lerskiffer

Lerskiffer is a term used in Swedish geology to describe a fine-grained sedimentary rock dominated by clay minerals, commonly known in English as clay shale. It forms from the slow accumulation of clay and silt in low-energy environments such as deep lakes, quiet marine basins, or slow-moving rivers.

The rock is typically fissile, meaning it splits easily along layering, and it often displays a dull to pearly luster on freshly broken surfaces. Color ranges from gray to dark gray, greenish, or black, especially when it contains organic matter or iron compounds.

Composition includes clay minerals such as illite, kaolinite, and chlorite, with varying amounts of quartz, carbonaceous material, and minor feldspar or detrital grains. Texture is very fine, with grain sizes generally less than 0.004 millimeters.

Lerskiffer forms through diagenesis of clay-rich sediments. Burial and compaction reduce porosity and drive chemical changes that can lead to the formation of illite or other clay minerals. If subjected to low-grade metamorphism, lerskiffer may transform into slate or phyllite while retaining fissility.

It appears in sedimentary basins worldwide and is common in marine, lacustrine, and deltaic successions. In petroleum geology, clay-rich shales can act as source rocks or cap rocks, depending on organic content and maturation. Regional uses vary.

See also: shale, argillaceous rock, slate, mudstone, sedimentary rock.