Sedimentaire gesteentes - Breccia

Page / - last modified by Roelof Zwaan on 2021/09/30 15:18

Breccia

last modified by Roelof Zwaan

on 2021/09/30 15:18

Table of Contents

Breccia is a clastic sedimentary rock that contains large rounded clasts with a diameter from 2 mm to 63 mm. These clasts are poorly organized and present in a finer matrix. The lithology of the clasts and the matrix can be different from each other, but there is a possibility they are the same. Breccias mainly are formed in or close to the area where the clasts were deposited. This rock type differs from conglomerate with its angular clasts.

There are five types of breccia, namely:

Sedimentary breccia

Fault breccia

Igneous breccia

Impact breccia

Hydrothermal breccia

Sedimentary breccia

Sedimentary breccia consist of cemented angular to subangular, randomly oriented clasts. The lithology of these clasts can be the same as the lithology of the cement, but can also be totally different. A sedimentary breccia is lithified colluvium or scree and are formed next to fault scraps in grabens. Moreover sedimentary breccia can be found close to high velocity streams or close to submarine debris flows. Other environments in which it can be found: close to turbidites, in karst terrain or close to evaporite beds.

Fault breccia

Fault breccia is a non cohesive rock type. This non cohesiveness is formed due to its origin. They are formed in a localized zone of brittle deformation where tectonic movement takes place. This causes parts of the parent rock to loosen and end up as large loose material. These zones can easily be infiltrated by groundwater which causes the cementation with the help of the present minerals in the groundwater.

Igneous breccia

Igneous breccia can be formed in two different ways. The volcanic way is made due to eruptions and the intrusive way due to a process within the volcano associated with plutons or porphyry stocks.

Volcanic

A volcanic breccia can be made by an explosive eruption which drags along some rock parts from the wall of the magma conduit. Moreover when a lava flow is solidifying and another lava flow passes and takes along the nearly solidified rock parts they are integrated into the new lava flow and create an uniform breccia. The last mentioned process is called autobrecciation.

Apart from autobrecciation there is another form of volcanic breccia. This form is called alloclastic volcanic breccia and made due to the intrusive and volcanic environments who meet in the volcanic conduits. There the lava sometimes solidifies, only to be shattered by a following eruption.

Intrusive

Intrusive breccia originate from multiple stages of intrusion. This happens when magma enters partly consolidated or solidified magma. The harder parts will then form the angular clasts in the more fine softer cement.

Impact breccia

Impact breccia can be found near or within craters. When a meteorite strikes it catapults loose pieces of rock in the air and breaks the rocks underneath its impact point. These two ways form an impact breccia.

Hydrothermal breccia

Hydrothermal breccias are made close to shallow crustal levels. When volcanic activity is present this can cause a void to open which is then filled with hot water. Due to the pressure difference the rocks on the side of the cavity can implode and the rock parts are mixed up in the boiling water. When the contents of the fluids change minerals rapidly precipitates which result in an hydrothermal breccia.

At greater depths hydrothermal breccias can form in-situ. This formation is caused by highly pressured hydrothermal fluids which are released during seismic activity. These fluids rise to the crust and on there way they fracture rocks creating in situ breccia.

References

Allaby, Michael, ed. (2013). "Breccia". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN 9780199653065.

Fisher, Richard V.; Schmincke, H.-U. (1984). Pyroclastic rocks. Berlin: Springer-Verlag. pp. 89–92. ISBN 3540127569.

Michel Jébrak (1997). "Hydrothermal breccias in vein-type ore deposits: A review of mechanisms, morphology and size distribution". Ore Geology Reviews. 12 (3): 111–134. doi:10.1016/S0169-1368(97)00009-7.

Breccia

Characterisation	Identification
Groun	Clastic sedimentary rock
Texture	Coarse-grained
Clasts	2mm - 63mm
Hardness	Varying much
Color	Varying much
Minerals	All kind of rock minerals
Touch	Rough

Contents

Sedimentary breccia

Fault breccia

Igneous breccia

Volcanic

Intrusive

Impact breccia

Hydrothermal breccia

References

Breccia