Matrix stone with tabulate coral

[Doug Von Gausig]

The fossils in this stone are probably Thamnopora sp., a tabulate coral common in the Verde Valley of Arizona. Normally it appears in dolomite from the late Devonian (Frasnian), This specimen was picked up way outside the normal range, in an ancient riverbed, where rocks generally come from the Martin and Redwall limestone beds to the west. 

 

What's weird (to me) is that this stone matrix is impervious to HCl. No reaction at all, as if it's chalcedony, but it doesn't look or feel like chert or any other chalcedony I'm familiar with. Does anyone have an idea what this stone may be and what that might say about its age? 

[Tidgy's Dad]

Fine grained sandstone with iron oxide? 

[Doug Von Gausig]

42 minutes ago, Tidgy's Dad said:

Fine grained sandstone with iron oxide? 

 

I should have looked in the scope before, but at high magnification it definitely looks like a sandstone. I'll have to see which sandstones around here fit the time frame.

 

Thanks!

[DPS Ammonite]

45 minutes ago, Doug Von Gausig said:

 

I should have looked in the scope before, but at high magnification it definitely looks like a sandstone. I'll have to see which sandstones around here fit the time frame.

 

Thanks!

It can be a sandstone and a carbonate such as dolomite or limestone. Sandstone just refers to the particle size and not to the minerals that make up the sand.

 

 Some of the Martin Formation rocks have enough dolomite in them that they do not dissolve in acid easily. They also can have a lot of non carbonate sand and silt in them that makes dissolution difficult. I suspect that there is at least a little carbonate in the rock since the silicified fossils stand out in relief. Dolomite will react to acid if you grind up a small piece up and put very warm acid on it. You may need a lens to see the bubbling.

 

I like to keep a small bottle of 10% HCl with me to see if the fossil that I might bring home can be dissolved out.

 

Dolomite vs limestone:

 

http://www.pattisonsand.com/people-say-that-dolomitic-limestone-is-better-but-what-exactly-is-dolomitic-limestone-and-how-why-is-it-better-than-regular-limestone/

Edited March 23, 2021 by DPS Ammonite

[Rockwood]

I see a bryozoan from 0 - 5 mm. It may have to be silicified, but that's no problem.

[FranzBernhard]

8 hours ago, Rockwood said:

I see a bryozoan from 0 - 5 mm.

That´s always difficult. Maybe they got it all wrong here during the last 100 years, I don´t know:

Plabutsch-formation (Eifelian) - 1

Plabutsch-formation (Eifelian) - 2

Franz Bernhard

[Rockwood]

14 hours ago, Doug Von Gausig said:

Does anyone have an idea what this stone may be and what that might say about its age? 

Ah ! Did I miss the point and stumble upon one of those hanging identification by what is supposed to be there deals ?