Sedimentary Rocks Form from Sediments
Sedimentary rocks are formed at Earth’s surface by cementing together:
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.
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: 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:
Lithification transforms loose sediment into solid rock: burial increases pressure, squeezes out air and water, and compacts grains.
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
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
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
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 forms in a unique depositional marine environment:
The CaCO3 in limestone comes from shells from a diversity of organisms (plankton, corals, clams, snails, etc.).
Biochemical Sedimentary Rocks: Chert
Chert: rock made of cryptocrystalline quartz derived from opalline silica (SiO2) from the skeletons of some marine plankton
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:
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