Sedimentary Rocks
Definition: Sedimentary Rock - rock formed from weathered products of pre-existing rocks that have
been transported, deposited and lithified
Definition: Weathering -the breakdown of rock at or near the surface of the Earth
-the weathered material is picked up, transported and deposited by agents of erosion
Two Types of Weathering:
1. Mechanical Weathering – the physical breakdown of rock, resulting in smaller fragments
2. Chemical Weathering – process where the internal structure of a mineral is changed by the
removal and/or addition of elements (chemical reactions occur)
Definition: Erosion -the transportation of material by a mobile agent, such as water,
wind or ice
Definition: Sediment -loose particles created by weathering and erosion of rock, by
chemical precipitation from solution in water, or from the
secretions of organisms, and transported by water, wind or ice
-sediment is deposited in beds and undergo lithification to form sedimentary rock
Definition: Lithification -the process of converting sediment to solid rock
-sediment is lithified into solid sedimentary rock by the processes of compaction and
cementation
Definition: Compaction – a type of lithification in which the weight of overlying material compresses
(or squeezes) more deeply buried sediment
Definition: Cementation – a type of lithification in which particles precipitate from water and filter
through sediment, filling open spaces and eventually a solid mass is formed
-geologists estimate that sedimentary rocks account for only about 5 percent (by volume) of Earth's
outer 16 kilometers
-about 75 % of all rock formations on the continents are sedimentary
The Importance of Sedimentary Rocks:
-sedimentary rock layers contain evidence of past events that occurred at the surface
-they contain clues of past environments in which their particles were deposited and the mechanisms
involved in their transport
-some sedimentary rock layers contain fossils that help geologists age Earth (Geologic Time Scale)
-many sedimentary rocks are very important economically, ie. coal is burned to provide a significant
portions of electrical energy in many countries and was mined in Cape Breton for many years
-other major energy sources associated with sedimentary rocks are petroleum and natural gas (ie.
offshore natural gas drilling near Sable Island)
-sedimentary rocks contain major sources of iron, aluminum, manganese, and fertilizer, plus
numerous materials essential to the construction industry
Types of Sedimentary Rocks:
1. Detrital Sedimentary Rocks – those formed from sediment that originally accumulated due to
mechanical and chemical weathering (all are CLASTIC)
2. Chemical Sedimentary Rocks
a) Organic Sedimentary Rocks – those containing materials generated by living
organisms (most are NON-CLASTIC)
b) Inorganic Sedimentary Rocks – those formed from sediment that is soluble and
originally formed from chemical weathering (most are NON-CLASTIC)
1. Detrital Sedimentary Rocks:
-contain CLASTS: they are composed of discrete particles that are compacted and
cemented together
-clay minerals and quartz are the chief constituents
-clay minerals are the most abundant product of the weathering of silicate minerals, especially the
feldspars
-clays are fine-grained minerals with sheet-like crystalline structures similar to the micas
-quartz is abundant because it is extremely durable and very resistant to chemical weathering
-when igneous rocks such as granite are attacked by weathering processes, individual quartz grains
are released
-other common minerals in detrital rocks are feldspars and micas
-because chemical weathering rapidly transforms these minerals into new substances, their presence
in sedimentary rocks indicates that erosion and deposition were fast enough to preserve some of the
primary minerals from the source rock before they could be decomposed
-particle size is the best characteristic for classifying detrital sedimentary rocks
-sizes of the component grains also provide useful information about environments of deposition, ie.
currents of water or air sort particles by size  the stronger the current, the larger the particle size
carried
-gravels are moved by swiftly flowing rivers as well as by landslides and glaciers  less energy is
required to transport sand, thus it is common to such features as windblown dunes and some river
deposits and beaches
-very little energy is needed to transport clay, so it settles very slowly
-accumulation of these tiny particles are generally associated with the quiet water of a lake, lagoon,
swamp or certain marine environments
Particle Size Classification for Detrital Rocks
Size Ranges
(millimeters)
Particle Name
>256
Boulder
64-256
Cobble
4-64
Pebble
2-4
Granule
1/16-2
Sand
1/256-1/16
Silt
<1/256
Clay
Common
Sediment Name
Detrital Rock
Gravel
Conglomerate or breccia
Sand
Sandstone
mud
Shale or mudstone
-Common detrital sedimentary rocks, in order of increasing particle size, are shale, sandstone, and
conglomerate or breccia
Shale
-a sedimentary rock consisting of silt- and clay-sized particles
-over half of all sedimentary rocks are shale
-particles in these rocks are so small that they cannot be readily identified without great magnification
 more difficult to study and analyze than most other sedimentary rocks
-tiny grains in shale indicate that deposition occurs as the result of gradual settling from relatively
quiet, non-turbulent currents like lakes, river floodplains, lagoons, and portions of the deep-ocean
basins
-usually precipitates only after larger grains have deposited on the bottom of the water
-sometimes the chemical composition of the rock provides additional information  ie. Black shale
which is black because it contains abundant organic matter (carbon) & deposition occurred in an
oxygen-poor environment such as a swamp, where organic materials do not decay quickly
- oil shale may be burned as an energy source
- can be used as a filler in paints, plastics, and roofing cements, and ground shale can be used as a
replacement for clay in the manufacture of bricks
Sandstone
composed mainly of sand-sized minerals or rock grains. Most sandstone is composed of quartz
and/or feldspar because these are the most common minerals in the Earth's crust.
Three major types of sandstone:
1. Quartz sandstone (also called quartz arenite)
2. Arkose
3. Litharenite or lithic sandstone (commonly but imprecisely called graywacke)
Roundness of grains is a good clue to a long time in the depositional environment. Rounding of
grains takes a long time; it is more likely in a tectonically passive situation
Conglomerate and breccia
-forms in many different environments and settings where the energy of transport is high enough to
move large grains.
- is much coarser than other clastic sedimentary rocks except for breccias.
- difference between conglomerates and breccias is the roundness of the grains. In conglomerates,
the grains are rounded and usually indicate that they have been transported or worked more than the
angular grains found in breccias.
2. a) Organic Sedimentary rocks
-include carbonate minerals created by organisms which cover the ocean floor with
layers of calcium carbonate which can later form limestone.
 Biochemical (skeletal) limestone is made up of wave broken fragments of algae,
corals, and shells. An example would be chalk.
 Coquina limestone is formed at the shoreline via cemented shells and shell
fragments.
 Inorganic limestones are a result of inorganic processes.
-Coal is another example:
Formation of Coal and Other fossil Fuels:
1.
2.
3.
4.
Plant matter accumulates at the bottom of a swamp
Because there is little oxygen, there is little decay
Increased pressure from overlying sediments expels water and air
Heat and pressure over time convert the plant matter to coal
2. b) Inorganic Sedimentary Rocks
-two processes which contribute to the formation of these are precipitation after supersaturation and
evaporation of water
Definition- evaporite: any of a variety of individual minerals found in the sedimentary deposit of
soluble salts that results from the evaporation of water



Halite – rock salt
Gypsum
Dolomite - Dolomites are usually formed by the partial replacement of calcium in calcite
(limestone) with magnesium. Usually as water solutions move through the limestone.