Concretion? Stromatolite? Cup and Rings marking? OOPArt?

[Zhen]

My extraordinarily gifted fossil finding son found this in the bedrock of Brushy Creek (Williamson County, Tx.) and I can’t figure out what it is. 
Apologies for not using a proper scale or for poor picture quality but it is ~2’ in diameter and has well defined concentric rings. 
 

My son would love to show anyone in the Cedar Park area where it is and will offer a free Moonie’s hamburger to anyone who can help solve this mystery. 
Any help is greatly appreciated!

[Wrangellian]

My best guess is it's a stump or log that has been set (cemented?) in place and you're looking at the eroded rings. What I'd like to know is, is that cement around it and was there any historical reason why an old post would have been placed there, for example? I don't know the area.

[Zhen]

No cement, it’s a part of the legit actual bedrock of the creek. 

Best I can tell it is Lower Cretaceous Edwards formation (Ked) or Comanche Peak (Kcp) formations.  

Copious amounts of fossils around that area but I have never seen anything like this. 

  

[Ludwigia]

Fascinating. It certainly does appear to be a cross section of a log at first glance. Any chance of extricating it to have a closer look?

[supertramp]

1 hour ago, Ludwigia said:

Fascinating. It certainly does appear to be a cross section of a log at first glance. Any chance of extricating it to have a closer look?

...that was also my first impression; but I wonder if this erosive pattern is actually more indicative of a hemispherical (spherical in origin?) shape rather than a cylindrical laminated object; a similar concave shape can be seen, emptied, on the left; so, probably not a fossil trunk.

ciao

[minnbuckeye]

8 hours ago, Zhen said:

Copious amounts of fossils around that area

 

Are they marine or terrestrial fossils? Very intriguing fossil!! 

 

 Mike

[Zhen]

Ludwigia-

It is part of the bedrock of the creek under about 2 feet of water. Only means of extracting it would be with a jackhammer......and that would most definitely be a sin against geology to do so. 

 

Minnibuckeye-

Marine fossils (Cretaceous echinoids, cephalopods, gastropods). My son has found really exceptional ones, the little turd has a gift for finding awesome things. 

[GeschWhat]

Very cool!  Welcome to the forum!

[FossilNerd]

Maybe something geological like Liesegang Rings? 

https://en.m.wikipedia.org/wiki/Liesegang_rings_(geology)#History

 

[Wrangellian]

I still don't see anything that would contradict wood... in fact the spacing of the rings appears more regular like wood growth rings, than Liesegang rings which seem to be more irregular.

So my best guess is, if a modern log cemented in place is out of the question, you've got a nice chunk of petrified wood. Too bad you can't get it out!

[Ludwigia]

20 hours ago, Zhen said:

Ludwigia-

It is part of the bedrock of the creek under about 2 feet of water. Only means of extracting it would be with a jackhammer......and that would most definitely be a sin against geology to do so. 

 

I can understand your feelings, but a geologist would have to extract the sample for further study and determination, so I guess if it's going to stay there, we will never get a proper answer above the speculations over the few photos which we see here.

[grandpa]

8 hours ago, FossilNerd said:

Liesegang Rings

Learned something new today.  Didn't even know these existed.  Looking at the link, I think @fossil nerd is definitely on to something.

[Pippa]

8 hours ago, FossilNerd said:

Maybe something geological like Liesegang Rings? 

https://en.m.wikipedia.org/wiki/Liesegang_rings_(geology)#History

 

That was my first thought actually. But Liesegang rings don't usually appear in isolation. They are typically surrounded by many wavy lines, more rings and incomplete or imperfect rings. 

 

Like this small rock with a couple of rings and one almost perfect bullseye and plenty of "waves", that through glacial drift made it from the Lake Superior area (where there are Liesgang outcrops) all the way to my Lake Michigan beach in Illinois: 

 

[FossilNerd]

4 minutes ago, Pippa said:

That was my first thought actually. But Liesegang rings don't usually appear in isolation. They are typically surrounded by many wavy lines, more rings and incomplete or imperfect rings. 

 

Definitely a guess on my part. I knew that Liesegang rings were concentric ring structures in geology, but little else. Just thought it was worth a look. Thanks for shedding some light on the subject!

 

I agree with @Wrangellian in that it has the look of wood, but concentric rings are not a sure indicator that the specimen is/was wood. I think that @Ludwigia is right and a closer inspection would be needed to know what it is. A very interesting piece for sure!  

[Zhen]

This is definitely a stand alone feature. Nothing even close to being similar to it that I have seen anywhere. The rings are crisp but not perfect. There are areas that look folded over or deformed while still retaining the defined ring shape. It’s part of the bedrock but domed up as the higher part of that section of bedrock and the rings taper down and meld right back into the bedrock layer. 

When I search for “concentric rings rocks/fossils/limestone” there isn’t too much to compare this with. A couple of stromatolite, cup and rings markings, and Liesegang pictures on google images look similar but not enough to say this has to be that, and none of those fit the geology of where this is (Austin, Tx. area). 

Thanks to everyone for taking the time to share their thoughts. I will get some better pictures and better descriptions soon....underwater photography forthcoming!....just need to get the camera first. 

 

[Tidgy's Dad]

Thin flat layers of sediment eroded into a dome shape. 

[supertramp]

4 hours ago, Zhen said:

none of those fit the geology of where this is (Austin, Tx. area). 

Maybe you've already answered the question, but: is the bedrock made up of limestone or sandstone (or something else)?

It could help ruling out some of the hypotheses

ciao 

[minnbuckeye]

I am not taking sides on what this is, but  here is a picture of Liesegang rings showing they can "appear in isolation".

 

 

From Wikimedia Commons

 

 

[Fossildude19]

22 hours ago, Pippa said:

That was my first thought actually. But Liesegang rings don't usually appear in isolation. They are typically surrounded by many wavy lines, more rings and incomplete or imperfect rings. 

 

However, there could be more isolated rings, like in Mike's picture above. The one on the right looks pretty close. 

They could be covered by rocks or silt or mud.  We have no way of knowing without investigating further.   

[Zhen]

[Zhen]

[Zhen]

Used an underwater camera, date is inaccurate but hopefully the pictures look ok. 

Have been told by the nice folks at the University of Texas and the Texas Memorial Museum (who are very helpful) it could be a broken carbonate concretion, stromatoporoid, or possibly Liesegang rings except the person said she has never encountered a concretion in central Texas limestone....but need better photos to really make a solid decision. 

I have more pictures coming. 

 

[JohnJ]

You might try taking a scrub brush and cleaning some of the algae from the feature and surrounding rock.  Have you observed any fossils in the adjoining rocks?

[Zhen]

I took pictures before and after cleaning off with a dish brush (wife isn’t happy), but I’m only able to add a few pictures at a time. 

We do find a lot of fossils in this creek, but mostly loose Echinoids and broken Ammonites. There is a section of gray marl (I think that is the correct formation) along the creek bank about 40’ downstream from this that holds a lot of in situ Gryphaea (devil’s toenails). Other than those no fossils around this. 

Funny thing is I’ve probably walked on top of this 100s of times and never noticed. 

[supertramp]

Don't know how to translate in English, but they resemble some strange calcitic concretions we call "scodelle del diavolo" ("devil's bowl").

ciao