Sedimentary RockSection 3.3

Sedimentary RocksAll sedimentary rocks begin to form when

existing rocks are broken down into sediments through a process called weathering

Weathering is the process in which rocks are physically and chemically broken down by water, air, ice and gravity

Sediments are transported, by water, gravity, glaciers or wind, and eventually become compacted and cemented together, forming sedimentary rocks

Formation of Sedimentary RocksSedimentary comes from the Latin word sedimentum, which means settling

There are several major processes that contribute to the formation of sedimentary rocks:Weathering, Erosion and DepositionCompaction and Cementation

Weathering, Erosion and DepositionWeathering is any process that breaks

rocks into sedimentsChemical weathering takes place when the

minerals in rocks change into new substancesErosion involves weathering and the

transportation of sedimentsDeposition is the process when water, wind,

ice or gravity loses energy causing sediments to dropLarger sediments are deposited first,

smaller sediments are dropped later

Compaction and CementationOnce sediments are deposited, they

become lithified or turned into rockCompaction is a process that squeezes,

or compacts, sediments, pushing water outCompaction is caused by the weight of

sedimentsCementation takes place when dissolved

minerals are deposited in the tiny spaces among the sediments

Classification of Sedimentary RocksSedimentary Rocks can be classified into

two main groups according to the way they formClastic – rocks that are made of

weathered bits of rocks and mineralsChemical – forms when dissolved minerals

precipitate from water solutions

Clastic Sedimentary RocksClastic sedimentary rocks are made of

weathered bits of rocks and mineralsClastic sedimentary rocks can be grouped

according to the size of the sediments in the rocksConglomerate rocks are made up of

rounded, gravel-size or larger particlesBreccia rocks have angular particlesSandstone rocks have sediments that are

sand-sized grains

Clastic Sedimentary RocksConglomerat

e

Sandstone

Breccia

Chemical and Biochemical Sedimentary RocksChemical sedimentary rocks forms when

dissolved minerals precipitate from water solutions

The precipitation generally occurs when water evaporates or boils off leaving a solid productExamples include limestone, rock salt,

chert, flint and rock gypsumBiochemical sedimentary rocks contain

shells and skeletal remains of organisms that settle to the ocean floor

Chemical Sedimentary Rocks

Rock Salt

Limestone

Chert

Flint

Biochemical Sedimentary Rocks

Features of some Sedimentary RocksSedimentary rocks give clues to how,

when and where the rocks formedLayers of sedimentary rock records a period

of sediment deposition. The oldest layers are found at the bottom

Ripple marks may indicate that the rock was formed along a beach or stream bed

Mud cracks are formed when wet mud or clay dried and shrank, leaving a record of a dry environment

Fossils found in sedimentary rocks can help answer questions about the rock

Ripple marks

Fossils

Layers of sedimentary rock

Mud cracks

Summary of Sedimentary RocksSedimentary rocks form as the result of

five major processWeathering produces particles called

sedimentsWind, water, ice and gravity erode and deposit sediments

Sediments are compacted and cemented to form sedimentary rocks

Sedimentary rocks can be classified according to how they form, by using rock formation, texture and composition

(Table 2 on page 79)