8.1: Diagenetic Processes
Compaction
Compaction is a reduction in the volume of sediment due to the weight of the overburden and resulting burial pressure. In terms of a starting point, loose sand has a theoretical maximum of 47.6% pore space and wet mud could be 60-90% pore space. Once buried, there can be a marked reduction in volume/pore space; some example values include:
- ~1.1 m sand = 1 m sandstone
- ~2.6 m mud = 1 m mudrock
- ~5 to 10 m peat = 1 m coal
Carbonates are extremely variable, with lime mudstones behaving like shales and reef-type boundstones experiencing very little compaction.
Although significant volume loss can occur simply by squeezing out water and gasses, deformation can also occur at the scale of an individual grain by a) ductile deformation where grains that behave plastically get deformed between more robust grains, b) flexible bending of grains like micas, and c) pressure induced changes in grain contacts. Specifically, grains start out with floating or tangential (point) contacts and that increased pressure with burial causes a progressive shift to long (appears like a straight-ish line), concavo-convex (appears like a curved line), and eventually complexly sutured contacts caused by dissolution and "welding" of grains.
Compaction generally increases with depth and is accompanied, at least initially, by a decrease in porosity caused by grain scale processes and cementation. Secondary porosity can be generated by dissolution of unstable components.
Cementation
Cementation is the second major diagenetic process and it occurs when minerals precipitate from fluids into open pore spaces. Well sorted sediments with open and interconnected pores are particularly prone. The most common cements are calcite (which forms from solutions with a pH >7) and silica cement which forms from solutions with a pH<7). Pores can also fill with water, gas, or oil.
Other processes
Although compaction and cementation are the most important processes, other diagenetic changes can also occur and may be locally significant. Examples include:
- Replacement happens when one mineral takes the place of another. Common examples include the alteration of feldspars to clay minerals and pyrite replacing calcite.
- Recrystallization is the transformation of a mineral to its polymorph or another form of itself (change in crystal size or morphology). An example would be the transformation of aragonite to calcite.
- Bioturbation is the disruption of sediment caused by the activity of organisms. It results in everything from discrete tracks, trails, and burrows to the complete destruction of bedding.
- Pedogenesis is a term used for all of the soil forming processes, including wetting and drying, the action of plants and animals, and weathering which can destroy original structure of a rock. During pedogenesis (and after) the movement of water and changes in oxidation may also cause mottling of rock.
- Dissolution happens when select components are dissolved from the rock.
Readings and Resources
- Taylor, J.M., 1950, Pore-Space Reduction in Sandstones. AAPG Bulletin ; 34 (4): 701–716.
- https://www.geological-digressions.com/tag/stylolites-stylolitization/