DEFINITION OF DIAGENESIS

A group of physical and chemical processes that

affect sediments from the time when they are deposited until

deformation and metamorphism begin to set in. It therefore

occurs at low temperature (T) and pressure (P) conditions,

with its upper PT limit defined as when the first metamorphic

minerals appear. Diagenesis typically changes the sediment

from a loose unconsolidated state to a rock that is cemented,

lithified, or indurated.

The style of diagenetic changes in a sediment are controlled

by several factors other than pressure and temperature,

including grain size, rate of deposition, composition of

the sediment, environment of deposition, nature of pore fluids,

porosity and permeability, and the types of surrounding

rocks. One of the most important diagenetic processes is

dewatering, or the expulsion of water from the pore spaces

by the weight of overlying, newly deposited sediments. These

waters may escape to the surface or enter other nearby more

porous sediments, where they can precipitate or dissolve soluble

minerals. Compaction and reduction of the thickness of

the sedimentary pile result from dewatering of the sediments.

For instance, many muds may contain 80 percent water

when they are deposited, and compaction is able to rearrange

the packing of the constituent mineral grains to reduce

the water-filled pore spaces to about 10 percent of the rock.

This process results in the clay minerals being aligned, forming

a bedding-plane parallel layering known as fissility.

Organic sediments also experience large amounts of compaction

during dewatering, whereas other types of sediments

including sands may experience only limited compaction.

Sands typically are deposited with about 50 percent porosity

and may retain about 30 percent even after deep burial. The

porosity of sandstone is reduced by the pressing of small

grains into the pore spaces between larger grains and the

addition of cement.

Chemical processes during diagenesis are largely controlled

by the nature of the pore fluids. Fluids may dissolve

or more commonly add material to the pore spaces in the

sediment, increasing or decreasing pore space, respectively.

These chemical changes may occur in the marine realm, or in

the continental realm with freshwater in the pore spaces.

Chemical diagenetic processes tend to be more effective at the

higher PT end of the diagenetic spectrum when minerals are

more reactive and soluble.

Organic material experiences special types of diagenesis,

as bacteria aid in the breakdown of the organic sediments to

form kerogen and release methane and carbon dioxide gas.

At higher diagenetic temperatures, kerogen breaks down to

yield oil and liquid gas. Humus and peat are progressively

changed into soft brown coal, hard brown coal, and then

bituminous coal during a diagenetic process referred to as

coalification. This increases the carbon content of the coal

and releases methane gas in the process.

Most sandstones and coarse-grained siliciclastic sediments

experience few visible changes during diagenesis, but

they may experience the breakdown of feldspars to clay minerals

and see an overall reduction in pore spaces. The pore

spaces in sandstones may become filled with cements such as

calcite, quartz, or other minerals. Cements may form at several

times in the diagenetic process. Carbonates are very susceptible

to diagenetic changes and typically see early and late

cements, and many are altered by processes such as replacement

by silica, dolomitization, and the transformation of

aragonite to calcite. Carbonates typically show an interaction

of physical and chemical processes, with the weight of overlying

sediments forming pressure solution surfaces known as

stylolites, where grains are dissolved against each other.

These stylolites are typically crinkly or wavy surfaces oriented

parallel to bedding. The material that is dissolved along

the stylolites then is taken in solution and expelled from the

system or more commonly reprecipitated as calcite or quartz

veins, often at high angles to the stylolites reflecting the

stresses induced by the weight of the overburden.

See also COAL; HYDROCARBON; STRUCTURAL GEOLOGY.

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