Ocala , Florida Concentric Circles In Limestone ?

[Smmessineo]

I recently moved to Ocala, Florida and I've been enjoying fossil hunting right in my neighborhood. I've learned a lot by reading these forums, and studying up on Florida's geologic history.

This one has me stumped. There are well- defined layers and concentric circles at two corner of this rock.

Any ideas ?

Some parts of it are brittle, crumbly limestone. But some parts are quite hard and seem to have chert inclusions. ( I've seen a bunch of that around here. Beautiful ! )

[abyssunder]

Welcome to the Forum!

The big size, the concentric layers and the overall shape makes me to believe it is a concretion,...with possible fossils inside.

[Sacha]

My sister in law has a farm Northwest of Ocala from which I collect truckloads of this stuff for garden edging. Under a surface layer of sand and clay is a layer of this deteriorated limestone with thick bands and swirls of chert. I have allot of this concentric or swirl pattern displayed in the limestone that, I think, reflect the depositional character of this large grained Ocala Limestone.

I wouldn't call them concretions in the normal sense since there isn't a difference in the hardness of these features and the surrounding rock. They are part of the grain exhibited in the layering of the rock. Not fossil remnant, but geologic.

[Smmessineo]

Thanks for the replies !

[Auspex]

I would not have learned about the chert layer there, nor its weathering characteristics, had you not asked this question; it may not be fossil, but it's pretty interesting geology! The native tribes certainly took advantage of it

[Sacha]

I'm in Ocala today with another truckload of this stuff. A local friend has an impressive collection of points/blades found nearby that are made from this local chert. Beautiful workmanship and impresive quantity of large artifacts.

We also do a fair amount of gardening for my sister in law. Digging a planting hole or a bed involves excavating a very greasy and dense clay with tons of these limey/cherty rocks/boulders to get space for planting soil. Great ceramic material and great tool material. Just very hard to plant a bush.

Nearby is "Forty Fathom Grotto" a sink hole utilized for cave diving training.

[Diceros]

I think maybe it is a fossil, but of the most primitive kind - a cyanobacteria (FKA "blue-green algae") stromatolite (a bacterial mat which stabilizes the calcareous mud in layers, and lives on the surface of it). They produce a concentric weathering, when the rock is soft enough. You might try emailing your photos to Roger Portell, at the Florida Museum of Natural History in Gainesville.

Ocala is in the middle of the Ocala Uplift of northern peninsular Florida, where shallow marine Eocene limestone (I think it's late Eocene Crystal River Fm., Ocala Grp.) is brought to the surface. It's the reason that area is so plagued with sinkholes (with houses and cars dissappearing into the ground) - it's karst topography.

My guess about the chert areas in the fossil is that the silica derives from the overlying e. Miocene Hawthorn Clay, which has siliceous volcanic ash (in the form of tiny opaline silica bubble shards) in it. The groundwater sucks the silica out of the ash, then dumps it into whatever is porous below it. That's how central Louisiana came to have so many silicified logs. More ash was deposited in the Tampa area, so it has semi-silicified e. Miocene coral heads.

[Harry Pristis]

I've not heard about Hawthorn Group sediments contributing SiO₂ in the Ocala Group limestones, but it may have made some limited contribution in some areas of the state. However, the source of the silica is not opaline volcanic ash . . . at least, there is no mention of volcanic ash in the Randazzo & Jones analysis of the Hawthorn sediments in The Geology of Florida (1997). See pp. 199-200.

Diatom frustules (opal-A silica) and diagenetic opal-CT and quartz chert occur in places.

The process of dissolution of the calcium carbonate and the precipitation of the cryptocrystalline quartz (chalcedony) in Ballast Point corals is described by Lund in his 1960 paper "Chalcedony and Quartz Crystals in Silicified Corals.' According to Lund,

the origin of the dissolved silica is plants and animals as diatoms, radiolarians, and silica-secreting sponges, as well as other siliceous matter occuring in the limestone matrix.

Florida springs continue to produce silica-laden water as the fossil silicate skeletons which compose much of the Ocala Group limestone are dissolved.

In the Ocala area, the gumbo clay in the subsoil is a product of decomposing Ocala limestone.

[Diceros]

I've heard of the biogenic source of the silicified Tampa corals for years, and never bought it. To me, there aren't enough diatom frustules, radiolarian tests, or sponge spicules in the world to silicify the e. Miocene Tampa corals.

I became interested in the source of the Tampa silica years back, in studying an 8 ft. thick ash bed (it's that thickness for miles) in the early Miocene Catahoula Fm. of Louisiana. I sampled it in 1988, at a place called Chalk Hill, in NW Catahoula Parish (LA doesn't have counties), NE LA. Oddly, Chalk Hill hasn't an ounce of carbonate in it - it's chalky white color comes from montmorillonite clay, the material remaining from the ash when the silica has been devitrified. Unlike stable forms of silica, like quartz sand, the ash bed contains unstable amorphous opaline silica, in the form of tiny bubbles. When the bubbles are broken, it's very easy for the groundwater to suck the silica out of the thin shards of amorphous opal. In LA, it produces silicified wood, a poor form of opal, and a sandstone ridge (they call it a "wold" there) which is the source of about the only topography in that part of the state

The source of the LA ash is the Trans-Pecos volcanic field, in N. Mexico and W. Texas, erupting in the early Miocene. By the time it crosses the TX/LA state line, all of the heavier fragments (and datable crystals) have dropped out of suspension, so what remains is tiny bubbles of amorphous opaline silica (opal-A), and broken shards of the bubbles. Light as they are, they can travel a long way on the wind. I always figured that, if you could get 8 ft. of ash in NE LA, you could surely get a foot of it across the Gulf of Mexico in Tampa. The Hawthorn is approximately contemporaneous with the Catahoula, and I suspect it did get that foot of ash, and that the silica was completely removed from it, and dumped in the corals (not enough to fill them, but only partially silicify them), in the rainy Florida weather. Just a theory, mind you, but a well-informed one.

Just a word about the "diagenetic opal-CT" you mentioned above. Do you know what the CT stands for? It stands for two forms of high-temperature silica, called Crystoballite and Tridymite. So how exactly do you get high-temperature minerals in Florida? Same way the chert-like opal-CT got into the silicified logs of Louisiana - by getting silica from from a high-temperature source: volcanic ash.

[Harry Pristis]

The Hawthorn Group is marine, unlike the Catahoula.

Between the Loop Current and the Yucatan Current merging to produce the Gulfstream over a mostly-submerged Florida, an accumulation of a foot of volcanic ash is unlikely. It's even less likely to expect the fines to accumulate. I don't know how "rainy Florida weather" figures into your theory.

I think it is likely that traces of fine volcanic dust are to be found most everywhere in the world through wind and water transport. I think your Florida silica theory suffers from a bias from your experience in LA and a lack of appreciation for the number of silicate skeletons in the Florida limestones . . . But, that's just my theory.

[Smmessineo]

Thanks ! All of this gives me more ideas to read up on. I find Florida's geologic history to be amazing.

[Khyssa]

You might find the book Roadside Geology of Florida by Jonathan Bryon, Thomas Scott, and Guy helpful. I recently bought a copy after one of the authors was a guest speaker for the Tampa Bay Fossil Club. While I've mainly just skimmed through the book so far it is pretty interesting and informative.

[Shellseeker]

When this thread was created a couple of weeks back, I had just found an interesting piece of limestone with concentric circles.. Never got the photos and forgot until today reading "New Content". Very interesting & educational read.. Here is the one I found in Desoto county, Peace River Formation.

[digit]

I've found small black round items that also are surrounded by rings like Saturn and the rest of our gas giant planets. The ones I kept were smaller--about the size of a large button. I originally thought they might be biological but have decided that they are likely of geological origin. When I return home I'll try to remember to photograph my fossilized "mud buttons" as I call them and post them here.

-Ken