Dear Michael Issigonis

Please specify the depth from ground level (G.L.). According to the following link it may be LIMESTONE:

https://en.m.wikipedia.org/wiki/Limestone

Best regards.

About 400 m below surface, but what is prominent is the alteration. I am wondering if it is an intrusion rich in calcite or silica?

At the first approximation, a sequence of events including Lithification-Fracturing-fracture fill-compaction to produce alignment of resistant grains can be envisaged.

Macroscopic and petrographic observations are necessary to narrow down the guess.

Many thanks Muthuvairavasamy Ramkumar for your contribution!

Are you implying that fracturing and fracture-filling could be due to igneous intrusion or hydrothermal fluid intrusion?

Best wishes

Considering the ellipsoidal nature of grains (which might be in all probabilities, similar to the ones in limestone in shape and appearance) in the fracture fill, intense compaction AFTER fracturing-infilling (infilling could be collapse breccia) could be presumed. Similarly, given cognizance to the intensity of insoluble material within the fracture fill, but inheritance of certain precursor characteristics, involvement of hydrothermal fluid circulation can be a hazardous, yet plausible guess.

Examination of polished slabs under reflected light, and large thin sections under polarized light, staining and examination may throw light.

All the best

Dear Mr. Issigonis, this is part of a flaser-bedded limestone and the structure may be called a stylolite in the broadest sense caused by pressure solutions. The dark gray layer represents the argillic restite of the pressure solution. There are several studies dealing with this phenomenon which have been performed by Dr. Trurnit and applied to sedimentological and mineralogical processes. Kind regards H.G.Dill

Thank you Dr. Dill and Muthuvairavasamy Ramkumar for your detailed interpretation and explanations towards these phenomena that I was not aware of before.

We encountered intense fracturing, intrusive breccia and xenocrysts(?) of olivine in altered rock higher up in the sequence and I thought the above-pictured specimen was related to these events.

Thanks again for your contribution and best wishes.

Could it not be boudinage? The trace fossils in the shale / mudtone appear to be stretched. The shale would be an incompitent lithology compared to the more cemented stratigraphy that sandwiches it.

I may be wrong, but I think this could be microbialite growth clots bounding against each other. My idea is that the fossil rich "wads" are algal mat constructs, a thrombolite, and the compressed incipient stylolite-like microbial mats between are rich in carbon, specifically bitumen from hydrocarbon degradation due to microbial activity, in which case you might find sulfur rimming any anhydrites (originally gypsum) present, and appearing as vuggy porosity after cleaning for core analysis by Dean-Stark methods. The fossils are likely floated into the algal constructs due to tidal/wave action, and the water depths during deposition should prove to be shallow (~10-15') in a likely aragonitic sea (Icehouse) at roughly the equator (low latitudes, no Coriolis effects (hurricanes, storms) to tear the algal buildups apart. Shallow water so the shoreface churning can round and roil the fossil debris back into the mats below wavebase, and shallow because algae requires sunlight and CO2, both depleting quickly in algal-rich waters.

I think the fact that the colour change between the "shale" (the dark lithology) and the carbonate (light lithology) is somewhat gradual when you zoom in and also the fact that some of the trace fossils appear to go between both lithologies (particularly at the upper interface) would indicate that this is not a pressure dissolution feature. I wouldnt be surprised, under simple binocular microscope analysis, that you'll observe that this is either a stretching feature or a compactional feature as a result of differential competence.

Thank you both Scott and Bret for your valuable interpretations.

All the distinguished contributors so far have described diagenetic processes taking place that resulted in the configuration shown in the picture above.

When our small exploration group first saw this phenomenon we thought it had something to do with pressure from the nearby intrusive breccia (we are not anywhere near specialists like sedimentologists). As Scott has said it could be either "a stretching feature or a compaction feature".

I wonder if such features can be attributed to the local intrusion or are independent of any intrusion process, because any of you have seen them before?

Is it a picture or some real rock? so that you can perform some chemical analysis.Best regards

yes a real rock cut by a rotating drill (with open steel tube) at high speed into the ground to look for mineral resources.

Yes, one can perform some test like putting a drop of hydrochloric acid on it to see if there is a chemical reaction (there was in this case). Other tests are very expensive and have to be done in a commercial lab.

Thanks for asking!

I would like to add to the discussion by saying that the interval in the center of the picture resembles very much a clay-rich interval within typical calcareous sediments deposited at the top of a sea-mount undergoing typical top of sea-mount diagenesis, resulting in vaguely nodular or mottled limestone beds interbedded with thinner stylolite-like clay-rich wavy beds due to selective limestone dissolution and differential compaction.

The famous nodular "Rosso Ammonitico" limestone of the Jurassic of the Eastern Alps is the result of a long period characterized by such "top of sea-mount" sedimentation (and diagenesis).

Going back to the pictured rock, the small pale elongated dots within the dark lithology could represent the cross section of feeding tubes filled by paler calcareous material that have been flattened less that the clay-rich matrix by the diagenetic compaction of the rock.

The darkest lamina at the centre of the dark interval would correspond to the lamina richest in clay minerals.

I note a second similar darker interval at the bottom of the picture, indicating another episode of clay-rich sedimentation. The limestone bed between the two clay-rich layers appears to be mottled by some form of biological activity, resulting in paler zones within a slightly darker micritic matrix.

I would suggest that the visible structure of the rock does not depend from nearby (later?) igneous influence, but it is the result of a period of more abundant clay sedimentation (or conversely less abundant micritic sedimentation) coupled with early diagenetic dissolution of calcareous sediment and differential compaction.

Thank you dear Roberto Bencini for your extensive contribution to the discussion.

The clay-rich sediment factor contributing to this structure is interesting and makes perfect sense. The conclusion of most contributors have explained this phenomenon taking place during diagenesis and there is no relation to a possible igneous intrusion, if there was such an event in the area.