I'm having trouble identifying the round bioclasts indicated by white arrows. Could they be transverse sections of the elements indicated by black arrows? If so, could they be some kind of spicule?
Hi Alvaro, I would say that they look like calcispheres and some of them could be forams chambers (e.g. lower left of your thin section image). Calcispheres are common in carbonate rocks from cretaceous of middle east and yours have similar features. Please find those examples from Iraq which could be helpful.
Article Petrography and source rock potential of Chia Gara Formation...
Article Facies analysis and sequence stratigraphy of Kometan Formati...
Article Oligosteginid assemblages of basinal limestone succession in...
https://dergipark.org.tr/tr/download/article-file/442349
The black ones and white ones look different, former are hollow whereas the later ones are solid.
I am not sure, but thinking the white ones could be calcispheres (Calcareous dinoflagellate cysts)..
The black arrow are most likely irregular echinoids spines. They are thin and hollow inside. Try to see if they have a regular extinction with crossed nichols, if they have regular extinction (like a single crystal) then they are echinoids.
They white one seem different. Maybe different cuts of planktic foraminifera. For example the couple of white arrows in the bottom left corner look like planktic foraminifera to me
I would follow Mike on that, with white arrows being sponge spicules. There is a kind of triradiate one in the upper left quarter of the image. 2 of the black one could also be sponge spicule with pyrite/oxides in the central zone.
WHite arrows: i seem to see a thin rim around some of these bioclasts ? If so then I would personnally disqualify the spicules choice. Planctonic foram?
Black arrows: Indeed often echinoids display Fe-Oxids. However, the structure doesnt seem monocristalline to me from the picture. Xnichols should be diagnostic. If not monocristalline, I would say some green algaes?
Hi. Firstly I'd like to thank everybody for their kind answers.
Some additional context: the sample was taken from an Aptian carbonate ramp succession from the Maestrat Basin (Spain). It's a mudstone-wackestone level located at the base of a series of cm-thick tempestite levels containing abundant crinoid and bivalve debris (see attached log).
Yes, Christian Perrin and Mike Simmons
,the bioclasts signaled by white arrows do have a micritic rim. They lack the flower-like internal structure of regular echinoid spines, but they do indeed appear to have monocrystalline extinction, i don't know how I overlooked that.The black arrow ones have an opaque (Fe-oxide?) filling. Other sections (not pictured) appear as long tubes with thin microcrystalline walls and a micrite matrix filling
Considering all of this, the black arrow bioclasts could be different cuts of irregular echinoid spines as Giovanni Coletti suggested, and possibly the white arrow ones too. Please let me know what you think.
Aptian? Then I would say it looks like Hensonella dinarica for the black arrows.
As observed by the yellowish radial calcite (rather unusual for algae unless problematicum)
But I reckon it would be more for Mike to answer that:
Article Hensonella dinarica, an originally calcitic early Cretaceous...
My guess, though I am 6000 km away from your outcrop...
Hi Mike, there's a scale bar in the corner of the pic, it's just cropped out in the preview.
And I'll be happy to post some additional photos.
Hi Alvaro, I would say that they look like calcispheres and some of them could be forams chambers (e.g. lower left of your thin section image). Calcispheres are common in carbonate rocks from cretaceous of middle east and yours have similar features. Please find those examples from Iraq which could be helpful.
Article Petrography and source rock potential of Chia Gara Formation...
Article Facies analysis and sequence stratigraphy of Kometan Formati...
Article Oligosteginid assemblages of basinal limestone succession in...
https://dergipark.org.tr/tr/download/article-file/442349
Thanks, Omar. Yes, there could definitely be a mix of different bioclasts. This is proving to be a tricky topic!
Hi Alvaro,
just two further comments:
1. obviously no one noticed the calcitic aggregates with rounded micrite-filled "pores" (just below the center). Magnification is too small, but they remind the form-genera Halycoryne Misik, 1987 or Patruliuspora Barattolo et al., 2019 (= gametophores of green algae).
2. definitely no Salpingoporella dinarica in your image!
Hi everybody. I'm posting some more pictures of the sample in question.
The first one is a scan of the whole thin section.
Photo 1 is the same area depicted in my original post, but at a lesser magnification.
Photo 2 is a new area of the sample at the same magnification as Photo 1.
Photo 3 is the same area as photo 2 at the same magnification as the original picture.
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