Blastoid?

[SilurianSalamander]

Are these both blastoids? The larger one is about 2 cm across at its widest point and the smaller is about half a cm across. Ordovician, Dane county Madison Wisconsin. Thanks!

[Tidgy's Dad]

The first one in particular does look very much like a blastoid. Pentremites in fact. 

But the colour and texture of the rock plus the presence of a fragment of a large fenestellid, suggests to me that this rock is Mississippian. 

[DPS Ammonite]

33 minutes ago, Tidgy's Dad said:

The first one in particular does look very much like a blastoid. Pentremites in fact. 

But the colour and texture of the rock plus the presence of a fragment of a large fenestellid, suggests to me that this rock is Mississippian. 

Here’s a geological map of Wisconsin; not seeing any Carboniferous rocks. Doesn’t meant that they don’t exist in small quantities. 
 

https://wgnhs.wisc.edu/pubshare/M078-paper.pdf

 

What sort of differences in “color and texture” generally are there between Mississippian and older Paleozoic rocks? I would tend to ID age of a Paleozoic rock more on the fossils than the color or texture of a rock.
 

 

[Tidgy's Dad]

1 minute ago, DPS Ammonite said:

Here’s a geological map of Wisconsin; not seeing any Carboniferous rocks. Doesn’t meant that they don’t exist in small quantities. 
 

https://wgnhs.wisc.edu/pubshare/M078-paper.pdf

 

What sort of differences in “color and texture” generally are there between Mississippian and older Paleozoic rocks? I would tend to ID age of a Paleozoic rock more on the fossils than the color or texture of a rock.
 

 

I quite agree. 

I understand the geology of Wisconsin doesn't seem to support much if anything in the way of Carboniferous deposits, but I am just saying what I am seeing, I may well be completely off base. 

Again, I have pieces and have seen Early Carboniferous USA rocks that have this very pale grey colour and a sort of "spiky" texture and am aware that earlier rocks may also have this form but I pointed out that this was in conjunction with a piece of what appears to be a large, fan or cone fenestellid which are fairly rare in the Ordovician as are blastoids.  The first fragment at least looks very much like a typical Mississipian Pentremites and not like any of the Ordovician blastoids that I have seen. 

I am just saying what this rock looks like to me, others may have a better idea, and as I say, I may well be wrong.

[FossilNerd]

I see where @Tidgy's Dad is coming from. The rock does look suspiciously Carboniferous (Mississippian) in age to me was well.  Like Adam, the fragment in question reminds me of Pentremites that I find in the Mississippian rock here. That coupled with the fenestellid bryozoan fragments, and what I think is the basal plate from a crinoid calyx commonly found in the Mississippian lead me to that conclusion. Not to mention the conglomerate of other bits that scream Carboniferous to me. I see that same “spiky” texture with lighter color here in the Carboniferous, but not in the Ordovician.

 

1 hour ago, DPS Ammonite said:

What sort of differences in “color and texture” generally are there between Mississippian and older Paleozoic rocks? I would tend to ID age of a Paleozoic rock more on the fossils than the color or texture of a rock.

From my experience, the color and texture of rock can be a good indication for a particular formation or time period. In my area I can tell the difference, with near certainty, between an Ordovician age exposure and Mississippian age by color and texture alone. Of course, I agree that other factors (such as fossils) need to be looked at to confirm, but I usually have a pretty good idea of the age of rock before I get near it. At least in this area where I have extensively studied and hunted.

 

All that being said, from what I can find in a quick internet search, there is no Carboniferous rock in Dane County, Wisconsin. So I concede that my Mississippian suspicions could well be wrong, and my lack of knowledge about Dane County’s geology has me incorrectly comparing it’s rock to that of my own hunting grounds.
 

Link to a document specific to Dane County: 

https://wgnhs.wisc.edu/pubshare/ES043.pdf

Edited September 17, 2023 by FossilNerd

[crinus]

I have to agree that the specimen is a Pentremites from a Mississippian locality.   No such thing as a Ordovician blastoid.  There was a find in Tennessee but my understanding is that now they are leaning towards some other echinoderm.  

Now that doesn't mean that you did not pick it up in Wisconsin.  Did someone dump excess rock at the locality?  It is not unusual for someone with excess material to dump it at a local locality rather than drive it back to the locality where it was picked up.

Joe

Edited September 18, 2023 by crinus
missing word

[SilurianSalamander]

On 9/18/2023 at 9:07 AM, crinus said:

I have to agree that the specimen is a Pentremites from a Mississippian locality.   No such thing as a Ordovician blastoid.  There was a find in Tennessee but my understanding is that now they are leaning towards some other echinoderm.  

Now that doesn't mean that you did not pick it up in Wisconsin.  Did someone dump excess rock at the locality?  It is not unusual for someone with excess material to dump it at a local locality rather than drive it back to the locality where it was picked up.

Joe

This was in a stone bench. Many of the buildings on the UW Madison are built with this rock. It was supposedly quarried in Dane County which is strange considering the rocks there are supposed to be Cambrian to Ordovician.