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Secondary Deposition


Kim Ellis

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Hello,

I have a question about fossils and secondary deposition....

I am assuming from what I have read that secondary deposition occurs when creature is preserved in sedimentary rock. Over time softer structures will decompose and be filled in by secondary sedimentary rock such as chert.

If this does occur does this alter the specimen and it is considered not a fossil???

Thank you for your help!

Kim

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2 hours ago, Kim Ellis said:

Hello,

I have a question about fossils and secondary deposition....

I am assuming from what I have read that secondary deposition occurs when creature is preserved in sedimentary rock. Over time softer structures will decompose and be filled in by secondary sedimentary rock such as chert.

If this does occur does this alter the specimen and it is considered not a fossil???

Thank you for your help!

Kim

 

Well that depends on what you define to be a fossil. Secondary deposition is a metamorphic process where the composition of the fossil is altered. This can result in petrification of a fossil where it permineralizes at the cellular level and this does indeed replace the organic composition of the organism. Coprolite for example is a fossil of a non-living organism as it is simply fossilized and petrified dung, but is a perfect example of petrification. @GeschWhat could certainly shed some more light on this subject.

 

Here's another source:

 

http://www.whatisafossil.net/

 

Also keep in mind there are three rock types:

* Igneous

* Metamorphic

* Sedimentary

 

Most fossils are preserved in an anaerobic environment where bacteria is limited to thrive and covered in sediment or other hard solids such as mud, sand, ice etc. Later the sediment forms into sedimentary rock such as shale, mudstone, sandstone, limestone, etc.

Do or do not. There is no try. - Yoda

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@Fossil-Hound, you give me much more credit than I deserve. I am not familiar with the term secondary deposition. When I think of secondary deposition, I think of materials eroding and then being covered up again. Kim, if I understand your question correctly, you referring to fossil casts (aka steinkerns). To be honest, I don't know if they would technically be considered fossils. They may be their own separate category.  Coprolites, themselves, are considered trace fossils (ichnofossils) because they are evidence of a behavior. However, they can contain fossil inclusions of undigested prey/vegetable matter which would, I guess, be considered fossils. Steinkerns on the otherhand represent the shape (all or in part) of the organism itself, with none of the original organism remaining. A while back @Carland I had an interesting conversation regarding fossilization. What I learned from that is terms like "fossils" and "petrification" are pretty ambiguous. As you hangout here in the forum, I'm sure you will hear a multitude of different views on subjects such as this. :)

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Thank you so much...

 I am probably using the term incorrectly. I did see the erosion discussion also. 

Being new to the forum I probably need to look a few things up.... 

So if I am reading this correctly - if the the organism is completely replaced by another type of rock then it ia not a fossil, even if the shape is preserved.

But if trace amounts of the original organism, even on a cellular level are present then it is a fossil.

Trying to gain knowledge... Got alot of experts here at my finger tips... Thank you

Kim

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Well as we said that depends on your definition of a fossil. Technically speaking it's not a fossil in terms of being derived from a biological organism but using that same logic then neither is coprolite but many would consider that to be a fossil (i.e. fossilized poop).

Do or do not. There is no try. - Yoda

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24 minutes ago, Fossil-Hound said:

Well as we said that depends on your definition of a fossil. Technically speaking it's not a fossil in terms of being derived from a biological organism but using that same logic then neither is coprolite but many would consider that to be a fossil (i.e. fossilized poop).

Some young ones I've encountered consider me a fossil too. Hey...why are you picking on poop? Always the butt of the conversation ;).  Seriously though, the purpose of words is to communicate a message. As long as the recipient of those words understand what you are talking about, you are ahead in the game.

 

I agree, Kim. There is rarely a day that goes by that I don't learn something new here. It seems like the more questions I ask, the more questions I have.

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19 hours ago, Kim Ellis said:

Hello,

I have a question about fossils and secondary deposition....

I am assuming from what I have read that secondary deposition occurs when creature is preserved in sedimentary rock. Over time softer structures will decompose and be filled in by secondary sedimentary rock such as chert.

If this does occur does this alter the specimen and it is considered not a fossil???

Thank you for your help!

Kim

In regards to chert and fossils. Replacement chert forms when other material is replaced by silica, e.g. petrified wood forms when silica rich fluids percolate through dead wood and the silica precipitates to replace the wood. Chert can also form through direct precipitation from silica rich fluids, e.g. agate is formed by the precipitation of silica in voids within a rock. Using the definition of "fossil", yes, its still a fossil.

...I'm back.

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Thank you!! That is very helpful!! I couldn't understand why no one could see the fossils I was seeing.  

I finally figured that out after much research in the weee hours of this morning..

What a concept!! I started out looking at molds, then casts, then external casts and then internal casts....  whew enough to make my head spin... I finally found a thread here on the forum explaining just that, chert mineral replacement. I guess it makes sense knowing how chert is formed from the mineral content in water and mineralization forming hard structures.

Thank you so much for the confirmation!

That knowledge alone is so very valuable to me as a newbie!

Kim

 

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So I assume opal will do the same thing since it is so closely related to chert.

I have seen pictures of petrified wood with opal inside.

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19 minutes ago, Kim Ellis said:

Thank you!! That is very helpful!! I couldn't understand why no one could see the fossils I was seeing.  

I finally figured that out after much research in the weee hours of this morning..

What a concept!! I started out looking at molds, then casts, then external casts and then internal casts....  whew enough to make my head spin... I finally found a thread here on the forum explaining just that, chert mineral replacement. I guess it makes sense knowing how chert is formed from the mineral content in water and mineralization forming hard structures.

Thank you so much for the confirmation!

That knowledge alone is so very valuable to me as a newbie!

Kim

 

Please keep in mind that just because you see a shape that resembles a leaf, it doesnt mean its a fossil. I did see your chert finds and I agree with the general consensus of geological origins. Chert can fracture and sheer in unique patterns. Another example is Chinese writing stone. This stone has white inclusions throughout the rock. When polished or split, sometimes you can find clusters of these inclusions that appear to be Chinese writing, hence the name. 

 

Pareidolia is a psychological phenomenon in which the mind responds to a stimulus (an image or a sound) by perceiving a familiar pattern where none exists.

 

Example

pareidolia.jpg.45c0ba5c5753a325a5d1ce378f8fb4a7.jpg

...I'm back.

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1 hour ago, doushantuo said:

Raggedy,your examples are wonderful:P

Thanks! I couldnt stop laughing. They're so spot on!

...I'm back.

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So I assume opal will do the same thing since it is so closely related to chert.

I have seen pictures of petrified wood with opal inside. 

And by the way I see faces in rocks all the time!! LOL!!

 

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Not that this has anything to do with chert, but I always thought secondary deposition was when a fossil was buried twice?   Being a crab concretion guy we use to find crab concretions where the outer part of the conc was filled with plant material and none on the inside with a perfect round line seperating the two. 

 

RB

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Unless i have my teminology all tangled up,what you just meant is called redeposition,or even reworking.

 

 

 

 

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The word fossil includes almost everything described here. Any evidence of a living organism (plant, animal, fungi, bacteria, etc.) recorded in the geological record (rocks and soils) can be called a fossil. Terms like cast, mold, steinkern, permimeralization, etc., etc. are just various kinds of "fossils", and it doesn't have to be the body, bones or shell. Tracks and traces are included. We call them ichno fossils.

 

A great resource for learning more would be a college-level intro to Earth Sciences textbook. Find a used copy on line or in a local bookstore. They always have a chapter or two on fossils. They also always have a great glossary section to explain all of these terms that get bantered about.

 

I am also not sure if your use of the term secondary deposition is what you think it is. I would do some more research on that.

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