Sedimentary Rocks

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.

Horizontal Beds of Sedimentary Rock

BedsBedding planes

Law of Superposition

states that younger rocks are deposited on top of older ones.

Sedimentary Rock Stratification

the arrangement of sedimentary rocks in distinct layers (strata)

layer represents the sediment deposited over a specific period

Sedimentary Rocks

How is a sedimentary rock formed?

Sediments get compacted and cemented together.

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

Marcellus Shale Map of PA

shale

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.

Coal Map of PA

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.

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