Geol 370: Sedimentology and Stratigraphy Topic 6: Classification of Mudstone

1. 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)

2. Mudstones are classified first by grain size and second by fissility.
Mudstone Classification

3. 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

4. How do you tell them apart in the field?
Photos by W. W. Little

5. Photo by W. W. Little
Shale
Shale is an indurated mudstone with well-developed fissility.

6. Types of Bedding in Shale

7. 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

8. Clay Mineralogy
Clay 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.

9. • 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 rocks
9
Significance of Mudstones