allogène

Allogène is a term used in geology to describe a rock fragment or mineral grain that is foreign to the sedimentary rock in which it is found. This means the allogeneic material originated from a different source rock or geological area than the surrounding matrix. Allogenes can be derived from a variety of sources, including pre-existing igneous, metamorphic, or sedimentary rocks. Their presence in a sedimentary rock provides valuable clues about the geological history of the depositional basin.

The study of allogenes, known as allogenic analysis, can help reconstruct the provenance of sediments. By identifying the type of rock or mineral and its characteristic features, geologists can infer the nature of the source rocks and the geological processes that transported the material to its current location. For example, the presence of specific types of metamorphic minerals might indicate that the source area contained metamorphic rocks that were eroded and transported. Similarly, the shape and rounding of allogenic grains can reveal information about the distance and duration of transport.

Allogenes can be found in various sedimentary rocks, including sandstones, conglomerates, and breccias. Their size can range from fine sand grains to large cobbles or boulders. The recognition and analysis of allogenes are fundamental techniques in sedimentary petrology and are crucial for understanding geological formations, basin evolution, and the history of Earth's crust.