Dear colleagues Good day. Can I ask if any one have ever seen such hexagonal cracks (about 1 t0 2 cm in diameter) in marine marlstones. I will be very much grateful if any reference or interpretation is provided. Regards, Sincerely yours Issa Makhlouf Prof. of Sedimentology Hashemite University Jordan
This is really strange
I think that this is basaltic pseudomorphic occurring in marlstone by unknown method
Yes, we have seen such a structure within the marle associated with Daba Marble. Ask Prof Hani Khori, Jordan University, he still have the sample
If they are present only at certain depth (few m) into the outcrop surface, they could be related to weathering effects
these look like polygonal faults often observed in marine sediments, but at a small outcrop scale. poly faults are usually observed on seismic data and finding outcrops is a bit of a struggle. yet, this could maybe be the case(?). these also seem to be bound within a specific interval (or tier), which is also part of the definition of the polygonal faults.
Although the suggestion of polygonal faults given by Davide Gamboa seems tempting. I suppose they are simply columnar joints similar to those described in Sandstones from the Island of Bute in Scotland by Young 2008.
These are joints in the rock stratum along which the rock body has been parted.
scale is the issue here for the comparison I made. not saying they are strictly the same, there could be some sort of identical process and these smaller scale features provide some insight into the large scale stuff? After all, for the columnar jointing to occur in sandstones there must be some sort of consolidation/agreggation...
Isotropic contraction cracks, in this case most probably dessication cracks.
I have seen in recent mud deposits exposed very locally along the low-lying coastal sector of the northern Nile Delta of Egypt, likely some settings which are periodically submerged by a small depth several centimeters of marine waters, and then periodically subaerially exposed almost at sea level and dry at which time I assume the sequence shrinks out enough at depth to leave cracks that may be as long (deep) as those you illustrate. Good luck with this. Jean-Daniel Stanley
attached some photos of similar columnar jointing within metamorphosed marle and marble from central jordan. The columnar jointing formed by heating and rapid cooling of the marble and marl
the reason of creation hexagonal cracks (about 1 to 2 cm in diameter) in marine marlstones is whenever compression is applied from the bottom to a hardened clay coating, fractures are created in 3 that are created by their conjugation 6-sided design and 6-point prism
Dear Dr. Reza
Thanks for your answer which is probably reasonable. Could you please send me any reference for the answer you provided regarding the mudcracks?
Issa
I can not answer the question, but I want to add some photos similar to your observation but larger in scale. This maybe can arise another question. Vertical extension of this features is around 70 m and are located 50 m below seafloor, at water depth of around 120 m. Figure one is from a depth slice, the circle like feature have diameter around 500m, The second one is extracted amplitude average of seafloor, and the feature shows hexagonal shape, around 700 m in diameter.
Do you have any vertical section across? These are likely honeycomb structures related to diagenetic fronts like Opal A/CT boundaries. In places like New Zealand, they are commonly interlayered with polygonal fault systems and are sued to result from bulk contraction of the sediment volume plus syneresis.
Article New style of honeycomb structures revealed on 3D seismic dat...
Thank you Kamal, very helpful. In the morning I find the paper you attached and I think it is just what it is saying in that paper. The data I have is just enough to see it partly, I will get more detail and update here.
These rocks are not marls, but tuff basalts that have been replaced by marl and sandstone over time.
Moslem Yazdani - what is your suggested mechanism by which tuff basalt might be replaced by marl and sandstone?
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