CHAPTER 6 - SEDIMENT AND SEDIMENTARY ROCKS
Processes of weathering produce SEDIMENT. Particles continue to
be broken down as they are TRANSPORTED and SORTED (by the processes of
EROSION). Following DEPOSITION, sediments become SEDIMENTARY
ROCKS by the processes of LITHIFICATION
LITHIFICATION - the processes of converting sediment into
sedimentary rock through COMPACTION and CEMENTATION. Most
important cements in sedimentary rocks: CALCITE, SILICA, IRON-
TWO TYPES OF SEDIMENTARY ROCKS
- DETRITAL SEDIMENTARY ROCKS - rocks that form from transported
solid material. Detritus - Latin for "worn down." Solid
rock fragments in sediment are defined by size of the fragments:
largest to smallest: BOULDERS, COBBLES, PEBBLES, SAND, SILT, and
- CHEMICAL SEDIMENTARY ROCKS - formed when dissolved substances
precipitate from water to form sediments and eventually rock;
include salts such as ROCK SALT, GYPSUM, and LIMESTONE, and biochemically mediated rocks
such as peat, and reef rock (an organic limestone).
TYPES OF DETRITAL SEDIMENTARY ROCKS
CONGLOMERATE - forms from cemented GRAVEL (a mixture of cobbles,
pebbles, and coarse sand), such as from a stream gravel bar. The
gravel particles are "rounded" by erosion processes.
BRECCIA - forms from cemented "gravel-sized" fragments of rock,
but the rock fragments are very "angular" in shape. Broken rock
fragments are common in fault zones and in volcanic areas.
SANDSTONE - a sedimentary rock formed by cemented SAND. (Sand
particles are between 1/16-2mm; sandstone usually displays a high
degree of SORTING due to wind and water transport processes.
SILTSTONE - forms from silt (particle sizes: 1/16-1/256mm).
SHALE - forms from clay(particles: <1/256mm); shale displays laminae (layering).
MUDSTONE - forms from "mud" - a mix of clay and silt; mudstone
generally lacks clear lamination.
CHEMICAL SEDIMENTARY ROCKS
Chemical sedimentary rocks can be subdivided into groups of
BIOCHEMICAL ORIGIN and INORGANIC PRECIPITATES.
BIOCHEMICAL ORIGIN - Rocks formed from the accumulation
of organic remains include various kinds of LIMESTONE, CHERT,
COAL, and TAR.
INORGANIC PRECIPITATES - mineral precipitates that do not involve
organic activity for their primary origin.
- LIMESTONE - accumulations of sediment of organic remains
(body fragments and fecal material) consisting dominantly of
CaCO3 (calcite): includes:
a) REEFS (buildup of marine organisms with a skeletal framework)
b) COQUINA - "shell hash" - most shells consist of CaCO3
c) CHALK - calcareous plankton,mostly a kind of planktonic algae called coccoliths, die and accumulate.
- COAL - accumulations of plant remains deposited in swamps.
- TAR: degraded oil & gas - asphalt from organic residues);
originally deposited as organic matter, mostly planktonic algae.
- LIMESTONE - inorganic varieties include:
a) TRAVERTINE - forms around springs (examples include facades
the Grace Building and Rockefeller Center;
b) OOLITIC LIMESTONE - white, round pellet-sized concretions of calcite formed
by wave turbulence in shallow, warm marine water.
- DOLOMITE - limestone altered by seawater becomes "enriched"
in magnesium: has a composition of(Ca,Mg)(CO3)2; dolomite is
important because it has "high porosity"; make good oil & gas
reservoirs:~50% of the oil in the world comes from dolomite in
- CHERT (SiO2) - a precipitate of silica. It has many forms,
including: FLINT, AGATE, JASPER, fossil wood and bone
replacement. Some chert is possibly organic. For instance, diatomite made of the silica
shells of diatoms can be altered and turned to chert.
- EVAPORITES: the name applied to salts that precipitate from
water by the process of concentration by evaporation; includes:
a) Rock Salt (Halite, NaCl)
b) Rock Gypsum (Gypsum CaSO4 + (2)H2O)
c) Potash (Sylvite, KCl)
Sedimentary rocks can be classified by TEXTURE characteristics.
If the rock consists of transported grains it is called CLASTIC -
"broken fragments." If individual crystals consist of
interlocking crystals formed from direct precipitation for water
it is considered NONCLASTIC.
FEATURES IN SEDIMENTARY ROCKS
- BEDDING PLANES - layers between sedimentation "events"
- RIPPLE MARKS - surface features cause by a moving fluid
- MUD CRACKS - drying, shrinking, and cracking of a muddy
- CROSS BEDDING - migration of sand dunes produces these
- FOSSILS - the preserved remains of dead organisms
Solar energy ultimately the source of all processes leading to
the accumulation of coal, oil, gas, and tar. Examples include:
COAL - Plant material accumulates in swamps (usually coastal
areas); coal goes through a series of changes as it undergoes
formation and burial as follows:
COAL RESERVES are huge; US has major reserves to last
current population for 100's of years. Coal burning releases
CO2, SO2, and other gasses responsible for acid rain. Pyrite
(FeS) in coal is responsible for acid mine drainage.
- PEAT - organic material in swamp deposits where
lack of oxygen prevents further decay; low energy content.
- LIGNITE - brown, poorly lithified plant material
(abundant in Texas and Maine)
- BITUMENOUS (soft) coal - Rocky Mountain & Appalachian coal
- ANTHRACITE (hard) coal - Metamorphosed coal; high energy
(forms by very high pressure deep within mountain ranges)
- GRAPHITE? - various origins, may or may not be
- DIAMONDS? - do NOT form from COAL - that's Hollywood!
OIL & NATURAL GAS consist of HYDROCARBONS. The difference is
that GAS is a vapor at standard surface temperatures and
TAR SANDS and OIL SHALES are vast deposits of hydrocarbon
resources that are too "thick" to be extracted by conventional oil
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