However, breccia is made up mostly of subangular to angular clasts. Sedimentary clasts can be angular or rounded. What determines the difference? They both start out at an outcrop a location where a rock unit is exposed at Earth's surface. This outcrop is known as the "source area" for the clasts. In the source area, chemical and physical weathering act upon the rock, causing it to break or disintegrate into smaller pieces. These pieces are usually subangular to angular.
If the clasts accumulate near the outcrop and form into a rock, that rock will have angular pieces and be a breccia. However, if the pieces are transported by a stream or the action of waves, the clasts will be abraded against one another and against other clasts on the bottom of the stream. That abrasion will - over time - cause their angular shapes to become subrounded to rounded. If the rounded clasts are deposited and formed into a rock, that rock with rounded clasts will be a conglomerate.
The difference between conglomerate and breccia is in the transportation history of their clasts. Red Conglomerate: This photograph shows a portion of a dimension stone slab that was cut from a red conglomerate. The conglomerate is composed of well-rounded clasts of quartz and sedimentary rocks of various sizes and kinds along with a fine-grained matrix. To work well as a dimension stone, this conglomerate would have to be bound tightly with a very competent cement. If competent, this material would make spectacular wall panels, flooring tiles, stair treads, and other architectural elements.
Conglomerate has very few commercial uses. Its inability to break cleanly makes it a poor candidate for dimension stone, and its variable composition makes it a rock of unreliable physical strength and durability. Conglomerate can be crushed to make a fine aggregate that can be used where a low-performance material is suitable.
Many conglomerates are colorful and attractive rocks, but they are rarely used as an ornamental stone. Analysis of conglomerate can sometimes be used as a prospecting tool. For example, most diamond deposits are hosted in kimberlite. If a conglomerate contains clasts of kimberlite, then the source of that kimberlite must be upstream of the location where the kimberlite clast was deposited.
That sounds simple, but the kimberlite clast might have been deposited a few million years ago in a different landscape - but people have been successful in using this type of clue to successfully locate a diamond deposit.
In rare instances, conglomerate can be a "fossil placer deposit" containing gold , diamonds, or other valuable minerals. These conglomerates are mined, crushed, and processed as ores. Puddingstone: Puddingstone is a conglomerate composed of clasts that contrast sharply with the rock matrix.
This rock in the photo is a specimen of the Hertfordshire puddingstone, from the lower Eocene of the London Basin. It consists of colorful flint pebbles in a white to brown silicate matrix. This type of rock is found at many locations in Hertfordshire County, England. Puddingstone is a nonscientific name for a conglomerate made up of clasts that contrast sharply with the color of the rock's matrix. People in what is today the United Kingdom were the first to use the name.
They claim that the rocks reminded them of a "plum pudding". Puddingstones are found in many parts of the world. Class - conglomerate can be divided into two broad classes: Clast supported - where the clasts touch each other and the matrix fills the voids; and Matrix supported - where the clasts are not in contact and the matrix surrounds each clast;.
Sorting - a conglomerate comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted;. Lithology - a conglomerate where the clasts represent more than one rock type is termed polymictic or petromictic , while one where the clasts are of a single rock type are monomictic or oligomictic. If these clasts consist of rocks and minerals that are identical to or consistent with the lithology of the enclosing matrix and, thus, penecontemporaneous and derived from within the basin of deposition, the conglomerate is known as an intraformational conglomerate.
Two recognized types of type of intraformational conglomerates are shale-pebble and flat-pebble conglomerates. A shale-pebble conglomerate is a conglomerate that is composed largely of clasts of rounded mud chips and pebbles held together by clay minerals and created by erosion within environments such as within a river channel or along a lake margin.
Flat-pebble conglomerates edgewise conglomerates are conglomerates that consist of relatively flat clasts of lime mud created by either storms or tsunami eroding a shallow sea bottom or tidal currents eroding tidal flats along a shoreline. Finally, conglomerates are often differentiated and named according to the dominant clast size comprising them.
In this classification, a conglomerate composed largely of granule-size clasts would be called a granule conglomerate ; a conglomerate composed largely of pebble-size clasts would be called a pebble conglomerate ; and a conglomerate composed largely of cobble-size clasts would be called a cobble conglomerate.
Deepwater marine In turbidites, the basal part of a bed is typically coarse-grained and sometimes conglomeratic. In this setting, conglomerates are normally very well sorted, well-rounded and often with a strong A-axis type imbrication of the clasts. Conglomerates are normally present at the base of sequences laid down during marine transgressions above an unconformity, and are known as basal conglomerates. They represent the position of the shoreline at a particular time and are diachronous.
Conglomerates deposited in fluvial environments are typically well rounded and well sorted. Clasts of this size are carried as bedload and only at times of high flow-rate.
The maximum clast size decreases as the clasts are transported further due to attrition, so conglomerates are more characteristic of immature river systems. In the sediments deposited by mature rivers, conglomerates are generally confined to the basal part of a channel fill where they are known as pebble lags.
Conglomerates deposited in a fluvial environment often have an AB-plane type imbrication. Alluvial deposits form in areas of high relief and are typically coarse-grained. At mountain fronts individual alluvial fans merge to form braidplains and these two environments are associated with the thickest deposits of conglomerates. The bulk of conglomerates deposited in this setting are clast-supported with a strong AB-plane imbrication.
Matrix-supported conglomerates, as a result of debris-flow deposition, are quite commonly associated with many alluvial fans. When such conglomerates accumulate within an alluvial fan, in rapidly eroding e. Glaciers carry a lot of coarse-grained material and many glacial deposits are conglomeratic. Tillites, the sediments deposited directly by a glacier, are typically poorly sorted, matrix-supported conglomerates. The matrix is generally fine-grained, consisting of finely milled rock fragments.
Waterlaid deposits associated with glaciers are often conglomeratic, forming structures such as eskers. An example of conglomerate can be seen at Montserrat, near Barcelona. Here, erosion has created vertical channels that give the characteristic jagged shapes the mountain is named for Montserrat literally means "jagged mountain".
The rock is strong enough to use as a building material, as in the Santa Maria de Montserrat Abbey. The Crestone Conglomerate consists of poorly sorted fanglomerates that accumulated in prehistoric alluvial fans and related fluvial systems.
0コメント