Jdeutsch Posted January 7, 2018 Posted January 7, 2018 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?
ynot Posted January 7, 2018 Posted January 7, 2018 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. 2
abyssunder Posted January 7, 2018 Posted January 7, 2018 It looks more like chert to me, rather than agate. " We are not separate and independent entities, but like links in a chain, and we could not by any means be what we are without those who went before us and showed us the way. " Thomas Mann My Library
ynot Posted January 7, 2018 Posted January 7, 2018 1 minute ago, abyssunder said: It looks more like chert to me, rather than agate. Wrong fracture pattern for agate or chert.
abyssunder Posted January 7, 2018 Posted January 7, 2018 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”. " 1 " We are not separate and independent entities, but like links in a chain, and we could not by any means be what we are without those who went before us and showed us the way. " Thomas Mann My Library
ynot Posted January 7, 2018 Posted January 7, 2018 3 minutes ago, abyssunder said: The agates are encased in chert nodules 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.
Recommended Posts