Fossil? In Hard, Dense Gravel

[fowells]

A piece of gravel used for landscaping on a San Antonio construction job.

Alluvial gravel, mined from river beds for concrete aggregate and xeriscape. This piece is very hard - 8 Mohs.

Turn it over and there appears to be a recognizable fossil cast in a face created by a relatively recent fracture.

I am used to seeing this sort of fossil in soft limestone.

What is it and how did it get cast in this hard rock?

[Auspex]

To me, it looks like it might be a piece of the shell ("test") of a sea urchin ("echinoid").

It was buried in sediment that turned to stone that then broke up through weathering and was (from the looks of the gravel) rolled around in a river for a while. The original material of the creature did not survive the later events.

[fowells]

To me, it looks like it might be a piece of the shell ("test") of a sea urchin ("echinoid").

It was buried in sediment that turned to stone that then broke up through weathering and was (from the looks of the gravel) rolled around in a river for a while. The original material of the creature did not survive the later events.

I betray my complete lack of knowledge of the geologic process but this has been bugging me for a while. So somewhere upstream, a layer of fossil bearing sediment is replaced somehow by silica then breaks up and is tumbled in the river and ends up jumbled up in this alluvial gravel where fossils from any period occuring upstream are represented?

[Guest solius symbiosus]

Basically, yes, but the silica wasn't replaced; it formed during diagenesis.

[fowells]

Now I've got to know what diagenesis is.

[tracer]

look, um, here's the thing. one foot follow the other, you know, and before long, you get so far down the path, that it's fairly hard to get back home again...

oh. i'm talking about trying to understand stuff like diagenesis. i'm being very serious here - the more i read, the less i know. and don't even get me started on chert. ok, ok, need convincing. alright, here. somebody invented a word called "chalcedony", ok? well, get this - it's pronounced "cal sed' nee". yes, really. and it gets worse. when you start reading up on what chert is, and then start reading up on what chalcedony is, you'll basically come to the conclusion that they're both the same and they're everything. yes, really. i think even the "eye sand" that you find in the corners of your eyes in the morning is chert. but don't research what i just said! you might find out about eyelash mites and look up a picture of them and sheesh.

oh, well! good thing i can keep it all in perspective! <dancing the snoopy dance in such a frantic manner as to make the mites get dizzy and fall out>

[fowells]

look, um, here's the thing. one foot follow the other, you know, and before long, you get so far down the path, that it's fairly hard to get back home again...

oh. i'm talking about trying to understand stuff like diagenesis. i'm being very serious here - the more i read, the less i know. and don't even get me started on chert. ok, ok, need convincing. alright, here. somebody invented a word called "chalcedony", ok? well, get this - it's pronounced "cal sed' nee". yes, really. and it gets worse. when you start reading up on what chert is, and then start reading up on what chalcedony is, you'll basically come to the conclusion that they're both the same and they're everything. yes, really. i think even the "eye sand" that you find in the corners of your eyes in the morning is chert. but don't research what i just said! you might find out about eyelash mites and look up a picture of them and sheesh.

oh, well! good thing i can keep it all in perspective! <dancing the snoopy dance in such a frantic manner as to make the mites get dizzy and fall out>

Thanks. Very helpful.

[MikeD]

Looks like a plate from an urchin. The pic is a little fuzzy. If it is, the circle is where a large spine is attached.

[Uncle Siphuncle]

Looks like an Archaeocidaris plate if it is Pennsylvanian in origin, Phyllacanthus if its Cretaceous.

[tracer]

Thanks. Very helpful.

well, it is relatively easy to speculate that an interamb plate with tubercle from a lower cretaceous echinoid had enough organic material associated with it upon deposition to begin a diagenetic process which facilitated the diagenetic formation of a chert concretion in the sedimentary process.

but attempting to understand the process, in my opinion, requires a grasp of organic chemistry beyond what i will be capable of reaching with my aging brain, and i therefore remain with a rudimentary understanding of it, and a bias toward compensating for that with humor.

i have fossil gastropod molds which i found in a similar-in-appearance piece of chert. it is less unusual than it seems, but the geology of the san antonio area would definitely create a strong if inaccurate impression in one's mind that limestone or at least a limey marl are the principle constituents of most fossils.

i am very fond of the central texas cretaceous and don't doubt that your learning curve will be quite a rapid one in that environment.

regards,

tracer

[Auspex]

Without shooting for dual degrees in chemistry and geology, diagenesis is (according to Wikipedia):

"...any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. The boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational.

After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization."

I usually just think of it as magic...

[fowells]

Without shooting for dual degrees in chemistry and geology, diagenesis is (according to Wikipedia):

"...any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. The boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational.

After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization."

I usually just think of it as magic...

I'll go with magic also. Thanks for your help.

[fowells]

well, it is relatively easy to speculate that an interamb plate with tubercle from a lower cretaceous echinoid had enough organic material associated with it upon deposition to begin a diagenetic process which facilitated the diagenetic formation of a chert concretion in the sedimentary process.

but attempting to understand the process, in my opinion, requires a grasp of organic chemistry beyond what i will be capable of reaching with my aging brain, and i therefore remain with a rudimentary understanding of it, and a bias toward compensating for that with humor.

i have fossil gastropod molds which i found in a similar-in-appearance piece of chert. it is less unusual than it seems, but the geology of the san antonio area would definitely create a strong if inaccurate impression in one's mind that limestone or at least a limey marl are the principle constituents of most fossils.

i am very fond of the central texas cretaceous and don't doubt that your learning curve will be quite a rapid one in that environment.

regards,

tracer

Tracer,

In that case, can we go upstream to these fossil bearing chert deposits (veins? layers?) and bust off hunks and go to cuttin and polishin? (I was a lapidary in a previous life)

And speaking of Eye Chert, here's one I found in the landscaping of a motel across the road from the seawall in Galveston:

Thanks for your help

fowells