Mudstone

Mudstone is a fine-grained sedimentary rock of clay and silt that, unlike shale, breaks into blocky pieces rather than thin sheets. It is the most abundant sedimentary rock type among mudrocks.

🪨 Geology Guide Specimen: Open Mudstone in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 6 subgroups of Sedimentary, and test your eye in LithoClade.

Shale splits because its clay flakes lie parallel. In mudstone, burrowing animals, roots and the way clay flocculates in water disturb that alignment, so the rock breaks in irregular blocks or nodules instead of sheets.

Oxidized iron in the form of hematite gives mudstones in well-aerated, often terrestrial settings their red and brown colors, as in the Chinle Formation of the Painted Desert. Where organic matter and low oxygen kept iron reduced, the rock is grey or black, and the color often records the environment of deposition.

Mudstones make up most of the sedimentary record and store organic carbon. Because they are tight, they form seals above oil and gas reservoirs and barriers to groundwater flow, and their clay minerals come from the weathering of continents, as covered in silicate weathering.

• Siltstone — the coarser, gritty relative
• Black Shale — the fissile, organic-rich relative
• Silicate Weathering — the source of clay minerals