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Keith,
Some Texas geology questions for you.  I found what I think is an iron concretion (next to a quarter for size reference)in Hamilton County, Texas.  Since the geology in this area hails from the Cretaceous period, I am more accustomed to finding fossil shells and ammonites.  Is this truly an iron concretion?  If so, how did it form and why did I find it in an ancient seabed?

Answer
Ross:

So you found it in Hamilton county.

Iron Concretions occur by the mobilization of ions and depostion of them in a porous matrix, cementing the matrix together.  It is very likely that your concretion started growing around some nuecleus while the sediment was not yet lithified and the saturating water was high in iron.  

The nucleus could be a piece of shell or fossile, such as what you commonly collect from the same formation.

Concretion formation has little to do with the original environment of deposition of the surrounding sediments. They can form long after the host sediments have been laid down and uplifted.   Their formation is more likely if the sediments have iron as part of their composition.  If the original iron was reduced, it could have been mobilized by a change in the pH of the water moving through it, and subsequently deposited to form the concretion.

Yours looks hematitic, or formed of Hematite, or iron oxide.

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Keith Patton

Expertise

I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

Experience

I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

Organizations
American Association of Petroleum Geologists
American Association of Photogrammetrists and Remote Sensing

Education/Credentials
Bachelor and Master of Science
Registered Geologist in State of Texas

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