Conglomérat (géologie)

Conglomerate (kənˈɡlɒmərɪt) is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay. Conglomerates form by the consolidation and lithification of gravel. They can be found in sedimentary rock sequences of all ages but probably make up less than 1 percent by weight of all sedimentary rocks. In terms of origin and depositional mechanisms, they are closely related to sandstones and exhibit many of the same types of sedimentary structures, e.g., tabular and trough cross-bedding and graded bedding. Fanglomerates are poorly sorted, matrix-rich conglomerates that originated as debris flows on alluvial fans and likely contain the largest accumulations of gravel in the geologic record. Conglomerates may be named and classified by the: Amount and type of matrix present Composition of gravel-size clasts they contain Size range of gravel-size clasts present The classification method depends on the type and detail of research being conducted. A sedimentary rock composed largely of gravel is first named according to the roundness of the gravel. If the gravel clasts that comprise it are largely well-rounded to subrounded, it is a conglomerate. If the gravel clasts that comprise it are largely angular, it is a breccia. Such breccias can be called sedimentary breccias to differentiate them from other types of breccia, e.g. volcanic and fault breccias. Sedimentary rocks that contain a mixture of rounded and angular gravel clasts are sometimes called breccio-conglomerate. Conglomerates contain at least 30% of rounded to subangular clasts larger than in diameter, e.g., granules, pebbles, cobbles, and boulders. However, conglomerates are rarely composed entirely of gravel-size clasts.