Tuffaceous

Tuffaceous is a geological term describing rocks or sediments that contain a significant amount of volcanic ash, or that resemble tuff in composition. It is most often applied to sedimentary rocks such as sandstones, siltstones, shales, or claystones in which ash particles have been incorporated and preserved after deposition. The ash originates from volcanic eruptions and may consist of fine glass shards, pumice fragments, and mineral crystals.

In practice, tuffaceous material can form from fall-out ash (tephra) or from reworking of pyroclastic deposits by water or wind. The degree of ash content can vary widely, and diagenetic processes may alter color, texture, and cementation, sometimes producing light gray to tan hues and a fine-grained matrix.

Tuffaceous rocks are important in stratigraphy and sedimentology because ash layers can serve as tephra markers for dating and correlation. The presence of volcanic ash can also influence porosity and permeability in sedimentary rocks, affecting interpretations of reservoir quality. While they share volcanic ash components with true igneous tuff, tuffaceous rocks are categorized by sedimentary origin and ash content rather than by igneous texture.

See also tephra, tephrochronology, tephra layers.