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Identity HelpLava Pillow

28th May 2017 16:21 UTCJennifer Cindrich

Hello.

I am in the llano uplift region in Central Texas.

I have what I think to be Lava pillows embedded in the limestone here. Is it?

It does not react to acid, is porous, hardness 6, streak tan.

Jennifer

28th May 2017 16:36 UTCWayne Corwin

Looks like a Stromatolite laying on it's side.

28th May 2017 16:41 UTCJennifer Cindrich

click on the photo. its verticle

28th May 2017 16:47 UTCJennifer Cindrich

Interesting. thank you.

28th May 2017 16:52 UTCJennifer Cindrich

I have broke some open. some contain quartz crystals and some have trilobites in them. cool find anyway.

28th May 2017 17:06 UTCGregg Little 🌟

Hi Jennifer;

Assuming the limestone ID is correct, it would highly unlikely (note I didn't say never) be lava in the limestone formation as the depositional settings are quite different. Limestone deposition usually is distant from a volcanic or sedimentary source, like lava, pyroclastics, sand, clay,etc. These structures appear to be more cylindrical than spherical which argues against pillow lava.

More likely, it is a silicified structure like a fossil coral (rugose or horn type) or stromatolitic structure (algal mound). If you can spare a piece, break it apart and see if there is any organized (organic?) structure or texture inside. If you are near a computer a search on fossil site in central Texas might give you some direction. A scale in the photo like a pen or lens cover or finger would help too.

28th May 2017 17:28 UTCJennifer Cindrich

This one was football shaped before I broke it.

28th May 2017 20:53 UTCWayne Corwin

Jennifer

Am I seeing fossils inside of it?

28th May 2017 21:09 UTCJennifer Cindrich

you are indeed seeing a trilobite

28th May 2017 21:14 UTCBob Harman

You must have a more active imagination than I do. Please point out the orientation and fossilized body parts from your photos.

Thanks and CHEERS.....BOB

28th May 2017 21:20 UTCDon Saathoff Expert

Hello Jennifer,

Where, relative to the town of Llano (or Mason or Burnet), did you find this?

Don S.

28th May 2017 21:29 UTCDon Saathoff Expert

Hello Bob, On a line at 1:00 from the center of the rounded mass you can see the rear portion of a possible trilobite - if the head & eyes could be found the species might be determined. Click on the image and then expand.

Don

28th May 2017 21:59 UTCJennifer Cindrich

Don,

In the southeastern part of the Llano uplift near the town of Round Mountain.

Bob- here is another photo for you, the broken half was sent to my geology professor in El paso who confirmed the trilobite. She also questioned if it was a volcanic tuft.

28th May 2017 22:02 UTCBob Harman

I'll take your word for it. It must be one of the larger species. I am only familiar with small examples from Ohio and SE Indiana. How hard was this mass to break; is it a true rock structure? It looks more like a compacted earthen clod like those turned up in plowed fields or during road construction. I guess I need to be convinced.

CHEERS.....BOB

28th May 2017 22:03 UTCBob Harman

Okay!!! Now I see it in your last photo. Thanks, BOB

28th May 2017 22:13 UTCRalph S Bottrill 🌟 Manager

I agree with Greg, it's most likely a siliceous concretion, which can incorporate fossils.

28th May 2017 22:48 UTCJennifer Cindrich

Bob, I took a rock bar to it. It is a rock.

28th May 2017 22:53 UTCPaul Brandes 🌟 Manager

Jennifer,

If you're near Round Mountain, TX, you are likely dealing with either Glen Rose Limestone or the San Saba Member of the Wilberns Formation which is made up of limestone and dolomite. To me, these look very much like the Wilberns Fm. based upon the colour and the fact it contains trilobites. Also, the Wilberns Fm. does contain stromatolitic bioherms and rare siliceous dolostone nodules; it is possible that you have found some of these in your top photo.

28th May 2017 23:58 UTCJennifer Cindrich

Thank you Paul. My exact location I am in is on the inside of the stribling formation where it meets the honeycut formation. It resides inside of the cusp of the Mississippian/Devonian age period.

a good resource for my area-

http://www.lib.utexas.edu/books/landscapes/publications/txu-oclc-2561072/txu-oclc-2561072.pdf

29th May 2017 00:03 UTCJennifer Cindrich

Paul, Why do you say 'rare silicous dolstone nodules and what do they look like?

29th May 2017 00:21 UTCJennifer Cindrich

Do you mean these nodules Paul?

29th May 2017 17:35 UTCEd Clopton 🌟 Expert

The last photos show nodules of chert (which can be called flint if they are grey like this). "Silicified dolostone nodules" (rare or otherwise) would seem to describe the rounded balls featured in your very first photos. Dolostone is a limestone look-alike containing more dolomite than calcite. Dolostone often is harder and more resistant to weathering than limestone, but that's not absolute.

29th May 2017 17:50 UTCJennifer Cindrich

Ed, thanks for clarifying that. Maybe I should stop breaking them open but the fossils inside are so interesting to see. Thanks again!!

29th May 2017 19:29 UTCWayne Corwin

Jennifer

I think you can rule out Lava pillows at this point !

29th May 2017 19:41 UTCJennifer Cindrich

Yes Wayne! Now I think I'll go hunt for something else that I can ask more dumb questions about! ;)

29th May 2017 20:13 UTCWayne Corwin

It's never dumb to ask questions Jennifer, Keep On Rockin' !

29th May 2017 21:54 UTCPaul Brandes 🌟 Manager

Never a bad idea to ask a question, no matter how dumb or ridiculous it may seem to be.

Remember Jennifer, we were all in your shoes at one time......

29th May 2017 22:18 UTCJennifer Cindrich

Thanks Wayne!

29th May 2017 22:18 UTCJennifer Cindrich

Thanks Paul!!

29th May 2017 23:59 UTCGregg Little 🌟

The hard chert or flint like nodules might be novaculite; the formation of siliceous nodules in sedimentary rocks. One more thing, in the smaller bioclastic debris forming the bulk of the structure there are rectangular to circular fragments, sometimes with holes in the center which might be disks of palmatozoan (primitive crinoid) stems. It seems these structures (biohermal?) have attracted the siliceous, and possibly dolomitizing, alteration fluids due to their relatively higher porosity. There could also be leached shell fragments forming the slit-like voids in the groundmass, gone now but likely brachiopod.

Keep the questions coming as I haven't seen everything ;>).

30th May 2017 03:13 UTCJennifer Cindrich

Gregg,

I love what you said. Thank you. Is there anyway on my own with minimal knowledge and tools that I could tell the difference between chert and novaculite? I read that it is indeed of a mississippian/devonian age period, which is here where I am.

30th May 2017 16:32 UTCGregg Little 🌟

Jennifer;

It would be difficult if you had in hand a piece of chert and novaculite as they are both composed almost completely of microcrystalline to cryptocrystalline quartz (basically microscopic to submicroscopic crystals). Since novaculite is a chemically precipitated sedimentary rock then if you find it in the sedimentary depositional environment then that is your best identification; sort of a guilt by association. If fossils (sponge spicules, conodonts, radiolaria, etc.) can be seen in the chert then that is positive too. You would need a hand lens which is one of your cheapest ID tools.

Two "rule of thumb" identifications are that it is said to "feel" a little more gritty than chert and have a lower luster. This would be due to its sedimentary origins and would require comparing sedimentary to non sedimentary chert but that would take time plus samples equaling experience so that not the best in your case but, over time you too could be a novaculite expert. You could seek out an archaeologist for an opinion (the stone was used by early people for tools) but she/he might just ask you where you found it, not getting you much further ahead. If it is in the shape of a tool or chipping fragment then that would be a strong positive ID.

Cheers, Gregg

31st May 2017 11:33 UTCJennifer Cindrich

Gregg,

Thank you so much for the information, it was very helpful. I hunted down some more nodules and inspected them for the fossils with my lens-I never thought to look, but there they were indeed! Also on my hunt for them I found a 'tool', also very cool!

Thanks for making my learning fun and most of all, for helping my feelings of stupidity wane a bit.

31st May 2017 12:12 UTCPaul Brandes 🌟 Manager

Jennifer,

It's not uncommon to find Native American tools and other artifacts in your neck of the woods. In fact, our most recent Society field trip to the Hill Country yielded a few. From your photo, it appears you found a very nice arrow point; congrats!

And thank you Gregg for explaining the nodules in greater detail; it is greatly appreciated...

31st May 2017 12:55 UTCJennifer Cindrich

Thank you Paul,

It was fun to find and unexpected.

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