What is Conglomerate
Conglomerate (/kənˈɡlɒmərɪt/) is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter.
Conglomerates form by the consolidation and lithification of gravel.
Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.
The size and composition of the gravel-size fraction of a conglomerate may or may not vary in composition, sorting, and size. In some conglomerates, the gravel-size class consist almost entirely of what were clay clasts at the time of deposition.
Conglomerates 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.
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 is 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.
Below is a transcription of the above sign found at the Valley of Fire Visitor Center.
Conglomerate is a coarse-grained sandstone, made from water-worn pebbles, sand, and mud deposited by ancient streams.
Over millions of years, these separate forms of rock were cemented together by minerals.
Conglomerate often looks like manufactured concrete, but the bond is generally much looser.
Rounded rocks in coarse sands are typical of conglomerate at Valley of Fire, laid down by streams in the Mesozoic Era when the land became dry after the seas disappeared.
Because conglomerate varies in appearance and hardness, there are few commercial uses for it.
Time Period: Mesozoic Era
Composition: Gravel, sand, and mud loosely cemented together by minerals.
Location: Below Aztec Sandstone in Muddy Mountains; can be seen across from Visitor Center.