extremesand

Extremesand is a term used in sedimentology and planetary geology to describe a sediment material characterized by an unusually wide range of grain sizes, spanning from fine clays to coarse gravels within a single deposit or sample. The concept emphasizes extreme polydispersity and imperfect sorting, distinguishing extremesand from more uniform sand, silt, or gravel deposits. While not a formal mineralogical category, extremesand appears in field journals and research as a descriptive label for heterogeneous sediments.

Its formation can arise from multiple processes that simultaneously introduce disparate grain sizes or from rapid reworking of earlier deposits. Examples include turbidity currents mixing fine and coarse material, river floods that truncate existing layers and deposit new fractions, wind-blown dust combined with downstream gravels, or anthropogenic backfills that blend materials on construction sites.

Physical properties of extremesand are variable and context dependent. The broad grain-size range can create heterogeneous packing and porosity, affecting hydraulic conductivity and shear strength. Sorting may be poor or bimodal, leading to localized zones of high permeability adjacent to coarse-grained lenses or fines filling pore spaces.

Extremesand is studied for insights into depositional history, sediment transport, and granular-material physics. It can serve as a test case in filtration design, hydrogeology, and civil engineering where non-uniform materials influence compaction and stability. In planetary science, analogs of extremesand can appear in regolith formed by mixed deposition and regolith gardening on small bodies.

See also: grain-size distribution, sedimentology, sorting, porosity.