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History of Sedimentary Rocks
Make up 75% of the Earth’s surface
In the past the continents have been covered by shallow, warm seas which deposited thick sequences of sediments.
Sedimentary Rocks
Rocks made from sediments, pieces of other rocks, or plant and animal remains.
Sediments pieces of solid material
deposited on the Earth’s surface.
Form in layers by water or wind.
Sedimentary Rock Stratification
the arrangement of sedimentary rocks in distinct layers (strata)
layer represents the sediment deposited over a specific period
Sedimentary Rocks
Clastic – made from fragments of other rocks
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.
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 RocksChemical – form due to evaporation
Dissolved minerals are left over
Rock salt, rock gypsum, halite, calcite (limestone)
Think about it: Where would these rocks form? Sea, lake, swamps, or underground waters
that contain dissolved minerals
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 Biological or Organic Rocks – formed from
the remains of plants and animals.
Shells of marine animals pile up, compact and cement to create fossiliferous limestone (coquina).
Plants pile up and compress over time to form coal.
Where would these form?
Swamps – large amount of build up of organic material.
Oceans
Coquina – cemented
aggregate of geologicallymodern 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 H2O, which impedes decay. H2 and O2 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.
Features: Stratification Fossils Ripple marks & crossbeds Mud cracks Nodules, concretions & geodes
Rock Stratification (layering) Bryce Canyon, 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
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.
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