DEFINITION OF CONGLOMERATE

A clastic sedimentary rock characterized by

rounded or subangular fragments that are larger than 0.079

inch (2 mm) in diameter, typically set in a finer grained matrix

of sand or silt-sized particles. It is the lithified or hardened

equivalent of gravel. The matrix and grains are held together by

a cementing agent such as calcium carbonate, silica, iron oxide,

or clay. The clasts may be made of a single type of parent rock,

in a monomictic conglomerate, or of several types of parent

rock, in a polymictic conglomerate. The amount of rounding of

clasts can in many cases be related to the amount that the clasts

have been transported from their source, with greater transport

distances equated with more rounding. Conglomerates may be

classified by the types of clasts present, the type of cement,

and, less commonly, by the inferred environment of deposition.

Many conglomerates are deposited in channels in fluvial

(stream) systems, whereas others are deposited at the bases of

submarine slopes, and still others are formed by storms ripping

up shallow shelf muds and depositing the fragments in

deeper water. Thick conglomeratic sequences are an indication

of the erosion of high topography, and the sequence of

erosion and exposure of different crustal levels can sometimes

be learned by examining the types of clasts throughout the

deposit. During mountain-building events, a sequence of conglomerates,

sandstones, and shales known as flysch is typically

deposited during active deformation, followed by a

sequence of conglomerates and stream deposits known as

molasse as the mountains get eroded.

See also BRECCIA; CLASTIC ROCKS; FLYSCH; MOLASSE;

SEDIMENTARY ROCKS.

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