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Author Topic: Ophiolitic exposures in Jamaica... Can anyone help me with a few more questions?  (Read 25171 times)

Offline susanshirleyjamaica

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Thanks for your follow up, Bass.  And Fretback... THANKS for the pictures and diagrams of sponges!  The chert nodules that make up the great bulk of our beach rocks were one of the greatest puzzles for me.

Back to ophiolites.  I was pleased to receive information from a prof at UWI regarding how ophiolitic rocks could end up in such good condition on my beach.  The ophiolite structure would have eroded and broken up long ago.  Some would have become part of conglomerates that made it to the sea via the two rivers near me, both of which go into frequent turbulent spate.  The sea would (and I suppose still does) erode the conglomerates, liberating the individual rocks.

How's that Simon, did I get it right?
 

Offline frethack

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I actually came across an article about a hydrothermal chert reservoir in Canada...supposedly the only one in the world.  I havent read anything about it except that article, so I couldnt give any details.

Its in the Parkland Field and it is Devonian in age.  As I said, I dont really know anything about it and havent fully even read the article.

http://aapgbull.geoscienceworld.org/cgi/content/abstract/85/1/51
 

Offline Bass

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BAM!
         bzzzzz
                   bzzzz
                           bzzzz
                                    bzzzz
                                              bzzzzzzzzzz
                                                           SPLAT! [xx(]

Frethack, you shot me down with that one.  Hydrothermal chert?  (I'd call it jasper)  What'll they think up next??
« Last Edit: 19/12/2008 04:22:03 by Bass »
 

Offline frethack

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But youre still right.  If it is hydrothermal it seems like there would HAVE to be some alteration of the country rock, as well as unusual isotopes. 

Ive been climbing trees and cutting branches all day (being a monkey is not an easy task for a big guy like myself!) as well as being a grease monkey and "fixing" my car (to be read "breaking it more"), so Ill save reading the article for another day.  I couldnt see how you would be wrong though.
 

Offline susanshirleyjamaica

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A couple of days ago frethack said...

"For the most part, sponges decompose into microscopic spicules (needles) of silica or carbonate material (usually aragonite).  There are a few species of sponges whose spicules are made from proteins instead of inorganic minerals.  Chert can also come from plankton such as radiolarians and diatoms, who build their tests from silica as well.  Im sure there are many other sources, including hydrothermal activity (Bass?), but the main sources that I know of are organic in origin."

and posted this diagram...



Below are local sponge skeletons and a couple of chert nodules. 



Makes sense to me!

 

Offline Barrettia

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Hi Susan,

The ophiolitic rocks sit between the Yalahs and Morant Rivers and cannot drain to the north.

There are two possibilities as to the origin of the chert. One is as everyone has said biological - from diatoms and spondes (the White Limestone does have a large proportion of sponde spicules). The other is volcanic. There are many bentonites in the White Limestone and remobilisation of silica has been suggested as a source.

As you see, many of your cherts probably come from the White Limestone. If you hunt hard, you can find larger benthic forams embedded in some of them (lens shaped fossils up to 4 or 5 mm in diameter, even larger sometimes).

Looks like you have a good eye for finding things,

Simon
 

Offline JimBob

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  You think JimBob might have more detailed information?

Classic! You are right to question that.

I'm sure he'll weigh in with his opinion whether or not he has more detailed information

That made my day.

What is this know-nothing think he is doing in an area where he has absolutely no knowledge? It appears that he is just hurling insults - behavior of what the monkeys at the zoo do when bored, throw sh*t at the people. But then, he is just a trained monkey so why should I be surprised?
 

Offline JimBob

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Oh, Grasshopper, there is so much to teach and so little time. Even David Caradine (the original "Grasshopper") learned faster. At least I don't use physical discipline.

Cherts do form from sponge spicules and siliceous sponges did at one time contribute considerably on occasion to silicate rocks. But the vast majority
The more likely source is your organic suggestion- radiolarians and diatoms, especially in or downwind from volcanically active areas.  Or the silica could be provided by nearby more silica-rich sediments during diagenesis.

Deep ocean chert layers (ie top of ophiolite sequences)are derived from radiolarian and diatom tests as they filter to the ocean bottom (plus lots of time!)
is the correct answer.

Siliceous sponges were thought to be extinct until recently and had been considered extinct since the end of the Paleozoic (I am not certain of this date and could be wrong.)
http://www.msnbc.msn.com/id/20055869/

Yes, there are hydrothermal cherts but they are very, very rare. Many of the cherts know are from solution deposition of from foramanifera (radiolarians, diatoms)but A COMMON source of large, blanket cherts is from volcanic eruptions.

There is another oil filed I can think of that was formed by quartz and several gas fields. The Kern River Oil Field, just outside of Bakersfield, California, is a trap formed by a volcanic ash-fall that opalized the interstitial voids in the dipping beds of quarts sand that was underneath it, forming the trap for well over 2 billlion BBLS OIL (in-place, unrecoverable oil is the majority of oil in an oil field.) This field is still producing and was discovered in 1899.

In Texas, Pinon Field in Pecos County, Texas has produced over 300 Trilon CFG. Isom Springs Oil and Gas field in Oklahoma has produce oil from cherts.

And shame, shame shame, Grasshopper, you have seen these last two fields, my maps of one them, published maps for the other and have had a lesson on them from yours truly.

Sigh! Sometime I wonder why I bother.  ::)

« Last Edit: 28/12/2008 04:21:04 by JimBob »
 

Offline susanshirleyjamaica

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Hi Simon...

earlier above, you said...

"The ophiolitic rocks sit between the Yalahs and Morant Rivers and cannot drain to the north."

You have burst my shiny bubble if what you are telling me is that my finds can not ophiolitic in origin.  What about the trilogy... chert, chalcedony and serpentine. (I hope I got that right).  Please tell me there is another way these rocks may have reached my beach?  Or do they not fall into "ophiolite" at all? 

I had extensive conversations related to this, mostly with Bass, in my previous posts "Tektites in Jamaica" and "geology of Jamaica"
 

Offline JimBob

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What is your speciality?  I have another mystery that I need solved. 


To not leave any unfinished business, the answer to the above questions is "sedimentologist." That means that any rock formed on the surface of the earth, including in the oceans, that are not of volcanic origin, are what I study and what i find oil and gas - and occasionally other things - in. Things such as radiative waste, Helium, CO2,  etc.

 

Offline frethack

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He also finds methane (CH4) in caves and crevasses.

Of course, the largest methane emitting crack he has to study is planted firmly in his computer chair.  ;D

My apologies, JimBob, if I was unclear, but I was not intending to say that chert forms mostly from sponges.  That was the topic at hand and was explaining HOW they can form from sponges.  Dont worry though, I have been properly self-flagellated and am reciting 100 "hail JimBob's" before nightfall. 

« Last Edit: 30/12/2008 20:49:24 by frethack »
 

Offline JimBob

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He also finds methane (CH4) in caves and crevasses.

Of course, the largest methane emitting crack he has to study is planted firmly in his computer chair.  ;D

My apologies, JimBob, if I was unclear, but I was not intending to say that chert forms mostly from sponges.  That was the topic at hand and was explaining HOW they can form from sponges.  Dont worry though, I have been properly self-flagellated and am reciting 100 "hail JimBob's" before nightfall. 



You are forgiven IF you have you said your Hail JimBob's while kneeling in rice grains.

 

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