I think concretion but I'd like to be sure...

[ToeKnee]

I found a drainage ravine with thousands of these these in them.  I'm almost certain they're an iron concretion of some type but I've gotten several different identifications.  I took a few of them to the MAPS expo last spring for an ID.  One person said michelinoceras, but then an expert on cephalopods said no, definitely not, but he had also never seen anything like them.

 

These were found on the north side of Dubuque, IA right at the top of the lower Galena dolomite just above the upper chert beds.  They are in a thick sticky grey clay which sits just above a thick iron rich encrusted layer that varies from 1/4 to 1/2 inch thick (blackend hardground?).  The clay layer is approximately 20' thick and contains thousands of these.   They are almost all vertically oriented, cylindrical in shape, and vary from 1/2 inch to as much as 6 inches in diameter, and vary in length from several inches to several feet long.  

 

Some of the smaller ones that have weathered out do look amazingly like cephalopods.  I had previously found a few pinky finger sized weathered ones farther down the ravine and thought cephalopod but then found the clay with the bigger ones. They have a center that resembles a siphuncle but I don't see anything that looks like septa or individual chambers.  There are too many of these to think they haven't been found before but I can't seem to find anything describes them specifically for this area.  I did find a paper from a study done in Finland titled "Ferruginous Concretions Around Root Channels and Fine Sand Deposits".   That paper seems to describe what these may be be but since I've gotten a couple different ID's and none of them concretion I was hoping someone with a little more knowledge can tell me for sure.

 

url to the research paper - https://doi.org/10.17741/bgsf/47.1-2.020

 

 

[westcoast]

I think these are rhizocretions, Iron mineral deposits that form around roots 

[ToeKnee]

24 minutes ago, westcoast said:

I think these are rhizocretions, Iron mineral deposits that form around roots 

Oooo, that's a new word I haven't heard before! Although I didn't see that term, it sounds like what the research paper from Findland is describing.  Off to google I go...