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Oxford Clay - Different Biotas

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@oxford clay keith @DE&i and anyone with any thoughts on the matter :) 


I go to two sites, within a few miles of each other, with exposures of the Oxford Clay. I've been puzzling at the difference between the biotas I find:


Yaxley - Lots and lots of crinoid ossicles, vertebralis and surpula. Also plenty of gryphaea, belemnites and ammonites, the latter three dimensional and pyratised. Basically a lot of benthic critters, with some pelagic.


King's dyke - No crinoids. No serpula. Haven't found verterbralis. There is evidence of pyrite, but this is usually a dusting over delicately preserved aragonite shells.  There are gryphaea, but the benthic fauna seems less diverse.


I'm baffled. I thought one possibility could be that Yaxley represents an oyster reef, glued together with serpula tubes. This would provide a firmer base for animals such as crinoids than the usual soupy mud of this period and location. Oyster reefs exist all over the world today, still exist off the coast of Britain, and there are very rare serpulid reefs off the coast of Scotland. Gryphaea are a form of oyster, and these unlike at King's dyke, are really encrusted with fantastic serpula tubes. I thought perhaps the Yaxley exposure was in the Jurassic slightly closer to a shore and sustaining reefs.


There are some other important differences between the sites. Yaxley is an eroding former brick quarry, so relatively static, whereas at King's dyke the clay is supplied and renewed by the brick company from different layers of the Oxford Clay, and put on a site for fossil hunters. The preservation is very different, and in part that will be because the latter is fresh clay, just quarried. I could be as much looking at different layers as different locations. Both sites produce marine reptile fossils. But one seems more pelagic than benthic, the other more benthic than pelagic. Preservation bias could be playing a big role. Delicate fossils would not survive at Yaxley in the same way, leaving mostly heavily mineralised, hard fossils.


All thought very welcome.

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The pyrite can be explained by sulphate reduction in a anoxic/dysaerobic environment.

However ,the subject is complicated.

You'd have to at least examine the carbon/sulfur isotope ratios in fossils AND surrounding sediment to get to the bottom of that .

Is the pyrite framboidal,BTW?

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Thank you for these, I shall get reading. I don't know if the pyrite framboidal, and don't think my microscope is powerful enough. I shall take a look.

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