Geol 135 Sedimentation

J Bret Bennington

Updated 9/99

Chemical and Pyroclastic Sedimentary Rocks

Carbonates (limestones and dolostones)

Carbonate minerals

Calcite (trigonal crystal)

Aragonite (orthorhombic crystal) - unstable, will recrystallize to calcite

Dolomite - calcium magnesium carbonate, forms as a replacement of calcite and aragonite

Carbonate Rock Classification - based on texture and grain type

Texture - based on a classification developed by Dunham in the early '60s.

Dunham's classification is particularly useful for describing carbonate rocks in hand specimen and in outcrop. In addition to the four main textures describing rocks with originally unbound sediments, there are a variety of textures used to describe carbonate strata with large, organic structures that trapped and bound sediment during deposition (boundstones).

Bafflestone

Bindstone (see chart handout for descriptions)

Framestone

Carbonate Grain Types (allochems)

Dunham's classification is primarily based on texture. It can be expanded by incorporating description of the types of carbonate grains present (see handout).

Skeletal grains - pieces of carbonate skeletal material (see below)

Non-skeletal grains:

Ooids - concentrically layered spheres of calcite less than 2mm in size

Pisoids - ooids larger than 2mm

Peloids - spherical to irregularly rounded pellets of micrite without internal structure

Oncoids - similar to pisoids, but with an irregular internal structure

Intraclasts - fragments of reworked, partially lithified carbonate

Micrite - lime mud; carbonate particles less than 4 mm in diameter.

Spar - a clear carbonate cement that precipitates between grains after deposition.

Carbonate intraformational conglomerates - these are called floatstone or rudstone in Dunham's classification, depending on the density of large intraclasts.

Folk's Classification

An alternative classification to Dunham's is sometimes used by sedimentologists studying carbonates using thin sections. Folk's classification combines a prefix based on the type of grain with a suffix denoting the type of matrix (micrite or spar). For example, an ooid grainstone cemented by spar would be called an oosparite.

 

Important Biological Producers of Carbonate Sediment

Animals (skeletal fragments)

Molluscs (bivalves, gastropods, cephalopods, etc.)

Brachiopods

Echinoids and crinoids (echinoderms)

Corals

Foraminifera

Algae

Red algae (encrusting)

Green algae (benthic and planktonic)

Yellow-green algae (coccolithophores)

Cyanobacteria - these organisms are important because they form mats that trap and bind carbonate mud, producing stromatolites (a type of bioherm or biostrome structure). Also, some types of cyanobacteria bore into skeletal debris and break down the shell material into tiny rods of calcite mud (micritization).

 

Evaporite Sedimentary Rocks

Sedimentary rocks can form from the deposition of inorganically precipitated salts, usually due to evaporation of lake or seawater in arid environments.

Evaporite Minerals (in order of precipitation from seawater)

Calcium Carbonate (.3%)

Calcium Sulphate (3.5%) - gypsum (saturated) and anhydrite (dehydrated)

Sodium Chlorite (78%)

Potash salts (eg. KCl) (18%)

 

Cherts

Cherts are hard, aphanitic rocks made up of silt-sized quartz crystals (micro- or cryptocrystaline quartz) and chalcedony, a form of silica made up of radiating fibers tens to hundreds of microns in length.

The dark, organic-rich form of chert is sometimes called flint.

Iron-rich, red chert is called jasper.

Chert in sedimentary rock can be either diagenetic or primary.

Diagenetic chert: generally occur as nodules or irregular beds, usually in limestones or sometimes in mudstones. Chert nodules probably form by mineral replacement around nuclei of crystalization, with additional dissolved silica arriving by diffusion.

Primary (depositional) chert: Layers of siliceous ooze can accumulate where the influx of siliceous particles (usually the microscopic silica tests of radiolarians or diatoms) is much greater than the influx of any other sedimentary particle. Most commonly this occurs in deep ocean environments below the depth at which calcium carbonate is insoluble (the carbonate compensation depth or CCD). Usually, primary chert occurs as thin beds, interbedded with clay layers.

 

Phosphorites (sedimentary deposits of calcium phosphate and hydroxyapatite)

Sedimentary deposits of phosphorite are rare, but economically important, sources of industrial fertilizer. Most appear to form as lag deposits of bone, teeth, and fecal material on continental shelves during maximum transgression, when the shelve is temporarily starved of other types of sediment.

 

Sedimentary Ironstones

Iron is a common element in the Earth' crust. Sedimentary rocks that contain more than 15% iron are called ironstones.

Common iron-rich minerals in sedimentary rocks

Hematite - iron oxide (Fe2O3), common in well-oxygenated environments, red to dark purple in color

Goethite - hydrous iron oxide (FeO.OH), forms in less oxidizing conditions, but often converts to hematite later. Yellowish in color, probably gives desert sands their distinctive color, altering to red after deposition in desert sandstones.

Pyrite - iron sulphide ((FeS2), forms in sulphur-rich anoxic (stagnant / low in O2) muds. Pyrite is common in marine sediments that are rich in organic material. Decaying organics use up oxygen and release sulphur, creating conditions ideal for pyrite formation.

Siderite - iron carbonate (FeCO3), forms where the absence of sulphur and anoxic conditions combine. Siderite often occurs as small, reddish nodules or thin sheets aligned along bedding planes. Generally, siderite is associated with freshwater and deltaic sediments, while pyrite is more common in marine sediments (marine waters are rich in sulfate, which precludes the formation of siderite in favor of pyrite).

Glauconite - an iron-rich sheet silicate which forms large sand-sized grains, probably from the alteration of clay particles in fecal pellets. Glauconite deposits are usually associated with sediment-starved continental shelves, although glauconite grains are common in a variety of nearshore, marine deposits (possibly reworked from previously deposited layers).

 

Carbonaceous (organic) deposits

Sedimentary layers rich in carbon (not to be confused with carbonate-rich deposits, which are called calcareous).

Peat - plant material accumulated under anoxic conditions, preventing oxidation of tissues. Peat deposits are generally associated with persistently wet conditions in a variety of climates where there is little clastic influx.

Coal - lithified peat composed of greater than 66% organic material. Most economically useful coals have less than 10% inorganic matter (ash).

The material making up the organic components of coal are called macerals.

Vitrinite - woody material - stems, leaves, branches, roots)

Exinite - spores, cuticles, resin, algae

Inertinite - partially oxidized material, including charcoal (fusinite)

With burial, humic peats (composed mainly of vitrain) are progressively altered during coalification to produce a sequence of increasing carbon concentration and therefore energy value:

Peat - lignite (brown coal) - subbituminous - bituminous - anthracite (metamorphic)

Oil shale - Mudrocks with a high proportion of organic material altered to hydrocarbon. Also called petroliferous shale.