Thursday, July 9, 2009

Two polished carbonate slabs

There is no time to write great posts these days because of my continued work on my diploma mapping report. Though I can share some stuff. Today I cut and polished a number of samples to enhance the illustration of my report. I didn't manage to polish them all but a few samples are done and I want to share two of the more spectacular ones.

First, I cut and polished a sample I already discussed here. Back then I thought it to be a carbonate tempestive with an erosive base. Right now I am not that sure. Either way the cement is entirely sparitic from the shells upward. Below one can also find micritic and sparitic cement. Very few ooids are cut and truncated by shells. So maybe it is still a storm layer or a very proximal debris flow of carbonate sand. You can find a wild mixture of grains from shells, snails, foraminifera even, ooids, coated grains, aggregate grains, etc.

Event layer? The upside is up in the image.

Second is a cut sample of an oncoid bed that I also discussed earlier. These wonderful and easy to recognise oncoids show a great, irregular laminae, one as a gastropod as a nucleus and two of them are obviously bored with internal boring sediment. I am not so sure how to interpret the contacts between the oncoids. First I thought them to be slightly deformed by each other. Meaning they would have been still soft enough for that to happen. Some partial laminae seem to have broken off. But then some contacts look a little like pressure solution seams and the laminae are abrubtly truncated. The matrix is a carbonate mud with shells, snails and other stuff that I didn't bother to check out, yet.

(My excuses, I accidentally messed-up the scale. The scale in the oncoid image is suppossed to be 4 cm not 6 cm.)

Bored oncoids with snail nucleus


Suvrat Kher said...

very nice shell layers and oncoids. I was curious about the cement in your shell layers. Is there any marine cement preserved? I notice you work on the Jurassic and since my interest is in cements I remembered that low Mg calcite marine cements are not uncommon in Jurassic carbonates (Jurassic was a time of calcite seas)as opposed to the more common aragonite and high Mg calcite in Cenozoic and other times in the past.

Lockwood said...

Wow! Those are spectacular in the full-size views! Carbonates are probably my weakest area... very few in Oregon, and most are pretty well metamorphosed.

Lost Geologist said...

Suvrat, I am not sure at this point. It will not be part of my work and since there are no thin-sections it will be hard to decide. However, with the help of the handlens and with some uncertainty the cement seems to be drusy to granular or blocky. Which does not sound like a typical marine cement to me. Do you have some comparision images for those cements?

Lost Geologist said...

I forgot: My diploma thesis will most likely be about the same carbonates, just from another location. I will certainly do thin-sections and basic geochemistry then. Perhaps in 3 months I have more concrete information.

Suvrat Kher said...

lost geologist-

the interesting part of some examples of Jurassic marine cements is that they often have a granular equant fabric! not unlike meteoric cements, which makes recognition tricky. Low mg calcite tends to grow in equant fashion as against high Mg, where Mg ion poisoning leads to slow crystal growth except along the C axis and hence acicular crystals elongated along the C axis.

sorry I am travelling but I will try to dig out some photos soon.

looking forward to your results

Lost Geologist said...

that is indeed very intersting! the succession is not known for pervasive exposure (those surfaces are very limited). so it might be possible that these cements are original marine cements. the limestones are from the rhine rift valley though and cross-cut by faults. a lot of fluids moved through them.

also the upper specimen was stinking like a gasoline station while cutting with into two with some tiny bit if black goo left behind on the cut surface in some spots. which i think originated from the underside that is cut my a stylolite.

Suvrat Kher said...

sounds like an interesting diagenetic project as well!

just a follow up, that Jurassic marine equant cements have mostly been recognized associated with hardgrounds and low energy facies, where possibly rates of supply of carbonate ion to nucleation sites is slower than high energy settings.

off there may be local variations to this theme, so it will be interesting to examine those cements in more detail.

Lost Geologist said...

There are few hardground in the formation. I did not sample them because it would have been far beyond the scope of the diploma mapping project.

Now I need to formulate an outline (my profs want one) for my thesis which will be in cooperation with a large, German company. The company wants a detailed stratigraphic documentation, detailed karst features on the surface and at depth if possible, tectonics and samples for determining the degree of whiteness and impurities (Fe, Mg, Mn, Qtz, clay, Ka, Na, SO4). That pretty basic, esp. as the field stuff can be done in less than 2 weeks.

So I need to put more to the company idea. I am thinking of thin-sections for all the samples to compare microfacies to whiteness and impurities, comparision to modern depositional environment (the Bahamas might come pretty close), and a comparison of the limestone quality to my current mapping area which is only 40 to 50km away from the other location. So I can take 1 or 2 days to collec some selected samples from of quarries.

There are some color variations, maybe with some geochemistry they can be resolved, too.

But I never did the analytical part before (apart of the thin-sections). So need to find out what geochemistry but also other methods are useful, doable and feasible.

Suvrat Kher said...

good luck!

Lost Geologist said...

Thanks a lot!