Chert

Chert is a form of quartz that occurs in both siliciclastic and carbonate rocks, usually in the form of discontinuous beds, lenses, or nodules. It’s origin can be either biologic (mostly siliciclastics) or diagenetic (mostly carbonates).

Photo by W. W. Little

Coal

Coal is a rock made up primarily of plant debris and is found in association with some siliciclastic rocks.

Macroscopic Description

On a macroscopic scale, coal can be divided into banded and non-banded textures. Non-banded beds are of either cannel (spores) or boghead (algal) composition. Banded beds range from dull (durain & fusain) to bright (clarain & vitrain) reflecting maceral (organic compound) content.

Components of Coal

Macerals: Dehydrogenated plant fragments consisting of vitrinites, inertnites, and liptinites. Serve the same function as minerals in other sedimentary rocks.

Ash: Mostly siliciclastic particles.

Yellow: sporesOrange: cuticlesBlack: charcoal and opaque mineralsWhite: holes in thin section

• Inertnite: charcoal• Vitrinite: Shiny, glass-like material composed of cellular plant material• Liptinite: Decayed leaf matter, spores, pollen, algal material, resin, and wax

Carbonaceous Sediment Environments

Carbonaceous (organic-rich) clastic sediment, including peat, forms in a variety of anaerobic (reducing) environments.

• Minerotrophic peat: mostly nutrient-rich, groundwater-fed mires (e.g. floodplains, delta plains, coastal plains)

• Ombrotrophic peat: mostly nutrient-poor, rainwater-fed mires (e.g., relatively high, flat terrains)

• Gyttja: organic-rich lake sediment• Sapropel: organic-rich marine sediment• Coal: organic accumulations in swamps• Carbonaceous shale: less organic material than coal• Oil shale: anaerobic lake and marine environments

containing organic matter that can be driven off as liquid or gas by heating

Coal Rank

Coal rank is based on the degree of metamorphism as peat is buried and subjected to elevated heat and pressure over long periods of time. With increasing rank, peat first dewaters and then becomes compressed, increasing the amount of energy contained per unit volume. Methane is an important byproduct of coal formation.

Coal Classification by Rank

Evaporites are rocks that form through inorganic chemical precipitation in closed basins with restricted circulation where the rate of evaporation exceeds the rate of precipitation. Each rock type is composed of a sintle mineral.

Evaporites

Modern Evaporite BasinIn comparison to ancient systems, modern evaporite basins are quite small and compositionally incomplete.

Slide modified from Gahn, 2006

Successions of evaporites, particularly gypsum and halite, thousands of feet thick are found in the ancient rock record. Similar successions are not being produced today.

Ancient Evaporite

Succession

Photo by W. W. Little

Evaporitic Sequence

Carbonates (mostly dolomite)

• Form when original seawater volume is reduced by 50%

Ca Sulfates (gypsum and anhydrite)

• Form when original seawater volume is reduced to 20%

Chlorides (halite, sylvite, etc.)

• Na salts form when the original seawater volume is reduced to 10%

• Mg and K salts form when the original seawater volume is reduced to 5%

No modern equivalent exists for thick evaporite successions found in the ancient rock record. However, through laboratory experiments, a similar pattern has been produced.

Evaporation of a 1-km column of seawater produces about a 12-m thick succession of evaporite sediment.

Gypsum

Gypsum is a common chemical precipitate that forms relatively early in the evaporative sequence.

Halite is a common chemical precipitate that forms late in the evaporative sequence.

Halite

Thick phosphate deposits are relatively uncommon in the rock record and are not currently being produced. They appear to form under upwelling conditions at low latitudes in conjunction with rapid sea-level rise and high phosphorus concentrations.

Phosphates

Ironstones and Iron Formations

Iron-rich sedimentary deposits, such as the Banded Iron Formation (BIF), formed commonly in the Pre-cambrian, prior to the advent of an oxygenated atmosphere. Such deposits are no longer produced.