siltstone

Siltstone is a clastic sedimentary rock in which silt-sized particles, roughly 4 to 62 micrometers in diameter, predominate. It is finer than sandstone and coarser than most shales, and it typically lacks the pronounced fissility of shale. The rock is usually well to moderately sorted and commonly cemented by quartz overgrowths, calcite, or clay minerals. Its fracture tends to be blocky rather than fissile, giving siltstone a dull, earthy appearance.

Formation and characteristics: Siltstone forms from the consolidation of suspended silt that settles in relatively low-energy environments, such as river floodplains, lakes, flood-basins, or distal delta-front settings. Bedding can be tabular or laminated, and diagenesis with cementation reduces primary porosity. The texture can be uniform or show fine lamination, and color varies with mineral content and oxidation.

Relation to other rocks: Siltstone occupies an intermediate position between sandstone (coarser grains) and shale (finer, clay-rich). It is distinguished by its fine but non-clay-dominant composition, generally non-fissile to weakly fissile character, and a consistency that is not as thinly bedded as typical shale. Petrographic analysis confirms grain size and cement type.

Uses and distribution: Siltstone is widespread in sedimentary basins worldwide and is studied for clues about past low-energy depositional environments. It is not a common building stone but can be used locally as a decorative or architectural material where available.