forearcbasseinid

Forearcbasseinid is a term used in some geological discussions to designate an informal lithostratigraphic unit within forearc basins—sedimentary basins that develop between a subduction trench and an overlying volcanic arc. The name combines forearc, basin, and the suffix -id, indicating a depositional package rather than a named formation. It is not a formal stratigraphic rank in the IUGS scheme, but is sometimes used to describe a characteristic facies association found in forearc basins.

Geological context and composition: Forearcbasseinid deposits form in convergent-margin settings under rapid subsidence and variable sediment supply. They typically comprise alternating siliciclastic turbidites and hemipelagic muds, with interbedded volcanic ash layers (tuffs), and occasional volcanogenic sands. Lithologies range from sandstone to siltstone and mudstone, with possible chert or carbonate beds in some successions. Diagenesis may lead to compaction and lithification into a range of rock types.

Interpretation and use: The identification of a forearcbasseinid facies can aid in reconstructing subsidence rates, volcanic activity, and sediment routing in subduction-related basins. Dating often relies on biostratigraphy, radiometric methods in interbedded ash, and regional tectonic correlation. Distribution is continental to oceanic margins where forearc basins are present; examples appear in multiple convergent-margin systems, though the term is not universally adopted.

See also: Forearc basin, Turbidite, Accretionary wedge, Subduction zone.