Classification of Mudstone

Photo by W. W. Little

Mudstone is defined as a clastic sedimentary rock in which the ratio of mud when mixed with sand is at least 50% and when mixed with gravel is at least 70%.

Mudstone (lutite/argillaceous)

Mudstones are classified first by grain size and second by fissility.

Mudstone Classification

If the predominant grain size is silt, the rock is a siltstone. If it is clay, it is a claystone.

Classification by Grain Size

Siltstone Claystone

Photo by W. W. LittlePhoto by W. W. Little

How do you tell them apart in the field?

Photos by W. W. Little

Photo by W. W. Little

Shale

Shale is an indurated mudstone with well-developed fissility.

Types of Bedding in Shale

Photo by W. W. Little

Argillite is an indurated mudstone with poor fissility. Can be considered to be a low-grade metamorphic rock without cleavage (metamudstone).

Argillite

Clay MineralogyClay mineralogy is important to many geological studies, in that it affects absorption, sorption flocculation, and cation exchange capacity. Because of the fine grain size, mineralogy is determined most effectively using SEM and XRD.

• The most abundant type of sedimentary rock

• The source of much organic material that forms fossil fuels

• Good indicators of chemical/biological conditions at the time of deposition

• Impermeable physical properties form barriers to subsurface fluid flow

• Form both primary and secondary minerals in sedimentary rocks9

Significance of Mudstones