packstone

Packstone is a carbonate rock type defined in the Dunham (1962) classification. It is characterized by a framework of grains, such as skeletal fragments, peloids, and ooids, that is bound together by a matrix of carbonate mud (micrite). The fabric is generally grain-supported, with the mud filling intergranular spaces and sometimes acting to bind the grains.

In terms of composition and texture, packstones contain a noticeable amount of fine-grained carbonate mud within the voids between grains. The mud matrix distinguishes packstone from grainstone (which has little to no mud) and from wackestone (which is mud-supported with grains dispersed within it). The intergranular porosity is typical of such rocks and can be affected by diagenetic processes like micritization and cementation.

Depositional environments for packstones are commonly shallow-water carbonate settings, including shelves and shoals, where moderate energy allows grain production and accumulation while fine mud can still settle and contribute to the cementing matrix. The presence of a micritic matrix often indicates limited early cementation and potential for later diagenetic alteration.

Summary: Packstone is a grain-supported carbonate rock with a noticeable carbonate mud matrix that binds grains together, lying between grainstone and wackestone in the Dunham framework. It provides insights into depositional energy and sediment dynamics in ancient carbonate environments.