How Are Concretions Created?

[RJB]

Ok, I know there are some purty dang smart peoples on this forum. Many years ago a fellow told me aobut how snarge concretions are made. He told me that the crab dies, gets buried, and as it rots, the gases, (sulphure dioxide) are trapped and on the molecular level that draws in iron oxides and cements the matrix around the critter? Then a few years later another fellow told me that it isnt sulphur dioxide but amonia that creates the concretions. Just today, somewhere on the net, while surfing around looking for placenticeras ammos, this site said that the more "cement" around each sand grain, the bigger and harder the concretion? They were talking about the Pierre Shale formation, and as far as I know, there is no sand in that formation? And what they meant by "cement", I do not know. Im assuming that with all the different sedimentary formations in the world that there must be a few ways that concretions are formed? Can anyone here clerifie this mystery for me? Its about time I learn this so I dont sound so stupid when I talk with someone about it. Im not too worried about my spelling though.

RB

[Northern Sharks]

snarge concretions- maybe you should be a little bit worried about your spelling

[Uncle Siphuncle]

Good thread, but I have nothing to add. I will however sit on the sidelines to hear what the experts have to say.

[Rain1950]

It's not sulphur dioxide; rotting gives off hydrogen sulphide; which readily (in contact with water) becomes sulphuric acid. And it isn't iron involved, but calcium, i.e. concrete is calcium carbonate.

[non-remanié]

It's not sulphur dioxide; rotting gives off hydrogen sulphide; which readily (in contact with water) becomes sulphuric acid. And it isn't iron involved, but calcium, i.e. concrete is calcium carbonate.

There are different types of concretions, including iron ones. In NJ Cretaceous deposits we often get layers of siderite (an iron carbonate mineral) concretions formed in shallow burial depths. I have also been interested in learning more about them because they are very helpful geologic indicators here. The wiki article on concretions gives a decent but brief overview: http://en.wikipedia.org/wiki/Concretion