packstones

Packstones are a textured category used in carbonate sedimentology to describe a type of limestone in which grains are held together by a calcareous mud matrix. In Dunham’s 1962 framework, a packstone is grain-supported with a mud matrix filling the spaces between grains; the grain framework provides physical support, while the mud binds and cements the grains to one another. This contrasts with grainstone, which has little or no mud between grains, and wackestone, which is mud-supported with dispersed grains.

The composition of packstones typically includes a mix of framework grains—such as fossil fragments, peloids, intraclasts, and sometimes ooids—embedded in a micritic or other calcareous mud matrix. The mud fraction is distinct enough to occupy the intergranular spaces but not so extensive as to obscure the grain framework entirely.

Depositional environments for packstones are commonly shallow marine settings where enough energy exists to transport and accumulate grains, yet conditions allow the production or preservation of micritic mud. They are often associated with shoal, reef, or subtidal ramp settings where grain supply and micrite production co-occur.

Porosity in packstones is variable. Cementation and diagenetic polishing can reduce porosity, while dissolution or secondary porosity development can enhance it. Packstones can serve as reservoir rocks in some basins, depending on porosity and diagenetic history.

See also: Dunham classification, Folk classification, grainstone, wackestone.