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Post by ammocarbsteve on Oct 30, 2012 12:54:44 GMT -5
a beautiful green Caloceras.... Attachments:
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Post by pleecan (Peter Lee) on Oct 30, 2012 14:17:02 GMT -5
Steve: Very nice specimen! Just beautiful!!
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Post by Joe Botting on Oct 31, 2012 10:21:19 GMT -5
That really is a beauty, Steve. It's amazing how often you seem to get aragonite preservation in these mudstones, for a mineral that is metastable...
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Post by ammocarbsteve on Nov 1, 2012 4:01:58 GMT -5
Thanks Guys.... Joe...It seems to be limited to the Hettangian Planorbis zone where the Psiloceras planorbis and Caloceras johnstoni come from... Conditions must of been perfect for this type of preservation... I had a wander over there yesterday at low tide after spending some time on another beach to see if I could find anymore 3D Psiloceras... No luck, but I couldnt leave this little double crushed Psiloceras... It should look very nice when its prepped although they seem to dull down more than the Caloceras...We shall see... Attachments:
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Post by Joe Botting on Nov 1, 2012 5:54:32 GMT -5
Yep, I can certainly see some potential there... ;D
So the aragonite is really restricted, then. I know someone who has been working on ammonite preservation a bit, and I'm pretty sure she said that she thinks anoxia preserves the aragonite better than aerobic conditions. Or maybe it was the other way round.
Any benthos in these beds? If so, more or less than in the adjacent biozones?
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Post by ammocarbsteve on Nov 1, 2012 8:36:18 GMT -5
'' Any benthos in these beds? If so, more or less than in the adjacent biozones? ''
Joe... I'm not sure on benthos fluctuations in and around this zone but being a shale as opposed to a limestone the organisms required to help form the limestone were obviously missing (or in short supply) so anaerobic conditions would be a pretty good assumption...it also suggests your friend is correct saying anaerobic conditions are needed for this type of preservation...if there was a good seawater column the usual calcitic preservation of the shells would of occured perhaps...
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Post by Joe Botting on Nov 1, 2012 18:38:54 GMT -5
Hmmm... 'tis a bit more complicated than that, Steve. There are many black mudstones with benthic fossils in as well; in the Mesozoic you'd expect to see things like gastropods, bivalves, ostracodes, maybe sponge spicules and crustacean remains... so it would be worth checking when you're next out collecting. On the other hand, I agree that it's most likely to be anoxic conditions that preserve aragonite, as recrystallisation to calcite seems to happen all the time in reef environments.
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Post by ammocarbsteve on Nov 2, 2012 7:41:10 GMT -5
Joe... Ive definitely noticed some quite highly populated beds of bivalves in layers around this zone and thanks for the information, I'll certainly keep my eyes peeled...please feel free to give me a geology lesson anytime, its much appreciated... Vertebrate material is very sparse in these beds I found out recently... Attachments:
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Post by reighan on Nov 3, 2012 4:43:06 GMT -5
Really nice, Steve! The green Caloceras, especially, is beautiful.
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Post by ammocarbsteve on Nov 4, 2012 4:04:36 GMT -5
reighan...Thanks... I picked this up the other day to that had been washing about in the surf... Psiloceras plicatulum... I particularily liked the specimen on the tip of the pyramid but it seemed such a shame to seperate them...Theres one that landed angular in the sediments and consequently got crushed into an eliptical shape... Attachments:
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Post by ammocarbsteve on Nov 4, 2012 4:06:00 GMT -5
Here it is finished... Attachments:
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Post by ammocarbsteve on Nov 4, 2012 4:11:12 GMT -5
I didnt do too bad really in a couple of hours hunting... I got a cracking ammonite in the limestone block... usually you find half the ammonite as the limestone is so geologically fragmented they end up in 2 or 3 chunks of rock and not always laying around... Attachments:
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Post by reighan on Nov 4, 2012 4:59:23 GMT -5
Nice. :-) You're getting me motivated again. (I'm dealing with assorted ageing issues and let them get the better of me sometimes.) There are probably no local ammonites but I've found a few cephs and luckily I'm still rather fond of corals. I did find a bit of flint echinoid amongst the predominantly Carboniferous drift, though, so who knows what else may fall out of those mudbanks. :-)
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Post by ammocarbsteve on Nov 5, 2012 4:49:48 GMT -5
reighan... I'm glad I'm getting you motivated... I used to have to travel a mnimum of 130 miles for my ammonites but now I'm 15 mins away from the coast... I need to get out looking for some upper carboniferous plant... ;D
Corals... Wow... I have an online friend who is putting a collection of corals together and who is polishing them to aid identification and to see the structures in detail... they look fabulous!...
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Post by reighan on Nov 5, 2012 5:31:55 GMT -5
Thank goodness I'm not the only one. :-) I get a lot of lithostrotions in black limestone that doesn't polish well. Repeated applications of dilute PVA worked pretty well, though.
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