animals pile up, compact and cement to create fossiliferous limestone (coquina).
Sedimentary rocks are the only rocks that contain fossils
Coquina – cemented aggregate of geologically modern shell fragments. Fossiliferous limestone – a cemented aggregate of original shell fragments, molds, and casts of ancient marine organisms. Note fossil mold of a shell in this specimen.
Peat , a mass of matted together plant material covered by H 2 O, which impedes decay. H 2 and O 2 are lost, concentrating carbon. Lignite , so called “brown coal”, a soft coal that forms when peat is compressed and aged, about 40% carbon.
Bituminous coal – soft coal formed when lignite is compacted and altered for millions of years, about 85% carbon, this coal is the most commonly mined and used for a fuel.
Sedimentary Rock Features
Features in sedimentary rock that reflect the sedimentary environment.
Not found in other rock types.
Ripple marks & crossbeds
Nodules, concretions & geodes
Rock Stratification (layering) Bryce Canyon, UT Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL
Ripple marks caused by wave action on the sandy bottom of a shallow bay Almost identical ripple marks on the surface of a sandstone millions of years old.
Mud cracks in drying mud Mud cracks preserved on the bedding surface of a shale.
Groundwater dissolves hollow spaces in sedimentary rock, typically limestone, and mineral material is deposited inside the hollow with crystal points growing toward the center. Geode Thunder Egg
Cross-bedding at Checkerboard Mesa Zion National Park, UT Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL
Dinosaur skeleton preserved in sedimentary rock - China Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL
Metamorphic rocks are formed a few kilometers below the earth’s surface and extend into the upper mantle
2 types of metamorphism
Contact - when magma forces its way into the rock
Regional -results in large scale deformation of rock, usually takes place during mountain building
Agents of metamorphism
Heat - the most important agent because it drives the energy needed for a chemical reaction
Pressure - results in a more compact rock
Hydrothermal solution- very hot water based solution. When solution comes in contact with rock it will dissolve crystals and deposit new ones (only agent that changes the overall chemical composition of rock)
Metamorphic rocks are classified by texture
Foliated-minerals recrystallize at right angles to the direction of the force, rocks are said to be folded or have bands that go around rocks
Example- Gneiss and Schist
Nonfoliated-rocks that does not have banded texture