DJ Fossil Posted April 14, 2011 Share Posted April 14, 2011 Hey forum, I have a deceptively simple question for you all. It has been posed to me several times by students that I sometimes give lectures to. Concretions. Almost all the fossillized beasties we find around here are encased inside concretions, and the question is, how did they get inside? How do concretions form around a shell or tooth or bone? And of course, that leads to the next question, why are so many of 'em EMPTY??? Link to comment Share on other sites More sharing options...
Scylla Posted April 14, 2011 Share Posted April 14, 2011 (edited) Just speaking off the top of my head, but as things decompose, especially in annoxic conditions, the pH of the area tends to drop. Different minerals can precipitate in these conditions than at other pH's. I also read somewhere that decaying creatures can emit ammonia that also can cause mineral precipitation. Why are there empty ones? Maybe there was better decomposition so that no recognisable structure was left? Edited April 14, 2011 by Scylla Link to comment Share on other sites More sharing options...
Auspex Posted April 14, 2011 Share Posted April 14, 2011 The organic material chemically alters the entombing sediment in the immediate area, causing it (the sediment) to form a harder mass (bacteria are usually big players in this). Empty ones are where the ogranics were "used up" and left no visible trace. Tell them that the T-rex Sue (as well as a couple others) was inside a biiiiiig concretion; just a little wow-factor to keep their attention "There has been an alarming increase in the number of things I know nothing about." - Ashleigh Ellwood Brilliant “Try to learn something about everything and everything about something.” - Thomas Henry Huxley >Paleontology is an evolving science. >May your wonders never cease! Link to comment Share on other sites More sharing options...
bear-dog Posted April 15, 2011 Share Posted April 15, 2011 Had to put my tin-foil hat on to post this cause everyone is readin my mind.The fossil gods hired some Keebler Elves that were laid off at that time.Due to automation combined with a bad economy,the little guys couldn't keep up. Luckily they had a strong union so they stayed working for a while.Now all that is done overseas and it became a mess.The keebler guys said their real sorry and will try harder next time.Now I will remove my tin-foil hat.Don't tell anyone our little secret. :lol: Bear-dog. Link to comment Share on other sites More sharing options...
tracer Posted April 15, 2011 Share Posted April 15, 2011 the question may be deceptively simple, but the answer is not. if you can master every nuance of understanding diagenesis, compaction, cementation, and how bacterial biochemistry operates in a reducing environment, then you're scary smart. compounding the issue of understanding them is that even though the concept of concretions may be a convenient generalization created by humans, geology does not apparently feel it necessary to operate in sync with concepts, so there is wide variance in the structure and composition of concretions. edit: just realized i didn't address the question very well. i think fossils are found in concretions sometimes because the presence of the organic material serves to "seed" a progressive precipitating/cementing process. the reason i think some concretions don't have fossils in them is because i don't think all concretions form based on only one type of process. some are "innies" and some are "outies", and some around the marlin, texas area are so freaking hard you just flat can't break them without wearing yourself to a frazzle. and even then probably not. just avoid the area and don't look for them. concretions can be quite annoying, now that i think about it. Link to comment Share on other sites More sharing options...
pleecan Posted April 15, 2011 Share Posted April 15, 2011 Here is the famed Devonian era Kettle Point concretions on shores of Lake Huron in Ontario Canada... http://www.thefossilforum.com/index.php/topic/16534-trip-to-kettle-point-on/ The carbonate concretions can reach 3-4 meters in diameter.... placoderm remains and other fossils have been found in these concretions. Link to comment Share on other sites More sharing options...
glacialerratic Posted April 15, 2011 Share Posted April 15, 2011 There is a good chapter in "Richardson's Guide to the Fossil Fauna of Mazon Creek" on diagenetic events. Without re-reading the chapter, I recall that the author (Baird) stated that preservation needed to occur within 21 days, or else the nodule forms empty. Peter, there is an area on Lake Michigan, just north of where I am, that has an exposure of the Norwood Shale, and there are concretions similar to those at Kettel Point. Link to comment Share on other sites More sharing options...
pleecan Posted April 15, 2011 Share Posted April 15, 2011 There is a good chapter in "Richardson's Guide to the Fossil Fauna of Mazon Creek" on diagenetic events. Without re-reading the chapter, I recall that the author (Baird) stated that preservation needed to occur within 21 days, or else the nodule forms empty. Peter, there is an area on Lake Michigan, just north of where I am, that has an exposure of the Norwood Shale, and there are concretions similar to those at Kettel Point. Yes I knew there was a similar place on the US side , thanks for the info Tim! Areas of geologic anomalies = potential interesting fossil hunting areas. PL Link to comment Share on other sites More sharing options...
Bullsnake Posted April 15, 2011 Share Posted April 15, 2011 In the ID section, Fossidude19 posted a good link to an OP by sok82; 'Need a little help with this guy'. Posted today. Steve Link to comment Share on other sites More sharing options...
pleecan Posted April 18, 2011 Share Posted April 18, 2011 Concretion paper: https://kb.osu.edu/dspace/bitstream/handle/1811/4432/V57N02_114.pdf;jsessionid=37DBB5C2A4DB2D55D6BADD435AA7E1D4?sequence=1 Link to comment Share on other sites More sharing options...
DJ Fossil Posted April 19, 2011 Author Share Posted April 19, 2011 the question may be deceptively simple, but the answer is not. if you can master every nuance of understanding diagenesis, compaction, cementation, and how bacterial biochemistry operates in a reducing environment, then you're scary smart. compounding the issue of understanding them is that even though the concept of concretions may be a convenient generalization created by humans, geology does not apparently feel it necessary to operate in sync with concepts, so there is wide variance in the structure and composition of concretions. edit: just realized i didn't address the question very well. i think fossils are found in concretions sometimes because the presence of the organic material serves to "seed" a progressive precipitating/cementing process. the reason i think some concretions don't have fossils in them is because i don't think all concretions form based on only one type of process. some are "innies" and some are "outies", and some around the marlin, texas area are so freaking hard you just flat can't break them without wearing yourself to a frazzle. and even then probably not. just avoid the area and don't look for them. concretions can be quite annoying, now that i think about it. I think the empty ones have been placed in my path, just to frustrate and annoy me! Actually, I think they annoy John more than they annoy me....next time budd, we will fill your pack with ammonites! Link to comment Share on other sites More sharing options...
jbswake Posted April 19, 2011 Share Posted April 19, 2011 I think the empty ones have been placed in my path, just to frustrate and annoy me! Actually, I think they annoy John more than they annoy me....next time budd, we will fill your pack with ammonites! Sorry, but I don't believe you anymore! "Blimey! Would you look at the size of that!"McAbee is the other woman! Link to comment Share on other sites More sharing options...
DJ Fossil Posted April 19, 2011 Author Share Posted April 19, 2011 Sorry, but I don't believe you anymore! I'm headed back there tomorrow morning....shall I leave some for you, or bring em here and store them? Link to comment Share on other sites More sharing options...
jbswake Posted April 19, 2011 Share Posted April 19, 2011 Yeah, yeah, yeah! "Blimey! Would you look at the size of that!"McAbee is the other woman! Link to comment Share on other sites More sharing options...
micropterus101 Posted April 19, 2011 Share Posted April 19, 2011 (edited) Dj, check out my information page. I have some information about how concretions in our area were formed. Regarding duds: Any organic material can start the process, the trick is for the "seed" to be preserved before it is destroyed. Its hard to explain but basically what I am saying is that for something to be preserved in this manner the process has to happen in a certain amount of time before the organic item that started the proces is completely broke down. Something like a crab, ammonite, gastropod etc.. will take longer to break down versus something like a sea slug, seaweed, piece of flesh etc... Therefore it is more likely you will find concretions bearing hard shelled organisms yet also find concretions bearing absolutely nothing but rest assured something organic started the process either way. Edited April 19, 2011 by micropterus101 fossil crabs website Link to comment Share on other sites More sharing options...
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