Cave sediments

[LCF]

I have a question about the difference in sediments between FL and MX. I've done a lot of diving in the latter, and only a little in the forner. But today, I got a lesson in siltouts, and came out of the cave with my harness packed with fine, red clay. The passage we swam didn't look that unforgiving; I've certainly been in places in MX that were equally small, and only stirred up a little, light dust. And it wasn't ever red.

Does anybody know why the primary sediments in MX seem to be sand, limestone silt, and mung, whereas the FL caves have this red clay in them? I really haven't ever seen clay like that down south.

[Webmaster]

Huh, there's plenty of sand in FL. Some of the sediments will be from organic matter sucked in from a sink in some places. Some will be weathering products from the limestone being broken down, so you essentially have whatever more or less insoluble part is leftover. Different rocks, different sediments.

That's how the amount of rock removal from stylolites is determined, by measuring the thickness of the residue compared to the amount in a given volume of that rock.

[jason]

The red clays come from dissolution of a particular part of the sand that you see all over the surface of Florida. As an interesting aside, this sand is actually responsible for Florida being connected to mainland USA.... For much of it's 'recent' geological history, Florida was an isolated carbonate platform similar to the Bahamas. Continued erosion of the Appalachian Mountains and transport of the sand created from erosion filled in the trench that separated Florida from the mainland and eventually covered much of the state. You'll notice the land surface in the Yucatan is mostly bare limestone. This is because the Yucatan lacked a source of (siliclastic) sand (and why it doesn't have much in the way of red clay - though you'll see some red dust in places that originated from the Sahara desert). The sands in Florida weather to clays that migrate through fractures in the rock and build up in layers on cave floors during low flow conditions. Now the really interesting question is why are there are laminated clay layers indicative of low flow in high flow caves like Cow or Ginnie (or others)..... (we're working on it.... and if we figure anything out - I'll be sure to let you know). You'll also get a lot of dissolution of sand on cave floors when the springs reverse flow during large river floods.

[JamieZ]

Interesting info, thanks

The red clays come from dissolution of a particular part of the sand that you see all over the surface of Florida. As an interesting aside, this sand is actually responsible for Florida being connected to mainland USA.... For much of it's 'recent' geological history, Florida was an isolated carbonate platform similar to the Bahamas. Continued erosion of the Appalachian Mountains and transport of the sand created from erosion filled in the trench that separated Florida from the mainland and eventually covered much of the state. You'll notice the land surface in the Yucatan is mostly bare limestone. This is because the Yucatan lacked a source of (siliclastic) sand (and why it doesn't have much in the way of red clay - though you'll see some red dust in places that originated from the Sahara desert). The sands in Florida weather to clays that migrate through fractures in the rock and build up in layers on cave floors during low flow conditions. Now the really interesting question is why are there are laminated clay layers indicative of low flow in high flow caves like Cow or Ginnie (or others)..... (we're working on it.... and if we figure anything out - I'll be sure to let you know). You'll also get a lot of dissolution of sand on cave floors when the springs reverse flow during large river floods.

[Webmaster]

Now the really interesting question is why are there are laminated clay layers indicative of low flow in high flow caves like Cow or Ginnie (or others)..... (we're working on it.... and if we figure anything out - I'll be sure to let you know).

So you don't think something within the range of observed flow is enough to account for the deposits? Is there any indication of seasonality in the laminations?

Higher rainfall locally, longer periods of low flow conditions?

[jason]

It's possible that the clay layers might form during a slackwater period during spring reversals in river floods. It's also possible the clay layers could have formed under lower water table conditions during lower sea levels when the caves would had negligible flow in them (and wouldn't have functioned as springs)... Some of the clay layers are curiously capped by a hard, orange 'cemented' layer (have a look at the floor in Devil's between the Keyhole and the Hill 400 - in places this is coated by a black manganese oxide - but if you brush it off, you can see the orange crust underneath) - but we haven't analyzed it yet.

I wouldn't expect their to be seasonal differences reflected in the clays because most of the springs do not have seasonal variations water chemistry.

[skip]

So you don't think something within the range of observed flow is enough to account for the deposits? Is there any indication of seasonality in the laminations?

Higher rainfall locally, longer periods of low flow conditions?

or the separation after deposit? stir it up and let it set and things tend to settle in like-kind layers. with millions of years to 'set' would it not layer-out - it is a kind of liquidy solid with some flow to it, right?

skip

[LCF]

Thank you! That's fascinating. I love learning things like that, because the next time I dive in Florida, I'll be thinking about the geology, and the experience will be that much richer. Thanks again!

[JahJahwarrior]

Thank you Jason, your one paragraph answered quite a lot of the questions I had after reading a book on Floridian geology.

LCF, care you share what cave you were diving? I have yet to exit with red clay, but I have stirred up plenty of silt.

[RN]

It was Twin.