Geodized crinoids from Brown County, IN

[AHoffman]

My wife and I recently traveled to Brown County, Indiana (Nashville) so she could attend a friend's baby shower and, while she was there, I slipped away to check out some local streams for fossils. I remembered finding lots of geodes and crinoids around Lake Monroe (to the west) as a kid attending scout camp so I was hoping I might have similar luck on this trip. I lucked out and the stream I checked had many, MANY geodes and a smattering of crinoids and other fossil material. After doing some searching on this forum and elsewhere online, it seems these geodes are frequently formed around (within?) fossils as is the case for a couple of the crinoids in the photo below. The second image looks like it could be some kind of geodized coral? I was wondering if the third and fourth image is of a crinoid crown? Lastly, one of the big geodes I brought back is similar in shape to what I'd expect from a crinoid crown and has some ridges that almost look like faded crinoid parts. Are these just normal annulations found on all geodes or is this possibly a crinoid crown too (I assume it is too big)? I believe all the fossils in this area should be Mississippian (based on websites discussing the fossils around nearby Lake Monroe). Also, is it likely that most of the geodes in this area formed in association with some kind of underlying fossil?

I had no idea about the association between geodes and fossils and am looking forward to learning more from anyone here who could enlighten me!

Thanks,
-Andrew

 

[doushantuo]

makra

 

outtake:

[doushantuo]

microstructures/petrography(still from the previous pdf):

[doushantuo]

the above is from:

A022 - Com2010.pdf

[doushantuo]

1908

[doushantuo]

replaced evaporites?

evapolabsaltw-silica-mob-replaced-noduopy.pdf

[Harry Pristis]

Other examples from Indiana:

 

 

[doushantuo]

Amazing specimens,Harry!

[AHoffman]

Thanks for all of the information on the process of geode formation; that certainly answers my questions about that! Any opinions on the identity of any of my fossil-shaped geodes? I assume if fossils were involved in the formation of those geodes they may not be detailed enough to tell what the fossils were?

 

-Andrew

[abyssunder]

The way how the fossils (crinoids, corals, brachiopods) become geodized is nicely explained and illustrated in Bassler's work. Sometimes they are hard to recognize due to the long crystallization process which makes them inflated, cracked and covered with crystals.
Specimens 2,3,4 are geodized corals, 5 is a geodized crinoid calyx.

[AHoffman]

Thanks abyssunder!

 

Im particularly interested in what makes you think the large piece is a crinoid calyx. That was my initial feeling due to its shape and form, but it just seemed too massive! I attached a couple more images below.

 

-Andrew

[abyssunder]

If I compare your specimen with the below ones, the resemblance might be good. It is big because it's inflated in the silicification process.

 

"In the case of the calyx of a crinoid, little or no fracturing is necessary to give rise to the deposition of silica, since the plate sutures furnish an excellent place for crystalline growth. This particular phase is sufficiently illustrated on Plates XIX and XXIV to require no further mention." - R.S. Bassler

 

 

color picture from here

 

[AHoffman]

Thanks again!

[jewelonly]

@AHoffman, I really enjoyed looking at your finds and reading the information on this thread since I am fascinated with fossils, first, and geodes, second. However, the two combined are amazing!!  

[Archimedes]

Geodized fossils are common in the lower Mississippian Strata, in the Keokuk and Fort Payne Formations 

[Tidgy's Dad]

Quote

 

Interesting. 

[Thebes]

Hello fossil fellow!  I would say the second image you have a question about is indeed a rugose coral.  Regarding the crown question, it is hard to say though the longitudinal arms do not seem apparent.  A large siliceous calyx pseudomorph can easily be 4 in x 4 in.  Depending on the geochemistry at deposition, the end result of a calyx at this time seems to come in many varieties.  I would say that is indeed a large calyx if indeed it is one.  Great finds!

[JessEileen]

I’m pretty new to learning the proper terms (but certainly not new to collecting!) but I believe that your second picture is a horn coral fossil as I have pulled many identical specimens from the area. 

[dalmayshun]

Love this thread. My name photo is a photo of a silicified crinoid stem...one end...it is fascinating to me to look at its structure.