Conglomerates and Breccias

The morphological investigation of certain details of quartz-cemented conglomerates and breccias leads to similar questions to those discussed above.

With the Precambrian conglomerates discussed here, which are completely hardened, the crystallinity also goes through the matrix material, so that the individual rocks can no longer be separated out.  Current petrology assumes here that river gravels sank deep into the earth into regions of high temperature, where under high pressure and with the combined working of silicatic solutions were metamorphically altered (recrystallized) and cemented together.  In part these metamorphoses are thought to have reached almost to the melting point.

With the quartzitic breccias, an analogous origin is assumed, with the difference that the starting material was not river deposits but rather broken material from avalanches (possibly underwater) or fracture zones, which again descended into warm or hot depths where metamorphosis occurred.

Two phenomena are of interest here:  The mutual deformation of component rocks, and evidence of dissolving.

Pictures: Conglomerates of quartzite, gneiss and magnetite with indentations and deformations (Bahia, northeast Brazil; Precambrian):


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The indentations show their difference from younger conglomerates of the Tertiary, which lack the mutual deformation.

One finds the same phenomenoon of mutual interpenetration in quartzitic breccias (Norway, presumably Precambrian):

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The following pictures show evidence of dissolution in the conglomerates:
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And corresponding evidence of dissolution in breccias:
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The following photos show that diffusion processes have played a significant role in the metamorphosis of the pictured conglomerates and breccias.  One can see a red and a green diffusion region spreading around two rocks, further diffusion rings within one rock, and yellow borders in the outer zone of another.  One could imagine the diffusion taking place due a hot-watery solution permeating them.
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During the metamorphosis, strong re-crystallization processes took place, which can be seen in the following pictures.
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What was the consistency of these gravels and breccias, however, when they mutually indented one another and broke up like a decaying material?  One has the general impression of a "hardened-colloidal" condition.  Naturally, with a pressure of 1 or 2 Kilobars one cannot think of a colloidality such as that of an Emmentaler cheese, but it is conceivable at least that a greater water content led to the characteristics displayed.

The following forms may have arisen through a stronger heating and softening, whereby here also, free or bound water may have played a role:

Flattened conglomerates (Brasil?):

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With the quartzitic breccias, the darker rocks dissolve in the lighter, flowing quartz, meaning that the mobile matrix consisted not only of an original sand, but also of dissolved rock substance:
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This streaming matrix, which even allowed rocks to penetrate into one another before it hardened again to a pure quartz, reminds one of substances like boiled butter or chocolate.  One can make these comparisons despite knowing of the huge pressure which prevailed, since the forms are corresponding.  The processes were, at the least, fluid on a small scale.

With the quartzite- and gneiss-conglomerates, the rocks dissolve in part into a granite-like mass, which means that here also the matrix consists not only of original erosion material, but also of dissolved rock substance:

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Furthermore, there are dissolution forms observable, which appear to have less to do with pure heating, as rather the same kind of fracturing and breaking-up which was already discussed with the limestone formations (compare the chapter on Colloids and Limestone).  For example, the gray-red rock in the left-hand photo was simply bent by pressure like heated glass, and also fractured, like glass which has been overstressed.  Here also the split forms indicate a consistency which was soft and cheese-like:
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In conclusion I would like to come back to the striking indentations and interpenetrations in the breccias:  the question of the consistency of the substance presents itself here with particular intensity, since with a regional metamorphosis or softening of a compacted, crystalline conglomerate body the individual objects could hardly have interpenetrated each other in this manner.  There must have been significant fluid movement on a small scale with locally-concentrated pressure effects.  In addition, the rocks must have been soft and at the same time able to hold their form, as is experienced with gelled colloidal conditions.  One has the impression that the phenomena in total show a mixture of plasticity due to warming, "sandy-fluid" mobility in which silicic acid or free water participated, and deformability which displays the character of a hardened colloid.  It could well be that pressure and water were the main factors responsible for processes of this kind.

On the detour through the red quartzite-conglomerates from north Brazil, one comes up against a further phenomenon, one which likewise involves the "water-question".  The red conglomerates come from the central uplands in the Brazilian state of Bahia.  In this region are also found fluvial (river-origin) and aeolian (wind-eroded) sandstone from the middle Proterozoic, that are penetrated by caves, and thus have a karst character similar to many limestone regions.  A parallel example appears in the Proterozoic and Archaen quartzite complexes of the uplands of the Brazilian state of Minas Gerais.  Caves are found there with lengths of up to three kilometers and 50 meters width, with stalagtites and stalagmites of quartz (in the amorphous form of opal), along with sinkholes, dolines and other karst phenomena in quartz!  This phenomena can be found in Australia and South Africa as well, making it a global phenomena that is nevertheless seldom seen.  Particularly puzzling is the fact that not only quartz-cemented sandstone, but also the more strongly-cemented quartzite can show clear karst phenomena, since quartzite is extremely resistant to dissolving.  The investigations made up tot he present have indicated water as the driving force behind the phenomena, and thus must assume extremely long periods of time to explain the large-scale features.  One is reminded of the Austrian story "A Second of Eternity", in which a mountain is carried away piece by piece by a bird, which only comes to the mountain once every thousand years. With the phenomenon of quartzite karst regions, one could ask in connection with the conglomerates already discussed: Was the consistency of these quartzites (and sandstones), due to a larger amount of chemically-bound and also free water, such that the karst processes could have proceeded in a much shorter time, similar to limestone karst regions?