23 July, 2009

Oncolite Rudstone from the "Mumienbank"

I am not getting ahead fast with my diploma mapping report and it keeps being a pain in the...well, you can imagine - a lot of headache. So there isn't much around that I have time or nervs to make some serious posting about. For your entertainment I decided to share another polished slab I made that will be part of my report. It is a piece of the "Mumienbank" - an oncolitic Rudstone from the Jurassic limestones that I am working on. You can observe some lovely oncolites, nucleus and laminae intact, bored and re-filled, some deformation on the grain contacts that are partially filled with some iron minerals (the black stuff) - guess haematite or stuff. Didn't yet have time to look at it with a binocular or microscope but the black stuff is actually quite shiny in the sun.

Oncolite Rudstone of the "Mumienbank"

10 July, 2009

Flowstone, cut and polished

Another of the samples I cut and polished: This flowstone originates from an exposed karst cavity. The sample is taken from the floor of the former cave that is now infilled with flowstone and other sediments. These deposits form where water flows over rock walls or floors. Looks pretty cool, no?

Flowstone deposit. Upside is down.

09 July, 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