Chapter 7: Sedimentary Rocks (Rocks From Bits & Dissolved Stuff)

Sedimentary Rock Formation

Weathering makes sediment → moved → deposited → lithified (squeezed & glued)

Sedimentary Rock

formed at or near the Earth's surface from the accumulation and cementation of sediment, which can be fragments of pre-existing rocks, organic matter, or minerals that precipitate from water

Sedimentary Rock Coverage

Covers about 75% of Earth’s surface in a thin layer

Importance of Sedimentary Rocks

Record Earth’s history, including past environments, climates, and life forms

3 kinds of sedimentary rocks 

• Clastic (detrital)

• Chemical

• Organic

Clastic (detrital) sedimentary rocks

pieces of older rock (quartz, clay, etc.)

Clastic (detrital) Sorted by grain size

conglomerate/breccia (big), sandstone, siltstone/shale (tiny)

Rounded & well-sorted

long transport (wind/beach)

Angular & poorly sorted

short, rapid dump (glaciers/landslides)

Chemical Sedimentary Rocks

Form when minerals precipitate from water, either inorganically or through biological activity by organisms

Limestone (CaCO₃)

The most common chemical sedimentary rock; forms from reefs, shells, or cave deposits like travertine

Dolostone

Chemical sedimentary rock formed when magnesium-rich water alters limestone through dolomitization

Chert

Hard chemical sedimentary rock made of microcrystalline quartz; includes varieties like flint and jasper.

Evaporites

Chemical sedimentary rocks formed by evaporation of saline water; includes halite (rock salt) and gypsum (rock gypsum)

Organic Sedimentary Rocks

Form from the accumulation of plant remains rich in carbon, creating coal

Stages of Coal Formation

Peat → Lignite → Bituminous Coal → Anthracite; increasing heat and pressure transform plant material into harder coal types

Diagenesis

all changes after burial (<~200°C)

Compaction

squeeze water out

Cementation

natural “glues” (calcite/silica/iron oxide) fill pores

Sedimentary structures are

clues to the past

Sedimentary structures

• Bedding/strata

• Ripple marks

• Cross-bedding

• Graded bending

• Mud cracks

• Fossils

Sedimentary structures - Bedding/strata

layers

Sedimentary structures - Ripple marks

Water/wind flow direction

Sedimentary structures - Cross-bedding

Migrating dunes/bars

Sedimentary structures - Graded bending

coarse → fine (underwater avalanches)

Sedimentary structures - Mud cracks

Dried wet mud (was exposed)

Sedimentary structures - Fossils

Life + environment clues

Sedimentary Environments

The different settings where sediments are deposited, including continental, transitional, and marine environments.

Continental Environments

Land-based settings where sediments form; include glaciers, rivers, and deserts.

Glacial Environments

Produce poorly sorted sediment called till because glaciers carry and drop a mix of all grain sizes.

River (Fluvial) Environments

Form sand and gravel in river channels and mud on floodplains as water flow sorts materials by size.

Desert Environments

Produce well-sorted sand dunes formed by wind action in dry regions.

Transitional Environments

Border land and sea; include deltas, beaches, and tidal flats where sediment is influenced by both water and land processes.

Delta Environments

Form where rivers enter standing bodies of water, depositing layers of sediment in fan-shaped formations.

Beach Environments

Shaped by wave action; typically produce well-sorted sand deposits.

Tidal Flat Environments

Areas alternately covered and exposed by tides; contain mud, silt, and ripple-marked layers.

Marine Environments

Ocean settings where most chemical and fine-grained sediments form; divided into shallow and deep zones.

Shallow Marine Environments

Continental shelf areas where limestone and sandstone commonly form from shells, reefs, and sand.

Deep Marine Environments

Deep ocean basins where fine mud and silica-rich sediments form rocks like shale and chert.

Carbon Cycle Link

The carbon cycle connects sedimentary processes to Earth's climate through the storage and movement of carbon.

Carbon Storage in Sedimentary Rocks

Carbon is stored in limestone (CaCO₃) and fossil fuels like coal, oil, and natural gas.

Weathering and Climate

Weathering of silicate rocks uses up CO₂ from the atmosphere over long periods, which can cool Earth's climate.