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Geology/Obsidian in Lake Charlevoix

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Question
When I was a child I collected several samples of black and green Obsidian from a concentrated area off Lake Charlevoix.  Do you think that is unique, and how do you suppose they originated?

Answer
Hi Jeff,
It would be unlikely to find obsidian here in Michigan.  I have received other questions about obsidian in Michigan and it's enough to make me wonder.  My researches still haven't come up with any obsidian deposits in Michigan.  But that doesn't mean they do not exist somewhere.  We have metavolcanic/sedimentary deposits in the western UP that are 7 miles thick!  You'd think that with all that volcanism (that's where obsidian comes from) we'd find some deposits that are in situ (in the rocck).  

But we haven't yet.  All we find are loose specimens, like your find.  Most experts believe that obsidian found in Michigan was taken in trade by the prehistoric Native Americans.  Rough samples as well as finished product have been found.

There is also the possibility that, somewhere, there is, or was, a deposit of obsidian, and the glaciers that shaped Michigan long ago (the last one was 10,000 years ago) scraped through the deposits and brought the loose specimens down to the Lower Peninsula.  After all, diamonds have been found in the glacial till of the LP, and those are from deposits in the UP and Canada.  Possible....unlikely though, since obsidian is more fragile than diamond, but we shouldn't discount it completely.

But your find might be something else.  It might be flint.  Very fine grained flint can look much like obsidian; black, glassy, conchoidal fracture.  Flint, and it's less showy cousin chert, are native and common in Michigan.  
Here's a couple of sites that show both flint and chert.

http://en.wikipedia.org/wiki/Flint

http://www.beg.utexas.edu/mainweb/publications/graphics/chert.htm

Without having seen your specimen I can't make a definitive identification.  But it IS easy to mistake obsidian for flint, sometimes.  

However, there is also the very real possibility that, leaving aside the prehistoric users, it could have been brought here by a modern-day collector who tired of it, or simply lost it.  

Hope this helps.
Bob

Geology

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C. Robert Reszka, Jr.

Expertise

I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.

Experience

I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Organizations
Michigan Basin Geological Society

Publications
Decade of North American Geology.
Bedrock Geology of Michigan

Education/Credentials
BS Wayne State University

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