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Jdeutsch

geology question

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Jdeutsch

I was in an agate field in South Dakota just south of Badlands Park.  Cretaceous era.

 

I collected a few not-so-special agates and some jasper, and the pictured rock, which I thought was wind polished jasper- but it broke and had what looks to be brachiopods inside.

I assume this is sedimentary or metamorphic. 

 

What is the relation between the agates in this region and the kind of specimen here?   Why are there so many agate fields in this area of S Dak?

 

 

sdakUntitled-1.jpg

sdakUntitled-2.jpg

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ynot

The rock with the brachiopods is most likely a quartzite sandstone. Similar in appearance but formed through different conditions.

Agate and jasper are very common throughout the western United States. Most likely because of all the volcanic activity which caused much hydrothermal activity that led to the deposition of the agates and jaspers.

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abyssunder

It looks more like chert to me, rather than agate.

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ynot
1 minute ago, abyssunder said:

It looks more like chert to me, rather than agate.

Wrong fracture pattern for agate or chert.

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abyssunder

I just read this :

" The agates are encased in chert nodules housed within the lower Minnelusa Formation (Paleozoic: Pennsylvanian).  I suppose these nodules are the result of silica-rich meteoric waters circulating through the unit with resulting diagenesis producing the chert.  Why some nodules are agatized—I don’t have the slightest idea.  Just as I am uncertain how/why agates really form!  The formation of agates in several types of rocks is extremely complicated, even for the “experts”. "

 

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ynot
3 minutes ago, abyssunder said:

The agates are encased in chert nodules

:headscratch:Never heard of this type of association before. Would like to hear more about it, because what I know of agates does not fit well with cherts.

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