Sedimentary Rocks





Sedimentary Rocks Form from Sediments


Sedimentary rocks are formed at Earth’s surface by cementing together:

  • weathered fragments of pre-existing rock,
  • fragments of shells, accumulation of organic matter, or
  • precipitation of minerals dissolved in water.


Sedimentary rocks can indicate the environment of deposition based on the sediment grains in the rock and the cements that bind those grains together.




Sedimentary Rocks Are a Thin Cover

Sedimentary rocks form layers, like the pages of a book.

The layers record a history of ancient events and ancient environments on the ever-changing face of planet Earth. Like a book, geologists can read this history.

Sedimentary rocks occur only in the uppermost part of the crust and cover igneous and metamorphic basement rocks.


Grand Canyon






Different Types of Sedimentary Rocks

Physical and chemical weathering provide the raw materials (particles and dissolved ions) for all sedimentary rocks.


Geologists define four sedimentary rock classes:

  • Clastic: loose rock fragments (clasts) cemented together

  • Biochemical: cemented shells of organisms

  • Organic: carbon-rich remains of once-living organisms

  • Chemical: minerals that crystallize directly from water





Clastic: Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock.
























Clastic Sedimentary Rocks

Clastic (or detrital) sedimentary rocks consist of mineral grains, rock fragments, and cementing material.







Creating Clastic Sedimentary Rocks

Clastic sedimentary rocks are created by several processes operating at Earth’s surface, including:

  • weathering (generation of detritus via rock disintegration),
  • erosion (removal of grains),
  • transportation (dispersal of solid particles and ions by:
    • gravity,
    • wind,
    • water, and
    • ice),
  • deposition (settling out of the transporting fluid), and
  • lithification (transformation into solid rock).









Lithification transforms loose sediment into solid rock: burial increases pressure, squeezes out air and water, and compacts grains.

Minerals (often quartz or calcite) precipitate from groundwater into pore spaces.

This cement glues the loose sediments together







Classifying Clastic Sedimentary Rocks: Grain Size and Composition

Grain size is a measure of the size of fragments or grains.

Size ranges from very coarse to very fine (gravel, sand, silt, and clay).

As transport distance increases, grain size decreases.

Grain composition refers to the mineral makeup of sediment grains.

Mineral composition yields clues about the original source rock.

A variety of different clast compositions (or a lack thereof) hints at source area and transport processes.

Clasts may be comprised of individual mineral grains or rock fragments containing several mineral types.

Smaller particles are more likely to be composed of single mineral grains.








Classifying Clastic Sedimentary Rocks: Sorting

Sorting is a measure of the uniformity of grain sizes in a sediment population. Degree of sorting increases with transport distance.









Clastic Sedimentary Rocks: Breccia

Breccia—coarse, angular rock fragments.

Angularity indicates the absence of rounding by transport, hence, these are deposited relatively close to clast source.








Clastic Sedimentary Rocks: Conglomerate

Conglomerate—rounded rock clasts.

Clasts are rounded as flowing water wears off corners and edges; these are deposited farther from the source than breccia.









Clastic Sedimentary Rocks: Arkose

Arkose—sand and gravel with abundant feldspar; commonly deposited in alluvial fans.

Feldspar indicates short transport and arid conditions.








Clastic Sedimentary Rocks: Sandstone

Sandstone—clastic rock made of sand-sized particles. Quartz is, by far, the most common mineral in sandstones.











Clastic Sedimentary Rocks: Shale and Mudstone

Fine clastics are composed of silt and clay.

Silt-sized sediments are lithified to form siltstone.

Clay-sized particles form mudstone or shale.










Biochemical Sedimentary Rocks: Limestone

Biochemical sedimentary rocks are made of sediments derived from the shells of once-living organisms.

Hard mineral skeletons accumulate after the death of the organisms.

Limestone is a sedimentary rock made almost entirely of calcite* or aragonite* (which are CaCO3 varients).

These minerals are the most common materials used by organisms that make seashells.

Limestone often preserves the shells of fossil organisms, sometimes in great abundance.










Biochemical Limestone

Biochemical limestone forms in a unique depositional marine environment:

  • warm,
  • tropical,
  • shallow,
  • clear,
  • O2-rich

The CaCO3 in limestone comes from shells from a diversity of organisms (plankton, corals, clams, snails, etc.).



Biochemical Limestone










Biochemical Sedimentary Rocks: Chert

Chert: rock made of cryptocrystalline quartz derived from opalline silica (SiO2) from the skeletons of some marine plankton

Chert can occur in beds or as nodules.

It has all of the properties of quartz, including hardness and conchoidal fracture.










Organic Sedimentary Rocks: Coal and Oil Shale

Organic sedimentary rocks are made of organic carbon, the soft tissues of living things.

Rocks of this type include:

  • coal,
  • altered vegetation, and
  • oil shale.









Chemical Sedimentary Rocks: Evaporites

Chemical sedimentary rocks are composed of minerals precipitated from water solution.

They have a crystalline (interlocking) texture developed from initial crystal growth from solution (which may be recrystallized during burial).

Evaporites are derived from evaporation of large volumes of sea or lake water, and evaporite minerals include halite (rock salt) and gypsum.














Chemical Sedimentary Rocks: Travertine

Travertine is calcium carbonate (CaCO3) precipitated from groundwater where it reaches the surface.

Dissolved calcium (Ca2+) reacts with bicarbonate (HCO3-).

When CO2 is expelled into the air, it causes CaCO3 to precipitate.

This process occurs in thermal (hot) springs and in caves.