Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: susanshirleyjamaica on 07/09/2008 20:15:52
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So sorry to submit this twice [ Invalid Attachment ] the easiest ones (chert and flint) The more I learn about those... the more I begin to learn about other beach stones.
However... A couple of years ago, while looking in the field above the cliffs for black glass remnants of 18th century bottles... I came across what appeared to be an oddly shaped blob of black glass. I figured it might some kind of primitive bottle stopper, but that didn't really make sense. So I put it up figuring someday I might get a clue.
So just yesterday when I was searching thru my new rock books... I came across something I had never heard of before... a Tektite.
Jamaica doesn't seem to be the right location for these phenomena... but my sample meets the pictures and description so very exactly that I must wonder. Can you give me any clues as to identifying this piece?
Much thanks for your kind help, [ Invalid Attachment ]
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Certainly resembles the few tektites I've seen- all of which were much smaller.
Were there any old mines, glassworks or smelters in the area where you found this? What sort of other rocks were nearby?
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Hi Bass... This object was found in a bulldozed field which extends back from the cliff at which base I am finding all the wonderful rocks. To my knowledge there were no glass works of any kind in this area. Early settlers were Spanish and then English. All bottles (and they sure left evidence of using ALOT of bottles) are European. It was found near an area where a house once stood, perhaps in the late 1600's. I'm going to see if I can add the other picture which I was having trouble with! [ Invalid Attachment ]
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Sorry, you asked what other kinds of rocks are in the area... up on the flatlands it is just limestone and small river-stone type rocks. I have never before in my life seen anything like this... until I saw it described in a book. Do you suppose a settler could have brought it along from Europe as a good luck charm... or something like that?
I'd like to show you some of the amazing rocks I have found on the beach. There are MANY breccias. Your explanation of plates grinding together would seem to be the perfect explanation for these. I am having trouble understanding the distinctions of chert, flint, smokey quartz and something that looks like quatz agate.
Thanks for your interest,
Susan
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Breccia!?
My favorite rocks. What type of breccias? Are they all the same? Any pictures?
Will post some pictures of breccias on my project thread later this week.
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I have found MANY breccias but I happened to have taken a picture of this one, my favourite, yesterday. It is like a finely banded rock brocken up and cemented in a red matrix. Has the look and feel of jasper. I am showing it both dry and wet to try to show the colours.
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Here is another breccia. Could you tell me the correct pronunciation? There are many more of different types... but I only just started collecting them.
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Nice ferruginous (iron oxide) breccia in top picture. Appears to be mostly clast supported (supported by large fragments) with a mix of clasts in an iron rich matrix. Definite rotation of the clasts- iron oxides are probably secondary (from weathering) since there is little alteration of the clasts.
Bottom picture- my guess would be karst collapse breccia. Definitely clast suppported, and given Jamaica's karst topography, would not be out of line. Does it bubble when you put acid on it (either the clasts or the matrix)?
pronunciation: http://www.merriam-webster.com/dictionary/breccia (http://www.merriam-webster.com/dictionary/breccia)
then press the pronunciation (audio) icon
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The only think I have that resembles acid is vinegar... so I dunked the designated breccia in a bath of it, but very little fizzing. Then I put another small breccia into the bath and it fizzed with gusto.
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ok... I'm having trouble putting more than one picture on a post... so this is continued below.
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Here's another pretty cool breccia... the matrix is quatz-like
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ok... this is what I think is a jasper group. The banded one looks very much like the clast material in one of the earlier pictures.
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So... speaking of banded... can you tell me anything about this? Texture is chert/quartz-like.
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ok... one more for this time. Apologies, but I am going to keep picking your brain until you get sick of me! (or until you decide to come to Jamaica and see for yourself!) :-)
This is a group that seems to contain jasper and... copper?... blue stuff?? The one on the far left is way cool with diagonal band meeting horzontal band.
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Maybe a stupid question... but how do I put more than one picture in a single post?
Thanks a WHOLE lot for the help you have been giving me. Every bit helps fill in the "big picture" for me. I'm not done with rocks, but I have some neat fossils from the cliff face and beach. Would you like to see them? I'll be very disappointed if you don't! :-)
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OK, first I am remiss in my promise, Susan, and I do apologize - I will eventually get the illustrations about the evolution of Jamaica up but see the subject for a temporary fix.
HUMM.... lets see, chert, green rocks, limestone and dolomite (the no phizz stuff) breccias, and possibly some of the green being originally a basalt-related rock, serpintine - sound suspiciously like the grand trinity of ophiolites. No, not ophiolitism but ophiolites.
the trinity - serpentine, pillow lava, and chert ("Steinmann's trinity") http://en.wikipedia.org/wiki/Ophiolite
I will venture a guess you live in eastern Jamaica. Why?, you may well ask.
An abstract of an article I am too cheap to purchase.
The ophiolitic Bath-Dunrobin Formation, Jamaica: significance for Cretaceous plate margin evolution in the north-western Caribbean
G. Wadge, T. A. Jackson, M. C. Isaacs and T. E. Smith
Two neighbouring areas of tholeiitic basalts, dolerites, isotropic gabbros and tonalites in eastern Jamaica are recognized as the upper section (~2.5 km thick) of an ophiolite. Limestones associated with this Bath-Dunrobin Formation contain a fauna of Upper Campanian and Lower Maestrichtian age. The basalts are overlain by a thick sequence of Maestrichtian volcaniclastic rocks and are faulted against a blueschist terrain. Since its emplacement the ophiolite has been subjected to uplift and major left-lateral strike-slip tectonism beginning in the Eocene. Recognition of this subduction zone complex and regional considerations of island arc polarity necessitates that ocean crust must have been consumed by a southward-dipping subduction zone. The overlying late Cretaceous/Palaeogene arc stretched from Guatemala through Jamaica to Oriente Province, Cuba. This arc separated the Yucatan Basin to the N, where active spreading accompanied subduction, from the Colombian Basin and the southern Nicaraguan Rise to the SE, where tholeiitic magmatism (B") penetrated older ocean crust. The Bath-Dunrobin Formation probably represents ocean crust from the Yucatan Basin obducted during the Maestrichtian. The major ophiolite terrains of Guatemala and Oriente, Cuba are by inference also derived from a proto-Yucatan Basin.
So now you know. If you do not live in Eastern Jamaica move there and don't make me look like a fool (again)!
Lastly, I doubt that it is a tektite BUT I am not sure.
Ok, I'll try to log on to PhotoBucket again to upload the fix for the other subject.
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Hi JimBob,
I still think it might very well be a tektite (I am so embarrassed to have spelled it incorrectly in my first post). I did some research and found that a tektite field originating from Cuba includes Jamaica. Also... this field seems to have heavier (larger?) tektites than others. Could be?
I need to digest the info you just sent then will get back to you. Yes, I live on the northeast coast of Jamaica. There is a cliff face right below us which is providing fossils and though I don't know where they originated, the beach is littered with many different and beautiful rock types.
Can I ask you about fossils? I have many whorls, both large and small... and I am trying to figure out what kind of shell they are from.
Thanks,
Susan
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Jamaica is a
stones throw tektites throw from Chicxulub (http://en.wikipedia.org/wiki/Chicxulub_crater)
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Oh! Thank you for that link! And JimBob... thanks for introducing me to the concept of the of ophiolite and the "trinity". I am starting to "get it"!
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Your "tektite" looks like the concretion (http://en.wikipedia.org/wiki/Concretion) in the green rectangle ...
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http://web.utah.edu/unews/releases/04/jun/marsmarbles.html
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Yes, that is it! Thanks. Now... is it unusual for such a band of ophialite to be exposed? Am I especially blessed to have such a variety on one rock beach?
So here are three more I picked up yesterday evening (my husband is beginning to think I am nuts!). The one on the left appears to have volcanic rock embedded in a sort of quartz. Next is just another breccia in paradise. On the right is an amazing rock with pink speckles and exposed quartz crystals.
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So now I am going to bring out my fossils. :-)
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The urchin was found on the beach and the other two were dug (very carefully) out of newly exposed limestone cliff face. Can you help me identify and date the two shells? I know one is a scallop of some sort... and the other is some sort of huge snail ? or something?
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and one more question below.
Thanks!
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I am fascinated by the little curly things I have been collecting... I have tentatively identified them as whorls. But could the big doughnut one be a whorl? or something else? Many, MANY beach rocks have holes in them, like the one on far right... but don't seem to be whorls. I'd really like to find out what animal these shell whorls might have come from... also, what could all the other holes be?
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As usual... thanks so much for all the info...
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This site may be able to ID your fossil shells ...
If you have a shell or a shell fossil at home and you just can't place an identification label to it, or is your snail doing something and you're not to sure just what, here's where you might get some help. I am but an amateur collector myself, but there are many good conchologists and malacologists that may be willing to help you.
Send your photos and collection data, if you have any, to me, Avril Bourquin . I will set up a file page for your shell or fossil shell then people can contact you personally in regards to their idea as to your shells proper identification. Please try to keep your jpg files small.
This kind of service is also available from some other GREAT shell collectors and dealers on their web sites. You may wish to seek out their assistance as well. Contact information forsome of these sites and people is located at the very bottom of this page.
Good luck in getting that definitive identification on your treasured shell or shell fossil.
http://www.manandmollusc.net/id_help.html
PS
some of your holey finds look a bit like vertebrae, the holes being where the spinal cord passes.
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Vertebrae? Really? That would be really neat... but I haven't found any other "body parts".
I will try that site, Thanks!
Susan
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The rock on the left is a Septarian nodule (http://en.wikipedia.org/wiki/Concretion#Septarian_concretions)
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Your find on the right looks like it formed under similar conditions.
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oh for a lapidary saw! This is really the first rock that I've really wanted to look inside!
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On the left are bits of casts made from the interior of ammonite (http://en.wikipedia.org/wiki/Ammonite) type shells, (i.e. shell is mould).
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your similar find is on the right
[Your smooth "whorl" finds are consistent with being casts of the entire interior of helical shells]
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The pink rock with the quarts looking vein running down the middle of it is a textbook example of contact metamorphism. The isolate pink piece in the middle with the white rind around it is typical of this intrusive type of contact metamorphism. See Bass's post for another example. Both of these are textbook examples of contact metamorphism - don't be thrown off by rounded nature of this particular specimen.
Bass's Post - http://www.thenakedscientists.com/forum/index.php?topic=16816.msg193185#msg193185
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Hi JimBob,
I really am beginning to understand why there are so many beautiful and diverse rocks on our beach. You said... don't be put off by the rounded nature of my rock. How might it (and almost every other rock on the beach) become rounded? Is it river abrasion? Or wave action rolling them in and out of the sea? Did they break off a rock face somewhere in the mountains? Our river, the Rio Nuevo, has a wide flood basin and often goes into spate... renewing beachcoming opportunities about every two months or so.
Below is a picture of cliff face and beach...
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This is a picture of the river mouth. It changes often.
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"By Jove, I think she's got it!"
------- Pygmalion
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Ya... I really am getting it... at least enough so that when I walk along the beach I can identify (in a very broad way) many of the rocks that had me completely perplexed not so long ago. Most helpful has been gaining a better understanding of how metamorphism works, and the very many different ways that it does work.
Thanks a whole lot... all you guys!
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Susan, I like this breccia
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Note that the clasts are not rounded, and how the layers, even though fragmented, are still continuous (black fragments in oval). Also very little in the way of reaction rims around the fragments.
My interpretation- this rock was fractured (broken apart) and fluids deposited quartz in the open spaces. Probably happened numerous times since the breccia appears to be matrix supported. While breaking apart, the two sides did not otherwise move relative to each other- if they had, the dark layer of fragments would have been disrupted.
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I also like that Breccia... I'm fascinated with quart matrix!
Actually, my favourite rock, at this time anyway, is the large... breccia (?) with crystalline matrix. It actually has in it broken pieces of what looks to me like volcanic rock, almost black and bubble pitted. There are some lighter volcanic pieces as well. They are obviously broken.
Am I correct in assuming the left hand piece is volcanic? It is palm size and quite light.
The dark smaller rock I think is basalt? The piece on the far right appears to be several large, amber coloured and fairly transparent crystals jammed up together. The detail doesn't really show its transparent nature.
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large rock may be flow breccia (chunks break off as the lava flows) that was cemented by later minerals. Hard to tell from pictures, but if the lower left rock is light and full of holes, as you describe, probably volcanic as well. Center looks like black rock- could be basalt. Can you see any crystals in it?
Las tork looks like some sort of jasper or chert. Is it harder than a knife?
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Yes, that is it! Thanks. Now... is it unusual for such a band of ophialite to be exposed? Am I especially blessed to have such a variety on one rock beach?
So here are three more I picked up yesterday evening (my husband is beginning to think I am nuts!). The one on the left appears to have volcanic rock embedded in a sort of quartz. Next is just another breccia in paradise. On the right is an amazing rock with pink speckles and exposed quartz crystals.
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So now I am going to bring out my fossils. :-)
Bottom breccia is definitely clast supported, perhaps jasper?
Love the cockscomb quartz in the "pink" rock. Obviously open spaces between the fragments allowed large quartz crystals to grow radially around each one- then a thinner band of darker (green?) minerals. Are the clasts corroded at all (replaced by the quartz)? Or do the clast boundaries well defined?
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Hi Bass,
I didn't think about the "pink rock" as having clasts... but I guess on close look, maybe it does. If so, the clasts are not distinct and their edges not well defined. The dark lines around the quartz crystals is black and has some very small jet black crystals, which I didn't notice before. Maybe these "other view" pictures might help.
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View of the other side
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The dark rock that I thought might be basalt has no visible crystals in it... in fact is has very finely scattered pitting.
In the "flow breccia" the clasts are well defined. There may be other small rock types in it, but the darker ones, as well as the larger lighter ones, seem most certainly volcanic, as far as I can tell anyway. Here is a close up.
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One more... the rock that I thought was crystals and you asked if it was jasper or chert-like. Maybe chert, but looks different. Here's two more details. The second detail show it with the other rock in my collection which it most resembles.. But the texture is different... maybe it is chert, if chert can be transparent? A knife will not scratch it unless I press real hard. I'm not sure I am doing the test right.
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Looks like chalcedony or opal. With all this quartz/silica, and mineral deposits nearby?
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shucks... the window doesn't want to accept my picture... even though it is only 83kb!
I'll try again later.
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My guess is quartz-chalcedony-opal; conchoidal fracture is a sure sign. Calcite scratches easily with a knife, quartz does not.
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Cool.... That sounds good to me. But is it the same sort of opal that they make semi-precious jewelry stones from?
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Chalcedony, agate, chert (also flint) and jasper are all varieties of cryptocrystalline (extremely fine-grained) quartz. Even though these are basically the same, I think of agate as banded, jasper as colored by impurities, and chert as forming by diagenetic processes. Opal is amourphous silica (a bit of water messes up the structure). There is opal, and there is precious opal- minor structural differences make the difference.
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I'm guessing these are chalcedony... can I get away with calling them Jamaican Opal?
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On to a completely different rock... The large one looks like a blobby conglomerate or nodule seems to be cemented with limestone. Forgot to put scale, it is about 8" across. The two small ones are different sides of the same palm rock that looks like two pieces of plasticine mushed together.
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Is the blue color partly due to the light? Or is the rock actually blue in color. It looks like a clastic mud-crack rip-up then redeposited rock but that doesn't fit with the rest of the beach.
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I thought it was pretty strange as well... I've never before seen anything like it. The clasts are dark gray with a suggestion of blue to blue-green. The matrix is yellowish limestone (the larger areas of matrix do not fizz, but the very fine cracks do) that sure looks like it was once mud! Tell me how these are usually formed and maybe it will give me a clue.
It has a cast hole that looks as if it might have held a fossil.
Blob-Rock 1
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Blob-Rock 2
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Blob-Rock 3
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Has anyone heard from SusanShirleyJamaic lately? I am fortunate enough to be stopping in Jamaica next month and need a local "Hook Up" for some rock hounding.
Tom