CHAPTER 6 - SEDIMENTARY ROCKS

Processes of weathering produce SEDIMENT. Particles continue to be broken down as they are TRANSPORTED (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- OXIDES.

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 CLAY.
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.

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.

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 microplankton 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.

INORGANIC PRECIPITATES - mineral precipitates that do not involve organic activity for their primary origin.

LIMESTONE - inorganic varieties include:
a) TRAVERTINE - forms around springs (examples include facades the Grace Building and Rockefeller Center;
b) OOLITIC LIMESTONE - white, round pellets of calcite formed by wave turbulence in shallow, warm marine water. Example: "Empire State Building" facade.
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 Mideast region).
CHERT (SiO2) - a precipitate of silica. It has many forms, including: FLINT, AGATE, JASPER, fossil wood and bone replacement. Some chert is possibly organic.
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 surface
CROSS BEDDING - migration of sand dunes produces these features
FOSSILS - the preserved remains of dead organisms

FOSSIL FUELS

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:

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? - unknown origin, may or may not be organic.
DIAMONDS? - do NOT form from COAL - that's Hollywood!

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. (map on p. 136).

OIL & NATURAL GAS consist of HYDROCARBONS. The difference is that GAS is a vapor at standard surface temperatures and pressures.

TAR SANDS and OIL SHALES are vast deposits of hydrocarbon resources that are to "thick" to be extracted by conventional oil well means.

Click here to return to the Class Website